MACROCYCLIZATION REACTIONS AND INTERMEDIATES AND OTHER FRAGMENTS USEFUL IN THE SYNTHESIS OF HALICHONDRIN MACROLIDES

Background

The invention relates to intermediates useful in the synthesis of pharmaceutically active macrolide compounds and methods of synthesizing macrolide compounds. Halichondrin B is a potent anticancer agent originally isolated from the marine sponge Halichondria okadai, and subsequently found in Axinella sp., Phakellia carteri, and Lissodendoryx sp. A total synthesis of halichondrin B was published in 1 992 (Aicher, T. D. et al., J. Am. Chem. Soc. 1 14:31 62-3164). Further synthetic and structure-activity relationship studies have been discloses in U.S. Patent Nos. 5,338,865 and 5,436,238 and in Towle et al., Annual Meeting of the American Association for Cancer Research, April 6-1 0, 2002, 5721 and Wang et al., Bioorg. Med. Chem. Lett, 10:1029-1032, 2000. Methods and intermediates for the synthesis of certain halichondrin B analogs and intermediates are described in International Publication Nos. WO 2005/1 1 8565, WO 2009/046308, WO 2009/064029, and WO 2009/124237; U.S. Patent No. 6,214,865; Austad et al., Synlett 24(3):333-337, 2013; Austad et al., Synlett. 24(3):327-332, 2013; and Chase et al., Synlett 24(3):323-326, 2013. New methods for the synthesis of halichondrin B and its analogs (e.g., macrolide analogs) are desirable.

Summary of the Invention

In general, the present invention provides methods for macrocyclization of intermediates in the synthesis of a halichondrin macrolide. The invention also provides intermediates that can be employed in the macrocyclization reactions described herein.

In a first aspect, the invention provides a method of preparing a macrocyclic intermediate in the synthesis of a halichondrin macrolide, the method involving performing a macrocyclization reaction on a non- macrocyclic intermediate, the macrocyclization reaction producing the macrocyclic intermediate by forming C.2-C.3, C.3-C.4, C.12-C.13, C.15-C.16, C.1 9-C.20, or C.26-C.27 bond in the structure of the halichondrin macrolide.

In some embodiments of the first aspect, performing the macrocyclization reaction involves contacting the non-macrocyclic intermediate (e.g., a compound of formula (IA)) with an organic base (e.g., DBU or triethylamine) and a Lewis acid (e.g., a salt of Li or Zn). The non-macrocyclic intermediate can be a compound of formula (IA):

or a salt or a tautomer thereof,

where

each R is independently optionally substituted alkyl or optionally substituted aryl;

each of D and D' is independently H, optionally substituted alkyl, or OPi , provided that only one of D and D' is OPi , where Pi is H, alkyl, or a hydroxyl protecting group; A is a group of formula (1 ) or a C1-6 saturated or C2-6 unsaturated hydrocarbon skeleton, the skeleton being unsubstituted or having from 1 to 10 substituents independently selected from the group consisting of cyano, halo, azido, oxo, and Qi , the group of formula (1 ) having the structure:

(1 ),

where

L is -(CH(OP ₂ ))-, -(C(OH)(OP ₂ ))-, or -C(O)-;

Ri is H, or Ri and Pi combine to form a bond;

R2 is H or -(CH2)nOP3, and each of P2 and P3 is independently H, optionally substituted alkyl, or a hydroxyl protecting group, or P2 and P3, together with the atoms to which each is attached, combine to form a ketal, a cyclic carbonate, a dicarbonyl-dioxo, or silylene- dioxo; or R2 and P2 combine to form an optionally substituted ethylene or a structure selected from the group consisting of:

, where each P' is independently H or a hydroxyl protecting group;

E is optionally substituted alkyl or optionally substituted alkoxy;

G is O, S, CH2, or NRN, where RN is H, an N-protecting group, or optionally substituted alkyl ; each Qi is independently ORA, SRA, S0 ₂ RA, OS0 ₂ RA, NRBRA, NRB(CO)RA, NRB(CO)(CO)RA, NRA(CO)NRBRA, NRB(CO)ORA, (CO)ORA, 0(CO)RA, (CO)NRBRA, or 0(CO)NRBRA, where each of RA and RB IS independently H, alkyl, haloalkyi, hydroxyalkyi , aminoalkyi , aryl , haloaryl, hydroxyaryl, alkoxyaryl, arylalkyi, alkylaryl, haloarylalkyi, alkylhaloaryl , (alkoxyaryl)alkyl, heterocyclic radical, or heterocyclic radical-alkyl ;

k is 0 or 1 ;

n is O, 1 , or 2;

Ai and R4 combine to form oxo, R3 is H or a hydroxyl protecting group, and R5 is H ; or

Ai is H or OP", where P" is H or a hydroxyl protecting group, and:

(i) R3 is H or a hydroxyl protecting group, and R4 and R5 combine to form a double bond;

or

(ii) R3 and R4 combine to form a bond, and R5 is H or OP";

each P4 is independently H or a hydroxyl protecting group, and Xi is oxo or Xi , together with the carbon atom to which it is attached, is -(CH(OPY))-, wherein Ργ is H or a hydroxyl protecting group; or both P4 groups and Xi , together with the atoms to which each is attached, combine to form ketal ; and

P5 is H or a hydroxyl protecting group; and

the macrocyclic intermediate in the synthesis of a halichondrin macrolide is a compound of formula (IB):

(IB)

or a salt or a tautomer thereof. In formula (IA) or (IB), each R can be optionally substituted C1-6 alkyl. In formula (IA) or (IB), both P4 groups and Xi , together with the atoms to which each is attached, can combine to form ketal. In formula (IA) or (IB), Xi , together with the carbon atom to which it is attached, can be -(CH(OPY))-, where PY is H or a hydroxyl protecting group. In formula (IA) or (IB), P5 can be a hydroxyl protecting group. In formula (IA) or (IB), R3 and R4 can combine to form a bond, and R5 can be H.

In particular embodiments of the first aspect, performing the macrocyclization reaction involves contacting the non-macrocyclic intermediate (e.g., a compound of formula (IIA), formula (MIA), formula (IVA), or formula (VA)) with an olefin metathesis catalyst (e.g., a ruthenium-carbene complex).

The non-macrocyclic intermediate can be a compound of formula (IIA):

(IIA)

or a salt or a tautomer thereof,

where

each of D and D' is independently H, optionally substituted alkyl, or OP1 , provided that only one of D and D' is OP1 , where Pi is H, alkyl, or a hydroxyl protecting group; A is a group of formula (1 ) or a C1-6 saturated or C2-6 unsaturated hydrocarbon skeleton, the skeleton being unsubstituted or having from 1 to 1 0 substituents independently selected from the group consisting of cyano, halo, azido, oxo, and Qi , the group of formula (1 ) having the structure:

(1 ),

where

L is -(CH(OP ₂ ))-, -(C(OH)(OP ₂ ))-, or -C(O)-;

Ri is H, or Ri and Pi combine to form a bond;

R2 is H or -(CH2)nOP3, and each of P2 and P3 is independently H, optionally substituted alkyl, or a hydroxyl protecting group, or P2 and P3, together with the atoms to which each is attached, combine to form a ketal, a cyclic carbonate, a dicarbonyl-dioxo, or silylene- dioxo; or R2 and P2 combine to form an optionally substituted ethylene or a structure selected from the group consisting of:

, where each P' is independently H or a hydroxyl protecting group;

E is optionally substituted alkyl or optionally substituted alkoxy;

G is O, S, CH2, or NRN, where RN is H, an N-protecting group, or optionally substituted alkyl ; each Qi is independently ORA, SRA, S0 ₂ RA, OS0 ₂ RA, NRBRA, NRB(CO)RA, NRB(CO)(CO)RA, NRA(CO)NRBRA, NRB(CO)ORA, (CO)ORA, 0(CO)RA, (CO)NRBRA, or 0(CO)NRBRA, where each of RA and RB IS independently H, alkyl, haloalkyi, hydroxyalkyi , aminoalkyi , aryl , haloaryl, hydroxyaryl, alkoxyaryl, arylalkyi, alkylaryl, haloarylalkyi, alkylhaloaryl , (alkoxyaryl)alkyl, heterocyclic radical, or heterocyclic radical-alkyl ;

k is 0 or 1 ;

n is O, 1 , or 2;

Ai and R4 combine to form oxo, R3 is H or a hydroxyl protecting group, and R5 is H ; or

Ai is H or OP", where P" is H or a hydroxyl protecting group, and:

(i) R3 is H or a hydroxyl protecting group, and R4 and R5 combine to form a double bond;

or

(ii) R3 and R4 combine to form a bond, and R5 is H or OP";

each P4 is independently H or a hydroxyl protecting group, and Xi is oxo or Xi , together with the carbon atom to which it is attached, is -(CH(OPY))-, wherein Ργ is H or a hydroxyl protecting group; or both P4 groups and Xi , together with the atoms to which each is attached, combine to form ketal ; and

P5 is H or a hydroxyl protecting group; and

The non-macrocyclic intermediate can be a compound of formula (MIA) :

or a salt or a tautomer thereof,

where

each of D and D' is independently H, optionally substituted alkyl, or OPi , provided that only one of D and D' is OPi , where Pi is H, alkyl, or a hydroxyl protecting group; A is a group of formula (1 ) or a C1-6 saturated or C2-6 unsaturated hydrocarbon skeleton, the skeleton being unsubstituted or having from 1 to 10 substituents independently selected from the group consisting of cyano, halo, azido, oxo, and Qi , the group of formula (1 ) having the structure:

(1 ),

where

L is -(CH(OP ₂ ))-, -(C(OH)(OP ₂ ))-, or -C(O)-;

Ri is H, or Ri and Pi combine to form a bond;

R2 is H or -(CH2)nOP3, and each of P2 and P3 is independently H, optionally substituted alkyl, or a hydroxyl protecting group, or P2 and P3, together with the atoms to which each is attached, combine to form a ketal, a cyclic carbonate, a dicarbonyl-dioxo, or silylene- dioxo; or R2 and P2 combine to form an optionally substituted ethylene or a structure selected from the group consisting of:

group;

E is optionally substituted alkyl or optionally substituted alkoxy;

G is O, S, CH2, or NRN, where RN is H, an N-protecting group, or optionally substituted alkyl ; each Qi is independently ORA, SRA, S0 ₂ RA, OS0 ₂ RA, NRBRA, NRB(CO)RA, NRB(CO)(CO)RA, NRA(CO)NRBRA, NRB(CO)ORA, (CO)ORA, 0(CO)RA, (CO)NRBRA, or 0(CO)NRBRA, where each of RA and RB IS independently H, alkyl, haloalkyi, hydroxyalkyi , aminoalkyi , aryl , haloaryl, hydroxyaryl, alkoxyaryl, arylalkyi, alkylaryl, haloarylalkyi, alkylhaloaryl , (alkoxyaryl)alkyl, heterocyclic radical, or heterocyclic radical-alkyl ;

k is 0 or 1 ;

n is O, 1 , or 2;

Ai and R4 combine to form oxo, R3 is H or a hydroxyl protecting group, and R5 is H ; or

Ai is H or OP", where P" is H or a hydroxyl protecting group, and:

(i) R3 is H or a hydroxyl protecting group, and R4 and R5 combine to form a double bond;

or

(ii) R3 and R4 combine to form a bond, and R5 is H or OP";

each P4 is independently H or a hydroxyl protecting group, and Xi is oxo or Xi , together with the carbon atom to which it is attached, is -(CH(OPY))-, wherein Ργ is H or a hydroxyl protecting group; or both P4 groups and Xi , together with the atoms to which each is attached, combine to form ketal ; and P5 is H or a hydroxyl protecting group; and where the macrocyclic intermediate in the synthesis of a halichondrin macrolide is a compound of formula (1MB):

(1MB)

or a salt or a tautomer thereof.

In formula (IIA), (MB), (MIA), or (1MB), both P4 groups and Xi , together with the atoms to which each is attached, can combine to form ketal. In formula (IIA), (MB), (IMA), or (1MB), P5 can be a hydroxyl protecting group. In formula (IIA), (MB), (MIA), or (1MB), Xi , together with the carbon atom to which it is attached, can be -(CH(OPY))-, where PY is H. In formula (IIA), (MB), (MIA), or (1MB), Xi can be oxo. the non-macrocyclic intermediate is a compound of formula (IVA):

or a salt thereof,

where

each of D and D' is independently H, optionally substituted alkyl, or OP1 , provided that only one of D and D' is OP1 , where Pi is H, alkyl, or a hydroxyl protecting group; A is a group of formula (1 ) or a C1-6 saturated or C2-6 unsaturated hydrocarbon skeleton, the skeleton being unsubstituted or having from 1 to 10 substituents independently selected from the group consisting of cyano, halo, azido, oxo, and Qi , the group of formula (1 ) having the structure:

(1 ),

where

L is -(CH(OP ₂ ))-, -(C(OH)(OP ₂ ))-, or -C(O)-; Ri is H, or Ri and Pi combine to form a bond;

R2 is H or -(CH2)nOP3, and each of P2 and P3 is independently H, optionally substituted alkyl, or a hydroxyl protecting group, or P2 and P3, together with the atoms to which each is attached, combine to form a ketal, a cyclic carbonate, a dicarbonyl-dioxo, or silylene- dioxo; or R2 and P2 combine to form an optionally substituted ethylene or a structure selected from the group consisting of:

group;

E is optionally substituted alkyl or optionally substituted alkoxy;

G is O, S, CH2, or NRN, where RN is H, an N-protecting group, or optionally substituted alkyl; each Qi is independently ORA, SRA, S0 ₂ RA, OS0 ₂ RA, NRBRA, NRB(CO)RA, NRB(CO)(CO)RA, NRA(CO)NRBRA, NRB(CO)ORA, (CO)ORA, 0(CO)RA, (CO)NRBRA, or 0(CO)NRBRA, where each of RA and RB IS independently H, alkyl, haloalkyi, hydroxyalkyi, aminoalkyi, aryl, haloaryl, hydroxyaryl, alkoxyaryl, arylalkyi, alkylaryl, haloarylalkyi, alkylhaloaryl, (alkoxyaryl)alkyl, heterocyclic radical, or heterocyclic radical-alkyl;

k is 0 or 1 ;

n is O, 1 , or 2;

0) R3 is H or a hydroxyl protecting group, R4 is alkyl ether, and R5 is H ;

(ϋ) R3 is H or a hydroxyl protecting group, and R4 and R5 combine to form a double bond; or

(iii) R3 and R4 combine to form a bond, and R5 is H;

each P4 and P5 is independently H or a hydroxyl protecting group;

R6 is H, and Ρβ is H or a hydroxyl protecting group; or R6 and Ρβ combine to form a double bond; and

In formula (IVA) or (IVB), at least one of P ₄ and P ₅ can be H. In formula (IVA) or (IVB), R ₆ and P ₆ can combine to form a double bond.

The non-macrocyclic intermediate can be a compound of formula (VA) :

(VA)

or a salt or a tautomer thereof,

where

each of D and D' is independently H, optionally substituted alkyl, or OPi , provided that only one of D and D' is OPi , where Pi is H, alkyl, or a hydroxyl protecting group; A is a group of formula (1 ) or a C1-6 saturated or C2-6 unsaturated hydrocarbon skeleton , the skeleton being unsubstituted or having from 1 to 10 substituents independently selected from the group consisting of cyano, halo, azido, oxo, and Qi , the group of formula (1 ) having the structure :

(1 ),

where

L is -(CH(OP ₂ ))-, -(C(OH)(OP ₂ ))-, or -C(O)-;

Ri is H, or Ri and Pi combine to form a bond;

R2 is H or -(CH2)nOP3, and each of P2 and P3 is independently H, optionally substituted alkyl, or a hydroxyl protecting group, or P2 and P3, together with the atoms to which each is attached, combine to form a ketal, a cyclic carbonate, a dicarbonyl-dioxo, or silylene- dioxo; or R2 and P2 combine to form an optionally substituted ethylene or a structure selected from the group consisting of:

group;

E is optionally substituted alkyl or optionally substituted alkoxy;

G is O, S, CH2, or NRN, where RN is H, an N-protecting group, or optionally substituted alkyl; each Qi is independently ORA, SRA, S0 ₂ RA, OS0 ₂ RA, NRBRA, NRB(CO)RA, NRB(CO)(CO)RA, NRA(CO)NRBRA, NRB(CO)ORA, (CO)ORA, 0(CO)RA, (CO)NRBRA, or 0(CO)NRBRA, where each of RA and RB IS independently H, alkyl, haloalkyi, hydroxyalkyi, aminoalkyi, aryl, haloaryl, hydroxyaryl, alkoxyaryl, arylalkyi, alkylaryl, haloarylalkyi, alkylhaloaryl, (alkoxyaryl)alkyl, heterocyclic radical, or heterocyclic radical-alkyl;

k is 0 or 1 ;

n is O, 1 , or 2;

(a1 ) R3 is H or a hydroxyl protecting group, R4 is alkyl ether, and R5 is H ;

(a2) R3 is H or a hydroxyl protecting group, and R4 and R5 combine to form a double bond; or

(a3) R3 and R4 combine to form a bond, and R5 is H;

(b1 ) Ai and R7 combine to form oxo, R6 is H or a hydroxyl protecting group, and Rs is H ; or

(b2) Ai is H or OP", where P" is H or a hydroxyl protecting group, and:

(i) R6 is H or a hydroxyl protecting group, and R7 and Rs combine to form a double bond;

or

(ii) R6 and R7 combine to form a bond, and Rs is H or OP";

(c1 ) R9 is H, and P4 is H or a hydroxyl protecting group; or

(c2) R9 and P4 combine to form a double bond;

R10 is H or -CH2Xi CH ₂ CH=CH2, where Xi is O, -C(Rn ) ₂ - or NP ₆ , and where each Rn is independently H or -COORi2, Ρβ is an /V-protecting group, and R12 is alkyl;

each P5 is independently H or a hydroxyl protecting group; and

where the macrocyclic intermediate in the synthesis of a halichondrin macrolide is a compound of formula (VB):

(VB)

or a salt or a tautomer thereof.

In formula (VA) or (VB), R9 can be H, and P4 can be H. In formula (VA) or (VB), R10 can be - CH2Xi CH2CH=CH2, and Xi can be O. In formula (VA) or (VB), R6 and R7 can combine to form a bond, and R8 can be H. In formula (VA) or (VB), each P5 can be independently a hydroxyl protecting group. In formula (VA) or (VB), at least one P5 can be H. In formula (VA) or (VB), R3 and R4 can combine to form a bond, and R5 can be H.

In certain embodiments of the first aspect, performing the macrocyclization reaction involves contacting the non-macrocyclic intermediate (e.g., a compound of formula (VIA) or (VIIA)) with a Cr(ll) salt and a Ni(ll) salt.

The non-macrocyclic intermediate can be a compound of formula (VIA):

where

Y is iodide, bromide, or trifluoromethanesulfonate;

b designates (/^-stereogenic center, and Z is a sulfonate, chloride, bromide, or iodide; or b designates (S)-stereogenic center, and Z is ΟΙ¾, where R6 is a hydroxyl protecting group;

each of D and D' is independently H, optionally substituted alkyl, or OPi , provided that only one of D and D' is OPi , where Pi is H, alkyl, or a hydroxyl protecting group; A is a group of formula (1 ) or a C1-6 saturated or C2-6 unsaturated hydrocarbon skeleton , the skeleton being unsubstituted or having from 1 to 10 substituents independently selected from the group consisting of cyano, halo, azido, oxo, and Qi , the group of formula (1 ) having the structure :

(1 ),

where

L is -(CH(OP ₂ ))-, -(C(OH)(OP ₂ ))-, or -C(O)-;

Ri is H, or Ri and Pi combine to form a bond;

R2 is H or -(CH2)nOP3, and each of P2 and P3 is independently H, optionally substituted alkyl, or a hydroxyl protecting group, or P2 and P3, together with the atoms to which each is attached, combine to form a ketal, a cyclic carbonate, a dicarbonyl-dioxo, or silylene- dioxo; or R2 and P2 combine to form an optionally substituted ethylene or a structure selected from the group consisting of:

group;

E is optionally substituted alkyl or optionally substituted alkoxy;

G is O, S, CH2, or NRN, where RN is H, an N-protecting group, or optionally substituted alkyl ; each Qi is independently ORA, SRA, S0 ₂ RA, OS0 ₂ RA, NRBRA, NRB(CO)RA, NRB(CO)(CO)RA, NRA(CO)NRBRA, NRB(CO)ORA, (CO)ORA, 0(CO)RA, (CO)NRBRA, or 0(CO)NRBRA, where each of RA and RB IS independently H, alkyl, haloalkyi, hydroxyalkyi, aminoalkyi, aryl, haloaryl, hydroxyaryl, alkoxyaryl, arylalkyi, alkylaryl, haloarylalkyi, alkylhaloaryl, (alkoxyaryl)alkyl, heterocyclic radical, or heterocyclic radical-alkyl;

k is 0 or 1 ;

n is O, 1 , or 2;

Ai and FU combine to form oxo, R3 is H or a hydroxyl protecting group, and R5 is H ; or

Ai is H or OP", where P" is H or a hydroxyl protecting group, and:

(i) P13 is H or a hydroxyl protecting group, and FU and R5 combine to form a double bond;

or

(ii) R3 and R4 combine to form a bond, and R5 is H or OP";

each P4 is independently H or a hydroxyl protecting group, and Xi is oxo; or both P4 groups and Xi , together with the atoms to which each is attached, combine to form ketal; and

where the macrocyclic intermediate in the synthesis of a halichondrin macrolide is a compound of formula (IVB):

(VIB)

or a salt or a tautomer thereof,

where

b designates (/^-stereogenic center, c designates (S)-stereogenic center, and Z is a sulfonate, chloride, bromide, or iodide;

or

b designates (S)-stereogenic center, c designates (/^-stereogenic center, and Z is OR6, where R6 is a hydroxyl protecting group.

In formula (VIA) or (VIB), Y can be bromide. In formula (VIA) or (VIB), R3 and R4 can combine to form a bond, and R5 can be H. In formula (VIA) or (VIB), both P4 groups and Xi , together with the atoms to which each is attached, can combine to form ketal. In formula (VIA) or (VIB), Z can be a sulfonate. In formula (VIA) or (VIB), Z can be OR6, where R6 is a hydroxyl protecting group. In formula (VIA) or (VIB), Z can be an ester, carbonate, or carbamate. The non-macrocyclic intermediate can be a compound of formula (VIIA) :

(VI I A)

or a salt thereof,

where

Y is iodide, bromide, or trifluoromethanesulfonate;

each of D and D' is independently H, optionally substituted alkyl, or OPi , provided that only one of D and D' is OPi , where Pi is H, alkyl, or a hydroxyl protecting group; A is a group of formula (1 ) or a C1-6 saturated or C2-6 unsaturated hydrocarbon skeleton, the skeleton being unsubstituted or having from 1 to 10 substituents independently selected from the group consisting of cyano, halo, azido, oxo, and Qi , the group of formula (1 ) having the structure:

(1 ),

where

L is -(CH(OP ₂ ))-, -(C(OH)(OP ₂ ))-, or -C(O)-;

Ri is H, or Ri and Pi combine to form a bond;

R2 is H or -(CH2)nOP3, and each of P2 and P3 is independently H, optionally substituted alkyl, or a hydroxyl protecting group, or P2 and P3, together with the atoms to which each is attached, combine to form a ketal, a cyclic carbonate, a dicarbonyl-dioxo, or silylene- dioxo; or R2 and P2 combine to form an optionally substituted ethylene or a structure selected from the group consisting of:

, where each P' is independently H or a hydroxyl protecting group;

E is optionally substituted alkyl or optionally substituted alkoxy;

G is O, S, CH2, or NRN, where RN is H, an N-protecting group, or optionally substituted alkyl; each Qi is independently ORA, SRA, S0 ₂ RA, OS0 ₂ RA, NRBRA, NRB(CO)RA, NRB(CO)(CO)RA, NRA(CO)NRBRA, NRB(CO)ORA, (CO)ORA, 0(CO)RA, (CO)NRBRA, or 0(CO)NRBRA, where each of RA and RB IS independently H, alkyl, haloalkyi, hydroxyalkyi, aminoalkyi, aryl, haloaryl, hydroxyaryl, alkoxyaryl, arylalkyi, alkylaryl, haloarylalkyi, alkylhaloaryl, (alkoxyaryl)alkyl, heterocyclic radical, or heterocyclic radical-alkyl;

k is 0 or 1 ;

n is O, 1 , or 2;

(a1 ) R3 is H or a hydroxyl protecting group, R4 and R5 combine to form a double bond, each P4 is independently H or a hydroxyl protecting group, and Xi , together with the carbon to which it is attached, forms a carbonyl or -(CH(ORs))-, where Rs is H or a hydroxyl protecting group;

or

(a2) R3 and R4 combine to form a bond, R5 is H, and

each P4 is independently H or a hydroxyl protecting group, and Xi , together with the carbon to which it is attached, forms a carbonyl or -(CH(ORs))-;

or

both P4 groups and Xi , together with the atoms to which each is attached, combine to form ketal;

(b1 ) Z is chloride, bromide, or iodide, and R6 and R7 combine to form a bond;

or

(b2) Z and R7 combine to form a double bond, and R6 is a hydroxyl protecting group;

and

where the macrocyclic intermediate in the synthesis of a halichondrin macrolide is a compound of formula (VIIB):

(VIIB)

or a salt thereof,

where each of Ps and Pi6 is independently H or a hydroxyl protecting group.

In formula (VMA), Z can be iodide. In formula (VMA) or (VIIB), Y can be trifluoromethanesulfonate. In formula (VMA) or (VIIB), R3 can be H or a hydroxyl protecting group, R4 and R5 can combine to form a double bond, each P4 can independently be H or a hydroxyl protecting group, and Xi , together with the carbon to which it is attached, can form -(CH(ORs))-, where Rs is H or a hydroxyl protecting group. In formula (VMA) or (VIIB), at least one of P3, P4, and R6 can be a hydroxyl protecting group. In formula (VMA), Z can be chloride, bromide, or iodide, and R6 and R7 combine to form a bond.

In a second aspect, the invention provides a method of preparing:

halichondrin macrolide

or a salt thereof,

where

each of Ai and A2 is independently H or OP", where each P" is independently H or a hydroxyl protecting group;

each of D and D' is independently H, optionally substituted alkyl, or OP1 , provided that only one of D and D' is OP1 , where Pi is H, alkyl, or a hydroxyl protecting group; A is a group of formula (1 ) or a C1-6 saturated or C2-6 unsaturated hydrocarbon skeleton, the skeleton being unsubstituted or having from 1 to 10 substituents independently selected from the group consisting of cyano, halo, azido, oxo, and Qi , the group of formula (1 ) having the structure:

(1 ),

where

L is -(CH(OP ₂ ))-, -(C(OH)(OP ₂ ))-, or -C(O)-;

Ri is H, or Ri and Pi combine to form a bond;

Pi2 is H or -(CH2)nOP3, and each of P2 and P3 is independently H, optionally substituted alkyl, or a hydroxyl protecting group, or P2 and P3, together with the atoms to which each is attached, combine to form a ketal, a cyclic carbonate, a dicarbonyl-dioxo, or silylene- dioxo; or R2 and P2 combine to form an optionally substituted ethylene or a structure selected from the group consisting of:

group;

E is optionally substituted alkyl or optionally substituted alkoxy;

G is O, S, CH2, or NRN, where RN is H, an N-protecting group, or optionally substituted alkyl; each Qi is independently ORA, SRA, S0 ₂ RA, OS0 ₂ RA, NRBRA, NRB(CO)RA, NRB(CO)(CO)RA, NRA(CO)NRBRA, NRB(CO)ORA, (CO)ORA, 0(CO)RA, (CO)NRBRA, or 0(CO)NRBRA, where each of RA and RB IS independently H, alkyl, haloalkyi, hydroxyalkyi, aminoalkyi, aryl, haloaryl, hydroxyaryl, alkoxyaryl, arylalkyi, alkylaryl, haloarylalkyi, alkylhaloaryl, (alkoxyaryl)alkyl, heterocyclic radical, or heterocyclic radical-alkyl;

k is 0 or 1 ;

n is O, 1 , or 2;

the method involving:

(A) producing a compound of formula (IB) from a compound of formula (IA), the compound of formula (IA) having the following structure:

or a salt or a tautomer thereof,

wherein

each R is independently optionally substituted alkyl or optionally substituted aryl ;

Ai and R4 combine to form oxo, R3 is H or a hydroxyl protecting group, and R5 is H ; or

Ai is H or OP", where P" is H or a hydroxyl protecting group, and:

(i) R3 is H or a hydroxyl protecting group, and R4 and R5 combine to form a double bond;

or

(ii) R3 and R4 combine to form a bond, and R5 is H or OP";

each P4 is independently H or a hydroxyl protecting group, and Xi is oxo or Xi , together with the carbon atom to which it is attached, is -(CH(OPY))-, wherein Ργ is H or a hydroxyl protecting group; or both P4 groups and Xi , together with the atoms to which each is attached, combine to form ketal ; and

P5 is H or a hydroxyl protecting group; and

the compound of formula (IB) having the following structure:

or a salt or a tautomer thereof;

and

(B) producing the halichondrin macrolide from compound (IB).

In some embodiments of the second aspect, the producing the compound of formula (IB) involves reacting the compound of formula (IA) with with an organic base and a Lewis acid. In certain embodiments of the second aspect, the producing the halichondrin macrolide inolves reacting the compound of formula (IB) with a hydroxyl protecting group removing agent. In particular embodiments of the second aspect, each R is optionally substituted alkyl.

In a third aspect, the invention provi

halichondrin macrolide

or a salt thereof,

where

each of Ai and A2 is independently H or OP", where each P" is independently H or a hydroxyl protecting group;

each of D and D' is independently H, optionally substituted alkyl, or OP1 , provided that only one of D and D' is OP1 , where Pi is H, alkyl, or a hydroxyl protecting group; A is a group of formula (1 ) or a C1-6 saturated or C2-6 unsaturated hydrocarbon skeleton , the skeleton being unsubstituted or having from 1 to 10 substituents independently selected from the group consisting of cyano, halo, azido, oxo, and Qi , the group of formula (1 ) having the structure :

(1 ),

where

L is -(CH(OP ₂ ))-, -(C(OH)(OP ₂ ))-, or -C(O)-;

Ri is H, or Ri and Pi combine to form a bond;

R2 is H or -(CH2)nOP3, and each of P2 and P3 is independently H, optionally substituted alkyl, or a hydroxyl protecting group, or P2 and P3, together with the atoms to which each is attached, combine to form a ketal, a cyclic carbonate, a dicarbonyl-dioxo, or silylene- dioxo; or R2 and P2 combine to form an optionally substituted ethylene or a structure selected from the group consisting of:

, where each P' is independently H or a hydroxyl protecting group;

E is optionally substituted alkyl or optionally substituted alkoxy;

G is O, S, CH2, or NRN, where RN is H, an N-protecting group, or optionally substituted alkyl; each Qi is independently ORA, SRA, S0 ₂ RA, OS0 ₂ RA, NRBRA, NRB(CO)RA, NRB(CO)(CO)RA, NRA(CO)NRBRA, NRB(CO)ORA, (CO)ORA, 0(CO)RA, (CO)NRBRA, or 0(CO)NRBRA, where each of RA and RB IS independently H, alkyl, haloalkyi, hydroxyalkyi, aminoalkyi, aryl, haloaryl, hydroxyaryl, alkoxyaryl, arylalkyi, alkylaryl, haloarylalkyi, alkylhaloaryl, (alkoxyaryl)alkyl, heterocyclic radical, or heterocyclic radical-alkyl;

k is 0 or 1 ;

n is O, 1 , or 2;

the method involving:

(A) producing a compound of formula (IB) from a compound of formula (IIA), the compound of formula (IIA) having the following structure:

or a salt or a tautomer thereof,

wherein Ai and FU combine to form oxo, R3 is H or a hydroxyl protecting group, and R5 is H ; or

Ai is H or OP", where P" is H or a hydroxyl protecting group, and:

(i) R3 is H or a hydroxyl protecting group, and R4 and R5 combine to form a double bond;

or

(ii) R3 and R4 combine to form a bond, and R5 is H or OP";

each P4 is independently H or a hydroxyl protecting group, and Xi is oxo or Xi , together with the carbon atom to which it is attached, is -(CH(OPY))-, wherein Ργ is H or a hydroxyl protecting group; or both P4 groups and Xi , together with the atoms to which each is attached, combine to form ketal ; and

P5 is H or a hydroxyl protecting group; and

the compound of formula (IB) having the following structure:

or a salt or a tautomer thereof;

and

(B) producing the halichondrin macrolide from compound (IB).

In some embodiments of the third aspect, the producing the compound of formula (IB) involves reacting the compound of formula (IIA) with an olefin metathesis catalyst (e.g. , a ruthenium-carbene complex). In some embodiments of the third aspect, the producing the halichondrin macrolide involves reacting the compound of formula (IB) with a hydroxyl protecting group removing agent.

In a fourth aspect, the invention provides a method of preparing :

halichondrin macrolide or a salt thereof,

wherein

each of Ai and A2 is independently H or OP", where each P" is independently H or a hydroxyl protecting group;

each of D and D' is independently H, optionally substituted alkyl, or OP1 , provided that only one of D and D' is OP1 , where Pi is H, alkyl, or a hydroxyl protecting group; A is a group of formula (1 ) or a C1-6 saturated or C2-6 unsaturated hydrocarbon skeleton , the skeleton being unsubstituted or having from 1 to 10 substituents independently selected from the group consisting of cyano, halo, azido, oxo, and Qi , the group of formula (1 ) having the structure :

(1 ),

where

L is -(CH(OP ₂ ))-, -(C(OH)(OP ₂ ))-, or -C(O)-;

Ri is H, or Ri and Pi combine to form a bond;

R2 is H or -(CH2)nOP3, and each of P2 and P3 is independently H, optionally substituted alkyl, or a hydroxyl protecting group, or P2 and P3, together with the atoms to which each is attached, combine to form a ketal, a cyclic carbonate, a dicarbonyl-dioxo, or silylene- dioxo; or R2 and P2 combine to form an optionally substituted ethylene or a structure selected from the group consisting of:

group;

E is optionally substituted alkyl or optionally substituted alkoxy;

G is O, S, CH2, or NRN, where RN is H, an N-protecting group, or optionally substituted alkyl ; each Qi is independently ORA, SRA, S0 ₂ RA, OS0 ₂ RA, NRBRA, NRB(CO)RA, NRB(CO)(CO)RA, NRA(CO)NRBRA, NRB(CO)ORA, (CO)ORA, 0(CO)RA, (CO)NRBRA, or 0(CO)NRBRA, where each of RA and RB IS independently H, alkyl, haloalkyi, hydroxyalkyi, aminoalkyi, aryl, haloaryl, hydroxyaryl, alkoxyaryl, arylalkyi, alkylaryl, haloarylalkyi, alkylhaloaryl, (alkoxyaryl)alkyl, heterocyclic radical, or heterocyclic radical-alkyl;

k is 0 or 1 ;

n is O, 1 , or 2;

the method involving:

(A) producing a compound of formula (NIB) from a compound of formula (IIIA), the compound of formula (IIIA) having the following structure:

or a salt or a tautomer thereof,

wherein

Ai and FU combine to form oxo, R3 is H or a hydroxyl protecting group, and R5 is H ; or

Ai is H or OP", where P" is H or a hydroxyl protecting group, and:

(i) is H or a hydroxyl protecting group, and FU and R5 combine to form a double bond;

or

(ii) R3 and R4 combine to form a bond, and R5 is H or OP";

each P4 is independently H or a hydroxyl protecting group, and Xi is oxo or Xi , together with the carbon atom to which it is attached, is -(CH(OPY))-, wherein Ργ is H or a hydroxyl protecting group; or both P4 groups and Xi , together with the atoms to which each is attached, combine to form ketal; and

P5 is H or a hydroxyl protecting group; and the compound of form

(1 MB),

or a salt or a tautomer thereof;

and

(B) producing the halichondrin macrolide from the compound of formula (1MB).

In some embodiments of the fourth aspect, the producing the compound of formula (1MB) involves reacting the compound of formula (IMA) with an olefin metathesis catalyst. In certain embodiments of the fourth aspect, the producing the halichondrin macrolide involves reacting the compound of formula (1MB) with a hydroxyl protecting group removing agent. In particular embodiments of the fourth aspect, both P4 groups and Xi , together with the atoms to which each is attached, combine to form ketal. In further embodiments of the fourth aspect, P5 is a hydroxyl protecting group.

In a fifth aspect, the invention provi

halichondrin macrolide

or a salt thereof,

where

each of Ai and A2 is independently H or OP", where each P" is independently H or a hydroxyl protecting group;

each of D and D' is independently H, optionally substituted alkyl, or OP1 , provided that only one of D and D' is OP1 , where Pi is H, alkyl, or a hydroxyl protecting group; A is a group of formula (1 ) or a C1-6 saturated or C2-6 unsaturated hydrocarbon skeleton , the skeleton being unsubstituted or having from 1 to 10 substituents independently selected from the group consisting of cyano, halo, azido, oxo, and Qi , the group of formula (1 ) having the structure :

(1 ),

where

L is -(CH(OP ₂ ))-, -(C(OH)(OP ₂ ))-, or -C(O)-;

Ri is H, or Ri and Pi combine to form a bond;

Pi2 is H or -(CH2)nOP3, and each of P2 and P3 is independently H, optionally substituted alkyl, or a hydroxyl protecting group, or P2 and P3, together with the atoms to which each is attached, combine to form a ketal, a cyclic carbonate, a dicarbonyl-dioxo, or silylene- dioxo; or R2 and P2 combine to form an optionally substituted ethylene or a structure selected from the group consisting of:

group;

E is optionally substituted alkyl or optionally substituted alkoxy;

G is O, S, CH2, or NRN, where RN is H, an N-protecting group, or optionally substituted alkyl; each Qi is independently ORA, SRA, S0 ₂ RA, OS0 ₂ RA, NRBRA, NRB(CO)RA, NRB(CO)(CO)RA, NRA(CO)NRBRA, NRB(CO)ORA, (CO)ORA, 0(CO)RA, (CO)NRBRA, or 0(CO)NRBRA, where each of RA and RB IS independently H, alkyl, haloalkyi, hydroxyalkyi, aminoalkyi, aryl, haloaryl, hydroxyaryl, alkoxyaryl, arylalkyi, alkylaryl, haloarylalkyi, alkylhaloaryl, (alkoxyaryl)alkyl, heterocyclic radical, or heterocyclic radical-alkyl;

k is 0 or 1 ;

n is O, 1 , or 2;

the method involving:

(A) producing a compound of formula (IVB) from a compound of formula (IVA), the compound of formula (IVA) having the following structure:

(IVA)

or a salt thereof,

where

(i) R3 is H or a hydroxyl protecting group, FU is alkyl ether, and R5 is H ;

(ii) R3 is H or a hydroxyl protecting group, and R4 and R5 combine to form a double bond; or

(iii) R3 and R4 combine to form a bond, and R5 is H;

each P4 and P5 is independently H or a hydroxyl protecting group;

R6 is H, and Ρβ is H or a hydroxyl protecting group; or R6 and Ρβ combine to form a double bond; and

the compound of formula (IVB) having the following structure:

(IVB)

or a salt thereof;

and

(B) producing the halichondrin macrolide from the compound of formula (IVB).

In some embodiments of the fifth aspect, the producing the compound of formula (IVB) involves reacting the compound of formula (IVA) with an olefin metathesis catalyst. In certain embodiments of the fifth aspect, the producing the halichondrin macrolide from the compound of formula (IVB) involves reacting the compound of formula (IVB) with a Bronsted acid. In other embodiments of the fifth aspect, the producing the halichondrin macrolide involves producing the compound of formula (IVC), the compound of formula (IVC) having the following structure:

(IVC)

or a salt thereof;

and producing the halichondrin macrolide from the compound of formula (IVC).

In yet other embodiments of the fifth aspect, the producing the compound of formula (IVC) involves reacting the compound of formula (IVB) with a hydroxyl protecting group removing agent. In still other embodiments of the fifth aspect, the producing the halichondrin macrolide from the compound of formula (IVC) involves reacting the compound of formula (IVC) with a Bronsted acid. In particular embodiments of the fifth aspect, at least one of P4 and P5 is H.

In a sixth aspect, the invention provides a method of preparing:

halichondrin macrolide

or a salt thereof,

where

each of Ai and A2 is independently H or OP", where each P" is independently H or a hydroxyl protecting group;

each of D and D' is independently H, optionally substituted alkyl, or OP1 , provided that only one of D and D' is OP1 , where Pi is H, alkyl, or a hydroxyl protecting group; A is a group of formula (1 ) or a C1-6 saturated or C2-6 unsaturated hydrocarbon skeleton, the skeleton being unsubstituted or having from 1 to 10 substituents independently selected from the group consisting of cyano, halo, azido, oxo, and Qi , the group of formula (1 ) having the structure:

(1 ), where

L is -(CH(OP ₂ ))-, -(C(OH)(OP ₂ ))-, or -C(O)-;

Ri is H, or Ri and Pi combine to form a bond;

Pi2 is H or -(CH2)nOP3, and each of P2 and P3 is independently H, optionally substituted alkyl, or a hydroxyl protecting group, or P2 and P3, together with the atoms to which each is attached, combine to form a ketal, a cyclic carbonate, a dicarbonyl-dioxo, or silylene- dioxo; or R2 and P2 combine to form an optionally substituted ethylene or a structure selected from the group consisting of:

group;

E is optionally substituted alkyl or optionally substituted alkoxy;

G is O, S, CH2, or NRN, where RN is H, an N-protecting group, or optionally substituted alkyl; each Qi is independently ORA, SRA, S0 ₂ RA, OS0 ₂ RA, NRBRA, NRB(CO)RA, NRB(CO)(CO)RA, NRA(CO)NRBRA, NRB(CO)ORA, (CO)ORA, 0(CO)RA, (CO)NRBRA, or 0(CO)NRBRA, where each of RA and RB IS independently H, alkyl, haloalkyi, hydroxyalkyi, aminoalkyi, aryl, haloaryl, hydroxyaryl, alkoxyaryl, arylalkyi, alkylaryl, haloarylalkyi, alkylhaloaryl, (alkoxyaryl)alkyl, heterocyclic radical, or heterocyclic radical-alkyl;

k is 0 or 1 ;

n is O, 1 , or 2;

the method involving:

(A) producing a compound of formula (VB) from a compound of formula (VA), the compound of formula (VA) having the following structure:

(VA)

or a salt or a tautomer thereof,

where

(a1 ) R3 is H or a hydroxyl protecting group, R4 is alkyl ether, and R5 is H ;

(a2) R3 is H or a hydroxyl protecting group, and R4 and R5 combine to form a double bond; or

(a3) R3 and R4 combine to form a bond, and R5 is H ;

(b1 ) Ai and R7 combine to form oxo, R6 is H or a hydroxyl protecting group, and Rs is H ; or

(b2) Ai is H or OP", where P" is H or a hydroxyl protecting group, and:

(i) R6 is H or a hydroxyl protecting group, and R7 and Rs combine to form a double bond;

or

(ii) R6 and R7 combine to form a bond, and Rs is H or OP";

(c1 ) R9 is H, and P4 is H or a hydroxyl protecting group;

or

(c2) R9 and P4 combine to form a double bond;

R10 is H or -CH2Xi CH ₂ CH=CH2, where Xi is O, -C(Rn ) ₂ - or NP ₆ , and where each Rn is independently H or -COORi2, Ρβ is an /V-protecting group, and R12 is alkyl;

each P5 is independently H or a hydroxyl protecting group; and

where the macrocyclic intermediate in the synthesis of a halichondrin macrolide is a compound of formula (VB):

(VB)

or a salt or a tautomer thereof; (B) producing a compound of formula (VC) from the compound of formula (VB), the compound of formula (VC) having the following structure:

(VC)

or a salt thereof,

where each of Ai and A2 is independently H or OP";

(C) producing a compound of formula (IVC) from the compound of formula (VC), the compound of formula (IVC) having the following structure:

(IVC)

or a salt thereof,

where each of Ai and A2 is independently H or OP"; and

(D) producing the halichondrin macrolide from the compound of formula (IVC). In some embodiments of the sixth aspect, the producing the compound of formula (VB) involves contacting the compound of formula (VA) with an olefin metathesis catalyst. In certain embodiments of the sixth aspect, P4 is H, R9 is H, and the producing the compound of formula (VC) involves oxidizing the compound of formula (VB) to produce a compound of formula (VBa):

or a salt thereof;

where each of Ai and A2 is independently H or OP"

and reacting the compound of formula (VBa) with a 1 ,4-reducing agent to produce the compound of formula (VC).

In particular embodiments of the sixth aspect, the producing the compound of formula (IVC) involves contacting the compound of formula (VC) with a hydroxyl protecting group removing agent. In further embodiments of the sixth aspect, the producing the halichondrin macrolide involves contacting the compound of formula (IVC) with a Bronsted acid.

In a seventh aspect, the invention provides a method of preparing:

halichondrin macrolide

or a salt thereof,

where

each of Ai and A2 is independently H or OP", where each P" is independently H or a hydroxyl protecting group;

each of D and D' is independently H, optionally substituted alkyl, or OP1 , provided that only one of D and D' is OP1 , where Pi is H, alkyl, or a hydroxyl protecting group; A is a group of formula (1 ) or a C1-6 saturated or C2-6 unsaturated hydrocarbon skeleton, the skeleton being unsubstituted or having from 1 to 10 substituents independently selected from the group consisting of cyano, halo, azido, oxo, and Qi , the group of formula (1 ) having the structure:

(1 ),

where

L is -(CH(OP ₂ ))-, -(C(OH)(OP ₂ ))-, or -C(O)-;

Ri is H, or Ri and Pi combine to form a bond;

Pi2 is H or -(CH2)nOP3, and each of P2 and P3 is independently H, optionally substituted alkyl, or a hydroxyl protecting group, or P2 and P3, together with the atoms to which each is attached, combine to form a ketal, a cyclic carbonate, a dicarbonyl-dioxo, or silylene- dioxo; or R2 and P2 combine to form an optionally substituted ethylene or a structure selected from the group consisting of:

group;

E is optionally substituted alkyl or optionally substituted alkoxy;

G is O, S, CH2, or NRN, where RN is H, an N-protecting group, or optionally substituted alkyl; each Qi is independently ORA, SRA, S0 ₂ RA, OS0 ₂ RA, NRBRA, NRB(CO)RA, NRB(CO)(CO)RA, NRA(CO)NRBRA, NRB(CO)ORA, (CO)ORA, 0(CO)RA, (CO)NRBRA, or 0(CO)NRBRA, where each of RA and RB IS independently H, alkyl, haloalkyi, hydroxyalkyi, aminoalkyi, aryl, haloaryl, hydroxyaryl, alkoxyaryl, arylalkyi, alkylaryl, haloarylalkyi, alkylhaloaryl, (alkoxyaryl)alkyl, heterocyclic radical, or heterocyclic radical-alkyl;

k is 0 or 1 ;

n is O, 1 , or 2;

the method involving:

(A) producing a compound of formula (VIB) from a compound of formula (VIA), the compound of formula (VIA) having the following structure:

(VIA)

or a salt or a tautomer thereof,

where

Y is iodide, bromide, or trifluoromethanesulfonate;

b designates (/^-stereogenic center, and Z is a sulfonate, chloride, bromide, or iodide; or b designates (S)-stereogenic center, and Z is ΟΙ¾, where R6 is a hydroxyl protecting group;

Ai and FU combine to form oxo, R3 is H or a hydroxyl protecting group, and R5 is H ; or

Ai is H or OP", where P" is H or a hydroxyl protecting group, and:

(i) R3 is H or a hydroxyl protecting group, and R4 and R5 combine to form a double bond;

or

(ii) R3 and R4 combine to form a bond, and R5 is H or OP";

each P4 is independently H or a hydroxyl protecting group, and Xi is oxo; or both P4 groups and

Xi , together with the atoms to which each is attached, combine to form ketal;

the compound of formula (VIB) having the following structure:

b designates (/^-stereogenic center, c designates (S)-stereogenic center, and Z is a sulfonate, chloride, bromide, or iodide;

or

b designates (S)-stereogenic center, c designates (/^-stereogenic center, and Z is OR6 where R6 is a hydroxyl protecting group; (B) producing the halichondrin macrolide from the compound of formula (VIB).

In some embodiments of the seventh aspect, the producing the compound of formula (VIB) involves reacting the compound of formula (VIA) with a Cr(ll) salt and a Ni(ll) salt. In particular embodiments of the seventh aspect, the producing the halichondrin macrolide involves the step of nucleophilic ring-closing of the compound of formula (VIB). In certain embodiments of the seventh aspect, both P4 groups and Xi , together with the atoms to which each is attached, combine to form ketal. In further embodiments of the seventh aspect, R3 and FU combine to form a bond, and R5 is H.

In an eighth aspect, the invention provides a method of preparing :

halichondrin macrolide

or a salt thereof,

where

each of Ai and A2 is independently H or OP", where each P" is independently H or a hydroxyl protecting group;

each of D and D' is independently H, optionally substituted alkyl, or OP1 , provided that only one of D and D' is OP1 , where Pi is H, alkyl, or a hydroxyl protecting group; A is a group of formula (1 ) or a C1-6 saturated or C2-6 unsaturated hydrocarbon skeleton, the skeleton being unsubstituted or having from 1 to 10 substituents independently selected from the group consisting of cyano, halo, azido, oxo, and Qi , the group of formula (1 ) having the structure:

(1 ),

where

L is -(CH(OP ₂ ))-, -(C(OH)(OP ₂ ))-, or -C(O)-;

Ri is H, or Ri and Pi combine to form a bond;

R2 is H or -(CH2)nOP3, and each of P2 and P3 is independently H, optionally substituted alkyl, or a hydroxyl protecting group, or P2 and P3, together with the atoms to which each is attached, combine to form a ketal, a cyclic carbonate, a dicarbonyl-dioxo, or silylene- dioxo; or R2 and P2 combine to form an optionally substituted ethylene or a structure selected from the group consisting of:

, where each P' is independently H or a hydroxyl protecting group;

E is optionally substituted alkyl or optionally substituted alkoxy;

G is O, S, CH2, or NRN, where RN is H, an N-protecting group, or optionally substituted alkyl; each Qi is independently ORA, SRA, S0 ₂ RA, OS0 ₂ RA, NRBRA, NRB(CO)RA, NRB(CO)(CO)RA, NRA(CO)NRBRA, NRB(CO)ORA, (CO)ORA, 0(CO)RA, (CO)NRBRA, or 0(CO)NRBRA, where each of RA and RB IS independently H, alkyl, haloalkyi, hydroxyalkyi, aminoalkyi, aryl, haloaryl, hydroxyaryl, alkoxyaryl, arylalkyi, alkylaryl, haloarylalkyi, alkylhaloaryl, (alkoxyaryl)alkyl, heterocyclic radical, or heterocyclic radical-alkyl;

k is 0 or 1 ;

n is O, 1 , or 2;

the method involving:

(A) producing a compound of formula (VIIB) from a compound of formula (VMA), the compound of formula (VMA) having the following structure:

(VI I A)

or a salt thereof,

where

Y is iodide, bromide, or trifluoromethanesulfonate; (a1 ) R3 is H or a hydroxyl protecting group, R4 and R5 combine to form a double bond, each P4 is independently H or a hydroxyl protecting group, and Xi , together with the carbon to which it is attached, forms a carbonyl or -(CH(OR8))-;

or

(a2) R3 and R4 combine to form a bond, R5 is H, and

each P4 is independently H or a hydroxyl protecting group, and Xi , together with the carbon to which it is attached, forms a carbonyl or -(CH(OR8))-;

or

both P4 groups and Xi , together with the atoms to which each is attached, combine to form ketal;

(b1 ) Z is chloride, bromide, or iodide, and R6 and R7 combine to form a bond;

or

(b2) Z and R7 combine to form a double bond, and R6 is a hydroxyl protecting group;

and

where Rs is H or a hydroxyl protecting group;

and

the compound of formul

(VIIB)

or a salt thereof,

where each of P5 and R6 is independently H or a hydroxyl protecting group;

(B) producing a compound of formula (VIIC) from the compound of formula (VIIB), the compound of formula (VIIC) having the following structure:

(VIIC)

or a salt or a tautomer thereof, where

Ai and FU combine to form oxo, R3 is H or a hydroxyl protecting group, and R5 is H ; or

Ai is H or OP", where P" is H or a hydroxyl protecting group, and:

(i) P13 is H or a hydroxyl protecting group, and FU and R5 combine to form a double bond;

or

(ii) P13 and FU combine to form a bond, and R5 is H or OP";

(C) producing a compound of formula (VII D) from the compound of formula (VIIC), the compound of formula (VII D) having the following structure:

(VI ID)

or a salt thereof,

where each of Ai and A2 is independently H or OP";

and

(D) producing the halichondrin macrolide from the compound of formula (VII D).

In some embodiments of the eighth aspect, the producing the compound of formula (VI IB) involves reacting the compound of formula (VI IA) with a Cr(l l) salt and a Ni(ll) salt. In certain embodiments of the eighth aspect, the producing the compound of formula (VIIC) involves the step of nucleophilic ring-closing of the compound of formula (VIIB). In particular embodiments of the eighth aspect, the producing the compound of formula (VII D) involves reacting the compound of formula (VI IC) with a hydroxyl protecting group removing agent. In further embodiments of the eighth aspect, the producing the halichondrin macrolide involves reacting the compound of formula (VI ID) with a Bronsted acid. In other embodiments of the eighth aspect, Z is iodide. In yet other embodiments of the eighth aspect, Y is

trifluoromethanesulfonate. In still embodiments of the eighth aspect, R3 is H or a hydroxyl protecting group, and R4 and R5 combine to form a double bond. In some embodiments of the eighth aspect, each P4 is independently H or a hydroxyl protecting group, and Xi , together with the carbon to which it is attached, forms -(CH(ORs))-, where Rs is H or a hydroxyl protecting group. In particular embodiments of the eighth aspect, P5 is H . In certain embodiments of the eighth aspect, Z is chloride, bromide, or iodide, and R6 and R7 combine to form a bond. In some embodiments of any aspect, a designates (S)-stereogenic center. In certain embodiments of any aspect, one and only one of D and D' is optionally substituted alkyl or OPi . In particular embodiments of any aspect, one and only one of D and D' is OPi , where Pi is H, alkyl, or a hydroxyl protecting group. In further embodiments of any aspect, A is a group of formula (1 ). In certain embodiments of any aspect, L is -(CH(OP2))-. In other embodiments of any aspect, Ri and Pi combine to form a bond. In yet other embodiments of any aspect, G is O. In still other embodiments of any aspect, E is optionally substituted alkyl. In some embodiments of any aspect, k is 1 . In certain embodiments of any aspect, R2 is - (CH2)nOP3. In particular embodiments of any aspect, at least one of P2 and P3 is a hydroxyl protecting group. In certain embodiments of any aspect, Ai is H. In some embodiments of any aspect, A2 is H.

In a ninth aspect, the invention provides a method of preparing an intermediate in the synthesis of a halichondrin macrolide. The method involves performing an allene-Prins reaction by contacting a compound of formula (VINA) with a compound of formula (VII IB) and R4OH,

where R4 is an optionally substituted acyl ;

where the compound of formula (VI NA) has the following structure:

(VI NA)

or a salt thereof,

where

each of D and D' is independently H, optionally substituted alkyl, or OPi , provided that only one of D and D' is OPi , where Pi is H, alkyl, or a hydroxyl protecting group; A is a group of formula (1 ) or a C1-6 saturated or C2-6 unsaturated hydrocarbon skeleton , the skeleton being unsubstituted or having from 1 to 10 substituents independently selected from the group consisting of cyano, halo, azido, oxo, and Qi , the group of formula (1 ) having the structure :

(1 ),

where

L is -(CH(OP ₂ ))-, -(C(OH)(OP ₂ ))-, or -C(O)-;

Ri is H, or Ri and Pi combine to form a bond;

R2 is H or -(CH2)nOP3, and each of P2 and P3 is independently H, optionally substituted alkyl, or a hydroxyl protecting group, or P2 and P3, together with the atoms to which each is attached, combine to form a ketal, a cyclic carbonate, a dicarbonyl-dioxo, or silylene- dioxo; or R2 and P2 combine to form an optionally substituted ethylene or a structure selected from the group consisting of: , where each Ρ' is independently Η or a hydroxyl protecting group;

E is optionally substituted alkyl or optionally substituted alkoxy;

G is O, S, CH2, or NRN, where RN is H, an N-protecting group, or optionally substituted alkyl; each Qi is independently ORA, SRA, S0 ₂ RA, OS0 ₂ RA, NRBRA, NRB(CO)RA, NRB(CO)(CO)RA, NRA(CO)NRBRA, NRB(CO)ORA, (CO)ORA, 0(CO)RA, (CO)NRBRA, or 0(CO)NRBRA, where each of RA and RB IS independently H, alkyl, haloalkyi, hydroxyalkyi, aminoalkyi, aryl, haloaryl, hydroxyaryl, alkoxyaryl, arylalkyi, alkylaryl, haloarylalkyi, alkylhaloaryl, (alkoxyaryl)alkyl, heterocyclic radical, or heterocyclic radical-alkyl;

k is 0 or 1 ;

n is O, 1 , or 2;

X is O, or X, together with the carbon atom to which it is attached, forms -(C(OPz)2)-, wherein each Pz is independently optionally substituted alkyl or optionally substituted aryl, or both Pz combine to form optionally substituted alkylene or optionally substituted arylene; and

P4 is H or a hydroxyl protecting group;

where the compound of formula (VIIIB) has the following structure:

(VIIIB), where

where P5 is H or a hydroxyl protecting group; each Ρβ is independently a hydroxyl protecting group, or both Ρβ groups, together with the atoms to which each is attached, combine to form a cyclic protected diol ; and R5 is is H or - where Xi is O, -CH2-, or NP ₇ , where P? is a sulfonyl ;

and where the intermediate is a compound of formula (VII IC) :

(VII IC)

or a salt thereof,

where a designates (/^-stereogenic center or (S)-stereogenic center.

In some embodiments of the ninth aspect, the performing a Prins reaction involves reacting the compound of formula (VINA) with a Lewis acid. In certain embodiments of the ninth aspect, k is 1 .

In a tenth aspect, the invention provides a compound of formula (IA) or formula (IB),

or a salt or a tautomer thereof,

where

each R is independently optionally substituted alkyl or optionally substituted aryl ; each of D and D' is independently H, optionally substituted alkyl, or OPi , provided that only one of D and D' is OPi , where Pi is H, alkyl, or a hydroxyl protecting group; A is a group of formula (1 ) or a C1-6 saturated or C2-6 unsaturated hydrocarbon skeleton, the skeleton being unsubstituted or having from 1 to 10 substituents independently selected from the group consisting of cyano, halo, azido, oxo, and Qi , the group of formula (1 ) having the structure:

(1 ),

where

L is -(CH(OP ₂ ))-, -(C(OH)(OP ₂ ))-, or -C(O)-;

Ri is H, or Ri and Pi combine to form a bond;

R2 is H or -(CH2)nOP3, and each of P2 and P3 is independently H, optionally substituted alkyl, or a hydroxyl protecting group, or P2 and P3, together with the atoms to which each is attached, combine to form a ketal, a cyclic carbonate, a dicarbonyl-dioxo, or silylene- dioxo; or R2 and P2 combine to form an optionally substituted ethylene or a structure selected from the group consisting of:

group;

E is optionally substituted alkyl or optionally substituted alkoxy;

G is O, S, CH2, or NRN, where RN is H, an N-protecting group, or optionally substituted alkyl; each Qi is independently ORA, SRA, S0 ₂ RA, OS0 ₂ RA, NRBRA, NRB(CO)RA, NRB(CO)(CO)RA, NRA(CO)NRBRA, NRB(CO)ORA, (CO)ORA, 0(CO)RA, (CO)NRBRA, or 0(CO)NRBRA, where each of RA and RB IS independently H, alkyl, haloalkyi, hydroxyalkyi, aminoalkyi, aryl, haloaryl, hydroxyaryl, alkoxyaryl, arylalkyi, alkylaryl, haloarylalkyi, alkylhaloaryl, (alkoxyaryl)alkyl, heterocyclic radical, or heterocyclic radical-alkyl;

k is 0 or 1 ; n is O, 1 , or 2;

Ai and FU combine to form oxo, R3 is H or a hydroxyl protecting group, and R5 is H ; or

Ai is H or OP", where P" is H or a hydroxyl protecting group, and:

(i) is H or a hydroxyl protecting group, and FU and R5 combine to form a double bond;

or

(ii) and FU combine to form a bond, and R5 is H or OP";

each P4 is independently H or a hydroxyl protecting group, and Xi is oxo or Xi , together with the carbon atom to which it is attached, is -(CH(OPY))-, wherein Ργ is H or a hydroxyl protecting group; or both P4 groups and Xi , together with the atoms to which each is attached, combine to form ketal; and

P5 is H or a hydroxyl protecting group.

In an eleventh aspect, the invention provides a compound of formula (IIA), formula (IIIA), or formula (1MB):

or a salt or a tautomer thereof,

where

each of D and D' is independently H, optionally substituted alkyl, or OP1 , provided that only one of D and D' is OP1 , where Pi is H, alkyl, or a hydroxyl protecting group; A is a group of formula (1 ) or a C1-6 saturated or C2-6 unsaturated hydrocarbon skeleton, the skeleton being unsubstituted or having from 1 to 10 substituents independently selected from the group consisting of cyano, halo, azido, oxo, and Qi , the group of formula (1 ) having the structure:

(1 ),

where

L is -(CH(OP ₂ ))-, -(C(OH)(OP ₂ ))-, or -C(O)-;

Ri is H, or Ri and Pi combine to form a bond;

Pi2 is H or -(CH2)nOP3, and each of P2 and P3 is independently H, optionally substituted alkyl, or a hydroxyl protecting group, or P2 and P3, together with the atoms to which each is attached, combine to form a ketal, a cyclic carbonate, a dicarbonyl-dioxo, or silylene- dioxo; or R2 and P2 combine to form an optionally substituted ethylene or a structure selected from the group consisting of:

group;

E is optionally substituted alkyl or optionally substituted alkoxy;

G is O, S, CH2, or NRN, where RN is H, an N-protecting group, or optionally substituted alkyl; each Qi is independently ORA, SRA, S0 ₂ RA, OS0 ₂ RA, NRBRA, NRB(CO)RA, NRB(CO)(CO)RA, NRA(CO)NRBRA, NRB(CO)ORA, (CO)ORA, 0(CO)RA, (CO)NRBRA, or 0(CO)NRBRA, where each of RA and RB IS independently H, alkyl, haloalkyi, hydroxyalkyi, aminoalkyi, aryl, haloaryl, hydroxyaryl, alkoxyaryl, arylalkyi, alkylaryl, haloarylalkyi, alkylhaloaryl, (alkoxyaryl)alkyl, heterocyclic radical, or heterocyclic radical-alkyl;

k is 0 or 1 ;

n is O, 1 , or 2;

Ai and R4 combine to form oxo, R3 is H or a hydroxyl protecting group, and R5 is H ; or

Ai is H or OP", where P" is H or a hydroxyl protecting group, and: (i) P13 is H or a hydroxyl protecting group, and FU and Rs combine to form a double bond;

or

(ii) P13 and FU combine to form a bond, and Rs is H or OP";

each P4 is independently H or a hydroxyl protecting group, and Xi is oxo or Xi , together with the carbon atom to which it is attached, is -(CH(OPY))-, wherein Ργ is H or a hydroxyl protecting group; or both P4 groups and Xi , together with the atoms to which each is attached, combine to form ketal; and

P5 is H or a hydroxyl protecting group.

In a twelfth aspect, the invention provides a compound of formula (IVA), formula (IVB), or formula (IVC):

(IVC)

or a salt thereof,

where

each of Ai and A2 is H or OP", where P" is H or a hydroxyl protecting group;

each of D and D' is independently H, optionally substituted alkyl, or OP1 , provided that only one of D and D' is OP1 , where Pi is H, alkyl, or a hydroxyl protecting group; A is a group of formula (1 ) or a C1-6 saturated or C2-6 unsaturated hydrocarbon skeleton, the skeleton being unsubstituted or having from 1 to 1 0 substituents independently selected from the group consisting of cyano, halo, azido, oxo, and Qi , the group of formula (1 ) having the structure:

(1 ), where

L is -(CH(OP ₂ ))-, -(C(OH)(OP ₂ ))-, or -C(O)-;

Ri is H, or Ri and Pi combine to form a bond;

R2 is H or -(CH2)nOP3, and each of P2 and P3 is independently H, optionally substituted alkyl, or a hydroxyl protecting group, or P2 and P3, together with the atoms to which each is attached, combine to form a ketal, a cyclic carbonate, a dicarbonyl-dioxo, or silylene- dioxo; or R2 and P2 combine to form an optionally substituted ethylene or a structure selected from the group consisting of:

group;

E is optionally substituted alkyl or optionally substituted alkoxy;

G is O, S, CH2, or NRN, where RN is H, an N-protecting group, or optionally substituted alkyl ; each Qi is independently ORA, SRA, S0 ₂ RA, OS0 ₂ RA, NRBRA, NRB(CO)RA, NRB(CO)(CO)RA, NRA(CO)NRBRA, NRB(CO)ORA, (CO)ORA, 0(CO)RA, (CO)NRBRA, or 0(CO)NRBRA, where each of RA and RB IS independently H, alkyl, haloalkyi, hydroxyalkyi , aminoalkyi , aryl , haloaryl, hydroxyaryl, alkoxyaryl, arylalkyi, alkylaryl, haloarylalkyi, alkylhaloaryl , (alkoxyaryl)alkyl, heterocyclic radical, or heterocyclic radical-alkyl ;

k is 0 or 1 ;

n is O, 1 , or 2;

(i) R3 is H or a hydroxyl protecting group, R4 is alkyl ether, and R5 is H ;

(ii) R3 is H or a hydroxyl protecting group, and R4 and R5 combine to form a double bond; or

(iii) R3 and R4 combine to form a bond, and R5 is H ;

each P4 and P5 is independently H or a hydroxyl protecting group; and

R6 is H, and Ρβ is H or a hydroxyl protecting group; or R6 and Ρβ combine to form a double bond. In a thirteenth aspect, the invention provides a compound of formula (VA), formula (VB), or formula (VC):

(VC)

or a salt or a tautomer thereof,

where

each of D and D' is independently H, optionally substituted alkyl, or OPi , provided that only one of D and D' is OPi , where Pi is H, alkyl, or a hydroxyl protecting group; A is a group of formula (1 ) or a C1-6 saturated or C2-6 unsaturated hydrocarbon skeleton , the skeleton being unsubstituted or having from 1 to 10 substituents independently selected from the group consisting of cyano, halo, azido, oxo, and Qi , the group of formula (1 ) having the structure :

(1 ),

where

L is -(CH(OP ₂ ))-, -(C(OH)(OP ₂ ))-, or -C(O)-;

Ri is H, or Ri and Pi combine to form a bond;

R2 is H or -(CH2)nOP3, and each of P2 and P3 is independently H, optionally substituted alkyl, or a hydroxyl protecting group, or P2 and P3, together with the atoms to which each is attached, combine to form a ketal, a cyclic carbonate, a dicarbonyl-dioxo, or silylene- dioxo; or R2 and P2 combine to form an optionally substituted ethylene or a structure selected from the group consisting of: , where each Ρ' is independently Η or a hydroxyl protecting group;

E is optionally substituted alkyl or optionally substituted alkoxy;

G is O, S, CH2, or NRN, where RN is H, an N-protecting group, or optionally substituted alkyl; each Qi is independently ORA, SRA, S0 ₂ RA, OS0 ₂ RA, NRBRA, NRB(CO)RA, NRB(CO)(CO)RA, NRA(CO)NRBRA, NRB(CO)ORA, (CO)ORA, 0(CO)RA, (CO)NRBRA, or 0(CO)NRBRA, where each of RA and RB IS independently H, alkyl, haloalkyi, hydroxyalkyi, aminoalkyi, aryl, haloaryl, hydroxyaryl, alkoxyaryl, arylalkyi, alkylaryl, haloarylalkyi, alkylhaloaryl, (alkoxyaryl)alkyl, heterocyclic radical, or heterocyclic radical-alkyl;

k is 0 or 1 ;

n is O, 1 , or 2;

(a1 ) R3 is H or a hydroxyl protecting group, R4 is alkyl ether, and R5 is H ;

(a2) R3 is H or a hydroxyl protecting group, and R4 and R5 combine to form a double bond; or

(a3) R3 and R4 combine to form a bond, and R5 is H;

(b1 ) Ai and R7 combine to form oxo, R6 is H or a hydroxyl protecting group, and Rs is H ; or

(b2) Ai is H or OP", where P" is H or a hydroxyl protecting group, and:

(i) R6 is H or a hydroxyl protecting group, and R7 and Rs combine to form a double bond;

or

(ii) R6 and R7 combine to form a bond, and Rs is H or OP";

(c1 ) R9 is H, and P4 is H or a hydroxyl protecting group;

or

(c2) R9 and P4 combine to form a double bond; Rio is H or -CH2Xi CH ₂ CH=CH2, where Xi is O, -C(Rn ) ₂ - or NP ₆ , and where each Rn is independently H or -COORi2, Ρβ is an /V-protecting group, and R12 is alkyl;

each P5 is independently H or a hydroxyl protecting group; and

In a fourteenth aspect, the inve :

(VIA)

or a salt or a tautomer thereof,

where

each of D and D' is independently H, optionally substituted alkyl, or OP1 , provided that only one of D and D' is OP1 , where Pi is H, alkyl, or a hydroxyl protecting group; A is a group of formula (1 ) or a C1-6 saturated or C2-6 unsaturated hydrocarbon skeleton, the skeleton being unsubstituted or having from 1 to 10 substituents independently selected from the group consisting of cyano, halo, azido, oxo, and Qi , the group of formula (1 ) having the structure:

(1 ),

where

L is -(CH(OP ₂ ))-, -(C(OH)(OP ₂ ))-, or -C(O)-;

Ri is H, or Ri and Pi combine to form a bond;

R2 is H or -(CH2)nOP3, and each of P2 and P3 is independently H, optionally substituted alkyl, or a hydroxyl protecting group, or P2 and P3, together with the atoms to which each is attached, combine to form a ketal, a cyclic carbonate, a dicarbonyl-dioxo, or silylene- dioxo; or R2 and P2 combine to form an optionally substituted ethylene or a structure selected from the group consisting of:

, where each P' is independently H or a hydroxyl protecting group;

E is optionally substituted alkyl or optionally substituted alkoxy;

G is O, S, CH2, or NRN, where RN is H, an N-protecting group, or optionally substituted alkyl; each Qi is independently ORA, SRA, S0 ₂ RA, OS0 ₂ RA, NRBRA, NRB(CO)RA, NRB(CO)(CO)RA, NRA(CO)NRBRA, NRB(CO)ORA, (CO)ORA, 0(CO)RA, (CO)NRBRA, or 0(CO)NRBRA, where each of RA and RB IS independently H, alkyl, haloalkyi, hydroxyalkyi, aminoalkyi, aryl, haloaryl, hydroxyaryl, alkoxyaryl, arylalkyi, alkylaryl, haloarylalkyi, alkylhaloaryl, (alkoxyaryl)alkyl, heterocyclic radical, or heterocyclic radical-alkyl;

k is 0 or 1 ;

n is O, 1 , or 2;

Y is iodide, bromide, or trifluoromethanesulfonate;

b designates (/^-stereogenic center, and Z is a sulfonate, chloride, bromide, or iodide; or b designates (S)-stereogenic center, and Z is OR6, where R6 is a hydroxyl protecting group;

Ai and R4 combine to form oxo, R3 is H or a hydroxyl protecting group, and R5 is H ; or

Ai is H or OP", where P" is H or a hydroxyl protecting group, and:

(i) R3 is H or a hydroxyl protecting group, and R4 and R5 combine to form a double bond;

or

(ii) R3 and R4 combine to form a bond, and R5 is H or OP";

each P4 is independently H or a hydroxyl protecting group, and Xi is oxo or Xi , together with the carbon atom to which it is attached, is -(CH(OPY))-, wherein Ργ is H or a hydroxyl protecting group; or both P4 groups and Xi , together with the atoms to which each is attached, combine to form ketal. In a fifteenth aspect, the invention provides a compound of formula (VIB) :

(VIB)

or a salt or a tautomer thereof,

where

b designates (/^-stereogenic center, c designates (S)-stereogenic center, and Z is a sulfonate, chloride, bromide, or iodide;

or

b designates (S)-stereogenic center, c designates (/^-stereogenic center, and Z is OR6, where Ι¾ is a hydroxyl protecting group;

each of D and D' is independently H, optionally substituted alkyl, or OPi , provided that only one of D and D' is OPi , where Pi is H, alkyl, or a hydroxyl protecting group; A is a group of formula (1 ) or a C1-6 saturated or C2-6 unsaturated hydrocarbon skeleton , the skeleton being unsubstituted or having from 1 to 10 substituents independently selected from the group consisting of cyano, halo, azido, oxo, and Qi , the group of formula (1 ) having the structure :

(1 ),

where

L is -(CH(OP ₂ ))-, -(C(OH)(OP ₂ ))-, or -C(O)-;

Ri is H, or Ri and Pi combine to form a bond;

R2 is H or -(CH2)nOP3, and each of P2 and P3 is independently H, optionally substituted alkyl, or a hydroxyl protecting group, or P2 and P3, together with the atoms to which each is attached, combine to form a ketal, a cyclic carbonate, a dicarbonyl-dioxo, or silylene- dioxo; or R2 and P2 combine to form an optionally substituted ethylene or a structure selected from the group consisting of:

, where each P' is independently H or a hydroxyl protecting group;

E is optionally substituted alkyl or optionally substituted alkoxy;

G is O, S, CH2, or NRN, where RN is H, an N-protecting group, or optionally substituted alkyl ; k is 0 or 1 ;

n is O, 1 , or 2;

Ai and R4 combine to form oxo, R3 is H or a hydroxyl protecting group, and R5 is H ; or

Ai is H or OP", where P" is H or a hydroxyl protecting group, and:

(i) R3 is H or a hydroxyl protecting group, and R4 and R5 combine to form a double bond;

or

(ii) R3 and R4 combine to form a bond, and R5 is H or OP";

each P4 is independently H or a hydroxyl protecting group, and Xi is oxo or Xi , together with the carbon atom to which it is attached, is -(CH(OPY))-, wherein Ργ is H or a hydroxyl protecting group; or both P4 groups and Xi , together with the atoms to which each is attached, combine to form ketal. In a sixteenth aspect, the invention provides a compound of formula (VIIA) or formula (VIIB):

(VIIA) (VIIB)

or a salt thereof,

where

each of D and D' is independently H, optionally substituted alkyl, or OPi , provided that only one of D and D' is OPi , where Pi is H, alkyl, or a hydroxyl protecting group; A is a group of formula (1 ) or a C1-6 saturated or C2-6 unsaturated hydrocarbon skeleton, the skeleton being unsubstituted or having from 1 to 10 substituents independently selected from the group consisting of cyano, halo, azido, oxo, and Qi , the group of formula (1 ) having the structure:

(1 ),

where

L is -(CH(OP ₂ ))-, -(C(OH)(OP ₂ ))-, or -C(O)-;

Ri is H, or Ri and Pi combine to form a bond;

R2 is H or -(CH2)nOP3, and each of P2 and P3 is independently H, optionally substituted alkyl, or a hydroxyl protecting group, or P2 and P3, together with the atoms to which each is attached, combine to form a ketal, a cyclic carbonate, a dicarbonyl-dioxo, or silylene- dioxo; or R2 and P2 combine to form an optionally substituted ethylene or a structure selected from the group consisting of:

, where each P' is independently H or a hydroxyl protecting group;

E is optionally substituted alkyl or optionally substituted alkoxy;

G is O, S, CH2, or NRN, where RN is H, an N-protecting group, or optionally substituted alkyl; each Qi is independently ORA, SRA, S0 ₂ RA, OS0 ₂ RA, NRBRA, NRB(CO)RA, NRB(CO)(CO)RA,

NRA(CO)NRBRA, NRB(CO)ORA, (CO)ORA, 0(CO)RA, (CO)NRBRA, or 0(CO)NRBRA, where each of RA and RB IS independently H, alkyl, haloalkyi, hydroxyalkyi, aminoalkyi, aryl, haloaryl, hydroxyaryl, alkoxyaryl, arylalkyi, alkylaryl, haloarylalkyi, alkylhaloaryl, (alkoxyaryl)alkyl, heterocyclic radical, or heterocyclic radical-alkyl;

k is O or l ;

n is O, 1 , or 2;

(a1 ) R3 is H or a hydroxyl protecting group, R4 and R5 combine to form a double bond, each P4 is independently H or a hydroxyl protecting group, and Xi , together with the carbon to which it is attached, forms a carbonyl or -(CH(ORs))-;

or

(a2) R3 and R4 combine to form a bond, R5 is H, and

each P4 is independently H or a hydroxyl protecting group, and Xi , together with the carbon to which it is attached, forms a carbonyl or -(CH(ORs))-;

or

both P4 groups and Xi , together with the atoms to which each is attached, combine to form ketal;

(b1 ) Z is chloride, bromide, or iodide, and R6 and R7 combine to form a bond;

(b2) Z and R7 combine to form a double bond, and R6 is a hydroxyl protecting group;

or

(b3) when Z and R7 are absent, R6 is H or a hydroxyl protecting group;

P5 is H or a hydroxyl protecting group;

where Rs is H or a hydroxyl protecting group. In a seventeenth aspect, the invention provides a compound of formula (VIIC):

(VIIC)

or a salt or a tautomer thereof,

where

each of D and D' is independently H, optionally substituted alkyl, or OPi , provided that only one of D and D' is OPi , where Pi is H, alkyl, or a hydroxyl protecting group; A is a group of formula (1 ) or a C1-6 saturated or C2-6 unsaturated hydrocarbon skeleton, the skeleton being unsubstituted or having from 1 to 10 substituents independently selected from the group consisting of cyano, halo, azido, oxo, and Qi , the group of formula (1 ) having the structure:

(1 ),

where

L is -(CH(OP ₂ ))-, -(C(OH)(OP ₂ ))-, or -C(O)-;

Ri is H, or Ri and Pi combine to form a bond;

R2 is H or -(CH2)nOP3, and each of P2 and P3 is independently H, optionally substituted alkyl, or a hydroxyl protecting group, or P2 and P3, together with the atoms to which each is attached, combine to form a ketal, a cyclic carbonate, a dicarbonyl-dioxo, or silylene- dioxo; or R2 and P2 combine to form an optionally substituted ethylene or a structure selected from the group consisting of:

, where each P' is independently H or a hydroxyl protecting group;

E is optionally substituted alkyl or optionally substituted alkoxy;

G is O, S, CH2, or NRN, where RN is H, an N-protecting group, or optionally substituted alkyl; each Qi is independently ORA, SRA, S0 ₂ RA, OS0 ₂ RA, NRBRA, NRB(CO)RA, NRB(CO)(CO)RA, NRA(CO)NRBRA, NRB(CO)ORA, (CO)ORA, 0(CO)RA, (CO)NRBRA, or 0(CO)NRBRA, where each of RA and RB IS independently H, alkyl, haloalkyi, hydroxyalkyi, aminoalkyi, aryl, haloaryl, hydroxyaryl, alkoxyaryl, arylalkyi, alkylaryl, haloarylalkyi, alkylhaloaryl, (alkoxyaryl)alkyl, heterocyclic radical, or heterocyclic radical-alkyl;

k is 0 or 1 ;

n is O, 1 , or 2;

each P4 is H or a hydroxyl protecting group, and Xi is oxo or Xi , together with the carbon atom to which it is attached, is -(CH(OPY))-, wherein PY is H or a hydroxyl protecting group; or both P4 groups and Xi , together with the atoms to which each is attached, combine to form a ketal; and

Ai and R4 combine to form oxo, R3 is H or a hydroxyl protecting group, and R5 is H ;

or

Ai is H or OP", where P" is H or a hydroxyl protecting group, and:

(i) R3 is H or a hydroxyl protecting group, and R4 and R5 combine to form a double bond;

or

(ii) R3 and R4 combine to form a bond, and R5 is H or OP".

In an eighteenth aspect, the invention provides a compound of formula (VIIIC):

(VIIIC)

or a salt thereof,

where

each of D and D' is independently H, optionally substituted alkyl, or OP1 , provided that only one of D and D' is OP1 , where Pi is H, alkyl, or a hydroxyl protecting group; A is a group of formula (1 ) or a C1-6 saturated or C2-6 unsaturated hydrocarbon skeleton, the skeleton being unsubstituted or having from 1 to 10 substituents independently selected from the group consisting of cyano, halo, azido, oxo, and Qi , the group of formula (1 ) having the structure:

(1 ),

where

L is -(CH(OP ₂ ))-, -(C(OH)(OP ₂ ))-, or -C(O)-;

Ri is H, or Ri and Pi combine to form a bond;

R2 is H or -(CH2)nOP3, and each of P2 and P3 is independently H, optionally substituted alkyl, or a hydroxyl protecting group, or P2 and P3, together with the atoms to which each is attached, combine to form a ketal, a cyclic carbonate, a dicarbonyl-dioxo, or silylene- dioxo; or R2 and P2 combine to form an optionally substituted ethylene or a structure selected from the group consisting of:

where each P' is independently H or a hydroxyl protecting group;

E is optionally substituted alkyl or optionally substituted alkoxy;

G is O, S, CH2, or NRN, where RN is H, an N-protecting group, or optionally substituted alkyl; each Qi is independently ORA, SRA, S0 ₂ RA, OS0 ₂ RA, NRBRA, NRB(CO)RA, NRB(CO)(CO)RA, NRA(CO)NR _B RA, NRB(CO)ORA, (CO)ORA, 0(CO)RA, (CO)NRBRA, or 0(CO)NRBRA, where each of RA and RB IS independently H, alkyl, haloalkyi, hydroxyalkyi, aminoalkyi, aryl, haloaryl, hydroxyaryl, alkoxyaryl, arylalkyi, alkylaryl, haloarylalkyi, alkylhaloaryl, (alkoxyaryl)alkyl, heterocyclic radical, or heterocyclic radical-alkyl;

k is 0 or 1 ;

n is O, 1 , or 2;

P4 is H or a hydroxyl protecting group; and

where Ps is H or a hydroxyl protecting group; each Ρβ is independently a hydroxyl protecting group, or both Ρβ groups, together with the atoms to which each is attached, combine to form a cyclic protected diol; and Rs is is H or -CH2Xi CH2CH=CH2, where Xi is

O, -CH2-, or NP7, where P7 is a sulfonyl.

The invention also features compounds of formula (IA), (IB), (IC), (IE), (IF), (MA), (MB), (IIC), (lICa), (MIA), (1MB), (IVA), (IVB), (IVC), (IVD), (IVE), (VA), (VB), (VBa), (VC), (VIA), (VIB), (VIC), (VIIA), (VIIB), (VIIC), (VIID), (VIIE), (VIIF), (VUG), (VIIIA), (VIIIB), (VIIIC), or (VIIID).

In certain embodiments of formula (IA), (IB), (IC), (MA), (MB), (MIA), (1MB), (VIA), (VIB), (VIC), (VIIA), (VIIB), and/or (VIIC) of the above aspects, each P4 is independently H or a hydroxyl protecting group, and Xi is oxo; or both P4 groups and Xi , together with the atoms to which each is attached, combine to form ketal (e.g., each P4 is independently H or a hydroxyl protecting group, and Xi is oxo).

In particular embodiments of formula (VIIIA) of the above aspects, X is O or X, together with the carbon to which it is attached, forms an acetal (e.g., cyclic acetal). Definitions

Compounds useful in the invention may be isotopically labeled compounds. Useful isotopes include hydrogen, carbon, nitrogen, and oxygen (e.g., ² H, ³ H, ¹³ C, ¹⁴ C, ¹⁵ N, ¹⁸ 0, and ¹⁷ 0). Isotopically-labeled compounds can be prepared by synthesizing a compound using a readily available isotopically-labeled reagent in place of a non-isotopically-labeled reagent.

For any of the following chemical definitions, a number following an atomic symbol indicates that total number of atoms of that element that are present in a particular chemical moiety. As will be understood, other atoms, such as hydrogen atoms, or substituent groups, as described herein, may be present, as necessary, to satisfy the valences of the atoms. For example, an unsubstituted C2 alkyl group has the formula -CH2CH3. When used with the groups defined herein, a reference to the number of carbon atoms includes the divalent carbon in acetal and ketal groups but does not include the carbonyl carbon in acyl, ester, carbonate, or carbamate groups. A reference to the number of oxygen, nitrogen, or sulfur atoms in a heteroaryl group only includes those atoms that form a part of a heterocyclic ring. By "acetal" is meant -0-(CHR)-0-, where R is H, optionally substituted alkyl, optionally substituted alkenyl, optionally substituted aryl, optionally substituted arylalkyl, or a bond to the chain of carbon atoms within an intermediate in the synthesis of the halichondrin macrolide. By "acyl" is meant -C(0)R, where R is H, alkyl, alkenyl, aryl, or arylalkyl. In exemplary acyl groups, R is H, C1-12 alkyl (e.g., C1-8, C1-6, C1-4, C2-7, C3-12, or C3-6 alkyl), C2-12 alkenyl (e.g., C2-8, C2-6, C2-4, C3-12, or C3-6 alkenyl), C6-20 aryl (e.g., Ce-15, Ce-ιο, C8-20, or Ce-is aryl), monocyclic C1-6 heteroaryl (e.g., monocyclic C1-4 or C2-6 heteroaryl), C4-19 heteroaryl (e.g., C4-10 heteroaryl), (C6-is)aryl(Ci-6)alkyl, (Ci -6)heteroaryl(Ci-6)alkyl, or (C4-i9)heteroaryl(Ci-6)alkyl. As defined herein, any heteroaryl group present in an acyl group has from 1 to 4 heteroatoms selected independently from O, N, and S. An acyl group can be unsubstituted or substituted (e.g., optionally substituted acyl). In the optionally substituted acyl group, the substituent R is H, optionally substituted alkyl, optionally substituted alkenyl, optionally substituted aryl, or optionally substituted arylalkyl. In some embodiments, acyl is C2-10 acyl.

By "acylating agent" is meant a compound that reacts with an amine or a hydroxyl group to produce an amide or an ester, respectively. An acylating agent has a formula R-LG, where R is acyl, and LG is halogen, carbonate, or -OR', where R' is acyl.

By "alkoxide" is meant an anionic compound RO ^_ , where R is alkyl. A counterion for alkoxide can be an alkali metal cation, an alkali earth metal cation, or a tetraalkylammonium cation. Alkoxide can be optionally substituted in the same manner as alkyl. By "alkoxy" is meant -OR, where R is alkyl. Alkoxy can be optionally substituted in the same manner as alkyl.

By "alkoxyalkyi" is meant -OR, where R is alkyl substituted by alkoxy. Each portion of the alkoxyalkyi can be optionally substituted in the same manner as alkyl.

By "alkoxyaryl" is meant -R'(R") _n , where n is 1 or 2, R' is arylene and R" is alkoxy, as defined herein. R' can be further optionally substituted in the same manner as aryl. R" can be optionally substituted in the same manner as alkyl. By "alkoxyarylalkyi" is meant -R'(R"(R'") _n ), where n is an integer from 1 to 3, R' is alkylene, R" is arylene, and R'" is alkoxy, as defined herein. R' can be optionally substituted in the same manner as alkyl. R" can be further optionally substituted in the same manner as aryl. R'" can be optionally substituted in the same manner as alkyl. By "alkyl" is meant a straight or branched chain saturated cyclic (i.e., cycloalkyl) or acyclic hydrocarbon group of from 1 to 12 carbons, unless otherwise specified. In some embodiments, alkyl is C1-6 alkyl. Exemplary alkyl groups include C1-8, C1-6, C1-4, C2-7, C3-12, and C3-6 alkyl. Specific examples include methyl, ethyl, 1 -propyl, 2-propyl, 2-methyl-1 -propyl, 1 -butyl, 2-butyl, and the like. Alkyl group can be optionally substituted with 1 , 2, 3, or 4 substituents selected from the group consisting of halogen, hydroxy, alkoxy, aryloxy, arylalkyloxy, amino, oxo, alkylthio, alkylenedithio, alkylamino, [alkenyl]alkylamino, [aryl]alkylamino, [arylalkyl]alkylamino, dialkylamino, silyl, sulfonyl, cyano, nitro, carboxyl, and azido.

By "alkylamino" is meant -NHR, where R is alkyl. By "[alkenyl]alkylamino" is meant -NRR', where R is alkyl, and R' is alkenyl. By "[aryl]alkylamino" is meant -NRR', where R is alkyl, and R' is aryl. By

"[arylalkyl]alkylamino" is meant -NRR', where R is alkyl, and R' is arylalkyl. By "dialkylamino" is meant - NR2, where each R is alkyl, selected independently.

By "alkylaryl" is meant -R'(R") _n , where n is an integer from 1 to 3, R' is arylene, and R" is alkyl. Alkylaryl can be optionally substituted in the same manner as defined for each R' and R" group.

By "alkylene" is meant a multivalent alkyl group. Alkylene groups can be optionally substituted in the same manner as alkyl groups. For example, a Ci alkylene group is -CH2-. By "alkylenedithio" is meant -S-alkylene-S-. Alkylenedithio can be optionally substituted in the same manner as an alkylene group.

By "alkylhaloaryl" is meant -R'(R") _n -R"', where n is an integer from 1 to 5 and R' is arylene, R" is halogen, and R'" is alkylene, as defined herein. R' can be further optionally substituted in the same manner as aryl. R'" can be further optionally substituted in the same manner as alkyl.

By "alkylthio" is meant -SR, where R is alkyl. Alkylthio can be optionally substituted in the same manner as an alkyl group. By "alkenyl" is meant a straight or branched chain cyclic or acyclic hydrocarbon group of, unless otherwise specified, from 2 to 12 carbons and containing one or more carbon-carbon double bonds. In some embodiments, alkenyl is C2-6 alkenyl. Exemplary alkenyl groups include C2-8, C2-7, C2-6, C2-4, C3-12, and C3-6 alkenyl. Specific examples include ethenyl (i.e., vinyl), 1 -propenyl, 2-propenyl (i.e., allyl), 2- methyl-1 -propenyl, 1 -butenyl, 2-butenyl (i.e., crotyl), and the like. Alkenyl group can be optionally substituted in the same manner as alkyl groups. Alkenyl groups, used in any context herein, may also be substituted with an aryl group.

By "amido" is meant -NHR, where R is acyl. Amido can be optionally substituted in the same manner as acyl.

By "aminal" is meant -O-CR2-NR'-, where each R is independently H, optionally substituted alkyl, optionally substituted alkenyl, optionally substituted aryl, or optionally substituted arylalkyl, or both R groups are together optionally substituted alkylene, and R' is H or an /V-protecting group. In particular, R' can be an /V-protecting group (e.g., Boc). By "amino" is meant -NR2, where N and R2 combine to form azido, or each R is independently H or an N- protecting group, or both R combine to form an /V-protecting group. Amino can be unmasked, when each R is H, or masked, when at least one R is not H. Thus, optionally masked amino can be masked or unmasked amino.

By "aminoalkyl" is meant -R'(R") _n , where n is 1 or 2, R' is alkylene, and R" is amino, as defined herein. R' can be optionally substituted in the same manner as an alkyl group.

By "aryl" is meant a monocyclic or multicyclic ring system having one or more aromatic rings, where the ring system is carbocyclic. Exemplary aryl groups include C6-20, Ce-15, Ce-ιο, C8-20, and Ce-is aryl. A preferred aryl group is a Ce-ιο aryl group. Specific examples of carbocyclic aryl groups include phenyl, indanyl, indenyl, naphthyl, phenanthryl, anthracyl, and fluorenyl. Aryl group can be optionally substituted with 1 , 2, 3, 4, or 5 substituents selected from the group consisting of alkyl, alkenyl, aryl, arylalkyi, halogen, alkoxy, aryloxy, arylalkyloxy, alkylthio, alkylenedithio, alkylamino, [alkenyl]alkylamino,

[aryl]alkylamino, [arylalkyl]alkylamino, dialkylamino, silyl, sulfonyl, cyano, nitro, carboxyl, and azido.

By "arylalkyi" is meant -R'R", where R' is alkylene, and R" is aryl. Arylalkyi can be optionally substituted in the same manner as defined for each R' and R" group. By "arylalkyloxy" is meant -OR, where R is arylalkyi. Arylalkyloxy can be optionally substituted in the same manner as defined for arylalkyi.

By "arylene" is meant a multivalent aryl group. Arylene groups can be optionally substituted in the same manner as aryl groups. For example, a Ce arylene group is phenylene.

By "aryloxy" is meant -OR, where R is aryl. Aryloxy can be optionally substituted in the same manner as aryl.

By "azido" is meant -N3.

By "boronate" is meant -OBRO-, where R is alkyl, alkenyl, aryl, arylalkyi, alkoxy, or 2,6- diacetamidophenyl. Boronate can be substituted, when R is a substituted alkyl, substituted alkenyl, substituted aryl, substituted arylalkyi, or substituted alkoxy. Alternatively, boronate can be unsubstituted, when R is unsubstituted alkyl, unsubstituted alkenyl, aryl, unsubstituted arylalkyi, unsubstituted alkoxy, or 2,6-diacetamidophenyl.

By "carbamate" is meant a group, when a hydroxyl protecting group, having the formula -OC(0)NR2, or, when an amine protecting group, having the formula -NR'-C(0)OR, where each R and R' is

independently H, optionally substituted alkyl, optionally substituted alkenyl, optionally substituted aryl, or optionally substituted arylalkyi. By "carbonate" is meant -OC(0)OR, where R is optionally substituted alkyl, optionally substituted alkenyl, optionally substituted aryl, or optionally substituted arylalkyl.

By "carbonyl" is meant -C(O)-.

By "carboxyl" is meant -C(0)OH, in free acid, ionized, or salt form. By "carboxylic acid" is meant R-OH, where R is optionally substituted acyl. By "carboxylic acid anhydride" is meant R-O-R, where each R is independently optionally substituted acyl. By "cyclic carbonate" is meant -OC(0)0- that is part of a ring. By "dicarbonyl" is meant -C(0)-C(0)-. Dicarbonyl-dioxo is -OC(0)-COO-.

By "ester" is meant -OC(0)R, where -C(0)R is an optionally substituted acyl group.

By "ether" is meant -OR, where R is alkyl, alkenyl, arylalkyl, silyl, or 2-tetrahydropyranyl. Ether can be optionally substituted as defined for each R group.

By "halichondrin macrolide" is meant a lactone including the structure of carbons 1 -30 as shown in Chart 1 , wherein carbons 29 and 30 form part of a five- or six-membered ring.

By "haloalkyl" is meant -R'(R") _n , where n is an integer from 1 to 5 and R' is alkylene and R" is halogen, as defined herein. R' can be further optionally substituted in the same manner as alkyl

By "haloaryl" is meant -R'(R") _n , where n is an integer from 1 to 5 and R' is arylene and R" is halogen, as defined herein. R' can be further optionally substituted in the same manner as aryl. By "haloarylalkyl" is meant -R'(R"(R"') _n ), where n is an integer from 1 to 5 and R' is alkylene, R" is arylene, and R'" is halogen, as defined herein. R' can be further optionally substituted in the same manner as alkyl. R" can be further optionally substituted in the same manner as aryl.

By "halogen" is meant fluoro, chloro, bromo, or iodo.

By "heterocyclic radical" is meant a 5-, 6- or 7-membered ring, unless otherwise specified, containing one, two, three, or four heteroatoms independently selected from the group containing nitrogen, oxygen, and sulfur. The 5-membered ring has zero to one double bonds, and the 6- and 7-membered rings have zero to two double bonds. Certain heterocyclyl groups include from 1 to 9 carbon atoms. Other such groups may include up to 12 carbon atoms. The term "heterocyclyl" also represents a heterocyclic compound having a bridged multicyclic structure in which one or more carbons and/or heteroatoms bridges two non- adjacent members of a monocyclic ring, e.g., a quinuclidinyl group. The term "heterocyclyl" includes bicyclic, tricyclic, and tetracyclic groups in which any of the above heterocyclic rings is fused to one, two, or three carbocyclic rings, e.g., an aryl ring, a cyclohexane ring, a cyclohexene ring, a cyclopentane ring , a cyclopentene ring, or another monocyclic heterocyclic ring, such as indolyl , quinolyl, isoquinolyl, tetrahydroquinolyl , benzofuryl, benzothienyl and the like. Examples of fused heterocyclyls include tropanes and 1 ,2,3,5,8,8a-hexahydroindolizine. Heterocyclics include pyrrolyl, pyrrolinyl, pyrrolidinyl, pyrazolyl, pyrazolinyl, pyrazolidinyl , imidazolyl , imidazolinyl, imidazolidinyl, pyridyl, piperidinyl, homopiperidinyl , pyrazinyl, piperazinyl , pyrimidinyl, pyridazinyl , oxazolyl, oxazolidinyl , isoxazolyl, isoxazolidiniyl , morpholinyl , thiomorpholinyl, thiazolyl, thiazolidinyl , isothiazolyl , isothiazolidinyl, indolyl , quinolinyl , isoquinolinyl , benzimidazolyl, benzothiazolyl, benzoxazolyl , furyl , thienyl, thiazolidinyl , isothiazolyl , isoindazoyl, triazolyl, tetrazolyl, oxadiazolyl, purinyl, thiadiazolyl (e.g., 1 ,3,4-thiadiazole), tetrahydrofuranyl , dihydrofuranyl , tetrahydrothienyl, dihydrothienyl, dihydroindolyl, tetrahydroquinolyl, tetrahydroisoquinolyl, pyranyl, dihydropyranyl , dithiazolyl , benzofuranyl , benzothienyl and the like. Still other exemplary heterocyclyls include: 2,3,4, 5-tetrahydro-2-oxo-oxazolyl ; 2,3-dihydro-2-oxo-1 H- imidazolyl ; 2,3,4,5-tetrahydro-5-oxo-1 H-pyrazolyl (e.g., 2,3,4,5-tetrahydro-2-phenyl-5-oxo-1 H-pyrazolyl) ; 2,3,4,5-tetrahydro-2,4-dioxo-1 H-imidazolyl (e.g., 2,3,4,5-tetrahydro-2,4-dioxo-5-methyl-5-phenyl-1 H- imidazolyl) ; 2,3-dihydro-2-thioxo-1 ,3,4-oxadiazolyl (e.g., 2,3-dihydro-2-thioxo-5-phenyl-1 ,3,4-oxadiazolyl) ; 4,5-dihydro-5-oxo-1 -/-triazolyl (e.g., 4,5-dihydro-3-methyl-4-amino 5-oxo-1 /-/-triazolyl) ; 1 ,2,3,4-tetrahydro- 2,4-dioxopyridinyl (e.g., 1 ,2,3,4-tetrahydro-2,4-dioxo-3,3-diethylpyridinyl) ; 2,6-dioxo-piperidinyl (e.g., 2,6- dioxo-3-ethyl-3-phenylpiperidinyl) ; 1 ,6-dihydro-6-oxopyridiminyl ; 1 ,6-dihydro-4-oxopyrimidinyl (e.g., 2- (methylthio)-l ,6-dihydro-4-oxo-5-methylpyrimidin-1 -yl) ; 1 ,2,3,4-tetrahydro-2,4-dioxopyrimidinyl (e.g., 1 ,2,3,4-tetrahydro-2,4-dioxo-3-ethylpyrimidinyl) ; 1 ,6-dihydro-6-oxo-pyridazinyl (e.g., 1 ,6-dihydro-6-oxo-3- ethylpyridazinyl) ; 1 ,6-dihydro-6-oxo-1 ,2,4-triazinyl (e.g., 1 ,6-dihydro-5-isopropyl-6-oxo-1 ,2,4-triazinyl) ; 2,3- dihydro-2-oxo-1 /- -indolyl (e.g., 3,3-dimethyl-2,3-dihydro-2-oxo-1 /- -indolyl and 2,3-dihydro-2-oxo-3,3'- spiropropane-1 /- -indol-1 -yl) ; 1 ,3-dihydro-1 -oxo-2/-/-iso-indolyl ; 1 ,3-dihydro-1 ,3-dioxo-2/-/-iso-indolyl ; I ff benzopyrazolyl (e.g., l -(ethoxycarbonyl)- 1 /-/-benzopyrazolyl) ; 2,3-dihydro-2-oxo-1 /-/-benzimidazolyl (e.g., 3-ethyl-2,3-dihydro-2-oxo-1 /-/-benzimidazolyl) ; 2,3-dihydro-2-oxo-benzoxazolyl (e.g., 5-chloro-2,3-dihydro- 2-oxo-benzoxazolyl) ; 2,3-dihydro-2-oxo-benzoxazolyl ; 2-oxo-2H-benzopyranyl ; 1 ,4-benzodioxanyl ; 1 ,3- benzodioxanyl ; 2,3-dihydro-3-oxo,4/-/-1 ,3-benzothiazinyl ; 3,4-dihydro-4-oxo-3/-/-quinazolinyl (e.g., 2- methyl-3,4-dihydro-4-oxo-3/-/-quinazolinyl) ; 1 ,2,3,4-tetrahydro-2,4-dioxo-3/-/-quinazolyl (e.g., 1 -ethyl- 1 ,2,3,4-tetrahydro-2,4-dioxo-3f/-quinazolyl) ; 1 ,2,3,6-tetrahydro-2,6-dioxo-7f/-purinyl (e.g., 1 ,2,3,6- tetrahydro-1 ,3-dimethyl-2,6-dioxo-7 H -purinyl) ; 1 ,2,3,6-tetrahydro-2,6-dioxo-1 -/ -purinyl (e.g., 1 ,2,3,6- tetrahydro-3,7-dimethyl-2,6-dioxo-1 /-/ -purinyl) ; 2-oxobenz[c,c/|indolyl ; 1 ,1 -dioxo-2H-naphth[1 ,8- c,c/|isothiazolyl ; and 1 ,8-naphthylenedicarboxamido. Heterocyclic groups also include groups of the where

F' is selected from the group consisting of -CH2-, -CH2O- and -0-, and G' is selected from the group consisting of -C(O)- and -(C(R')(R")) _V -, where each of FT and R" is, independently, selected from the group consisting of hydrogen or alkyl of one to four carbon atoms, and v is one to three and includes groups, such as 1 ,3-benzodioxolyl , 1 ,4-benzodioxanyl , and the like. Any of the heterocyclyl groups mentioned herein may be optionally substituted with one, two, three, four or five substituents

independently selected from the group consisting of : (1 ) alkanoyl (e.g., formyl, acetyl, and the like ) ; (2) alkyl (e.g., alkoxyalkylene, alkylsulfinylalkylene, aminoalkylene, azidoalkylene, acylalkylene, haloalkylene (e.g., perfluoroalkyl), hydroxyalkylene, nitroalkylene, or thioalkoxyalkylene) ; (3) alkenyl ; (4) alkynyl ; (5) alkoxy (e.g., perfluoroalkoxy) ; (6) alkylsulfinyl ; (7) aryl ; (8) amino; (9) aryl-alkylene; (1 0) azido; (1 1 ) cycloalkyl ; (12) cycloalkyl-alkylene; (13) cycloalkenyl ; (14) cycloalkenyl-alkylene ; (1 5) halo; (1 6) heterocyclyl (e.g., heteroaryl) ; (1 7) (heterocyclyl)oxy; (1 8) (heterocyclyl)aza; (1 9) hydroxy; (20) oxo; (21 ) nitro; (22) sulfide; (23) thioalkoxy; (24) -(CH2) _q C02R ^A , where q is an integer from zero to four, and R ^A is selected from the group consisting of (a) alkyl, (b) aryl, (c) hydrogen, and (d) aryl-alkylene; (25) - (CH2)qCONR ^B R ^c , where q is an integer from zero to four and where R ^B and R ^c are independently selected from the group consisting of (a) hydrogen, (b) alkyl, (c) aryl, and (d) aryl-alkylene; (26) - (CH2) _q S02R ^D , where q is an integer from zero to four and where R ^D is selected from the group consisting of (a) alkyl, (b) aryl , and (c) aryl-alkylene; (27) -(CH2) _q S02NR ^E R ^F , where q is an integer from zero to four and where each of R ^E and R ^F is, independently, selected from the group consisting of (a) hydrogen, (b) alkyl, (c) aryl , and (d) aryl-alkylene ; (28) thiol ; (29) aryloxy; (30) cycloalkoxy; (31 ) arylalkoxy; (31 ) heterocyclyl-alkylene (e.g., heteroaryl-alkylene) ; (32) silyl ; (33) cyano; and (34) -S(0)R ^H where R ^H is selected from the group consisting of (a) hydrogen, (b) alkyl, (c) aryl, and (d) aryl-alkylene. In some embodiments, each of these groups can be further substituted as described herein . For example, the alkylene group of an aryl-Ci -alkylene or a heterocyclyl-Ci -alkylene can be further substituted with an oxo group to afford the respective aryloyl and (heterocyclyl)oyl substituent group. In addition, when a heterocyclyl group is present in a bioreversible group of the invention it may be substituted with an ester, thioester, or disulfide group that is bound to a conjugating moiety, a hydrophilic functional group, or an auxiliary moiety as defined herein. By "heterocyclic radical alkyl," as used herein, represents an alkyl group substituted with a heterocyclic radical. The heterocyclic radical and alkyl portions may be substituted as the individual groups as described herein.

By "hydroxyalkyl" is meant -R'(R") _n , where n 1 or 2, R' is alkylene and R" is hydroxyl, as defined herein. R' can be further optionally substituted in the same manner as alkyl.

By "hydroxyaryl" is meant -R'(R") _n , where n is 1 or 2, R' is arylene and R" is hydroxyl, as defined herein. R' can be further optionally substituted in the same manner as aryl. By "hydroxyl" is meant -OH .

By "hydroxyl protecting group" is meant any group capable of protecting the oxygen atom to which it is attached from reacting or bonding. Hydroxyl protecting groups are known in the art, e.g. , as described in Wuts, Greene's Protective Groups in Organic Synthesis, Wiley-lnterscience, 4th Edition, 2006.

Exemplary protecting groups (with the oxygen atom to which they are attached) are independently selected from the group consisting of esters, carbonates, carbamates, sulfonates, and ethers. In exemplary ester hydroxyl protecting groups, R of the acyl group is C1-12 alkyl (e.g., Ci-e, C1-6, C1-4, C2-7, C3- 12, and C3-6 alkyl), C2-12 alkenyl (e.g., C2-8, C2-6, C2-4, C3-12, and C3-6 alkenyl), carbocyclic C6-20 aryl (e.g., Ce- is, C6-10, C8-20, and Ce-15 aryl), monocyclic C1-6 heteroaryl (e.g., C1-4 and C2-6 heteroaryl), C4-19 heteroaryl (e.g., C4-10 heteroaryl), (Ce-ι s)aryl(Ci -6)alkyl , (C4-i 9)heteroaryl(Ci-6)alkyl, or (Ci-6)heteroaryl(Ci-6)alkyl. Specific examples of acyl groups for use in esters include formyl, benzoylformyl, acetyl (e.g.,

unsubstituted or chloroacetyl, trifluoroacetyl, methoxyacetyl, triphenylmethoxyacetyl, and p- chlorophenoxyacetyl), 3-phenylpropionyl, 4-oxopentanoyl, 4,4-(ethylenedithio)pentanoyl, pivaloyl (Piv), vinylpivaloyl, crotonoyl, 4-methoxy-crotonoyl, naphthoyl (e.g., 1 - or 2-naphthoyl), and benzoyl (e.g., unsubstituted or substituted, e.g., p-methoxybenzoyl, phthaloyl (including salts, such a triethylamine and potassium), p-bromobenzoyl, and 2,4,6-trimethylbenzoyl). As defined herein, any heteroaryl group present in an ester group has from 1 to 4 heteroatoms selected independently from O, N, and S. In exemplary carbonate hydroxyl protecting groups, R is C1-12 alkyl (e.g., Ci-e, C1-6, C1-4, C2-7, C3-12, and C3-6 alkyl), C2-12 alkenyl (e.g., C2-8, C2-6, C2-4, C3-12, and C3-6 alkenyl), carbocyclic C6-20 aryl (e.g., Ce-15, Ce-ιο, Ce- 20, and Ce-is aryl), monocyclic C1-6 heteroaryl (e.g., C1-4 and C2-6 heteroaryl), C4-19 heteroaryl (e.g., C4-10 heteroaryl), (C6-is)aryl(Ci-6)alkyl, (C4-i 9)heteroaryl(Ci-6)alkyl, or (Ci-6)heteroaryl(Ci-6)alkyl. Specific examples include methyl, 9-fluorenylmethyl, ethyl, 2,2,2-trichloroethyl, 2-(trimethylsilyl)ethyl, 2- (phenylsulfonyl)ethyl, vinyl, allyl, t-butyl, p-nitrobenzyl, and benzyl carbonates. As defined herein, any heteroaryl group present in a carbonate group has from 1 to 4 heteroatoms selected independently from O, N, and S. In exemplary carbamate hydroxyl protecting groups, each R is independently H, C1-12 alkyl (e.g., C1-8, C1-6, C1-4, C2-7, C3-12, and C3-6 alkyl), C2-12 alkenyl (e.g., C2-8, C2-6, C2-4, C3-12, and C3-6 alkenyl), carbocyclic C6-20 aryl (e.g., Ce-15, Ce-ι ο, Ce-20, and Ce-15 aryl), monocyclic C1-6 heteroaryl (e.g., C1-4 and C2-6 heteroaryl), C4-19 heteroaryl (e.g., C4-10 heteroaryl), (C6-is)aryl(Ci-6)alkyl, (C4-i9)heteroaryl(Ci-6)alkyl, or (Ci- 6)heteroaryl(Ci-6)alkyl. Specific examples include N-phenyl and N-methyl-N-(o-nitrophenyl) carbamates. As defined herein, any heteroaryl group present in a carbamate group has from 1 to 4 heteroatoms selected independently from O, N, and S. Exemplary ether hydroxyl protecting groups include C1-12 alkyl (e.g., C1-8, C1-6, C1-4, C2-7, C3-12, and C3-6 alkyl), C2-12 alkenyl (e.g., C2-e, C2-6, C2-4, C3-12, and C3-6 alkenyl), (Ce-i 5)aryl(Ci -e)alkyl, (C4-i 9)heteroaryl(Ci-e)alkyl, (Ci-e)heteroaryl(Ci-e)alkyl, (Ci-e)alkoxy(Ci-e)alkyl, (Ci- 6)alkylthio(Ci-6)alkyl, (C6-io)aryl(Ci-6)alkoxy(Ci-6)alkyl, and silyl (e.g., tri(Ci-e alkyl)silyl, tri(Ce-io aryl or C1-6 heteroaryl)silyl, di(C6-io aryl or C1-6 heteroaryl)(Ci-6 alkyl)silyl, and (Ce-ιο aryl or C1-6 heteroaryl)di(Ci-6 alkyl)silyl). Specific examples of alkylethers include methyl and t-butyl, and an example of an alkenyl ether is allyl. Ether hydroxyl protecting groups can be used to protect a carboxyl group (e.g., with a C1-12 alkyl (e.g., Ci-e, Ci-e, C1-4, C2-7, C3-12, and C ₃ -e alkyl), (Ce-i s)aryl(Ci -e)alkyl , (Ci-e)alkoxy(Ci-e)alkyl, (Ci- 6)alkylthio(Ci-6)alkyl, or (C6-io)aryl(Ci-6)alkoxy(Ci-6)alkyl). Examples of alkoxyalkyls and alkylthioalkyls that can be used as ether hydroxyl protecting groups include methoxymethyl, methylthiomethyl, (2- methoxyethoxy)methyl, and p-(trimethylsilyl)ethoxymethyl. Examples of arylalkyl groups that can be used as ether hydroxyl protecting groups include benzyl, p-methoxybenzyl (MPM), 3,4-dimethoxybenzyl, triphenylmethyl (trityl), o-nitrobenzyl, p-nitrobenzyl, p-halobenzyl, 2,6-dichlorobenzyl, p-cyanobenzyl, naphthylmethyl, and 2- and 4-picolyl ethers. Specific examples of silylethers include trimethylsilyl (TMS), triethylsilyl (TES), t-butyldimethylsilyl (TBS), t-butyldiphenylsilyl (TBDPS), triisopropylsilyl (TIPS), and triphenylsilyl (TPS) ethers. An example of an arylalkyloxyalkylether is benzyloxymethyl ether. As defined herein, any heteroaryl group present in an ether group has from 1 to 4 heteroatoms selected independently from O, N, and S. Vicinal or 1 ,3-diols may be protected with a diol protecting group (e.g., to produce a "cyclic protected diol"), such as acetal (e.g., containing C1-6 alkylene), ketal (e.g., containing C3-6 alkylene or C3-6 cycloalkyl), cyclic silylene, cyclic carbonate, and cyclic boronate. Examples of acetal and ketal groups include methylene-dioxo, ethylidene-dioxo, benzylidene-dioxo, isopropylidene-dioxo, cyclohexylidene-dioxo, and cyclopentylidene-dioxo. An example of a cyclic silylene is di-t-butylsilylene. Another diol protecting group is 1 ,1 ,3,3-tetraisopropylsiloxanediyl. Examples of cyclic boronates include methyl, ethyl, phenyl, and 2,6-diacetamidophenyl boronates. Protecting groups may be substituted as is known in the art; for example, aryl and arylalkyi groups, such as phenyl, benzyl, naphthyl, or pyridinyl, can be substituted with C1-6 alkyl, C1-6 alkoxy, nitro, cyano, carboxyl, or halogen. Alkyl groups, such as methyl, ethyl, isopropyl, n-propyl, t-butyl, n-butyl, and sec-butyl, and alkenyl groups, such as vinyl and allyl, can also be substituted with oxo, arylsulfonyl, halogen, and trialkylsilyl groups. Preferred protecting groups are TBS and Piv. Protecting groups that are orthogonal are removed under different conditions, as in known in the art. By "imido" is meant -NR2, where each R is independently optionally substituted acyl.

By "ketal" is meant -O-CR2-O-, where each R is independently optionally substituted alkyl, optionally substituted alkenyl, optionally substituted aryl, optionally substituted arylalkyi, or a bond to the chain of carbon atoms within an intermediate in the synthesis of the halichondrin macrolide; or both R groups are together optionally substituted alkylene.

By "macrocyclization" is meant a reaction converting a non-macrocyclic compound into a compound containing at least one n-membered ring, where n is equal to or greater than 16. By "MNBA" is meant 2-methyl-6-nitrobenzoic anhydride.

By "non-enolizable" is meant a group that, either alone or in combination with a group to which it is attached, cannot form an enol through a deprotonation/reprotonation sequence. For example, a "non- enolizable alkyl" can be bonded to a sulfone group or to a carbonyl group through a quaternary carbon atom (i.e., the carbon atom that is not bonded to a hydrogen atom).

By "non-macrocyclic" is meant a compound not containing rings or containing one or more m-membered rings, where m is less than or equal to 1 5. By "/V-protecting group" is meant a group protecting a nitrogen atom in a molecule from participating in one or more undesirable reactions during chemical synthesis (e.g., oxidation reactions, or certain nucleophilic and electrophilic substitutions). Commonly used /V-protecting groups are disclosed in Wuts, Greene's Protective Groups in Organic Synthesis, Wiley-lnterscience, 4th Edition, 2006. Exemplary N- protecting groups include acyl (e.g., formyl, acetyl, trifluoroacetyl, propionyl, pivaloyl, t-butyl acetyl, 2- chloroacetyl, 2-bromoacetyl, trifluoroacetyl, trichloroacetyl, phthalyl, o-nitrophenoxyacetyl, a-chlorobutyryl, benzoyl, 4-chlorobenzoyl, and 4-bromobenzoyl); sulfonyl-containing groups (e.g., benzenesulfonyl, p- toluenesulfonyl, o-nitrobenzenesulfonyl, and p-nitrobenzenesulfonyl); carbamate forming groups (e.g., benzyloxycarbonyl, p-chlorobenzyloxycarbonyl, p-methoxybenzyloxycarbonyl, p-nitrobenzyloxycarbonyl, 2-nitrobenzyloxycarbonyl, p-bromobenzyloxycarbonyl, 3,4-dimethoxybenzyloxycarbonyl,

3,5-dimethoxybenzyl oxycarbonyl, 2,4-dimethoxybenzyloxycarbonyl, 4-methoxybenzyloxycarbonyl, 2- nitro-4,5-dimethoxybenzyloxycarbonyl, 3,4,5-trimethoxybenzyloxycarbonyl, 1 -(p-biphenylyl)-1 - methylethoxycarbonyl, a,a-dimethyl-3,5-dimethoxybenzyloxycarbonyl, benzhydryloxy carbonyl, t- butyloxycarbonyl, diisopropylmethoxycarbonyl, isopropyloxycarbonyl, ethoxycarbonyl, methoxycarbonyl, allyloxycarbonyl, 2,2,2,-trichloroethoxycarbonyl, phenoxycarbonyl, 4-nitrophenoxy carbonyl, fluorenyl-9- methoxycarbonyl, cyclopentyloxycarbonyl, adamantyloxycarbonyl, cyclohexyloxycarbonyl, and phenylthiocarbonyl), arylalkyl (e.g., triphenylmethyl); silyl groups (e.g., trimethylsilyl); and imine-forming groups (e.g., diphenylmethylene). Preferred /V-protecting groups are acetyl, benzoyl, phenylsulfonyl, p- toluenesulfonyl, p-nitrobenzenesulfonyl, o-nitrobenzenesulfonyl, t-butyloxycarbonyl (Boc), and benzyloxycarbonyl (Cbz). By "oxo" or (O) is meant =0.

By "pharmaceutically acceptable salt" is meant a salt within the scope of sound medical judgment, suitable for use in contact with the tissues of humans and animals without undue toxicity, irritation, allergic response and the like and commensurate with a reasonable benefit/risk ratio. Pharmaceutically acceptable salts are well known in the art. For example, pharmaceutically acceptable salts are described in: Berge et al., J. Pharmaceutical Sciences 66:1 -19, 1977 and in Pharmaceutical Salts: Properties, Selection, and Use, (Eds. P.H. Stahl and C.G. Wermuth), Wiley-VCH, 2008. Representative acid addition salts include acetate, adipate, alginate, ascorbate, aspartate, benzenesulfonate, benzoate, bisulfate, borate, butyrate, camphorate, camphorsulfonate, citrate, cyclopentanepropionate, digluconate, dodecylsulfate, ethanesulfonate, fumarate, glucoheptonate, glycerophosphate, hemisulfate, heptonate, hexanoate, hydrobromide, hydrochloride, hydroiodide, 2-hydroxy-ethanesulfonate, lactobionate, lactate, laurate, lauryl sulfate, malate, maleate, malonate, methanesulfonate, 2-naphthalenesulfonate, nicotinate, nitrate, oleate, oxalate, palmitate, pamoate, pectinate, persulfate, 3-phenylpropionate, phosphate, picrate, pivalate, propionate, stearate, succinate, sulfate, tartrate, thiocyanate, toluenesulfonate, undecanoate, valerate salts and the like. A preferred salt is the mesylate salt.

By "silyl" is meant -S1R3, where each R is independently alkyl, alkenyl, aryl, or arylalkyl. Examples of silyl groups include tri(Ci-e alkyl)silyl, tri(C6-io aryl or C1-6 heteroaryl)silyl, di(C6-io aryl or C1-6 heteroaryl)(Ci-6 alkyl)silyl, and (C6-10 aryl or C1-6 heteroaryl)di(Ci-6 alkyl)silyl. It will be understood that, when a silyl group includes two or more alkyl, alkenyl, aryl, heteroaryl, or arylalkyl groups, these groups are independently selected. As defined herein, any heteroaryl group present in a silyl group has from 1 to 4 heteroatoms selected independently from O, N, and S. Silyl can be optionally substituted in the same manner as defined for each R group. By "silylene" is meant -S1R2-, where each R is independently alkyl, alkenyl, aryl, arylalkyl, or alkoxy. By "dialkylsilylene" is meant a silylene, where each R is alkyl. Silylene can be optionally substituted in the same manner as defined for each R group. Silylene-dioxo is a group having the formula -O-S1R2-O-. By "strong base" is meant a Bronsted base, the conjugate acid of which has pKa that is greater than or equal to 13. Non-limiting examples of strong bases include alkyl alkali metals (e.g., butyl lithium or Schlosser's base), Grignard reagents (e.g., alkyl magnesium halide), alkoxides (e.g., tertiary alkoxides, such as f-butoxide), amides (e.g., diisopropylamide, tetramethylpiperidide, or bis(trimethylsilyl)amide), and phosphazene bases (e.g., Schwesinger base).

By "sulfonamide" is meant -NR, where R is sulfonyl.

By "sulfonate" is meant -OS(0)2R, where R is optionally substituted alkyl, optionally substituted alkenyl, optionally substituted aryl, or optionally substituted arylalkyl. In exemplary sulfonates, R is C1-12 alkyl (e.g., C1-8, C1-6, C1-4, C2-7, C3-12, or C3-6 alkyl), C2-12 alkenyl (e.g., C2-8, C2-6, C2-4, C3-12, or C3-6 alkenyl), carbocyclic C6-20 aryl (e.g., Ce-15, Ce-ι ο, C8-20, or Cs-is aryl), monocyclic C1-6 heteroaryl (e.g., C1 -4 and C2-6 heteroaryl), C4-19 heteroaryl (e.g., C4-10 heteroaryl), (C6-is)aryl(Ci-6)alkyl, (C4-i9)heteroaryl(Ci-6)alkyl, or (Ci- 6)heteroaryl(Ci-6)alkyl. As defined herein, any heteroaryl group present in a sulfonate group has from 1 to 4 heteroatoms selected independently from O, N, and S.

By "sulfonyl" is meant -S(0)2R, where R is optionally substituted alkyl, optionally substituted alkenyl, optionally substituted aryl, optionally substituted arylalkyl, or silyl. Preferred R groups for sulfonyl are the same as those described above for sulfonates. By "thioacetal" is meant -S-(CHR)-S- where R is H, optionally substituted alkyl, optionally substituted alkenyl, optionally substituted aryl, or optionally substituted arylalkyl.

By "thioketal" is meant -S-(CR2)-S- where each R is independently optionally substituted alkyl, optionally substituted alkenyl, optionally substituted aryl, or optionally substituted arylalkyl.

By "trif late" is meant trifluoromethanesulfonate.

The pKa values recited herein refer to the pKa values of a conjugate Bronsted acid in water at room temperature, unless stated otherwise.

Brief Description of the Drawings

Figure 1 is a scheme showing preparation of a halichondrin macrolide involving a C.2-C.3 bond-forming macrocyclization through Horner-Wadsworth-Emmons olefination.

Figure 2 is a scheme showing preparation of a halichondrin macrolide involving a C.2-C.3 bond-forming macrocyclization through ring-closing olefin metathesis. Figure 3 is a scheme showing preparation of a halichondrin macrolide involving a C.3-C.4 bond-forming macrocyclization through ring-closing olefin metathesis. Figure 4 is a scheme showing preparation of a halichondrin macrolide involving a C.12-C.13 bond- forming macrocyclization through ring-closing olefin metathesis.

Figure 5 is a scheme showing preparation of a halichondrin macrolide involving a C.15-C.16 bond- forming macrocyclization through ring-closing olefin metathesis.

Figure 6 is a scheme showing preparation of a halichondrin macrolide involving a C.19-C.20 bond- forming macrocyclization through Nozaki-Hiyama-Kishi reaction.

Figure 7 is a scheme showing preparation of a halichondrin macrolide involving a C.26-C.27 bond- forming macrocyclization through Nozaki-Hiyama-Kishi reaction.

Detailed Description

The present invention provides methods for the synthesis of a halichondrin macrolide (see Chart 1 ) through a macrocyclization. Preferably, the halichondrin macrolide is a halichondrin B macrolide. The macrocyclizations of the present invention involve subjecting a non-macrocyclic intermediate to a carbon- carbon bond-forming reaction (e.g., an olefination reaction (e.g., Horner-Wadsworth-Emmons olefination), catalytic Ring-Closing Olefin Metathesis, or Nozaki-Hiyama-Kishi reaction) to afford a macrocyclic intermediate (e.g., a compound of formula (IB), (1MB), (IVB), (VB), (VIB), or (VIIB)). The carbon-carbon bond forming reaction provides C.2-C.3, C.3-C.4, C.12-C.13, C.15-C.16, C.19-C.20, or C.26-C.27 bond in the structure of a halichondrin macrolide. The carbon-atom numbering scheme for a halichondrin macrolide is shown in Chart 1 .

in which

each of D and D' is independently H, optionally substituted alkyl, or OPi , provided that only one of D and D' is OPi , where Pi is H, alkyl, or a hydroxyl protecting group; A is a group of formula (1 ) or a C1-6 saturated or C2-6 unsaturated hydrocarbon skeleton, the skeleton being unsubstituted or having from 1 to 1 0 substituents independently selected from the group consisting of cyano, halo, azido, oxo, and Qi , the group of formula (1 ) having the structure:

(1 ),

where

L is -(CH(OP ₂ ))-, -(C(OH)(OP ₂ ))-, or -C(O)-;

Ri is H, or Ri and Pi combine to form a bond;;

R2 is H or -(CH2)nOP3, and each of P2 and P3 is independently H, optionally substituted alkyl, or a hydroxyl protecting group, or P2 and P3, together with the atoms to which each is attached, combine to form a ketal, a cyclic carbonate, a dicarbonyl-dioxo, or silylene- dioxo; or R2 and P2 combine to form an optionally substituted ethylene or a structure selected from the group consisting of:

where each P' is independently H or a hydroxyl protecting group;

each of Ai and A2 is independently H or OP", where each P" is independently H or a hydroxyl protecting group;

E is optionally substituted alkyl or optionally substituted alkoxy;

G is O, S, CH2, or NRN, where RN is H, an N-protecting group, or optionally substituted alkyl; each Qi is independently ORA, SRA, S0 ₂ RA, OS0 ₂ RA, NRBRA, NRB(CO)RA, NRB(CO)(CO)RA,

NRA(CO)NRBRA, NRB(CO)ORA, (CO)ORA, 0(CO)RA, (CO)NRBRA, or 0(CO)NRBRA, where each of RA and RB IS independently H, alkyl, haloalkyi, hydroxyalkyi, aminoalkyi, aryl, haloaryl, hydroxyaryl, alkoxyaryl, arylalkyi, alkylaryl, haloarylalkyi, alkylhaloaryl, (alkoxyaryl)alkyl, heterocyclic radical, or heterocyclic radical-alkyl;

k is 0 or 1 ;

n is O, 1 , or 2. In some embodiments of the halichondrin macrolide, C and D' are not the same, and a designates (S)- stereogenic center. In certain embodiments of the halichondrin macrolide, one of D and D' is optionally substituted alkyl or OPi , the other of D and D' is H, and G is O. In particular embodiments of the halichondrin macrolide, one and only one of D and D' is OPi , where Pi is H, alkyl, or a hydroxyl protecting group. In one embodiment of the halichondrin macrolide, k is 1 , E is optionally substituted alkyl (e.g., Me); and D ise a group of formula (1 ); in further embodiments, L is -(CH(OP2))-, Ri and Pi combine to form a bond, R2 is -(CH2)nOP3, each of P2 and P3 is H, and n is 1 or 2 (e.g., n is 2).

The invention also provides intermediates in the synthesis of halichondrin and analogs of the macrolide in the structure of halichondrin and methods of preparing the same, as described herein.

C.2-C.3 Bond-forming Macrocyclization

The macrocyclization reaction of the invention can be a carbon-carbon bond-forming reaction (e.g., Horner-Wadsworth-Emmons olefination) that provides a C.2-C.3 bond in a halichondrin macrolide. The general synthetic sequence including Horner-Wadsworth-Emmons olefination that can be used to prepare a halichondrin macrolide is shown in Figure 1 . As shown in Figure 1 , the non-macrocyclic intermediate in the synthesis of the halichondrin macrolide can be a compound of formula (IA):

(IA)

or a salt or a tautomer thereof,

in which

each R is independently optionally substituted alkyl or optionally substituted aryl;

each of D and D' is independently H, optionally substituted alkyl, or OPi , provided that only one of D and D' is OPi , where Pi is H, alkyl, or a hydroxyl protecting group; A is a group of formula (1 ) or a C1-6 saturated or C2-6 unsaturated hydrocarbon skeleton, the skeleton being unsubstituted or having from 1 to 10 substituents independently selected from the group consisting of cyano, halo, azido, oxo, and Qi , the group of formula (1 ) having the structure:

(1 ),

where L is -(CH(OP ₂ ))-, -(C(OH)(OP ₂ ))-, or -C(O)-;

Ri is H, or Ri is H, or Ri and Pi combine to form a bond;

Pi2 is H or -(CH2)nOP3, and each of P2 and P3 is independently H, optionally substituted alkyl, or a hydroxyl protecting group, or P2 and P3, together with the atoms to which each is attached, combine to form a ketal, a cyclic carbonate, a dicarbonyl-dioxo, or silylene- dioxo; or R2 and P2 combine to form an optionally substituted ethylene or a structure selected from the group consisting of:

group;

E is optionally substituted alkyl or optionally substituted alkoxy;

G is O, S, CH2, or NRN, where RN is H, an N-protecting group, or optionally substituted alkyl ; each Qi is independently ORA, SRA, S0 ₂ RA, OS0 ₂ RA, NRBRA, NRB(CO)RA, NRB(CO)(CO)RA, NRA(CO)NRBRA, NRB(CO)ORA, (CO)ORA, 0(CO)RA, (CO)NRBRA, or 0(CO)NRBRA, where each of RA and RB IS independently H, alkyl, haloalkyi, hydroxyalkyi , aminoalkyi , aryl , haloaryl, hydroxyaryl, alkoxyaryl, arylalkyi, alkylaryl, haloarylalkyi, alkylhaloaryl , (alkoxyaryl)alkyl, heterocyclic radical, or heterocyclic radical-alkyl ;

k is 0 or 1 ;

n is O, 1 , or 2;

Ai and R4 combine to form oxo, R3 is H or a hydroxyl protecting group, and R5 is H ; or

Ai is H or OP", where P" is H or a hydroxyl protecting group, and:

(i) R3 is H or a hydroxyl protecting group, and R4 and R5 combine to form a double bond;

or

(ii) R3 and R4 combine to form a bond, and R5 is H or OP"; each P4 is independently H or a hydroxyl protecting group, and Xi is oxo or Xi , together with the carbon atom to which it is attached, is -(CH(OPY))-, where PY is H or a hydroxyl protecting group; or both P4 groups and Xi , together with the atoms to which each is attached, combine to form ketal; and

P5 is H or a hydroxyl protecting group.

In certain embodiments of the compound of formula (IA), R3 is H or a hydroxyl protecting group, R4 and R5 combine to form a double bond, P4 is independently H or a hydroxyl protecting group, and Xi is oxo. In other embodiments of the compound of formula (IA), R3 and R4 combine to form a bond, and R5 is H.

A macrocyclic intermediate in the synthesis of a halichondrin macrolide can be a compound of formula (IB), which can be produced by reacting the compound of formula (IA) with an organic base and a Lewis acid. The compound of formul

or a salt or a tautomer thereof,

where all variables are as defined for compound of formula (IA).

The reaction conditions that can be used to convert the compound of formula (IA) into the compound of formula (IB) include those known in the art for Horner-Wadsworth-Emmons reaction, e.g., Masamune- Roush conditions or Helquist protocol. In particular, the compound of formula (IA) can be reacted with an organic base (e.g., an organic base, the conjugate acid of which has a pKa of from 1 1 ± 2) and a Lewis acid (e.g., a salt of Li, Mg, or Zn). Non-limiting examples of an organic base that can be used in the Horner-Wadsworth-Emmons reaction include trialkylamines (e.g., triethylamine or Hunig's base), DBU, and DBN. Non-limiting examples of Lewis acids that can be used in the Horner-Wadsworth-Emmons reaction include LiCI, Zn(OTf)2, and MgC .

The compound of formula (IB), in which, e.g., P5 is a hydroxyl protecting group, can be converted into a halichondrin macrolide by reacting the compound of formula (IB) with a hydroxyl protecting group removing agent (e.g., a fluoride source, if the hydroxyl protecting group is a silyl group).

If, in the compound of formula (IB), each P4 is H, and Xi is oxo, the synthesis may further involve a reaction with a Bronsted acid (e.g., a Bronsted acid having a pKa of 5 ± 3) after the reaction of the compound of formula (IB) with a hydroxyl protecting group removing agent (e.g., to convert P4 from a hydroxyl protecting group into H).

If, in the compound of formula (IB), Xi is oxo, R3 is a hydroxyl protecting group, and R4 and R5 combine to form a double bond, treatment with a hydroxyl protecting group removing agent can provide the compound of formula (IB), in which R3 and R4 combine to form a bond, and R5 is H.

The compound of formula (IA) or formula (IB), in which Xi , together with the carbon atom to which it is attached, is -(CH(OPY))-, where PY is H, may be treated with an oxidizing agent capable of oxidizing hydroxyl to a carbonyl group (e.g., Dess-Martin periodinane or a dimethylsulfonium compound) to give the compound of formula (IA) or formula (IB), in which Xi is oxo.

Preparation of certain compounds of formula (IB) may further involve conversion of the compound of formula (IB), in which Ai is H, and R4 and R5 combine to form a double bond, into the compound of formula (IB) in which R4 and Ai combine to form oxo. In a non-limiting example, the enone in the compound of formula (IB), in which R4 and R5 combine to form a double bond can be converted into a C.12-C.13 epoxide using a nucleophilic peroxide agent, e.g., f-butyl hydroperoxide, which can then be converted into the compound of formula (IB), in which Ai and R4 combine to form oxo, using methods known in the art, e.g., by reacting with a bidentate phosphine ligand and a source of Pd(0) (see, e.g., Muzart, J., Eur. J. Org. Chem., 4717-4741 , 201 1 ). Thus, the compound of formula (IB), in which Ai is OP", can be prepared. Other transformations may involve a-oxygenation to produce the compound of formula (VIB), in which R ₅ is OP".

As described herein, one of skill in the art can identify the sequence of reactions involving hydroxyl protecting group removing agents and Bronsted acids to convert the compound of formula (IB) into a halichondrin macrolide.

C.2-C.3 Bond-forming Macrocyclization

The macrocyclization reaction of the invention can be a carbon-carbon bond-forming reaction (e.g., catalytic Ring-Closing olefin Metathesis (RCM)) that provides a C.2-C.3 bond in a halichondrin macrolide. The general synthetic sequence including RCM that can be used to prepare the halichondrin macrolide is shown in Figure 2. As shown in Figure 2, the non-macrocyclic intermediate in the synthesis of the halichondrin macrolide can be a compound of formula (MA):

(MA)

or a salt or a tautomer thereof,

in which

each of D and D' is independently H, optionally substituted alkyl, or OPi , provided that only one of D and D' is OPi , where Pi is H, alkyl, or a hydroxyl protecting group; A is a group of formula (1 ) or a C1-6 saturated or C2-6 unsaturated hydrocarbon skeleton, the skeleton being unsubstituted or having from 1 to 10 substituents independently selected from the group consisting of cyano, halo, azido, oxo, and Qi , the group of formula (1 ) having the structure:

(1 ),

where

L is -(CH(OP ₂ ))-, -(C(OH)(OP ₂ ))-, or -C(O)-;

Ri is H, or Ri and Pi combine to form a bond;

R2 is H or -(CH2)nOP3, and each of P2 and P3 is independently H, optionally substituted alkyl, or a hydroxyl protecting group, or P2 and P3, together with the atoms to which each is attached, combine to form a ketal, a cyclic carbonate, a dicarbonyl-dioxo, or silylene- dioxo; or R2 and P2 combine to form an optionally substituted ethylene or a structure selected from the group consisting of:

, where each P' is independently H or a hydroxyl protecting group;

E is optionally substituted alkyl or optionally substituted alkoxy;

G is O, S, CH2, or NRN, where RN is H, an N-protecting group, or optionally substituted alkyl; each Qi is independently ORA, SRA, S0 ₂ RA, OS0 ₂ RA, NRBRA, NRB(CO)RA, NRB(CO)(CO)RA,

NRA(CO)NRBRA, NRB(CO)ORA, (CO)ORA, 0(CO)RA, (CO)NRBRA, or 0(CO)NRBRA, where each of RA and RB IS independently H, alkyl, haloalkyl, hydroxyalkyl, aminoalkyl, aryl, haloaryl, hydroxyaryl, alkoxyaryl, arylalkyl, alkylaryl, haloarylalkyl, alkylhaloaryl, (alkoxyaryl)alkyl, heterocyclic radical, or heterocyclic radical-alkyl;

k is O or l ;

n is O, 1 , or 2;

Ai and R4 combine to form oxo, R3 is H or a hydroxyl protecting group, and R5 is H ; or

Ai is H or OP", where P" is H or a hydroxyl protecting group, and:

(i) R3 is H or a hydroxyl protecting group, and R4 and R5 combine to form a double bond;

or

(ii) R3 and R4 combine to form a bond, and R5 is H or OP";

each P4 is independently H or a hydroxyl protecting group, and Xi is oxo or Xi , together with the carbon atom to which it is attached, is -(CH(OPY))-, where PY is H or a hydroxyl protecting group; or both P4 groups and Xi , together with the atoms to which each is attached, combine to form ketal; and

P5 is H or a hydroxyl protecting group.

In certain embodiments of the compound of formula (IIA), R3 is H or a hydroxyl protecting group, R4 and R5 combine to form a double bond, P4 is independently H or a hydroxyl protecting group, and Xi is oxo. In particular embodiments of formula (IIA), Xi , together with the carbon atom to which it is attached, is - (CH(OPY))-, where PY is H. In some embodiments of formula (IIA), Xi is oxo. In other embodiments of the compound of formula (IIA), R3 and R4 combine to form a bond, and R5 is H. A macrocyclic intermediate in the synthesis of a halichondrin macrolide can be a compound of formula (IB), which can be produced by reacting the compound of formula (IIA) with an olefin metathesis catalyst. The compound of formula (IB) has the following structure:

(IB)

or a salt or a tautomer thereof,

where all variables are as defined for compound of formula (IIA).

The catalysts that can be used to convert the compound of formula (IIA) to the compound of formula (IB) can be those known in the art. Olefin metathesis catalysts include Ru-carbene complexes (e.g., Grubbs and Hoveyda-Grubbs catalysts). Olefin metathesis-competent catalysts that may be used in this reaction are known in the art (e.g., second generation Hoveyda-Grubbs-type catalysts, e.g., those in which the Ru- benzylidene moiety is modified to include electron-withdrawing and/or electron-donating groups).

The compound of formula (IIA) or formula (IB), in which Xi , together with the carbon atom to which it is attached, is -(CH(OPY))-, where PY is H, may be treated with an oxidizing agent capable of oxidizing hydroxyl to a carbonyl group (e.g., Dess-Martin periodinane or a dimethylsulfonium compound) to give the compound of formula (IIA) or formula (IB), in which Xi is oxo.

The halichondrin macrolide can be prepared from the compound of formula (IB) as described herein.

C.3-C.4 Bond-forming Macrocyclization

The macrocyclization reaction of the invention can be a carbon-carbon bond-forming reaction (e.g., catalytic Ring-Closing olefin Metathesis (RCM)) that provides a C.3-C.4 bond in a halichondrin macrolide. The general synthetic sequence including RCM that can be used to prepare the halichondrin macrolide is shown in Figure 3. As shown in Figure 3, the non-macrocyclic intermediate in the synthesis of the halichondrin macrolide can be a compound of formula (MIA):

(MIA)

or a salt or a tautomer thereof,

where

each of D and D' is independently H, optionally substituted alkyl, or OPi , provided that only one of D and D' is OPi , where Pi is H, alkyl, or a hydroxyl protecting group; A is a group of formula (1 ) or a C1-6 saturated or C2-6 unsaturated hydrocarbon skeleton, the skeleton being unsubstituted or having from 1 to 10 substituents independently selected from the group consisting of cyano, halo, azido, oxo, and Qi , the group of formula (1 ) having the structure:

(1 ),

where

L is -(CH(OP ₂ ))-, -(C(OH)(OP ₂ ))-, or -C(O)-;

Ri is H, or Ri and Pi combine to form a bond;

R2 is H or -(CH2)nOP3, and each of P2 and P3 is independently H, optionally substituted alkyl, or a hydroxyl protecting group, or P2 and P3, together with the atoms to which each is attached, combine to form a ketal, a cyclic carbonate, a dicarbonyl-dioxo, or silylene- dioxo; or R2 and P2 combine to form an optionally substituted ethylene or a structure selected from the group consisting of:

, where each P' is independently H or a hydroxyl protecting group;

E is optionally substituted alkyl or optionally substituted alkoxy;

G is O, S, CH2, or NRN, where RN is H, an N-protecting group, or optionally substituted alkyl; each Qi is independently ORA, SRA, S0 ₂ RA, OS0 ₂ RA, NRBRA, NRB(CO)RA, NRB(CO)(CO)RA,

NRA(CO)NRBRA, NRB(CO)ORA, (CO)ORA, 0(CO)RA, (CO)NRBRA, or 0(CO)NRBRA, where each of RA and RB IS independently H, alkyl, haloalkyl, hydroxyalkyl, aminoalkyl, aryl, haloaryl, hydroxyaryl, alkoxyaryl, arylalkyl, alkylaryl, haloarylalkyl, alkylhaloaryl, (alkoxyaryl)alkyl, heterocyclic radical, or heterocyclic radical-alkyl;

k is O or l ;

n is O, 1 , or 2;

Ai and R4 combine to form oxo, R3 is H or a hydroxyl protecting group, and R5 is H ; or

Ai is H or OP", where P" is H or a hydroxyl protecting group, and:

(i) R3 is H or a hydroxyl protecting group, and R4 and R5 combine to form a double bond;

or

(ii) R3 and R4 combine to form a bond, and R5 is H or OP";

each P4 is independently H or a hydroxyl protecting group, and Xi is oxo or Xi , together with the carbon atom to which it is attached, is -(CH(OPY))-, wherein Ργ is H or a hydroxyl protecting group; or both P4 groups and Xi , together with the atoms to which each is attached, combine to form ketal; and P5 is H or a hydroxyl protecting group.

In certain embodiments of the compound of formula (IIIA), R3 is H or a hydroxyl protecting group, R4 and R5 combine to form a double bond, P4 is independently H or a hydroxyl protecting group, and Xi is oxo. In other embodiments of the compound of formula (IIIA), R3 and R4 combine to form a bond, and R5 is H. In particular embodiments of formula (IIIA), Xi , together with the carbon atom to which it is attached, is - (CH(OPY))-, where PY is H. In some embodiments of formula (IIIA), Xi is oxo. A macrocyclic intermediate in the synthesis of a halichondrin macrolide can be a compound of formula (NIB), which can be produced by reacting the compound of formula (IIIA) with an olefin metathesis catalyst. The compound of formula (1MB) has the following structure:

The catalysts that can be used to convert the compound of formula (IMA) to the compound of formula (NIB) can be those known in the art. Olefin metathesis catalysts include Ru-carbene complexes (e.g., Grubbs and Hoveyda-Grubbs catalysts). Olefin metathesis-competent catalysts that may be used in this reaction are known in the art (e.g., second generation Hoveyda-Grubbs-type catalysts, e.g., those in which the Ru-benzylidene moiety is modified to include electron-withdrawing and/or electron-donating groups).

The halichondrin macrolide can be prepared from the compound of formula (NIB), as described for the synthesis involving the compound of formula (IB), as the C.3-C.4 double bond can undergo isomerization to give the compound of formula (IB) described herein upon exposure to basic (e.g., isomerization mediated by a hydroxyl protecting group removing agent, such as a fluoride source) or acidic (e.g., isomerization mediated by a Bronsted acid) conditions.

The compound of formula (IMA) or formula (NIB), in which Xi , together with the carbon atom to which it is attached, is -(CH(OPY))-, where PY is H, may be treated with an oxidizing agent capable of oxidizing hydroxyl to a carbonyl group to give the compound of formula (MIA) or formula (1MB), in which Xi is oxo.

Preparation of the halichondrin macrolide may further involve conversion of the compound of formula (1MB), in which Ai is H, and R4 and R5 combine to form a double bond, into the compound of formula (VIB) in which R4 and Ai combine to form oxo. In a non-limiting example, the enone in the compound of formula (1MB), in which R4 and R5 combine to form a double bond can be converted into a C.12-C.13 epoxide using a nucleophilic peroxide agent, e.g., f-butyl hydroperoxide, which can then be converted into the compound of formula (1MB), in which Ai and R4 combine to form oxo, using methods known in the art, e.g., by reacting with a bidentate phosphine ligand and a source of Pd(0) (see, e.g., Muzart, J., Eur. J. Org. Chem., 4717-4741 , 201 1 ). Thus, the halichondrin macrolide, in which Ai is OP", can be prepared. Other transformations may involve a-oxygenation to produce the halichondrin macrolide, in which R5 is OP". C.12-C.13 Bond-forming Macrocyclization

The macrocyclization reaction of the invention can be a carbon-carbon bond-forming reaction (e.g., catalytic Ring-Closing olefin Metathesis (RCM)) that provides a C.12-C.13 bond in a halichondrin macrolide. The general synthetic sequence including RCM that can be used to prepare the halichondrin macrolide is shown in Figure 4. As shown in Figure 4, the non-macrocyclic intermediate in the synthesis of the halichondrin macrolide can be a compound of formula (IVA):

or a salt thereof,

where

each of D and D' is independently H, optionally substituted alkyl, or OPi , provided that only one of D and D' is OPi , where Pi is H, alkyl, or a hydroxyl protecting group; A is a group of formula (1 ) or a C1-6 saturated or C2-6 unsaturated hydrocarbon skeleton, the skeleton being unsubstituted or having from 1 to 10 substituents independently selected from the group consisting of cyano, halo, azido, oxo, and Qi , the group of formula (1 ) having the structure:

(1 ),

where

L is -(CH(OP ₂ ))-, -(C(OH)(OP ₂ ))-, or -C(O)-;

Ri is H, or Ri and Pi combine to form a bond;

R2 is H or -(CH2)nOP3, and each of P2 and P3 is independently H, optionally substituted alkyl, or a hydroxyl protecting group, or P2 and P3, together with the atoms to which each is attached, combine to form a ketal, a cyclic carbonate, a dicarbonyl-dioxo, or silylene- dioxo; or R2 and P2 combine to form an optionally substituted ethylene or a structure selected from the group consisting of:

, where each P' is independently H or a hydroxyl protecting group;

E is optionally substituted alkyl or optionally substituted alkoxy;

G is O, S, CH2, or NRN, where RN is H, an N-protecting group, or optionally substituted alkyl; each Qi is independently ORA, SRA, S0 ₂ RA, OS0 ₂ RA, NRBRA, NRB(CO)RA, NRB(CO)(CO)RA, NRA(CO)NRBRA, NRB(CO)ORA, (CO)ORA, 0(CO)RA, (CO)NRBRA, or 0(CO)NRBRA, where each of RA and RB IS independently H, alkyl, haloalkyi, hydroxyalkyi, aminoalkyi, aryl, haloaryl, hydroxyaryl, alkoxyaryl, arylalkyi, alkylaryl, haloarylalkyi, alkylhaloaryl, (alkoxyaryl)alkyl, heterocyclic radical, or heterocyclic radical-alkyl;

k is 0 or 1 ;

n is O, 1 , or 2;

(i) R3 is H or a hydroxyl protecting group, R4 is alkyl ether, and R5 is H ;

(ii) R3 is H or a hydroxyl protecting group, and R4 and R5 combine to form a double bond; or

(iii) R3 and R4 combine to form a bond, and R5 is H;

each P4 and P5 is independently H or a hydroxyl protecting group; and

R6 is H, and Ρβ is H or a hydroxyl protecting group; or R6 and Ρβ combine to form a double bond.

In certain embodiments of the compound of formula (IVA), P5 is H. In particular embodiments of the compound of formula (IVA), R3 and R4 combine to form a bond, and R5 is H. In some embodiments, R6 is H, and Ρβ is H or a hydroxyl protecting group. In other embodiments, R6 and Ρβ combine to form a double bond.

A macrocyclic intermediate in the synthesis of a halichondrin macrolide can be a compound of formula (IVB), which can be produced by reacting the compound of formula (IVA) with an olefin metathesis catalyst. The compound of formula (IVB) has the following structure:

(IVB)

or a salt thereof,

where all variables are as defined for the compound of formula (IVA).

The catalysts that can be used to convert the compound of formula (IVA) to the compound of formula (IVB) can be those known in the art. Olefin metathesis catalysts include Ru-carbene complexes (e.g., Grubbs and Hoveyda-Grubbs catalysts). Olefin metathesis-competent catalysts that may be used in this reaction are known in the art (e.g., second generation Hoveyda-Grubbs-type catalysts, e.g., those in which the Ru-benzylidene moiety is modified to include electron-withdrawing and/or electron-donating groups).

The compound of formula (IVB) (e.g., the compound of formula (IVB), in which R6 and Ρβ combine to form a double bond) can be converted to the halichondrin macrolide by a reaction with a Bronsted acid (if each of P4 and P5 is independently H or a hydroxyl protecting group capable of being deprotected by the

Bronsted acid). Alternatively, if at least one P4 or P5 is a hydroxyl protecting group that is not capable of being deprotected by the Bronsted acid, the compound of formula (IVB) can be reacted with a hydroxyl protecting group removing agent to effect the removal of at least one hydroxyl protecting group, provided that the product of the reaction is the compound of formula (IVC):

(IVC)

or a salt thereof,

where each of Ai and A2 is H or OP", where P" is H or a hydroxyl protecting group;

and the remaining variables are as defined for the compound of formula (IVA). In a non-limiting example of the transformation of the compound of formula (IVB) to provide the compound of formula (IVC), at least one of P4 and P5 is a silyl, and the hydroxyl protecting group removing agent is a fluoride source. The compound of formula (IVC) can then be converted to the halichondrin macrolide by a reaction with a Bronsted acid.

If, in the compound of formula (IVB) or (IVC), R3 is a hydroxyl protecting group, and R4 and R5 combine to form a double bond, treatment with a hydroxyl protecting group removing agent can provide the compound of formula (IVB) or (IVC), in which R3 and R4 combine to form a bond, and R5 is H.

Preparation of the compound of formula (IVC) from the compound of formula (IVB) may further involve reaction with a hydroxyl protecting group removing agent to convert Ρβ in the compound of formula (IVB) to H and subsequent reaction with an oxidizing agent capable of converting a hydroxyl group into a carbonyl (e.g., capable of oxidizing allylic alcohol into an enone). In a non-limiting example, the enone in the compound of formula (IVB) formed through the oxidation can then be converted into a C.12-C.13 epoxide using a nucleophilic peroxide agent, e.g., f-butyl hydroperoxide, which can then be converted into the compound of formula (IVB), in which Ai and R4 combine to form oxo, using methods known in the art, e.g., by reacting with a bidentate phosphine ligand and a source of Pd(0) (see, e.g., Muzart, J., Eur. J. Org. Chem., 4717-4741 , 201 1 ). Thus, the compound of formula (IVC), in which Ai is OP", can be prepared. Other transformations may involve a-oxygenation to produce the compound of formula (IVC), in which A2 is OP".

As described herein, one of skill in the art can identify the sequence of reactions involving the reactions described above to convert the compound of formula (IVB) into a halichondrin macrolide with or without the isolation of the compound of formula (IVC).

C.15-C.16 Bond-forming Macrocyclization

The macrocyclization reaction of the invention can be a carbon-carbon bond-forming reaction (e.g., catalytic Ring-Closing olefin Metathesis (RCM)) that provides a C.15-C.16 bond in a halichondrin macrolide. The general synthetic sequence including RCM that can be used to prepare the halichondrin macrolide is shown in Figure 5. As shown in Figure 5, the non-macrocyclic intermediate in the synthesis of the halichondrin macrolide can be a compound of formula (VA):

(VA)

or a salt or a tautomer thereof,

where

each of D and D' is independently H, optionally substituted alkyl, or OPi , provided that only one of D and D' is OPi , where Pi is H, alkyl, or a hydroxyl protecting group; A is a group of formula (1 ) or a C1-6 saturated or C2-6 unsaturated hydrocarbon skeleton, the skeleton being unsubstituted or having from 1 to 10 substituents independently selected from the group consisting of cyano, halo, azido, oxo, and Qi , the group of formula (1 ) having the structure:

(1 ),

where

L is -(CH(OP ₂ ))-, -(C(OH)(OP ₂ ))-, or -C(O)-;

Ri is H, or Ri and Pi combine to form a bond;

P12 is H or -(CH2)nOP3, and each of P2 and P3 is independently H, optionally substituted alkyl, or a hydroxyl protecting group, or P2 and P3, together with the atoms to which each is attached, combine to form a ketal, a cyclic carbonate, a dicarbonyl-dioxo, or silylene- dioxo; or R2 and P2 combine to form an optionally substituted ethylene or a structure selected from the group consisting of:

, where each P' is independently H or a hydroxyl protecting group;

E is optionally substituted alkyl or optionally substituted alkoxy;

G is O, S, CH2, or NRN, where RN is H, an N-protecting group, or optionally substituted alkyl; each Qi is independently ORA, SRA, S0 ₂ RA, OS0 ₂ RA, NRBRA, NRB(CO)RA, NRB(CO)(CO)RA, NRA(CO)NRBRA, NRB(CO)ORA, (CO)ORA, 0(CO)RA, (CO)NRBRA, or 0(CO)NRBRA, where each of RA and RB IS independently H, alkyl, haloalkyi, hydroxyalkyi, aminoalkyi, aryl, haloaryl, hydroxyaryl, alkoxyaryl, arylalkyl, alkylaryl, haloarylalkyl, alkylhaloaryl, (alkoxyaryl)alkyl, heterocyclic radical, or heterocyclic radical-alkyl;

k is 0 or 1 ;

n is O, 1 , or 2;

(a1 ) R3 is H or a hydroxyl protecting group, R4 is alkyl ether, and R5 is H ;

(a2) R3 is H or a hydroxyl protecting group, and R4 and R5 combine to form a double bond;

or

(a3) R3 and R4 combine to form a bond, and R5 is H;

(b1 ) Ai and R7 combine to form oxo, R6 is H or a hydroxyl protecting group, and Rs is

H;

or

(b2) Ai is H or OP", where P" is H or a hydroxyl protecting group, and:

(i) R6 is H or a hydroxyl protecting group, and R7 and Rs combine to form a double bond;

or

(ii) R6 and R7 combine to form a bond, and Rs is H or OP";

(c1 ) R9 is H, and P4 is H or a hydroxyl protecting group;

or

(c2) R9 and P4 combine to form a double bond;

R10 is H or -CH2Xi CH ₂ CH=CH2, where Xi is O, -C(Rn ) ₂ - or NP ₆ , and where each Rn is independently H or -COORi2, Ρβ is an /V-protecting group, and R12 is alkyl; and

each P5 is independently H or a hydroxyl protecting group.

In certain embodiments of the compound of formula (VA), R6 and R7 combine to form a bond, and Rs is H. In particular embodiments of the compound of formula (VA), R3 and R4 combine to form a bond, and R5 is H. In some embodiments of the compound of formula (VA), each P5 is independently a hydroxyl protecting group. In further embodiments of the compound of formula (VA), R9 is H, and P4 is a hydroxyl protecting group. In other embodiments, Ai is H.

A macrocyclic intermediate in the synthesis of a halichondrin macrolide can be a compound of formula (VB), which can be produced by reacting the compound of formula (VA) with an olefin metathesis catalyst. The compound of form

(VB)

or a salt or a tautomer thereof,

where all variables are as defined for the compound of formula (VA).

The catalysts that can be used to convert the compound of formula (VA) to the compound of formula (VB) can be those known in the art. Olefin metathesis catalysts include Ru-carbene complexes (e.g., Grubbs and Hoveyda-Grubbs catalysts). Olefin metathesis-competent catalysts that may be used in this reaction are known in the art (e.g., second generation Hoveyda-Grubbs-type catalysts, e.g., those in which the Ru- benzylidene moiety is modified to include electron-withdrawing and/or electron-donating groups).

The compound of formula (VB) can be reacted with a hydroxyl protecting group removing agent and, optionally, an oxidizing agent (e.g., when R9 is H in the compound of formula (VB)) capable of converting an alcohol to a carbonyl group (e.g., capable of converting an allylic alcohol to an enone) to afford the compound of formula (VBa) :

(VBa)

or a salt thereof, where A2 is H or OP", and the remaining variables are as defined for the compound of formula

(VA).

The compound of formula (VBa) can be reacted with a 1 ,4-reducing agent to furnish the compound of formula (VC):

(VC)

or a salt thereof,

where all variables are as defined for the compound of formula (VBa).

The 1 ,4-reducing agents include copper (I) hydrides, which can be isolated (e.g., Stryker's reagent) or prepared in situ (e.g., from a copper (I) or copper (II) salt and a hydride source). Catalytic quantities of a copper salt (either copper (I) or copper (II) salt) in combination with stoichiometric or superstoichiometric quantities of a hydride source (e.g., a borohydride salt, borane, PMHS, or a hydrosilane (e.g., Ph2SiH2)). A non-limiting example of the reaction conditions that can be used in the reaction sequence from the compound of formula (VB) to the compound of formula (VC) are described, e.g., in Baker et al., Org. Lett, 10:289-292, 2008, the disclosure of which is incorporated herein by reference. Other metals can be used to catalyze 1 ,4-reduction, e.g., Ru, Pd, and Ir compounds.

Alternatively, if R9 and P4 combine to form a double bond in the compound of formula (VB), the reaction of the compound of formula (VB) with a hydroxyl protecting group removing agent can furnish the compound of formula (VC) directly.

If the compound of formula (VC) includes hydroxyl protecting groups as P5, these hydroxyl protecting groups can be removed with a hydroxyl protecting group removing agent to give the compound of formula (IVC):

(IVC)

or a salt thereof,

where all variables are as defined for the compound of formula (VBa). The halichondrin macrolide can be prepared from the compound of formula (IVC) as described herein. C.19-C.20 Bond-forming Macrocyclization

The macrocyclization reaction of the invention can be a carbon-carbon bond-forming reaction (e.g., Nozaki-Hiyama-Kishi reaction (NHK)) that provides a C.19-C.20 bond in a halichondrin macrolide. The general synthesis sequence including NHK that can be used to prepare the halichondrin macrolide is shown in Figure 6. As shown in Figure 6, the non-macrocyclic intermediate in the synthesis of the halichondrin macrolide can be a compound of formula (VIA):

(VIA),

or a salt or a tautomer thereof,

where

b designates (/^-stereogenic center, and Z is a sulfonate, chloride, bromide, or iodide; or b designates (S)-stereogenic center, and Z is OR6, where R6 is a hydroxyl protecting group;

each of D and D' is independently H, optionally substituted alkyl, or OPi , provided that only one of D and D' is OPi , where Pi is H, alkyl, or a hydroxyl protecting group; A is a group of formula (1 ) or a C1-6 saturated or C2-6 unsaturated hydrocarbon skeleton, the skeleton being unsubstituted or having from 1 to 10 substituents independently selected from the group consisting of cyano, halo, azido, oxo, and Qi , the group of formula (1 ) having the structure:

(1 ),

where

L is -(CH(OP ₂ ))-, -(C(OH)(OP ₂ ))-, or -C(O)-;

Ri is H, or Ri and Pi combine to form a bond;

Pi2 is H or -(CH2)nOP3, and each of P2 and P3 is independently H, optionally substituted alkyl, or a hydroxyl protecting group, or P2 and P3, together with the atoms to which each is attached, combine to form a ketal, a cyclic carbonate, a dicarbonyl-dioxo, or silylene- dioxo; or R2 and P2 combine to form an optionally substituted ethylene or a structure selected from the group consisting of:

group;

E is optionally substituted alkyl or optionally substituted alkoxy;

G is O, S, CH2, or NRN, where RN is H, an N-protecting group, or optionally substituted alkyl; each Qi is independently ORA, SRA, S0 ₂ RA, OS0 ₂ RA, NRBRA, NRB(CO)RA, NRB(CO)(CO)RA, NRA(CO)NRBRA, NRB(CO)ORA, (CO)ORA, 0(CO)RA, (CO)NRBRA, or 0(CO)NRBRA, where each of RA and RB IS independently H, alkyl, haloalkyi, hydroxyalkyi, aminoalkyi, aryl, haloaryl, hydroxyaryl, alkoxyaryl, arylalkyi, alkylaryl, haloarylalkyi, alkylhaloaryl, (alkoxyaryl)alkyl, heterocyclic radical, or heterocyclic radical-alkyl;

k is 0 or 1 ;

n is O, 1 , or 2;

Y is iodide, bromide, or trifluoromethanesulfonate;

Ai and R4 combine to form oxo, R3 is H or a hydroxyl protecting group, and R5 is H ; or

Ai is H or OP", where P" is H or a hydroxyl protecting group, and: (i) R3 is H or a hydroxyl protecting group, and FU and R5 combine to form a double bond;

or

(ii) R3 and R4 combine to form a bond, and R5 is H or OP";

and

each P4 is independently H or a hydroxyl protecting group, and Xi is oxo; or both P4 groups and Xi , together with the atoms to which each is attached, combine to form ketal.

In particular embodiments of the compound of formula (VIA), R3 and R4 combine to form a bond, and R5 is H. In some embodiments of the compound of formula (VIA), both P4 groups and Xi , together with the atoms to which each is attached, combine to form ketal.

An intermediate in the synthesis of the halichondrin macrolide can be a compound of formula (VIB), which can be produced by subjecting the compound of formula (VIA) to Nozaki-Hiyama-Kishi reaction conditions (e.g., by reacting with a Cr(ll) salt and a Ni(ll) salt). The compound of formula (VIB) has the following structure:

b designates (/^-stereogenic center, c designates (S)-stereogenic center, and Z is a sulfonate, chloride, bromide, or iodide;

or

b designates (S)-stereogenic center, c designates (/^-stereogenic center, and Z is OR6, where R6 is a hydroxyl protecting group;

and the remaining variables are as defined for the compound of formula (VIA).

Nozaki-Hiyama-Kishi reaction conditions that may be used to prepare the compound of formula (VIB) from the compound of formula (VIA) can be those known in the art. Nozaki-Hiyama-Kishi reaction on the compound of formula (VIA) can include reacting the compound of formula (VIA) with a Cr(ll) salt and a Ni(ll) salt. Ancillary ligands can be used in combination with the metal salts. In a non-limiting example, a substituted 1 ,10-phenanthroline can be used in combination with a Ni(ll) salt. Chiral ancillary ligands can be used to render the reaction stereoselective. In a non-limiting example, chiral N-(dihydrooxazolyl- phenyl)-sulfonamides can be used with a Cr(ll) salt to control the stereochemistry of the carbonyl carbon, to which a vinyl nucleophile is added in the course of Nozaki-Hiyama-Kishi reaction.

The compound of formula (VIB) can be converted to the halichondrin macrolide using the step of nucleophilic ring-closing of the compound of formula (VIB).

In particular, the compound of formula (VIB) (in which Z is a sulfonate, chloride, bromide, or iodide; b designates (/^-stereogenic center; and c designates (S)-stereogenic center) can be converted to the halichondrin macrolide directly, e.g., upon isolation from the Nozaki-Hiyama-Kishi reaction mixture (e.g., by treatment with a base) or by contacting a mixture containing the product of the Nozaki-Hiyama-Kishi reaction with silica gel. Additionally, the compound of formula (VIB) can be reacted with a hydroxyl protecting group removing agent (before or after the step of nucleophilic ring-closing) if, e.g., each of P4 is a hydroxyl protecting group, optionally, with a subsequent reaction with a Bronsted acid to form a compound, in which both P4 groups and Xi , together with the atoms to which each is attached, combine to form ketal.

Alternatively, if, in the compound of formula (VIB), b designates (S)-stereogenic center, c designates (R)- stereogenic center, and Z is OR6, where Ι¾ is a hydroxyl protecting group, the step of nucleophilic ring- closing of the compound of formula (VIB) can be effected after the hydroxyl group attached to the carbon atom designated with c is converted into a leaving group (e.g., a sulfonate by a reaction with a sulfonyl electrophile, such as a sulfonyl chloride or a sulfonyl anhydride). In these embodiments, the step of nucleophilic ring-closing can be carried out by, e.g., first, reacting the compound of formula (VIB), after the conversion of the hydroxyl into the leaving group, with a hydroxyl protecting group removing agent to convert P16 into H and then reacting the product with a base.

Preparation of certain compounds of formula (VIB) may further involve conversion of the compound of formula (VIB), in which Ai is H, and FU and R5 combine to form a double bond, into the compound of formula (VIB) in which FU and Ai combine to form oxo. In a non-limiting example, the enone in the compound of formula (VIB), in which FU and R5 combine to form a double bond can be converted into a C.12-C.13 epoxide using a nucleophilic peroxide agent, e.g., f-butyl hydroperoxide, which can then be converted into the compound of formula (VIB), in which Ai and FU combine to form oxo, using methods known in the art, e.g., by reacting with a bidentate phosphine ligand and a source of Pd(0) (see, e.g., Muzart, J., Eur. J. Org. Chem., 4717-4741 , 201 1 ). Thus, the compound of formula (VIB), in which Ai is OP", can be prepared. Other transformations may involve a-oxygenation to produce the compound of formula (VIB), in which R ₅ is OP".

One of skill in the art will recognize that any unprotected hydroxyl groups can be protected with a hydroxyl protecting group, if the unprotected hydroxyl groups are not intended for reacting in the step of nucleophilic ring-closing. C.26-C.27 Bond-forming Macrocyclization

The macrocyclization reaction of the invention can be a carbon-carbon bond-forming reaction (e.g., Nozaki-Hiyama-Kishi reaction (NHK)) that provides a C.26-C.27 bond in a halichondrin macrolide. The general synthesis sequence including NHK that can be used to prepare the halichondrin macrolide is shown in Figure 7. As shown in Figure 7, the non-macrocyclic intermediate in the synthesis of the halichondrin macrolide can be a compound of formula (VIIA):

(VIIA)

where

each of D and D' is independently H, optionally substituted alkyl, or OPi , provided that only one of D and D' is OPi , where Pi is H, alkyl, or a hydroxyl protecting group; A is a group of formula (1 ) or a C1-6 saturated or C2-6 unsaturated hydrocarbon skeleton, the skeleton being unsubstituted or having from 1 to 10 substituents independently selected from the group consisting of cyano, halo, azido, oxo, and Qi , the group of formula (1 ) having the structure:

(1 ),

where

L is -(CH(OP ₂ ))-, -(C(OH)(OP ₂ ))-, or -C(O)-;

Ri is H, or Ri and Pi combine to form a bond;

R2 is H or -(CH2)nOP3, and each of P2 and P3 is independently H, optionally substituted alkyl, or a hydroxyl protecting group, or P2 and P3, together with the atoms to which each is attached, combine to form a ketal, a cyclic carbonate, a dicarbonyl-dioxo, or silylene- dioxo; or R2 and P2 combine to form an optionally substituted ethylene or a structure selected from the group consisting of: , where each Ρ' is independently Η or a hydroxyl protecting group;

E is optionally substituted alkyl or optionally substituted alkoxy;

G is O, S, CH2, or NRN, where RN is H, an N-protecting group, or optionally substituted alkyl; each Qi is independently ORA, SRA, S0 ₂ RA, OS0 ₂ RA, NRBRA, NRB(CO)RA, NRB(CO)(CO)RA, NRA(CO)NRBRA, NRB(CO)ORA, (CO)ORA, 0(CO)RA, (CO)NRBRA, or 0(CO)NRBRA, where each of RA and RB IS independently H, alkyl, haloalkyi, hydroxyalkyi, aminoalkyi, aryl, haloaryl, hydroxyaryl, alkoxyaryl, arylalkyi, alkylaryl, haloarylalkyi, alkylhaloaryl, (alkoxyaryl)alkyl, heterocyclic radical, or heterocyclic radical-alkyl;

k is 0 or 1 ;

n is O, 1 , or 2;

Y is iodide, bromide, or trifluoromethanesulfonate; and

(a1 ) R3 is H or a hydroxyl protecting group, R4 and R5 combine to form a double bond, each P4 is independently H or a hydroxyl protecting group, and Xi , together with the carbon to which it is attached, forms a carbonyl or -(CH(ORs))-, where Rs is H or a hydroxyl protecting group;

or

(a2) R3 and R4 combine to form a bond, R5 is H, and

each P4 is independently H or a hydroxyl protecting group, and Xi , together with the carbon to which it is attached, forms a carbonyl or -(CH(ORs))-;

or

both P4 groups and Xi , together with the atoms to which each is attached, combine to form ketal;

Z is chloride, bromide, or iodide, and R6 and R7 combine to form a bond;

Z and R7 combine to form a double bond, and R6 is H or a hydroxyl protecting group. In particular embodiments of the compound of formula (VMA), Z and R7 combine to form a double bond, and R6 is a hydroxyl protecting group. In some embodiments of the compound of formula (VMA), each P4 is independently H or a hydroxyl protecting group, and Xi , together with the carbon to which it is attached, forms a carbonyl or -(CH(ORs))-. In certain embodiments of the compound of formula (VMA), R3 is H or a hydroxyl protecting group, R4 and R5 combine to form a double bond.

An intermediate in the synthesis of the halichondrin macrolide can be a compound of formula (VIIB), which can be produced by subjecting the compound of formula (VMA) to Nozaki-Hiyama-Kishi reaction conditions (e.g., by reacting with a Cr(ll) salt and a Ni(ll) salt). The compound of formula (VIIB) has the following structure:

(VIIB)

or a salt thereof,

where each of P5 and R6 is independently H or a hydroxyl protecting group, and the remaining variables are as defined for the compound of formula (VMA).

Nozaki-Hiyama-Kishi reaction conditions that may be used to prepare the compound of formula (VIIB) from the compound of formula (VMA) can be those known in the art. Nozaki-Hiyama-Kishi reaction on the compound of formula (VMA) can include reacting the compound of formula (VMA) with a Cr(ll) salt and a Ni(ll) salt. Ancillary ligands can be used in combination with the metal salts. In a non-limiting example, a substituted 1 ,10-phenanthroline can be used in combination with a Ni(ll) salt. Chiral ancillary ligands can be used to render the reaction stereoselective. In a non-limiting example, chiral N-(dihydrooxazolyl- phenyl)-sulfonamides can be used with a Cr(ll) salt to control the stereochemistry of the carbonyl carbon, to which a vinyl nucleophile is added in the course of Nozaki-Hiyama-Kishi reaction.

The compound of formula (VIIB) can be converted to a compound of formula (VIIC) through the step of nucleophilic ring-closing of the compound of formula (VIIB), where the compound of formula (VIIC) has the following structure:

(VIIC)

or a salt or a tautomer thereof,

where each P4 is H or a hydroxyl protecting group, and Xi is oxo; or both P4 groups and Xi , together with the atoms to which each is attached, combine to form a ketal;

Ai and FU combine to form oxo, R3 is H or a hydroxyl protecting group, and R5 is H ;

or

Ai is H or OP", where P" is H or a hydroxyl protecting group, and:

(i) R3 is H or a hydroxyl protecting group, and R4 and R5 combine to form a double bond;

or

(ii) R3 and R4 combine to form a bond, and R5 is H or OP";

and the remaining variables are as defined for the compound of formula (VIIA). The step of nucleophilic ring-closing of the compound of formula (VIIB) can be effected after the hydroxyl group produced by the NHK reaction is converted into a leaving group (e.g., a sulfonate by a reaction with a sulfonyl electrophile, such as a sulfonyl chloride or a sulfonyl anhydride). In these embodiments, the step of nucleophilic ring-closing can be carried out by reacting the compound of formula (VIB), in which R6 is H, after the conversion of the hydroxyl into the leaving group, with a strong base (e.g., alkoxide).

Preparation of the compound of formula (VIIC) can involve Vasella fragmentation prior to the step of nucleophilic ring-closing, if Z and R7 combine to form a double bond, and R6 is a hydroxyl protecting group in the compound of formula (VIIB).

The compound of formula (VIIC) can be reacted with a hydroxyl protecting group removing agent to give a compound of formula (VIID):

for the compound of formula (VIIC).

The compound of formula (VIID) can be converted to the halichondrin macrolide by reacting with a Bronsted acid (e.g., a pyridinium salt, such as pyridinium p-toluene sulfonate). In some embodiments, the preparation of the halichondrin macrolide from the compound of formula (VIIA) can further involve an oxidation reaction (e.g., contacting the compound of formula (VIIA), (VIIB), or (VIIC), in which Xi is -(CH(ORs))- and Rs is H, with an oxidizing agent capable of oxidizing an alcohol to a carbonyl group). Preparation of the compound of formula (VIID) from the compound of formula (VIIB) may further involve conversion of the compound of formula (VIIB), in which R4 and R5 combine to form a double bond into the compound of formula (VIIC) in which R4 and Ai combine to form oxo. In a non-limiting example, the enone in the compound of formula (VIIB), in which R4 and R5 combine to form a double bond can be converted into a C.12-C.13 epoxide using a nucleophilic peroxide agent, e.g., f-butyl hydroperoxide, which can then be converted into the compound of formula (VIIC), in which Ai and R4 combine to form oxo, using methods known in the art, e.g., by reacting with a bidentate phosphine ligand and a source of Pd(0) (see, e.g., Muzart, J., Eur. J. Org. Chem., 471 7-4741 , 201 1 ). Thus, the compound of formula (VIID), in which Ai is OP", can be prepared. Other transformations may involve a-oxygenation to produce the compound of formula (VIID), in which A2 is OP".

One of skill in the art will recognize that any unprotected hydroxyl groups can be protected with a hydroxyl protecting group, if the unprotected hydroxyl groups are not intended for reacting in the step of nucleophilic ring-closing. Synthesis of Non-macrocyclic intermediates

The compounds of formula (IA), (MA), (IMA), (IVA), (VA), (VIA), and (VIIA) can be prepared using methods and intermediates disclosed, e.g., in U.S. Patent Nos. 5,338,865 and 5,436,238; in International Patent application No. PCT/US2014/063960; and in Towle et al., Annual Meeting of the American Association for Cancer Research, April 6-1 0, 2002, 5721 ; Wang et al., Bioorg. Med. Chem. Lett., 10:1029-1 032, 2000; Aicher et al., J. Am. Chem. Soc, 1 14:3162-3164, 1992; Ueda et al., J. Am. Chem. Soc, 136:5171 -5176; and Yamamoto et al., J. Am. Chem. Soc, 134:893-896, 2012; each of which is incorporated herein by reference in its entirety.

Compound (I A)

The compound of formula (IA) can be prepared using methods described in the references incorporated by reference above. In particular, the compound of formula (IA) can be prepared from compound (IC), in which Px is H, and compound (ID), e.g., using esterification reaction conditions (e.g., Shiina esterification reaction conditions (e.g., using MNBA) or Yamaguchi esterification reaction conditions, e.g., the reaction conditions described in Aicher et al., J. Am. Chem. Soc, 1 14:3162-3164, 1992).

The compound of formula (IC)

(IC),

where

each of D and D' is independently H, optionally substituted alkyl, or OPi , provided that only one of D and D' is OPi , where Pi is H, alkyl, or a hydroxyl protecting group; A is a group of formula (1 ) or a C1-6 saturated or C2-6 unsaturated hydrocarbon skeleton, the skeleton being unsubstituted or having from 1 to 10 substituents independently selected from the group consisting of cyano, halo, azido, oxo, and Qi , the group of formula (1 ) having the structure:

(1 ),

where

L is -(CH(OP ₂ ))-, or -C(O)-;

Ri is H, or Ri and Pi combine to form a bond;

R2 is H or -(CH2)nOP3, and each of P2 and P3 is independently optionally substituted alkyl or a hydroxyl protecting group, or P2 and P3, together with the atoms to which each is attached, combine to form a ketal, a cyclic carbonate, a dicarbonyl-dioxo, or silylene- dioxo; or R2 and P2 combine to form an optionally substituted ethylene or a structure selected from the group consisting of:

, where each P' is independently a hydroxyl protecting group;

E is optionally substituted alkyl or optionally substituted alkoxy;

G is O, S, CH2, or NRN, where RN is H, an N-protecting group, or optionally substituted alkyl ; each Qi is independently ORA, SRA, S0 ₂ RA, OS0 ₂ RA, NRBRA, NRB(CO)RA, NRB(CO)(CO)RA, NRA(CO)NRBRA, NRB(CO)ORA, (CO)ORA, 0(CO)RA, (CO)NRBRA, or 0(CO)NRBRA, where each of RA and RB IS independently H, alkyl, haloalkyi, hydroxyalkyi , aminoalkyi , aryl , haloaryl, hydroxyaryl, alkoxyaryl, arylalkyi, alkylaryl, haloarylalkyi, alkylhaloaryl , (alkoxyaryl)alkyl, heterocyclic radical, or heterocyclic radical-alkyl ;

Px is H or a hydroxyl protecting group;

k is 0 or 1 ;

n is O, 1 , or 2;

Ai and R4 combine to form oxo, R3 is H or a hydroxyl protecting group, and R5 is H ; or

(i) R3 is H or a hydroxyl protecting group, and R4 and R5 combine to form a double bond;

or

(ii) R3 and R4 combine to form a bond, and R5 is H or OP";

R6 is OP" and R7 is H, or R6 and R7 combine to form a double bond;

each P4 is independently a hydroxyl protecting group, and Xi is oxo or Xi , together with the carbon atom to which it is attached, is -(CH(OPY))-, wherein Ργ is H or a hydroxyl protecting group; or both P4 groups and Xi , together with the atoms to which each is attached, combine to form ketal ; and

P5 is a hydroxyl protecting group; and

where each P" is independently H or a hydroxyl protecting group. The compound of formula (IC), in which Xi is oxo, can be prepared by reacting an oxidizing agent capable of oxidizing hydroxyl to a carbonyl group (e.g., Dess-Martin periodinane) with the compound of formula (IC), in which Xi , together with the carbon atom to which it is attached, is -(CH(OPY))-, where PY is H.

The compound of formula (ID) has the following structure: (RO)2P(0)CH2COOH, where each R is independently optionally substituted alkyl or optionally substituted aryl.

The preparation of the compound of formula (IA) from the compound of formula (IC) and the compound of formula (ID) may involve a hydroboration/oxidation reaction to furnish the aldehyde group in the compound of formula (IA) after the esterification reaction, if R6 and R7 combine to form a double bond in the compound of formula (IC). Alternatively, the preparation of the compound of formula (IA) from the compound of formula (IC) and the compound of formula (ID) may involve reacting the esterification product with a hydroxyl protecting group removing agent and then reacting the product with an oxidizing agent capable of converting a hydroxyl group into a carbonyl to furnish the aldehyde group in the compound of formula (IA), if, in the compound of formula (IC) R6 is OP and R7 is H.

The compound of formula (IC) can be prepared from a compound of formula (IE) and the compound of formula (IF), e.g., using Nozaki-Hiyama-Kishi reaction conditions, as described herein. The hydroxyl group used in the subsequent esterification reaction can be deprotected using a hydroxyl protecting group removing agent. Specifically, the preparation of the compound of formula (IC), in which Px is H, may further involve treating the compound of formula (IC), in which Px is a hydroxyl protecting group, with a hydroxyl protecting group removing agent. The compound of formula (IE) has the following structure:

(IE),

where

each of D and D' is independently H, optionally substituted alkyl, or OP1 , provided that only one of

D and D' is OP1 , where Pi is H, alkyl, or a hydroxyl protecting group; A is a group of formula (1 ) or a C1-6 saturated or C2-6 unsaturated hydrocarbon skeleton, the skeleton being unsubstituted or having from 1 to 10 substituents independently selected from the group consisting of cyano, halo, azido, oxo, and Qi , the group of formula (1 ) having the structure: (1 ),

where

L is -(CH(OP ₂ ))-, or -C(O)-;

Ri is H, or Ri and Pi combine to form a bond;

Pi2 is H or -(CH2)nOP3, and each of P2 and P3 is independently optionally substituted alkyl or a hydroxyl protecting group, or P2 and P3, together with the atoms to which each is attached, combine to form a ketal, a cyclic carbonate, a dicarbonyl-dioxo, or silylene- dioxo; or R2 and P2 combine to form an optionally substituted ethylene or a structure selected from the group consisting of:

each P' is independently a hydroxyl protecting group;

E is optionally substituted alkyl or optionally substituted alkoxy;

G is O, S, CH2, or NRN, where RN is H, an N-protecting group, or optionally substituted alkyl; each Qi is independently ORA, SRA, S0 ₂ RA, OS0 ₂ RA, NRBRA, NRB(CO)RA, NRB(CO)(CO)RA, NRA(CO)NRBRA, NRB(CO)ORA, (CO)ORA, 0(CO)RA, (CO)NRBRA, or 0(CO)NRBRA, where each of RA and RB IS independently H, alkyl, haloalkyi, hydroxyalkyi, aminoalkyi, aryl, haloaryl, hydroxyaryl, alkoxyaryl, arylalkyi, alkylaryl, haloarylalkyi, alkylhaloaryl, (alkoxyaryl)alkyl, heterocyclic radical, or heterocyclic radical-alkyl;

Px is a hydroxyl protecting group;

k is 0 or 1 ;

n is 0, 1 , or 2; and

Xi is oxo. The compound of formula (IF) has the following structure:

(IF),

where

each R3, P4, and P5 is independently a hydroxyl protecting group;

Ai is H or OP", where P" is a hydroxyl protecting group;

Pi6 is OP5, and R7 is H, or and R7 combine to form a double bond;

and

Y is chloro, bromo, iodo, trifluoromethanesulfonate, or trialkylsilane.

The compound of formula (IE), in which Xi is oxo, can be reacted with the compound of formula (IF) under the Nozaki-Hiyama-Kishi reaction conditions, as described herein.

The compound of formula (IE) can be accessed, e.g., from the compound of formula (VIIID).

The compound of formula (IF) can be prepared using methods known in the art, e.g., those described in WO 2015/066729.

The preparation of compound (IA) may further involve reactions deprotecting protected hydroxyl groups or protecting unprotected hydroxyl groups using methods described herein.

Compound (IIA)

The compound of formula (IIA) can be prepared using methods described in the references incorporated by reference above. In particular, the compound of formula (MIA) can be prepared from acrylic acid a compound of formula (MB), e.g., using esterification reaction conditions (e.g., Shiina esterification reaction conditions (e.g., using MNBA) or Yamaguchi esterification reaction conditions, e.g., the reaction conditions described in Aicher et al., J. Am. Chem. Soc, 1 14:3162-3164, 1992). The compound of formula (MB) has the following structure:

(MB),

where

each of D and D' is independently H, optionally substituted alkyl, or OPi , provided that only one of D and D' is OPi , where Pi is H, alkyl, or a hydroxyl protecting group; A is a group of formula (1 ) or a C1-6 saturated or C2-6 unsaturated hydrocarbon skeleton, the skeleton being unsubstituted or having from 1 to 10 substituents independently selected from the group consisting of cyano, halo, azido, oxo, and Qi , the group of formula (1 ) having the structure:

(1 ),

where

L is -(CH(OP ₂ ))-, or -C(O)-;

Ri is H, or Ri and Pi combine to form a bond;

R2 is H or -(CH2)nOP3, and each of P2 and P3 is independently optionally substituted alkyl or a hydroxyl protecting group, or P2 and P3, together with the atoms to which each is attached, combine to form a ketal, a cyclic carbonate, a dicarbonyl-dioxo, or silylene- dioxo; or R2 and P2 combine to form an optionally substituted ethylene or a structure selected from the group consisting of:

, where each P' is independently a hydroxyl protecting group;

E is optionally substituted alkyl or optionally substituted alkoxy;

G is O, S, CH2, or NRN, where RN is H, an N-protecting group, or optionally substituted alkyl ; each Qi is independently ORA, SRA, S0 ₂ RA, OS0 ₂ RA, NRBRA, NRB(CO)RA, NRB(CO)(CO)RA, NRA(CO)NRBRA, NRB(CO)ORA, (CO)ORA, 0(CO)RA, (CO)NRBRA, or 0(CO)NRBRA, where each of RA and RB IS independently H, alkyl, haloalkyi, hydroxyalkyi , aminoalkyi , aryl , haloaryl, hydroxyaryl, alkoxyaryl, arylalkyi, alkylaryl, haloarylalkyi, alkylhaloaryl , (alkoxyaryl)alkyl, heterocyclic radical, or heterocyclic radical-alkyl ;

Px is H or a hydroxyl protecting group;

k is 0 or 1 ;

n is O, 1 , or 2;

Ai and R4 combine to form oxo, R3 is H or a hydroxyl protecting group, and R5 is H ; or

Ai is H or OP", where P" is H or a hydroxyl protecting group, and:

(i) R3 is H or a hydroxyl protecting group, and R4 and R5 combine to form a double bond;

or

(ii) R3 and R4 combine to form a bond, and R5 is H or OP";

each P4 is independently a hydroxyl protecting group, and Xi is oxo or Xi , together with the carbon atom to which it is attached, is -(CH(OPY))-, wherein Ργ is H or a hydroxyl protecting group; or both P4 groups and Xi , together with the atoms to which each is attached, combine to form ketal ; and

P5 is a hydroxyl protecting group.

The compound of formula (MB), in which Xi is oxo, can be prepared by reacting an oxidizing agent capable of oxidizing hydroxyl to a carbonyl group (e.g. , Dess-Martin periodinane or a dimethylsulfonium compound) with the compound of formula (MB), in which Xi , together with the carbon atom to which it is attached, is -(CH(OPY))-, where PY is H.

The compound of formula (MB), in which Px is H, can be prepared by reacting a hydroxyl protecting group removing agent with the compound of formula (MB), in which Px is a hydroxyl protecting group.

In non-limiting examples, the compound of formula (MB) can be prepared from a compound of formula (IE) and a compound of formula (IIC) (e.g., via the compound of formula (lICa). The preparation of the compound of formula (MB) may further involve hydroboration/oxidation reaction with subsequent olefination (e.g., Wittig reaction (e.g., with PhsPChte) to provide a terminal alkene). For example, compound of formula (IIC) may be subjected to hydroboration/oxidation sequence to produce an aldehyde, which upon treatment with PhsPChb, produces the compound of formula (lICa). The compound of formula (IIC) has the following structure:

(IIC),

where

Ai is H or OP", where P" is H or a hydroxyl protecting group;

each Pi3, P4, and P5 is independently a hydroxyl protecting group; and

Y is chloro, bromo, iodo, or trifluoromethanesulfonate.

The compound of formula (lICa) has the following structure:

(lICa),

where all variables are as defined for formula (IIC).

The compound of formula (IE), in which Xi is oxo, can be reacted with the compound of formula (IIC) under the Nozaki-Hiyama-Kishi reaction conditions, as described herein.

The compound of formula (IIC) can be prepared using methods known in the art, e.g., those described in WO 2015/066729. The preparation of compound (MA) may also involve reactions deprotecting protected hydroxyl groups or protecting unprotected hydroxyl groups using methods described herein.

Compound (IIIA)

The compound of formula (IIIA) can be prepared using methods described in the references incorporated by reference above. In particular, the compound of formula (IIIA) can be prepared from but-3-enoic acid a compound of formula (IC), in which Ι¾ and R7 combine to form a double bond, e.g., using esterification reaction conditions (e.g., Shiina esterification reaction conditions (e.g., using MNBA) or Yamaguchi esterification reaction conditions, e.g., the reaction conditions described in Aicher et al., J. Am. Chem. Soc, 1 14:3162-3164, 1992). The preparation of compound (IIIA) may also involve reactions deprotecting protected hydroxyl groups or protecting unprotected hydroxyl groups using methods described herein.

Compound (IVA)

The compound of formula (IVA) can be prepared using methods described in the references incorporated by reference above. In particular, the compound of formula (IVA) can be prepared from a compound of formula (IVD) and a compound of formula (IVE), e.g., using esterification reaction conditions (e.g., Shiina esterification reaction conditions (e.g., using MNBA) or Yamaguchi esterification reaction conditions, e.g., the reaction conditions described in Aicher et al., J. Am. Chem. Soc, 1 14:31 62-3164, 1992). The compound of formula (IVD) has the following structure:

(IVD),

where

(i) R3 is a hydroxyl protecting group, FU is alkyl ether, and R5 is H;

(ϋ) R3 is a hydroxyl protecting group, and R4 and R5 combine to form a double bond;

or

(iii) R3 and R4 combine to form a bond, and R5 is H;

each P4 and P5 is independently a hydroxyl protecting group. The compound of formula (IVE) has the following structure:

(IVE),

where

each of D and D' is independently H, optionally substituted alkyl, or OP1 ; A is a group of formula (1 ) or a Ci-6 saturated or C2-6 unsaturated hydrocarbon skeleton, the skeleton being unsubstituted or having from 1 to 10 substituents independently selected from the group consisting of cyano, halo, azido, oxo, and Qi , the group of formula (1 ) having the structure:

(1 ),

where

L is -(CH(OP ₂ ))- or -C(O)-;

Ri and Pi combine to form a bond; or Ri is H, and Pi is alkyl;

R2 is H or -(CH2)nOP3, and each of P2 and P3 is independently optionally substituted alkyl or a hydroxyl protecting group, or P2 and P3, together with the atoms to which each is attached, combine to form a ketal, a cyclic carbonate, a dicarbonyl-dioxo, or silylene- dioxo; or R2 and P2 combine to form an optionally substituted ethylene or a structure selected from the group consisting of:

Me. ^H

P'O

H Me

, where each P' is independently a hydroxyl protecting group;

E is optionally substituted alkyl or optionally substituted alkoxy;

G is O, S, CH2, or NRN, where RN is H, an N-protecting group, or optionally substituted alkyl; each Qi is independently ORA, SRA, S0 ₂ RA, OS0 ₂ RA, NRBRA, NRB(CO)RA, NRB(CO)(CO)RA,

NRA(CO)NRBRA, NRB(CO)ORA, (CO)ORA, 0(CO)RA, (CO)NRBRA, or 0(CO)NRBRA, where each of RA and RB IS independently H, alkyl, haloalkyl, hydroxyalkyl, aminoalkyl, aryl, haloaryl, hydroxyaryl, alkoxyaryl, arylalkyl, alkylaryl, haloarylalkyl, alkylhaloaryl, (alkoxyaryl)alkyl, heterocyclic radical, or heterocyclic radical-alkyl;

k is O or l ;

n is 0, 1 , or 2; and

R6 is H, and Ρβ is H or a hydroxyl protecting group; or R6 and Ρβ combine to form a double bond.

The compound (IVE) can be prepared using methods known in the art or using the allene-Prins method described herein. In particular, after the allene-Prins reaction and allylic reduction, the substrate, in which

agents capable of oxidizing an alcohol to a carbonyl and vinyl addition to carbonyl.

The preparation of compound (IVA) may also involve reactions deprotecting protected hydroxyl groups or protecting unprotected hydroxyl groups using methods described herein.

Compound (VA)

The compound of formula (VA) can be prepared using methods described in the references incorporated by reference above. In particular, the compound of formula (VA) can be prepared from a compound of formula (VE) and a compound of formula (VD), in which Px is H, e.g., using esterification reaction conditions (e.g., Shiina esterification reaction conditions (e.g., using MNBA) or Yamaguchi esterification reaction conditions, e.g., the reaction conditions described in Aicher et al., J. Am. Chem. Soc, 1 14:3162- 3164, 1992). The compound of formula (VE) has the following structure:

(VE)

where

(a1 ) R3 is H or a hydroxyl protecting group, R4 is alkyl ether, and R5 is H ;

(a2) R3 is H or a hydroxyl protecting group, and R4 and R5 combine to form a double bond; or

(a3) R3 and R4 combine to form a bond, and R5 is H;

(b1 ) R6 is a hydroxyl protecting group, and R7 and Rs combine to form a double bond;

or

(b2) R6 and R7 combine to form a bond, and Rs is H;

(c1 ) R9 is H, and P4 is a hydroxyl protecting group;

or

(c2) R9 and P4 combine to form a double bond;

each P5 is independently a hydroxyl protecting group; and

Ai is H or OP", where P" is a hydroxyl protecting group;.

The compound of formula (VD) has the following structure:

(VD),

where

each of D and D' is independently H, optionally substituted alkyl, or OP1 ; A is a group of formula (1 ) or a C1-6 saturated or C2-6 unsaturated hydrocarbon skeleton, the skeleton being unsubstituted or having from 1 to 10 substituents independently selected from the group consisting of cyano, halo, azido, oxo, and Qi , the group of formula (1 ) having the structure:

(1 ),

where

L is -(CH(OP ₂ ))-, -(C(OH)(OP ₂ ))-, or -C(O)-;

Ri and Pi combine to form a bond; or Ri is H, and Pi is a hydroxyl protecting group or alkyl; Pi2 is H or -(CH2)nOP3, and each of P2 and P3 is independently optionally substituted alkyl or a hydroxyl protecting group, or P2 and P3, together with the atoms to which each is attached, combine to form a ketal, a cyclic carbonate, a dicarbonyl-dioxo, or silylene- dioxo; or R2 and P2 combine to form an optionally substituted ethylene or a structure selected from the group consisting of:

group;

E is optionally substituted alkyl or optionally substituted alkoxy;

G is O, S, CH2, or NRN, where RN is H, an N-protecting group, or optionally substituted alkyl ; each Qi is independently ORA, SRA, S0 ₂ RA, OS0 ₂ RA, NRBRA, NRB(CO)RA, NRB(CO)(CO)RA, NRA(CO)NRBRA, NRB(CO)ORA, (CO)ORA, 0(CO)RA, (CO)NRBRA, or 0(CO)NRBRA, where each of RA and RB IS independently H, alkyl, haloalkyi, hydroxyalkyi , aminoalkyi , aryl , haloaryl, hydroxyaryl, alkoxyaryl, arylalkyl, alkylaryl, haloarylalkyl, alkylhaloaryl , (alkoxyaryl)alkyl, heterocyclic radical, or heterocyclic radical-alkyl ;

Px is H or a hydroxyl protecting group;

k is 0 or 1 ;

n is 0, 1 , or 2; and

R10 is H or -CH2Xi CH2CH=CH2, where Xi is O, -C(Rn ) ₂ -, or NP ₆ , and where each Rn is independently H or -COOR12, Ρβ is an /V-protecting group, and R12 is alkyl.

The compound of formula (VD), in which Px is H , can be prepared by reacting a hydroxyl protecting group removing agent with the compound of formula (VD), in which Px is a hydroxyl protecting group.

The compound of formula (VD) can be prepared using methods known in the art or using the allene-Prins method described herein. For example, the compound of formula (VD), in which R10 is - CH2Xi CH2CH=CH2, can be prepared from the compound of formula (VF) :

(VF).

where RG is -CHO, -CH=CH-COORH, where RH is H or a hydroxyl protecting group, and all remaining variables are as defined for for formula (VD).

Thus, in a non-limiting example, the compound of formula (VF), in which RG is -CHO, may be subjected to Horner-Wadsworth-Emmons reaction conditions (e.g., with (EtO)2P(0)-CH2-COORi-i) to produce the compound of formula (VF), in which RG is -CH=CH-COORH. The latter compound may be treated with a 1 ,2-reducing agent (e.g., DIBAL) followed by a quench with allyl halide (e.g., allyl bromide) to give the compound of formula (VD).

The compound of formula (VF) may be prepared from the compound of formula (VIIID). In a non-limiting example, the compound of formula (VIIID), in which R3 is -CH2-OP5, and P5 is a hydroxyl protecting group, may be treated with a hydroxyl protecting group removing agent (e.g., if P5 is Piv, the hydroxyl protecting group removing agent may be DIBAL), followed by an oxidizing agent capable of oxidizing hydroxyl to a carbonyl group (e.g., Dess-Martin periodinane), to give the compound of formula (VF), in which RG is -CHO.

The preparation of compound (VA) may also involve reactions deprotecting protected hydroxyl groups or protecting unprotected hydroxyl groups using methods described herein.

Compound (VIA)

The compound of formula (VIA) can be prepared using methods described in the references incorporated by reference above. In particular, the compound of formula (VIA) can be prepared from a compound of formula (VIC) and a compound of formula (VID), e.g., using esterification reaction conditions (e.g., Shiina esterification reaction conditions (e.g., using MNBA) or Yamaguchi esterification reaction conditions, e.g., the reaction conditions described in Aicher et al., J. Am. Chem. Soc, 1 14:31 62-3164, 1992). The compound of formula (VIC) has the following structure:

(VIC),

or a tautomer thereof

where

b designates (/^-stereogenic center, and Z is a sulfonate, chloride, bromide, or iodide; or b designates (S)-stereogenic center, and Z is ΟΙ¾, where R6 is a hydroxyl protecting group;

Y is iodide, bromide, or trifluoromethanesulfonate;

(i) P13 is H or a hydroxyl protecting group, and FU and Rs combine to form a double bond; or

(ii) R3 and R4 combine to form a bond, and R5 is H ;

each P4 is independently a hydroxyl protecting group, and Xi is oxo or Xi , together with the carbon atom to which it is attached, is -(CH(OPY))-, wherein Ργ is H or a hydroxyl protecting group; or both P4 groups and Xi , together with the atoms to which each is attached, combine to form ketal.

The compound of formula (VID) has the following structure:

(VID),

where

each of D and D' is independently H, optionally substituted alkyl, or OP1 , provided that only one of D and D' is OP1 , where Pi is H, alkyl, or a hydroxyl protecting group; A is a group of formula (1 ) or a C1-6 saturated or C2-6 unsaturated hydrocarbon skeleton , the skeleton being unsubstituted or having from 1 to 10 substituents independently selected from the group consisting of cyano, halo, azido, oxo, and Qi , the group of formula (1 ) having the structure :

(1 ),

where

L is -(CH(OP ₂ ))-, -(C(OH)(OP ₂ ))-, or -C(O)-;

Ri and Pi combine to form a bond; or Ri is absent or H, and Pi is absent, a hydroxyl protecting group, or alkyl ;

R2 is H or -(CH2)nOP3, and each of P2 and P3 is independently H, optionally substituted alkyl, or a hydroxyl protecting group, or P2 and P3, together with the atoms to which each is attached, combine to form a ketal, a cyclic carbonate, a dicarbonyl-dioxo, or silylene- dioxo; or R2 and P2 combine to form an optionally substituted ethylene or a structure selected from the group consisting of:

group;

E is optionally substituted alkyl or optionally substituted alkoxy;

G is O, S, CH2, or NRN, where RN is H, an N-protecting group, or optionally substituted alkyl; each Qi is independently ORA, SRA, S0 ₂ RA, OS0 ₂ RA, NRBRA, NRB(CO)RA, NRB(CO)(CO)RA, NRA(CO)NR _B RA, NRB(CO)ORA, (CO)ORA, 0(CO)RA, (CO)NRBRA, or 0(CO)NRBRA, where each of RA and RB IS independently H, alkyl, haloalkyi, hydroxyalkyi, aminoalkyi, aryl, haloaryl, hydroxyaryl, alkoxyaryl, arylalkyl, alkylaryl, haloarylalkyl, alkylhaloaryl, (alkoxyaryl)alkyl, heterocyclic radical, or heterocyclic radical-alkyl;

k is 0 or 1 ;

n is 0, 1 , or 2; and

P5 is a hydroxyl protecting group

The compound of formula (VIC) can be prepared using methods described in WO 2015/066729. The preparation of compound (VIA) may also involve reactions deprotecting protected hydroxyl groups or protecting unprotected hydroxyl groups using methods described herein.

Compound (VI I A)

The compound of formula (VMA) can be prepared using methods described in the references incorporated by reference above. In particular, the compound of formula (VMA) can be prepared from a compound of formula (VIIE):

(VIIE)

or a salt or a tautomer thereof,

where

each of D and D' is independently H, optionally substituted alkyl, or OPi , provided that only one of D and D' is OPi , where Pi is H, alkyl, or a hydroxyl protecting group; A is a group of formula (1 ) or a C1-6 saturated or C2-6 unsaturated hydrocarbon skeleton, the skeleton being unsubstituted or having from 1 to 10 substituents independently selected from the group consisting of cyano, halo, azido, oxo, and Qi , the group of formula (1 ) having the structure:

(1 ),

where

L is -(CH(OP ₂ ))-, -(C(OH)(OP ₂ ))-, or -C(O)-;

Ri is H, or Ri and Pi combine to form a bond;

R2 is H or -(CH2)nOP3, and each of P2 and P3 is independently H, optionally substituted alkyl, or a hydroxyl protecting group, or P2 and P3, together with the atoms to which each is attached, combine to form a ketal, a cyclic carbonate, a dicarbonyl-dioxo, or silylene- dioxo; or R2 and P2 combine to form an optionally substituted ethylene or a structure selected from the group consisting of:

Me. ^H

H Me

, where each P' is independently H or a hydroxyl protecting group;

E is optionally substituted alkyl or optionally substituted alkoxy;

G is O, S, CH2, or NRN, where RN is H, an N-protecting group, or optionally substituted alkyl; each Qi is independently ORA, SRA, S0 ₂ RA, OS0 ₂ RA, NRBRA, NRB(CO)RA, NRB(CO)(CO)RA,

NRA(CO)NRBRA, NRB(CO)ORA, (CO)ORA, 0(CO)RA, (CO)NRBRA, or 0(CO)NRBRA, where each of RA and RB IS independently H, alkyl, haloalkyi, hydroxyalkyi, aminoalkyi, aryl, haloaryl, hydroxyaryl, alkoxyaryl, arylalkyl, alkylaryl, haloarylalkyl, alkylhaloaryl, (alkoxyaryl)alkyl, heterocyclic radical, or heterocyclic radical-alkyl;

k is O or l ;

n is O, 1 , or 2;

Y is iodide, bromide, or trifluoromethanesulfonate;

P5 is H or a hydroxyl protecting group; and

(a1 ) R3 is H or a hydroxyl protecting group, R4 and R5 combine to form a double bond, each P4 is independently H or a hydroxyl protecting group, and Xi , together with the carbon to which it is attached, forms a carbonyl or -(CH(ORs))-, where Rs is H or a hydroxyl protecting group;

or

(a2) R3 and R4 combine to form a bond, R5 is H, and

each P4 is independently H or a hydroxyl protecting group, and Xi , together with the carbon to which it is attached, forms a carbonyl or -(CH(ORs))-;

or

both P4 groups and Xi , together with the atoms to which each is attached, combine to form ketal;

(b1 ) Z is chloride, bromide, or iodide, and R6 and R7 combine to form a bond;

or

(b2) Z and R7 combine to form a double bond, and R6 is H or a hydroxyl protecting group.

The preparation of the compound of formula (VMA) can involve oxidizing the compound of formula (VIIE) (e.g., the compound of formula (VIIE), in which P5 is H or a hydroxyl protecting group), e.g., by reacting with an oxidizing agent capable of oxidizing hydroxyl to a carbonyl group. The mixture containing the oxidizing agent can also act as a hydroxyl protecting group removing agent.

The compound of formula (VIIE) can be formed by reacting a compound of formula (VIIF) with the compound of formula (VUG). The compound of formula (VIIF) has the following structure:

(VI IF)

or a salt or a tautomer thereof,

where

Y is iodide, bromide, or trifluoromethanesulfonate; and

(a1 ) R3 is H or a hydroxyl protecting group, FU and R5 combine to form a double bond, each P4 is independently H or a hydroxyl protecting group, and Xi , together with the carbon to which it is attached, forms a carbonyl or -(CH(ORs))-, where Rs is H or a hydroxyl protecting group;

or

(a2) R3 and R4 combine to form a bond, R5 is H, and

each P4 is independently H or a hydroxyl protecting group, and Xi , together with the carbon to which it is attached, forms a carbonyl or -(CH(ORs))-;

or

both P4 groups and Xi , together with the atoms to which each is attached, combine to form ketal;

(b1 ) Z is chloride, bromide, or iodide, and R6 and R7 combine to form a bond;

or

(b2) Z and R7 combine to form a double bond, and R6 is H or a hydroxyl protecting group. The compound of formula (VI IG) has the following structure:

(VIIG),

or a salt thereof,

each of D and D' is independently H, optionally substituted alkyl, or OP1 , provided that only one of D and D' is OP1 , where Pi is H, alkyl, or a hydroxyl protecting group; A is a group of formula (1 ) or a C1-6 saturated or C2-6 unsaturated hydrocarbon skeleton, the skeleton being unsubstituted or having from 1 to 10 substituents independently selected from the group consisting of cyano, halo, azido, oxo, and Qi , the group of formula (1 ) having the structure:

(1 ),

where

L is -(CH(OP ₂ ))-, -(C(OH)(OP ₂ ))-, or -C(O)-;

Ri is H, or Ri and Pi combine to form a bond;

Pi2 is H or -(CH2)nOP3, and each of P2 and P3 is independently H, optionally substituted alkyl, or a hydroxyl protecting group, or P2 and P3, together with the atoms to which each is attached, combine to form a ketal, a cyclic carbonate, a dicarbonyl-dioxo, or silylene- dioxo; or R2 and P2 combine to form an optionally substituted ethylene or a structure selected from the group consisting of:

group;

E is optionally substituted alkyl or optionally substituted alkoxy;

G is O, S, CH2, or NRN, where RN is H, an N-protecting group, or optionally substituted alkyl; each Qi is independently ORA, SRA, S0 ₂ RA, OS0 ₂ RA, NRBRA, NRB(CO)RA, NRB(CO)(CO)RA, NRA(CO)NRBRA, NRB(CO)ORA, (CO)ORA, 0(CO)RA, (CO)NRBRA, or 0(CO)NRBRA, where each of RA and RB IS independently H, alkyl, haloalkyi, hydroxyalkyi, aminoalkyi, aryl, haloaryl, hydroxyaryl, alkoxyaryl, arylalkyi, alkylaryl, haloarylalkyi, alkylhaloaryl, (alkoxyaryl)alkyl, heterocyclic radical, or heterocyclic radical-alkyl;

k is 0 or 1 ;

n is 0, 1 , or 2; and

P5 is H or a hydroxyl protecting group. Allene-Prins Reaction in the Preparation of Fragments (e.g., C.14-C.38 Fragment) of Halichondrin

The invention further features a method of preparing a fragment of halichondrin macrolide (e.g., C.20- C.35, C.20-C.38, C.16-C.35, C.16-C.38, C.14-C35, and C.14-C.38 fragments), which can be an intermediate in the synthesis of halichondrin macrolide or a salt thereof. The advantages of the process relative to the current synthesis of C.14-C.35, C.14-C.38, C.16-C.35, C.16-C.38, C.20-C.35, and C.20- C.38 fragments of halichondrin include non-metal mediated assembly that does not employ a C.23 leaving group. Additionally, the process obviates the need for a chiral ligand. The method involves performing an allene-Prins reaction on a compound of formula (VINA), a compound of formula (VIMB), and PHOH to afford the intermediate in the synthesis of halichondrin macrolide,

where FU is an optionally substituted acyl;

where the compound of formula (VINA) has the following structure:

(VINA)

or a salt thereof,

where

each of D and D' is independently H, optionally substituted alkyl, or OP1 , provided that only one of D and D' is OP1 , where Pi is H, alkyl, or a hydroxyl protecting group; A is a group of formula (1 ) or a C1-6 saturated or C2-6 unsaturated hydrocarbon skeleton, the skeleton being unsubstituted or having from 1 to 10 substituents independently selected from the group consisting of cyano, halo, azido, oxo, and Qi , the group of formula (1 ) having the structure:

(1 ),

where

L is -(CH(OP ₂ ))-, -(C(OH)(OP ₂ ))-, or -C(O)-;

Ri is H, or Ri and Pi combine to form a bond;

P12 is H or -(CH2)nOP3, and each of P2 and P3 is independently H, optionally substituted alkyl, or a hydroxyl protecting group, or P2 and P3, together with the atoms to which each is attached, combine to form a ketal, a cyclic carbonate, a dicarbonyl-dioxo, or silylene- dioxo; or R2 and P2 combine to form an optionally substituted ethylene or a structure selected from the group consisting of: , where each Ρ' is independently Η or a hydroxyl protecting group;

E is optionally substituted alkyl or optionally substituted alkoxy;

G is O, S, CH2, or NRN, where RN is H, an N-protecting group, or optionally substituted alkyl ; each Qi is independently ORA, SRA, S0 ₂ RA, OS0 ₂ RA, NRBRA, NRB(CO)RA, NRB(CO)(CO)RA, NRA(CO)NRBRA, NRB(CO)ORA, (CO)ORA, 0(CO)RA, (CO)NRBRA, or 0(CO)NRBRA, where each of RA and RB IS independently H, alkyl, haloalkyi, hydroxyalkyi , aminoalkyi , aryl , haloaryl, hydroxyaryl, alkoxyaryl, arylalkyi, alkylaryl, haloarylalkyi, alkylhaloaryl , (alkoxyaryl)alkyl, heterocyclic radical, or heterocyclic radical-alkyl ;

k is 0 or 1 ;

n is O, 1 , or 2;

X is O, or X, together with the carbon atom to which it is attached, forms -(C(OPz)2)-, wherein each Pz is independently optionally substituted alkyl or optionally substituted aryl , or both Pz combine to form optionally substituted alkylene or optionally substituted arylene; and

P4 is H or a hydroxyl protecting group;

where the compound of formula (VI IIB) has the following structure:

(VI IIB),

where

R ₃ is -CH2-OP5, -CH=CH ₂ , , or

where Ps is H or a hydroxyl protecting group; each Ρβ is independently a hydroxyl protecting group, or both Ρβ groups, together with the atoms to which each is attached, combine to form a cyclic protected diol; and Rs is is H or - CH ₂ Xi CH ₂ CH=CH2, where Xi is O, -CH ₂ -, or NP ₇ , where P? is a sulfonyl;

and where the intermediate is a compound of formula (VIIIC):

In some embodiments, a designates (S)-stereogenic center. In certain embodiments, one and only one of D and D' is optionally substituted alkyl or OPi . In other embodiments, one and only one of D and D' is OPi , where Pi is H, alkyl, or a hydroxyl protecting group. In yet other embodiments, A is a group of formula (1 ). In still other embodiments, L is -(CH(OP2))-. In particular embodiments, Ri and Pi combine to form a bond. In certain embodiments, G is O. In further embodiments, E is optionally substituted alkyl (e.g., methyl). In some embodiments, k is 1 . In particular embodiments, R2 is -(CH2)nOP3 (e.g., n is 1 or 2). In further embodiments, at least one of P2 and P3 is a hydroxyl protecting group. The reaction conditions that can be used to prepare the compound of formula (VIIIC) using allene-Prins reaction are those known in the art for Prins reaction and can include reacting the compound of formula (VINA), the compound of formula (VIIIB), and R4OH with a Lewis acid (e.g., boron trifluoride or a solvate thereof). The compound of formula (VIIIC) can be subjected to allylic reducing conditions to afford a compound of formula (VIIID):

(VIIID)

or a salt thereof, where

each of D and D' is independently H, optionally substituted alkyl, or OPi , provided that only one of D and D' is OPi , where Pi is H, alkyl, or a hydroxyl protecting group; A is a group of formula (1 ) or a C1-6 saturated or C2-6 unsaturated hydrocarbon skeleton, the skeleton being unsubstituted or having from 1 to 10 substituents independently selected from the group consisting of cyano, halo, azido, oxo, and Qi , the group of formula (1 ) having the structure:

(1 ),

where

L is -(CH(OP ₂ ))-, -(C(OH)(OP ₂ ))-, or -C(O)-;

Ri is H, or Ri and Pi combine to form a bond;

R2 is H or -(CH2)nOP3, and each of P2 and P3 is independently H, optionally substituted alkyl, or a hydroxyl protecting group, or P2 and P3, together with the atoms to which each is attached, combine to form a ketal, a cyclic carbonate, a dicarbonyl-dioxo, or silylene- dioxo; or R2 and P2 combine to form an optionally substituted ethylene or a structure selected from the group consisting of:

group;

E is optionally substituted alkyl or optionally substituted alkoxy;

G is O, S, CH2, or NRN, where RN is H, an N-protecting group, or optionally substituted alkyl; each Qi is independently ORA, SRA, S0 ₂ RA, OS0 ₂ RA, NRBRA, NRB(CO)RA, NRB(CO)(CO)RA, NRA(CO)NRBRA, NRB(CO)ORA, (CO)ORA, 0(CO)RA, (CO)NRBRA, or 0(CO)NRBRA, where each of RA and RB IS independently H, alkyl, haloalkyi, hydroxyalkyi, aminoalkyi, aryl, haloaryl, hydroxyaryl, alkoxyaryl, arylalkyi, alkylaryl, haloarylalkyi, alkylhaloaryl, (alkoxyaryl)alkyl, heterocyclic radical, or heterocyclic radical-alkyl; k is 0 or 1 ;

n is O, 1 , or 2;

and

P4 is H or a hydroxyl protecting group; and

where P5 is H or a hydroxyl protecting group; each Ρβ is independently a hydroxyl protecting group, or both Ρβ groups, together with the atoms to which each is attached, combine to form a cyclic protected diol; and R5 is is H or -CH2Xi CH2CH=CH2, where Xi is O, -CH2-, or NP7, where P? is a sulfonyl.

Preparation of a compound of formula (VII IB) is described in detail in International Patent Application No. PCT/US2014/063960, and the disclosure of the preparation is incorporated herein by reference in its entirety.

Compounds

The present invention also provides compounds that can be used in the synthesis of a halichondrin macrolide, e.g., the compounds of formula (IA), (IB), (IC), (IE), (IF), (MA), (MB), (IIC), (lICa), (III A), (1MB), (IVA), (IVB), (IVC), (IVD), (IVE), (VA), (VB), (VBa), (VC), (VIA), (VIB), (VIC), (VIIA), (VIIB), (VIIC), (VIID), (VIIE), (VIIF), (VUG), (VINA), (VIIIB), (VIIIC), or (VIIID).

Oxidizing Agents Capable of Converting an Alcohol to a Carbonyl Group

Oxidizing agents capable of converting an alcohol to a carbonyl group are known in the art. Non-limiting examples of these oxidizing agents include Dess-Martin periodinane, TEMPO (in the presence of bleach or BAIB), a dimethylsulfonium compound (e.g., dimethylchlorosulfonium chloride), aluminum trialkoxide with an excess of a ketone (e.g., acetone), and catalytic tetrapropylammonium perruthenate (TPAP) (in the presence of /V-methylmorpholine oxide). The dimethylsulfonium compound can be prepared in situ under the conditions known for Parikh-Doering oxidation, Swern oxidation, Corey-Kim oxidation, or Pfitzner-Moffatt oxidation. An oxidation reaction of an alcohol to a carbonyl group (e.g., a ketone) can be performed using aluminum trialkoxide and an excess of a ketone (e.g., acetone) under the conditions known in the art for Oppenauer oxidation. Allylic and benzylic alcohols can also be oxidized with Mn02.

Reducing Agents

Reducing agents that can be used in the methods of the invention are those known in the art. A reducing agent can be an electron-transfer reducing agent, a metal hydride, or a metalloid hydride. Non-limiting examples of electron-trannsfer reducing agent include alkali metals in oxidation state (0), alkali earth metals in oxidation state (0), alkali arenides, lanthanide (II) salts (e.g., Sm ), Zn(0), Fe(0), and Mn(0). Non-limiting examples of metal hydrides and metalloid hydrides include boron hydride compounds (e.g., NaBhU, L1BH4, LiB(Et)3H, selectrides (e.g., L-selectride), and boranes (e.g., 9-BBN and alpine borane)), aluminum hydride compounds (e.g., LiAlhU, Red-AI®, and alanes (e.g., DIBAL)), hydrosilanes (e.g., PMHS and Ph2SiH2), hydrostannanes (e.g., BusSnH), copper hydride complexes (e.g., Stryker's reagent), palladium hydride complexes, platinum hydride complexes, iridium hydride complexes, rhodium hydride complexes, and ruthenium hydride complexes. Reducing agents can be formed in situ, e.g., a copper hydride complex can be formed by a reaction of a copper salt with, e.g., a boron hydride compound or a hydrosilane. Thus, some reducing reagents (e.g., boron hydride compounds, hydrosilanes, and hydrostannanes) can be used in combination with a catalytic quantity of a metal salt (e.g., Cu, Pd, Pt, Ir, Rh, or Ru salt). Alternatively, catalytic reducing agents can be metal salts (e.g., aluminum isopropoxide or a ruthenium complex) in combination with an alcohol, which undergo transfer hydrogenation of carbonyl-containing compounds without intermediacy of a metal hydride. Non-limiting examples of transfer hydrogenation reactions include Meerwein-Ponndorf-Verley reduction (e.g., using aluminum isopropoxide/isopropanol) and Ru-catalyzed transfer hydrogenation (e.g., Hashiguchi et al., J. Am. Chem. Soc, 1 17:7562-7563, 1995).

When a substrate is an α,β-unsaturated carbonyl compound (e.g., an α,β-enone), a reducing agent can be a 1 ,2-reducing agent or a 1 ,4-reducing agent. For example, a reaction between an α,β-unsaturated carbonyl compound and a 1 ,2-reducing agent can afford, e.g., an allylic alcohol (or an allylic amine, if the starting compound is an enamide), whereas a reaction between an α,β-unsaturated carbonyl compound and a 1 ,4-reducing agent can afford an α,β-saturated compound and can leave the carbonyl group intact after work up of the reaction mixture. Non-limiting examples of 1 ,2-reducing agents include metal hydrides and metalloid hydrides, e.g., aluminum hydride compounds, boron hydride compounds (e.g., CeC with NaBhU), and ruthenium hydride complexes. Non-limiting examples of 1 ,4-reducing agents include boron hydride compounds, hydrostannanes, copper hydride complexes (e.g., Stryker's reagent), palladium hydride complexes, platinum hydride complexes, iridium hydride complexes, rhodium hydride complexes, and ruthenium hydride complexes.

A compound having an allylic leaving group (e.g., a carboxylate, a halide, or a sulfonate) can be treated with an allylic reducing agent to replace the leaving group with a hydrogen atom. A non-limiting example of allylic reducing agent is a palladium salt in combination with a formic acid salt (e.g., trialkylammonium formate).

Hydroxyl Protecting Groups and Hydroxyl Protecting Group Removing Agents

Hydroxyl protecting groups can be as defined herein. In particular, a hydroxyl protecting group can be an acyl, a sulfonyl, an arylalkyl (e.g., benzyl or p-methoxybenzyl), an aryl (e.g., p-methoxyphenyl), or an optionally substituted silyl (e.g., TMS, TES, TBS, TIPS, TBDPS, DMPS, or TPS). Hydroxyl protecting groups, hydroxyl protecting agents, and hydroxyl protecting reaction conditions can be selected to protect selectively certain hydroxyl groups in a compound, while leaving other hydroxyl groups unprotected. The choice of hydroxyl protecting groups for a compound can facilitate subsequent deprotection strategies, as some hydroxyl protecting groups can be removed in the presence of others using appropriate hydroxyl protecting group removing agents. Some of these strategies involving the choice of silyl hydroxyl protecting groups are discussed in, e.g., Silicon-Based Blocking Agents, Gelest, Inc., 201 1 .

Hydroxyl protecting group removing agents are those agents that can react with a compound having a protected hydroxyl group to afford the compound with a deprotected hydroxyl group. Hydroxyl protecting group removing agents and deprotection reaction conditions can be those known in the art. In a non- limiting example, hydroxyl masked as silyl ether can be unmasked by a reaction with a fluoride source (e.g., a fluoride salt, such as KF or TBAF). Alternatively, hydroxyl protected as TMS or TES ether can be deprotected by a reaction with a Bronsted acid (e.g., a carboxylic acid). In another non-limiting example, hydroxyl protected as an ester can be deprotected by a reaction with a C1-6 alkoxide (e.g., alkali C1-6 alkoxide or alkali earth C1-6 alkoxide). In yet another non-limting example, hydroxyl protected as an arylalkyl ether (e.g., 1 -arylalk-1 -yl ether) can be deprotected using a reduction reaction, e.g., with Pd/C and H2 or with Na/NH3. Alternatively, hydroxyl protected as an alkoxy-arylalkyl ether (e.g., MPM ether) can be deprotected by a reaction with 2,3-dichloro-5,6-dicyano-1 ,4-benzoquinone (DDQ). In still another non-limiting example, hydroxyl protected as alkoxyalkyl ether (e.g., 1 -alkoxyalk- -yl) or THP ether can be deprotected by a reaction with a Bronsted acid. Cyclic protected diols, such as acetals or ketals (e.g., as 2-alkyl-1 ,3-dioxolane, 2,2-dialkyl-1 ,3-dioxolane, 2-alkyl-1 ,3-dioxane, or 2,2-dialkyl-1 ,3-dioxane), can be deprotected by a reaction with a Bronsted acid (e.g., a carboxylic acid).

Dihydroxylation and Oxidative Carbon-Carbon Bond Cleavage

Dihydroxylation reaction produces diols from olefins. The reaction conditions can be those known in the art. Dihydroxylation reaction may be direct (e.g., Sharpless dihydroxylation, Upjohn dihydroxylation, or Milas dihydroxylation) or indirect (e.g., through epoxidation/hydrolysis, diborylation/oxidation, or carboxylation/hydrolysis (e.g., Woodward reaction or Prevost reaction)). A direct dihydroxylation reaction can be performed using an osmium complex (e.g., OsC or osmate salt). A catalytic quantity of the osmium complex can be used in the dihydroxylation reactions in combination with an additional oxidizing agent (e.g., /V-methylmorpholine /V-oxide or hydrogen peroxide). The dihydroxylation reaction can be rendered stereoselective by including a chiral ligand (e.g., in AD-mix, which is a mixture of osmate and a chiral ligand).

When a dihydroxylation reaction is performed with an osmium complex and a periodate salt as oxidizing agent, the carbon-carbon double bond of an olefin can be cleaved to afford two carbonyl groups. The transformation of an olefin to two carbonyl groups is referred to herein as a cleavage reaction. Cleavage reaction conditions can be those known in the art. Non-limiting examples of cleavage reaction conditions can be ozonolysis reaction conditions or a reaction with an osmium complex (e.g., OsC or osmate salt) and a periodate salt. A vicinal diol can be cleaved by sodium periodate or potassium permanganate.

Epimerizations

Epimerization reactions can be used to invert a stereogenic center having an undesired stereochemical identity. For example, through epimerization, R stereogenic center can be converted to S stereogenic center and vice versa. Epimerization of a stereogenic sp ³ -carbon bonded to one hydrogen atom and to one hydroxyl group can be achieved through a reaction sequence involving oxidation of the hydroxyl group to a carbonyl group followed by a 1 ,2-reduction reaction. The 1 ,2-reduction reaction can provide the desired stereochemical identity diastereoselectively, or the reaction can be carried out using a chiral catalyst, chiral auxiliary, or a chiral reducing agent. Non-limiting examples of chiral reducing agents include alpine borane and prapine borane. Non-limiting examples of 1 ,2-reduction reactions involving chiral catalysts are Corey-Bakshi-Shibata reduction, Noyori hydrogenation, and Noyori transfer hydrogenation, The oxidation/reduction reaction sequence can be carried out in situ using dynamic kinetic resolution. A dynamic kinetic resolution can further involve a reaction with a hydroxyl protecting agent, which removes the desired stereoisomer from the reduction/oxidation equilibrium. In a non-limiting example, a dynamic kinetic resolution of chiral secondary alcohols can involve reduction/oxidation equilibration using n ⁵ -PhsCpRu(CO)2H in combination with enantioselective esterification using isopropenyl acetate catalyzed by a lipase enzyme (e.g., lipase B from Candida Antarctica, see, e.g., Marti n-Matute et al., J. Am. Chem. Soc, 127:8817-8825, 2005). Epimerization can also be carried out on a compound containing a tetrahydropyran-2-yl-acetaldehyde moiety, in which carbon 2 of the pyran ring exhibits an undesired stereochemical identity. Contacting this compound with L-proline can provide equilibrium between two stereoisomers. If other, non-equilibrating stereogenic centers are present in the compound, the most stable stereoisomer will be present in a larger quantity relative to other stereoisomer(s) in equilibrium with the most stable stereoisomer.

The following examples are meant to illustrate the invention. They are not meant to limit the invention in any way.

Examples

Example 1 - C.2-C.3 Macrocyclization through Horner-Wadsworth-Emmons Olefination

A halichondrin macrolide 3 can be prepared according to the below sequence.

TBAF THF, rt

Compound 1 can be converted to compound 2 through the Horner-Wadsworth-Emmons olefination (e.g., under Masamune conditions or according to Helquist protocol). Compound 2 can be converted to compound 3 after global deprotection (e.g., removal of silyl groups with a fluoride source, such as TBAF).

Example 2 - C.2-C.3 Macrocyclization through Ring-Closing Olefin Metathesis

A halichondrin macrolide 3 was prepared according to the below sequence.

5-trans

Compound 4 (0.047 g, 0.027 mmol) was dissolved in toluene (57.0 mL) and 1 ,4-benzoquinone (1 mg, 10 μιηοΙ), and Hoveyda-Grubbs 2nd generation catalyst (3 mg, 5.3 μιηοΙ) was added. The reaction mixture was warmed to 70 °C. Upon completion (2h), DMSO (9.51 μΙ_, 0.134 mmol) was added, and the solution was allowed to stir at room temperature for 16 h. After concentration, the resulting green residue was purified by column chromatography (Biotage Ultra cartridge, 10g, heptane/EtOAc) to provide two RCM adducts, 5-cis and 5-trans with cis impurity (19 mg total, 0.01 1 mmol, 41 %, 3:1 ratio cis/trans).

5-cis: Ή NMR (400MHz, BENZENE-de) δ = 7.97 - 7.88 (m, 2H), 7.80 (dd, J=2.9, 6.4 Hz, 2H), 7.76 - 7.66 (m, 4H), 7.28 - 7.17 (m, 12H), 6.66 - 6.55 (m, 1 H), 6.04 (d, J=15.6 Hz, 1 H), 5.30 (dd, J=3.7, 6.8 Hz, 1 H), 5.02 (s, 1 H), 4.92 (t, J=1 .0 Hz, 1 H), 4.89 (br. s., 1 H), 4.87 (s, 1 H), 4.77 - 4.73 (m, 1 H), 4.77 - 4.73 (m, 1 H), 4.75 (s, 1 H),4.65 (br. s., 1 H), 4.47 (br. s., 1 H), 4.20 (dd, J=3.7, 6.8 Hz, 1 H), 4.1 6 - 4.09 (m, 2H), 4.03 - 3.89 (m, 3H), 3.80 - 3.66 (m, 3H), 3.43 (d, J=9.4 Hz, 4H), 3.33 (d, J=3.5 Hz, 1 H), 3.22 (d, J=9.4 Hz, 1 H), 2.96 - 2.90 (m, 1 H), 2.85 - 2.75 (m, 1 H), 2.63 - 2.44 (m, 1 H), 2.31 (d, J=5.5 Hz, 1 H), 2.25 - 1 .97 (m, 2H), 1 .96 - 1 .57 (m, 1 H), 1 .52 - 1 .23 (m, 1 H), 1 .21 (s, 1 H), 1 .15 - 1 .13 (m, 1 H), 1 .08 (s, 1 H), 1 .15 (s, 1 H), 0.99 - 0.97 (m, 1 H), 0.93 (s, 1 H), 0.85 - 0.85 (m, 1 H), 0.85 (s, 1 H),0.92 (s, 1 H), 0.81 (br. s., 1 H), 0.39 (s, 1 H), 0.32 - 0.30 (m, 1 H), 0.30 (s, 1 H), 0.30 - 0.29 (m, 1 H), 0.30 (s, 1 H), 1 .21 (s, 9H), 1 .14 (s, 9H), 1 .13 - 1 .12 (m, 3H), 1 .08 (s, 9H), 0.98 (s, 9H), 0.97 - 0.95 (m, J=2.0 Hz, 3H),0.93 (s, 9H), 0.85 (s, 9H), 0.30 (s, 3H), 0.30 (s, 3H), 0.18 (s, 3H), 0.14 (s, 3H), 0.13 (s, 3H), 0.1 1 (s, 3H), 0.01 (s, 3H), -0.1 1 (s, 3H)

The mixture of compounds 5-cis and 5-trans (0.019 g, 0.01 1 mmol) was dissolved in N,N- dimethylacetamide (0.304 ml_, 3.27 mmol). Imidazole hydrochloride (5.76 mg, 0.055 mmol) and TBAF (0.1 10 mL, 0.1 1 mmol) were added, and the reaction monitored by LCMS. Upon completion, water (5 mL) was added. Extraction with heptane removed lypophilic impurities. The aqeous layer was then extacted with toluene (5 mL) followed by toluene/THF (3:1 v/v, 4 x 8 mL). Brine was added to the aqueous layer to assist the recovery after the third extraction. The organic layers were combined and washed with saturated NaCI (2 x 6 mL). The organic layers were then dried over MgSC , filtered, and concentrated. The colorless residue was azeotroped with acetonitrile/water (3:1 , v/v) at 30 °C to provide compound 6 (27 mg, 310% mass recovery) as a pale tan solid that was used directly in the next step. LCMS (CI): calcd. 790, found, Na ion 813 (M + 1 + 22(Na)). Crude 6 was dissolved in DCM (0.087 mL, 1 .352 mmol), and PPTS (0.028 g, 0.1 1 mmol) was added. The reaction was monitored by LCMS. At 2 h, product forms as determined by M+1 of 773.

The residue (8.70 mg, 1 1 .00 μιηοΙ) was dissolved in dichloromethane (0.5 mL) at ambient temperature and PPTS (18 mg, 72 μιτιοΙ) was added. The reaction mixture was filtered through a S1O2 plug (0.75 cm x 2 cm) using EtOAc/MeOH to provide 5.5 mg of residue after concentration. The structure was confirmed by LCMS comparison with an authentic standard. UPLC column: Waters Acquity UPLC HSS T3 1 .8 μιτι, 2.1 ^* 50 mm,

Part no. 1 86003538, Serial no. 01753602015820

Temperature: 50 °C

Flow rate: 0.8 mL/min

Gradient: Time, %-Solvent A %-Solvent B

min

Initial 97 3

0.1 97 3

2.0 1 99

2.5 1 99

2.51 97 3

3.0 97 3

Injection volume: 1 μΐ

UV Detection: PDA 210-400 nm

Mass Detection(SQD): ES(+), mass scan 100.00 to 2000.00

Run time: 3 min

Compound 3 standard, RT 1 .90 min, ES+(M+1 ) 773

Found, RT 1 .90 min, ES+ (M+1 ) 773

4

(6S,E)-6-((2S,3R,4S,5S,6S)-6-(but-3-en-1-yl)-3,4,5-tris((ter t-butyldimethylsilyl)oxy)tetrahydro-2H- pyran-2-yl)-6-((tert-butyldimethylsilyl)oxy)-1-((2S,5S)-5-(2 -((2S,4R,6R)-6- (((2S,3R,4R,4aS,6R,7R,8aS)-7-((tert-butyldiphenylsilyl)oxy)- 6-(2-((tert-butyldiphenylsilyl)oxy)ethyl)- 3-((4-methoxybenzyl)oxy)-4-methyloctahydropyrano[3,2-b]pyran -2-yl)methyl)-4-methyl-5- methylenetetrahydro-2H-pyran-2-yl)ethyl)-4-methylenetetrahyd rofuran-2-yl)hex-4-en-3-ol

76%

Compound 8 (0.200 g, 0.242 mmol) and compound 7 (0.175 g, 0.1 56 mmol) were dissolved in N2 sparged THF (0.5 mL). In a separate flask, the ligand (0.255 g, 0.86 mmol) and chromium(ll) chloride (0.106 g, 0.862 mmol) were suspended in N ₂ sparged THF (1 .5 mL) at 30 °C. Triethylamine (0.12 mL, 0.861 mmol) was added, and the green solution was stirred for 30 minutes before cooling to 0 °C, at which time, Ni-DMP complex (8.00 mg, 0.024 mmol) was added. The solution of compounds 7 and 8 was added, and the reaction warmed to roomt temperature. Upon completion, the reaction was cooled to 0 °C, and ethylenediamine (0.158 ml, 2.345 mmol) was added and the stirred for 1 h. Water (2 mL) was added followed by heptane (10 mL) after 10 min. The phases were separated, and the aqueous phase was extracted with MTBE twice. The combined extracts were sequentially washed with HCI (1 N), water, and brine. After drying with MgSC , the filtrates were concentrated to provide a pale green solid.

Heptane trituration provided a solution that was loaded directly on a Redisep cartridge (4 g,

heptane/EtOAc) and provided the compound 9 (217 mg, 0.1 19 mmol, 76%, (est. 3:1 ratio of

diastereomers)). The product was used without further purification in the next step.

(S,E)-6-((2S,3R,4S,5S,6S)-6-(but-3-en-1-yl)-3,4,5-tris((t ert-butyldimethylsilyl)oxy)tetrahydro-2H- pyran-2-yl)-6-((tert-butyldimethylsilyl)oxy)-1-((2S,5S)-5-(2 -((2S,4R,6R)-6- (((2S,3R,4R,4aS,6R,7R,8aS)-7-((tert-butyldiphenylsilyl)oxy)- 6-(2-((tert-butyldiphenylsilyl)oxy)ethyl)- 3-((4-methoxybenzyl)oxy)-4-methyloctahydropyrano[3,2-b]pyran -2-yl)methyl)-4-methyl-5- methylenetetrahydro-2H-pyran-2-yl)ethyl)-4-methylenetetrahyd rofuran-2-yl)hex-4-en-3-one

Compound 9 (0.217 g, 0.1 1 9 mmol) was dissolved in DCM (2.1 70 mL), and water (0.107 μί) was added followed by Dess-Martin periodinane (0.061 g, 0.143 mmol). Upon completion, aqueous sodium bicarbonate (2 mL) was added, and the mixtuire was stirred for 15 minutes before extracting with DCM (3 x 2 mL). The combined extracts were washed with water and dried (Na2S04) to provide 246 mg of crude product as a white foam. The material was purified using a Redisep cartridge (4 g, conditioned with 9:1 ::heptane/EtOAc, loaded with 1 :1 helptane/DCM, and eluted as a single 30 mL fraction of

heptane/EtOAc (9:1 , (v/v)). After concentration, 15 (21 1 mg, 0.1 16 mmol, 97%) was provided as a white crunchy foam. ¹ H NMR (400MHz, CDC ) δ = d = 7.92 - 7.84 (m, 2H), 7.79 (dd, J=2.9, 6.4 Hz, 2H), 7.77 - 7.70 (m, 4H), 7.34 - 7.16 (m, 14H), 6.78 (d, J=1 .0 Hz, 2H), 6.62 (d, J=1 .0 Hz, 1 H), 5.96 - 5.81 (m, 1 H), 5.88 (d, J=5.9 Hz, 1 H), 5.39 - 5.30 (m, 1 H), 5.35 (dd, J=4.1 , 6.8 Hz, 1 H), 5.1 1 - 5.03 (m, 1 H), 5.12 - 5.02 (m, 1 H), 4.96 (d, J=10.9 Hz, 1 H), 5.00 - 4.92 (m, 1 H), 4.85 (s, 1 H), 4.82 - 4.79 (m, 1 H), 4.79 - 4.76 (m, 1 H), 4.79 (d, J=8.2 Hz, 1 H), 4.50 (t, J=5.5 Hz, 1 H), 4.40 (d, J=7.8 Hz, 1 H), 4.25 - 4.19 (m, 1 H), 4.18 - 4.1 1 (m, 1 H), 4.1 1 - 4.00 (m, 1 H), 3.96 (dd, J=5.1 , 9.4 Hz, 1 H), 3.85 - 3.67 (m, 1 H), 3.61 - 3.41 (m, 2H), 3.34 - 3.25 (m, 3H), 3.20 - 3.10 (m, 2H), 2.85 - 2.57 (m, 1 H), 2.55 - 2.30 (m, 2H), 2.28 - 1 .89 (m, 7H), 1 .89 - 1 .76 (m, 4H), 1 .76 - 1 .28 (m, 3H), 1 .1 8 (s, 7H), 1 .24 - 1 .16 (m, 6H), 1 .15 (s, 10H), 1 .09 (s, 9H), 0.97 (s, 1 1 H), 0.96 (br. s., 4H), 0.93 (s, 9H), 0.91 (s, 9H), 0.85 (t, J=6.8 Hz, 4H), 0.33 (br. s., 3H), 0.32 (br. s., 3H), 0.19 - 0.17 (m, 3H), 0.14 (br. s., 3H), 0.14 - 0.13 (m, 3H), 0.12 (s, 6H), 0.07 - 0.05 (m, 3H), 0.07 - 0.05 (m, 3H), 0.01 - -0.03 (m, 1 H), -0.01 (s, 3H).

(S,E)-6-((2R,3R,4S,5S,6S)-6-(but-3-en-1-yl)-3,4,5-tris((t ert-butyldimethylsilyl)oxy)tetrahydro-2H- pyran-2-yl)-6-((tert-butyldimethylsilyl)oxy)-1-((2S,5S)-5-(2 -((2S,4R,6R)-6- (((2S,3R,4R,4aS,6R,7R,8aS)-7-((tert-butyldiphenylsilyl)oxy)- 6-(2-((tert-butyldiphenylsilyl)oxy)ethyl)- 3-hydroxy-4-methyloctahydropyrano[3,2-b]pyran-2-yl)methyl)-4 -methyl-5-methylenetetrahydro-2H- pyran-2-yl)ethyl)-4-methylenetetrahydrofuran-2-yl)hex-4-en-3 -one

10 11 Compound 10 (0.21 1 g, 0.1 16 mmol) was dissolved in DCM (4.22 ml_), f-BuOH (0.042 ml_) and pH 7 phosphate buffer were added (0.422 ml_). DDQ (53 mg, 0.23 mmol) was added. Upon completion, aqueous sodium bicarbonate (10 mL) was added, and the solution stirred for 10 minutes before it was extracted with heptane (3 x 5 mL). The combined extracts were washed with aqueous sodium bicarbonate and water before drying with Na2S04. The filtrate was concentrated, and residue was purified by column chromatography (Biotage Ultra cartridge, 10 g, heptane/EtOAc) to provide 16 (139 mg, 0.083 mmol, 70%). ¹ H NMR (400MHz, BENZENE-d6) δ = 7.93 - 7.84 (m, 2H), 7.84 - 7.67 (m, 7H), 7.34 - 7.14 (m, 1 1 H), 6.62 (d, J=1 .0 Hz, 1 H), 5.96 - 5.82 (m, 1 H), 5.35 (dd, J=4.3, 7.4 Hz, 1 H), 5.08 (d, J=15.6 Hz, 1 H), 4.97 (d, J=10.2 Hz, 2H), 4.92 (s, 2H), 4.89 (s, 3H), 4.86 (s, 3H), 4.75 (s, 3H), 4.48 - 4.34 (m, 2H), 4.25 - 4.18 (m,1 H), 4.16 (s, 1 H), 4.13 - 4.07 (m, 1 H), 4.07 - 3.95 (m, 2H), 3.95 - 3.85 (m, 1 H), 3.85 - 3.75 (m, 1 H), 3.75 - 3.60 (m, 3H), 3.52 - 3.42 (m, 4H), 3.33 - 3.26 (m, 1 H), 3.12 - 3.08 (m, 1 H), 2.86 - 2.72 (m, 2H), 2.72 - 2.58 (m, 3H), 2.54 - 2.00 (m, 1 1 H), 1 .98-1 .20 (m, 1 1 H), 1 .12 (s, 9H), 1 .28 - 1 .1 1 (m, 60H), 1 .10 (s, 9H), 0.98 (s, 9H), 0.93- 0.91 (m, 3H), 0.87 (s, 9H), 0.85 (s, 9H), 0.83 (s, 9H), 0.33 (br. s., 3H), 0.32 (br. s., 3H), 0.18 (s, 3H), 0.15 (s, 3H), 0.14 (br. s., 3H), 0.12 (s, 3H), 0.06 (s, 3H), -0.01 (s, 3H) (2S,3R,4S,4aS,6R,7R,8aS)-2-(((2R,4R,6S)-6-(2-((2S,5S)-5-((S, E)-6-((2R,3R,4S,5S,6S)-6-(but-3-en-1- yl)-3,4,5-tris((tert-butyldimethylsilyl)oxy)tetrahydro-2H-py ran-2-yl)-6-((tert-butyldimethylsilyl)oxy 3-oxohex-4-en-1-yl)-3-methylenetetrahydrofuran-2-yl)ethyl)-4 -methyl-3-methylenetetrahydro-2H- pyran-2-yl)methyl)-7-((tert-butyldiphenylsilyl)oxy)-6-(2-((t ert-butyldiphenylsilyl)oxy)ethyl)-4- methyloctahydropyrano[3,2-b]pyran-3-yl acrylate

Compound 11 (0.082 g, 0.048 mmol) was dissolved in DCM (820 μΙ_), and the resulting solution was cooled to 0 °C. Triethylamine (0.05 mL, 0.359 mmol) was added, followed by acryoyl chloride (4 μί, 0.049 mmol). Additional 0.05 mL TEA and 6 μΙ_ acryoyl chloride were added. Upon completion of the reaction, saturated sodium bicarbonate (5 mL) was added, and the reaction mixture was extracted with heptane (3 x 3 mL). The combined extacts were washed with water and dried (Na2S04). After concentration, the residue was purified by column chromatography (Biotage Ultra cartridge, 10 g, heptane/EtOAc) to provide compound 4 (48 mg, 0.027 mmol, 57%) and a mixture of compounds 16 and 17 (35 mg). ¹ H NMR (400MHz, BENZENE-cfe) δ = 7.95 - 7.86 (m, 3H), 7.79 (dd, J=3.1 , 6.6 Hz, 3H), 7.73

- 7.66 (m, 7H), 7.34 - 7.27 (m, 1 H), 7.26 - 7.15 (m, 7H), 6.66 - 6.57 (m, 1 H), 6.34 (dd, J=1 .6, 17.6 Hz, 1 H), 5.99 (dd, J=10.2, 1 7.2 Hz, 1 H), 5.94 - 5.80 (m, 1 H), 5.35 (dd, J=4.1 , 7.2 Hz, 1 H), 5.31 - 5.25 (m, 1 H), 5.15

- 4.92 (m, 1 H), 4.91 - 4.83 (m, 3H), 4.77 (s, 1 H), 4.51 - 4.41 (m, 1 H), 4.34 (t, J=1 .0 Hz, 1 H), 4.25 - 4.19 (m, 1 H), 4.16 (s, 1 H), 4.09 - 3.95 (m, 4H), 3.89 (dt, J=4.7, 9.4 Hz, 1 H), 3.80 (t, J=8.8 Hz, 1 H), 3.75 - 3.68

(m, 1 H), 3.66 (br. s., 2H), 3.60 - 3.49 (m, 1 H), 3.42 - 3.35 (m, 2H), 3.29 (d, J=1 .0 Hz, 1 H), 2.95 (t, J=1 .6 Hz, 1 H), 2.86 - 2.61 (m, 2H), 2.56 - 2.33 (m, 3H), 2.31 - 1 .34 (m, 15H), 2.30-2.00 (m, 2H),1 .12 (s, 9H), 1 .09 (s, 9H), 1 .06 (d, J=1 .0 Hz, 3H), 0.97 (s, 9H), 0.95 (d, J=1 .0 Hz, 3H), 0.93 (s, 97H), 0.91 (s, 9H), 0.85 (s, 9H), 0.32 (br. s., 3H), 0.18 (s, 3H), 0.14 (s, 3H), 0.13 (br. s., 3H), 0.12 (s, 3H), 0.07 - 0.05 (m, 3H), 0.06 (s, 3H), -0.01 (s, 3H)

(2S,3S,4S,5R,6S)-4,5-bis((tert-butyldimethylsilyl)oxy)-6-((S ,E)-1-((tert-butyldimethylsilyl)oxy)-3- (trimethylsilyl)allyl)-2-(3-hydroxypropyl)tetrahydro-2H-pyra n-3-yl benzoate

Compound 15 (1 .160 g, 1 .548 mmol) was dissolved in THF (10 mL), and 9-borabicyclo[3.3.1 ]nonane (5.0 mL, 2.50 mmol) was added. Upon completion, the solution was cooled to 0 °C, and water (10 mL) was added followed by sodium perborate tetrahydrate (1 .429 g, 9.288 mmol). The mixture was stirred for 12 h. Aqueous sodium bicarbonate was added, and the mixture was extracted twice with heptane/MTBE (1 :1 (v/v)). The combined organic layers were washed with aqueous sodium bicarbonate followed by water and then dried over sodium sulfate. The resulting solution was concentrated, and the resulting residue was purified by column chromatgography to provide 0.820 g (1 .13 mmol, 73%) of compound 8A. ¹ H NMR (400MHz, CDCb) δ = 8.09 (br. s., 1 H), 7.44 - 7.36 (m, 1 H), 7.27 - 7.22 (m, 2H), 7.18 - 7.13 (m, 2H), 5.96 (br. s., 1 H), 5.21 (br. s., 1 H), 4.47 (br. s., 1 H), 4.1 1 (br. s., 2H), 4.01 - 3.91 (m, 3H), 3.65 (br. s., 3H), 3.30 (br. s., 1 H), 0.96 (s, 9H), 0.89 (s, 9H), 0.87 (s, 9H), 0.16 - 0.08 (m, 27 H). (2S,3S,4S,5R,6S)-2-(3-acetoxypropyl)-4,5-bis((tert-butyldime thylsilyl)oxy)-6-((S,E)-1-((tert- butyldimethylsilyl)oxy)-3-(trimethylsilyl)allyl)tetrahydro-2 H-pyran-3-yl benzoate

8A 8B

Compound 8A (0.870 g, 1 .134 mmol) was dissolved in DCM (8.70 ml) and cooled to 0 °C. Triethylamine (1 mL, 7.175 mmol) and DMAP (0.014 g, 0.1 13 mmol) were added followed by Ac ₂ 0 (0.214 mL, 2.268 mmol). Upon completion, saturated aqueous sodium bicarbonate (30 mL) was added. The mixture was extracted with MTBE (3 x 20 mL). The combined extracts were washed with saturated aqueous sodium bicarbonate, then water, and then brine. After drying over Na2S04, the solution was concentrated to provide compound 8B (0.895 g, 1 .106 mmol, 98%) as a colorless oil. ¹ H NMR (400MHz, CDCb) δ = 8.12 - 8.04 (m, 1 H), 7.55 - 7.48 (m, 1 H), 7.43 - 7.34 (m, 2H), 6.49 - 6.44 (m, 1 H), 5.97 - 5.89 (m, 1 H), 5.18 - 5.09 (m, 1 H), 4.46 - 4.33 (m, 1 H), 4.14 - 4.01 (m, 3H), 3.96 - 3.89 (m, 2H), 3.20 (s, 3H), 3.00 - 2.97 (m, 1 H), 2.35 - 2.33 (m, 1 H), 2.06 - 1 .99 (m, 2H), 1 .81 - 1 .74 (m, 1 H), 1 .64 - 1 .54 (m, 2H), 0.96 (s, 9H), 0.88 (s, 9H), 0.86 (s, 9H), 0.18 - 0.08 (m, 9H), 0.07 (s, 6H), 0.04 (s, 6H), -0.05 - -0.1 1 (m, 23) (2S,3S,4S,5R,6S)-2-(3-acetoxypropyl)-4,5-bis((tert-butyldime thylsilyl)oxy)-6-((S,E)-1-((tert- butyldimethylsilyl)oxy)-3-iodoallyl)tetrahydro-2H-pyran-3-yl benzoate

Compound 8B (0.895 g, 1 .106 mmol) was dissolved in toluene (5.37 ml) and acetonitrile (10.74 mL, 205.633 mmol). f-Butyldimethylchlorosilane (0.017 g, 0.1 1 1 mmol) was added, and the solution was warmed to 30 °C. NIS (1 .990 g, 8.846 mmol) was added. Upon completion, the reaction mixture was poured into a solution of sodium thiosulfate (2 g) in saturated aqueous sodium bicarbonate (40 mL). MTBE (20 mL) was added, and the mixture stirred for 45 minutes. The MTBE layer was removed in vacuo, and the aqueous layer was extracted with MTBE (2 x 20 mL). The combined MTBE extacts were washed with 10% (w/v) aqueuous sodium thiosulfate, followed by water and then by brine. The solution was dried over MgSC and concentrated to provide crude 8C (857 g, 0.993 mmol, 90%) as pale yellow oil. ¹ H NMR (400MHz, CDC ) δ = 8.06 (br. s., 2H), 7.53 (br. s., 1 H), 7.40 (br. s., 2H), 6.73 (br. s., 1 H), 6.38 (d, J=14.8 Hz, 1 H), 5.20 (br. s., 1 H), 5.01 (br. s., 1 H), 4.67 - 4.56 (m, 1 H), 4.54 - 4.46 (m, 1 H), 4.44 - 4.36 (m, 1 H), 4.1 7 - 3.98 (m, 3H), 3.98 - 3.82 (m, 2H), 3.34 - 3.1 9 (m, 1 H), 2.72 (s, 1 H), 2.33 (s, 1 H), 2.14 - 1 .88 (m, 3H), 1 .88 - 1 .50 (m, 4H), 1 .37 - 1 .17 (m, 2H), 0.94 (s, 9H), 0.88 (s, 9H), 0.87 (s, 1 0H), 0.05 (s, 18H)

(2S,3S,4S,5R,6S)-4,5-bis((tert-butyldimethylsilyl)oxy)-6- ((S,E)-1-((tert-butyldimethylsilyl)oxy)-3- iodoallyl)-2-(3-hydroxypropyl)tetrahydro-2H-pyran-3-ol

8C 8D

Compound 8C (0.814 g, 0.943 mmol) was dissolved in toluene (8.14 mL) and cooled to -10 °C. DIBAL (4.72 ml, 4.716 mmol) was added. After 15 minutes, methanol (0.191 mL, 4.716 mmol) was added followed by aqueous HCI (0.1 N, 20 mL). The mixture was stirred for 15 minutes and the extracted with MTBE three times. The combined extracts were washed with water and brine before drying over Na2S04 The resulting solution was concentrated in vacuo, and the residue was purified by column

chromatography (Biotage Ultra cartridge, 25 g; heptane/EtOAc) to provide compound 8D (601 g, 0.838 mmol, 89%). (2S,3S,4S,5R,6S)-4,5-bis((tert-butyldimethylsilyl)oxy)-6-((S ,E)-1-((tert-butyldimethylsilyl)oxy)-3- iodoallyl)-2-(3-((triethylsilyl)oxy)propyl)tetrahydro-2H-pyr an-3-ol

Step 1 . Compound 8D (0.601 g, 0.838 mmol) in DCM (6.01 mL) was cooled to 0 °C. 2,4-Lutidine (0.291 mL, 2.515 mmol) was added followed by chlorotriethylsilane (0.148 mL, 0.88 mmol). Upon completion, saturated aqueous sodium bicarbonate (20 mL) was added, and the mixture was extracted with DCM (3 x 20 mL). The combined extracts were washed with water. After drying over Na2S04, the organic layers were concentrated to provide crude TES-ether (0.697 g).

Step 2. The TES-ether (0.697 g) was dissolved in DCM (6.97 mL) and cooled to 0 °C. 2,4- Dimethylpyridine (0.388 mL, 3.354 mmol) was added followed by f-butyldimethylsilyl

trifluoromethanesulfonate (0.231 mL, 1 .006 mmol). Upon completion, saturated aqueous sodium bicarbonate (20 mL) was added, and the mixture was extracted with MTBE (3 x 20 mL). The combined extracts were washed with water, HCI (0.1 N), water, and then aqueous sodium bicarbonate. After drying, the filtrate was concentrated to a colorless oil and purified by column chromatography (Biotage Snap 25 cartridge, heptane/EtOAc) to provided compound 8E (0.650 g, 0.687 mmol, 82%). ¹ H NMR (400MHz, CDC ) δ = 6.80 - 6.57 (m, 1 H), 6.26 (dd, J=0.8, 14.5 Hz, 1 H), 4.58 - 4.25 (m, 1 H), 3.92 - 3.66 (m, 3H), 3.64-3.55 (m, 3H), 3.05 (d, J=8.6 Hz, 1 H), 1 .68 - 1 .55 (m, 1 H), 1 .51 - 1 .38 (m, 1 H), 1 .49 - 1 .33 (m, 2H), 1 .00 - 0.83 (m, 36H), 0.60 (q, J=8.2 Hz, 6H), 0.13 - -0.03 (m, 33H)

(((2S,3R,4S,5S,6S)-2-((S,E)-1-((tert-butyldimethylsilyl)o xy)-3-iodoallyl)-6-(3- ((triethylsilyl)oxy)propyl)tetrahydro-2H-pyran-3,4,5-triyl)t ris(oxy))tris(tert-butyldimethylsilane)

Step 1 (Swern oxidation): DMSO (0.19 mL, 1 .685 mmol) in DCM (0.4 mL) was added to oxalyl chloride (2 M in DCM, 0.357 mL, 0.715 mmol) in DCM (1 mL) at -78 °C. After 10 minutes, a solution of compound 8E (0.169 g, 0.179 mmol) in DCM (1 mL with a 1 mL rinse) was added dropwise at -78 °C. The solution was warmed to -40 °C and stirred for 30 min. The solution was cooled to -78 °C and triethylamine (0.299 mL, 2.145 mmol) was added. The reaction mixture was warmed to 0 °C and HCI (0.1 N) was added. The mixture was extracted with DCM three times. The organic layers were combined, washed with water and then dried with Na2S04. Concentration provided the aldehyde (171 mg, 134% mass recovery) as a pale yellow oil that was used directly in step 2.

Step 2. (Wittig reaction): Methyltriphenylphosphonium bromide (0.255 g, 0.715 mmol) was suspended in THF (1 .6 mL) and cooled to 0 °C. Butyllithium (2M in cyclohexane, 0.349 ml, 0.697 mmol) was added, and the resultig solution was stirred for 30 min. The aldehyde was dissolve THF (1 .6 mL) and added to the ylide mixture. Upon completion, HCI (0.1 N) was added and the mixture extracted with heptane (3 x 20 mL). The combined organics were washed with water followed by aqueous sodium bicarbonate. After drying with Na2S04, the filtrate was concentrated to provide crude compound 8F (0.155 g, 0.217 mmol, 122% mass recovery, contaminated with PPh ₃ ). ¹ H NMR (400MHz, CDC ) δ = 6.70 (q, J=6.3 Hz, 1 H), 6.38 - 6.22 (m, 1 H), 5.88 - 5.71 (m, 1 H), 5.13 - 4.96 (m, 2H), 4.39 - 4.28 (m, 1 H), 3.78 (s, 3H), 3.69 - 3.62 (m, 2H), 3.06 - 3.01 (m, 1 H), 2.21 - 2.01 (m, 1 H), 2.20 - 2.00 (m, 1 H), 1 .82 - 1 .63 (m, 1 H), 1 .45 - 1 .20 (m, 1 H) , 0.92 (s, 9H), 0.90 (s, 9H), 0.88 (s, 9H), 0.1 1 (s, 3H), 0.10 (s, 3H), 0.09 (s, 3H), 0.07 (s, 3H), 0.05 (s, 3H), 0.04 (s, 3H)

(((2S,3S,4S,5R,6S)-2-(but-3-en-1-yl)-6-((S,E)-1-((tert-bu tyldimethylsilyl)oxy)-3-iodoallyl)tetrahyd -pyran-3,4,5-triyl)tris(oxy))tris(tert-butyldimethylsilane)

Compound 8F (0.127 g, 0.1 78 mmol) was dissolved in dichloromethane (1 .270 mL) and cooled to 0 °C. 2,4-Dimethylpyridine (0.057 g, 0.534 mmol) was added followed by f-butyldimethylsilyl

trifluoromethanesulfonate (0.049 ml, 0.214 mmol). After 3 h, additional 2,4-dimethylpyridine (0.4 mL) and f-butyldimethylsilyl trifluoromethanesulfonate (0.1 mL) were added. Upon completion, aqueous sodium bicarbonate was added, and the reaction was stirred for 15 min. The product was extracted with heptane (3 x 10 mL). The combined extracts were washed with water, HCI (1 N, 2 x 10 mL), and water. After drying with Na2S04/MgS04, the filtrate was concentrated to provide a pale yellow oil that was purifed by column chromatography (Biotage Ultra cartridge, 10 g, H/EtOAc) to provide compound 8 (141 mg, 0.17 mmol, 95% over 3-steps (Swern, Wittig, TBS-protection)).

Example 3 - C.3-C.4 Macrocyclization through Ring-Closing Olefin Metathesis A halichondrin macrolide 3 can be prepared according to the below sequence.

the following sequence:

16

TBSOTf

2,4-lutidine, DCM

Example 4 - C. 12-C. 13 Macrocyclization through Ring-Closing Olefin Metathesis

A halichondrin macrolide 3 can be prepared according to the below sequence.

Compound 21 can be converted to compound 22 through the catalytic ring-closing olefin metathesis (e.g., with catalytic quantities of Hoveyda-Grubbs 2 ^nd generation catalyst). Compound 22 can be converted to compound 3 after global deprotection (e.g., removal of silyl groups with a fluoride source, such as TBAF).

Example 5 - C. 15-C. 16 Macrocyclization through Ring-Closing Olefin Metathesis

A halichondrin macrolide 3 can be prepared according to the below sequence.

2-((2R,3S,3aR,4aS,7R,8aS,9S,9aR)-3,9-bis((tert-butyldiphe nylsilyl)oxy)-2-(2-((triethylsilyl)oxy)but-3- en-1-yl)decahydrofuro[3,2-b]pyrano[2,3-e]pyran-7-yl)acetic acid

2-((2R,3S,3aR,4aS,7R,8aS,9S,9aR)-3,9-bis((tert-butyldiphenyl silyl)oxy)-2-(2-((triethylsilyl)oxy)but-3-en-1 - yl)decahydrofuro[3,2-b]pyrano[2,3-e]pyran-7-yl)ethanol (compound 23. 70 mg, 0.076 mmol) and TEMPO (1 .187 mg, 7.597 μιτιοΙ) were dissolved in toluene (2 ml_) at ambient temperature. To the resulting solution were added pH 7 phosphate buffer (1 ml_), a solution of sodium chlorite (42.9 mg, 0.38 mmol) in water (1 ml_), and 4% bleach (a solution of sodium hypochlorite in water) (0.012 ml_, 7.597 μιτιοΙ) slowly. After being stirred overnight, the reaction mixture was treated with sodium bisulfite (63.2 mg, 0.608 mmol) and water (5 mL). The resulting mixture was extracted twice with MTBE (10 mL each time). The combined organic layers were washed with 30% aqueous NaCI (2.0 mL) and dried over MgS04. Filtration, concentration in vacuo, and purification by silica gel column chromatography using a 25 - 66% gradient of ethyl acetate in n-heptane as eluent provided 54 mg of the target product as a white foam solid. ¹ H NMR (1 :1 diastereomeric mixture, 400 MHz, CDCb) δ ppm 0.50 - 0.66 (m, 6 H) 0.86 - 1 .01 (m, 9 H) 1 .05 - 1 .1 7 (m, 18 H) 1 .30 - 1 .45 (m, 3 H) 1 .55- 1 .65 (m, 1 .5H) 1 .68 - 1 .79 (m, 1 .5H) 1 .87 - 1 .99 (m, 1 H) 2.09 - 2.25 (m, 1 .5 H) 2.38 (d, J=5.5 Hz, 1 H) 2.94 (dd, J=9.4, 4.3 Hz, 0.5 H) 3.1 1 (dd, J=9.6, 6.4 Hz, 0.5 H) 3.34 (t, J=4.9 Hz, 0.5 H) 3.46-3.54 (m, 1 H) 3.58 (t, J=4.7 Hz, 0.5 H) 3.62 - 3.78 (m, 2 H) 3.88 (t, J=6.3 Hz, 0.5 H) 3.95 - 4.09 (m, 1 H) 4.16 - 4.25 (m, 2 H) 4.26-4.42 (m, 1 .5 H) 4.95 - 5.18 (m, 2 H) 5.71 (m, J=6.3 Hz, 1 H) 7.27 - 7.45 (m, 12 H) 7.64 - 7.77 (m, 8 H) ((2R,5S)-5-(2-((2S,4R,6R)-6-(((2S,3R,4R,4aS,6R,7R,8aS)-7-((t ert-butyldiphenylsilyl)oxy)-6-(2-((tert- butyldiphenylsilyl)oxy)ethyl)-3-hydroxy-4-methyloctahydropyr ano[3,2-b]pyran-2-yl)methyl)-4- methyl-5-methylenetetrahydro-2H-pyran-2-yl)ethyl)-4-methylen etetrahydrofuran-2-yl)methyl pivalate

A solution of 2-((2S,3R,4R,4aS,6R,7R,8aS)-7-((tert-butyldiphenylsilyl)oxy) -6-(2-((tert- butyldiphenylsilyl)oxy)ethyl)-3-((4-methoxybenzyl)oxy)-4-met hyloctahydropyrano[3,2-b]pyran-2- yl)acetaldehyde (compound 25; 340 mg, 0.347 mmol) and ((2R,5S)-5-((S)-5-methyl-3- ((triethylsilyl)oxy)hepta-5,6-dien-1 -yl)-4-methylenetetrahydrofuran-2-yl)methyl pivalate (compound 26; 197 mg, 0.452 mmol) in dichloromethane (10 mL) was cooled to -37 °C. Methoxyacetic acid (0.533 mL, 6.946 mmol) and BF3.0Et2 (0.132 mL, 1 .042 mmol) were added, and the resulting solution was stirred for 3 h between -20 to -35 °C. The reaction mixture was then treated with sat. NaHC03 (15 mL) and extracted twice with MTBE (30.3 mL each). The organic layers were combined, washed with 30% aqueous NaCI (15 mL), and dried over MgSC . Filtration, concentration in vacuo, and purification by silica gel column chromatography using a 20 - 50% gradient of ethyl acetate in n-heptane as eluent provided 0.367 g of a mixture of the target product and byproducts, identified as compound 25 that underwent MPM

deprotection and compound 26 that underwent TES deprotection. Without further purification, the mixture was dissolved in THF (7.5 mL). In another reactor, Pd(PPh3)4 (0.037 g, 0.032 mmol) and triphenylphosphine (0.034 g, 0.128 mmol) were dissolved in THF (2 mL) and treated with formic acid (0.123 mL, 3.204 mmol) and triethylamine (0.447 mL, 3.204 mmol). The resulting solution was added to the substrate solution prepared above. After being stirred for 20 h at 60 to 65 °C, the reaction mixture was diluted with MTBE (20 mL) and washed with sat. aq. NaHCC (5 mL) and 30% aqueous NaCI (5 mL). The organic layer was dried over MgSC , filtered, and concentrated in vacuo. The residue was purified by silica gel column chromatography using a 20 - 100% gradient of ethyl acetate in n-heptane as eluent to give 98 mg of the target product. ¹ H NMR (400 MHz, CDCb) δ ppm 1 .01 (s, 9 H) 1 .05 (s, 9 H) 1 .10 (d, J=6.3 Hz, 3 H) 1 .13 (d, J=7.8 Hz, 3 H) 1 .20 (s, 9 H) 1 .28 - 1 .37 (m, 1 H) 1 .45 - 1 .73 (m, 8 H) 1 .77 - 1 .85 (m, 1 H) 1 .91 - 2.15 (m, 5 H) 2.24 - 2.33 (m, 1 H) 2.37 - 2.47 (m, 1 H) 2.63 - 2.74 (m, 1 H) 3.22 (t, J=2.5 Hz, 1 H) 3.29 - 3.38 (m, 1 H) 3.49 (d, J=8.2 Hz, 1 H) 3.55 - 3.64 (m, 2 H) 3.68 (d, J=2.0 Hz, 1 H) 3.72 - 3.82 (m, 3 H) 3.91 (t, J=5.9 Hz, 1 H) 4.01 (dd, J=1 1 .7, 4.7 Hz, 1 H) 4.05 - 4.10 (m, 1 H) 4.13 (dd, J=1 1 .7, 4.7 Hz, 1 H) 4.22 - 4.34 (m, 1 H) 4.41 - 4.52 (m, 1 H) 4.77 - 4.84 (m, 2 H) 4.90 (d, J=2.0 Hz, 1 H) 4.99 (d, J=2.3 Hz, 1 H) 7.28 - 7.44 (m, 12 H) 7.60 (dq, J=8.1 , 1 .5 Hz, 4 H) 7.66 - 7.69 (m, 2 H) 7.75 - 7.78 (m, 2 H)

((2R,5S)-5-(2-((2S,4R,6R)-6-(((2S,3R,4S,4aS,6R,7R,8aS)-7- ((tert-butyldiphenylsilyl)oxy)-6-(2-((tert- butyldiphenylsilyl)oxy)ethyl)-4-methyl-3-((triethylsilyl)oxy )octahydropyrano[3,2-b]pyran-2- yl)methyl)-4-methyl-5-methylenetetrahydro-2H-pyran-2-yl)ethy l)-4-methylenetetrahydrofuran-2- yl)methyl pivalate

((2R,5S)-5-(2-((2S,4R,6R)-6-(((2S,3R,4R,4aS,6R,7R,8aS)-7- ((tert-butyldiphenylsilyl)oxy)-6-(2-((tert- butyldiphenylsilyl)oxy)ethyl)-3-hydroxy-4-methyloctahydrop

methylenetetrahydro-2H-pyran-2-yl)ethyl)-4-methylenetetra hydrofuran-2-yl)methyl pivalate (compound 27; 98 mg, 0.093 mmol) was dissolved in dichloromethane (5 mL) and treated with imidazole (31 .5 mg, 0.463 mmol) and chlorotriethylsilane (0.047 mL, 0.278 mmol) at ambient temperature. After 15 h, the reaction was quenched with saturated aqueous NH4CI (27% (w/v)) (10 mL). The resulting mixture was extracted twice with MTBE (20 mL). The combined organic layers were washed with 30% aqueous NaCI (5.0 mL) and dried over MgSC . Filtration, concentration in vacuo, and purification by silica gel column chromatography using a 10 - 50% gradient of ethyl acetate in n-heptane as eluent provided 94 mg of the target product. ¹ H NMR (400 MHz, CDC ) δ ppm 0.65 (q, J=7.8 Hz, 6 H) 0.97 (t, J=7.8 Hz, 9 H) 1 .02 (s, 9 H) 1 .04 (s, 9 H) 1 .06 - 1 .10 (m, 6 H) 1 .20 (s, 9 H) 1 .44 - 1 .52 (m, 2 H) 1 .58 - 1 .68 (m, 4 H) 1 .71 - 1 .84 (m, 3 H) 1 .85 - 2.01 (m, 2 H) 2.07 - 2.23 (m, 2 H) 2.35 - 2.45 (m, 1 H) 2.63 - 2.75 (m, 1 H) 3.18 - 3.28 (m, 2 H) 3.43 - 3.51 (m, 2 H) 3.57 - 3.61 (m, 2 H) 3.64 - 3.72 (m, 1 H) 3.74 - 3.85 (m, 3 H) 4.01 (dd, J=1 1 .7, 4.7 Hz, 1 H) 4.12 (dd, J=1 1 .3, 5.1 Hz, 1 H) 4.23 - 4.31 (m, 1 H) 4.39 - 4.47 (m, 1 H) 4.78 (s, 1 H) 4.84 (br. s., 1 H) 4.86 (br. s., 1 H) 4.97 (s, 1 H) 7.29 - 7.44 (m, 12 H) 7.61 - 7.71 (m, 8 H) ((2R,5S)-5-(2-((2S,4R,6R)-6-(((2S,3R,4S,4aS,6R,7R,8aS)-7-((t ert-butyldiphenylsilyl)oxy)-6-(2-((tert- butyldiphenylsilyl)oxy)ethyl)-4-methyl-3-((triethylsilyl)oxy )octahydropyrano[3,2-b]pyran-2- yl)methyl)-4-methyl-5-methylenetetrahydro-2H-pyran-2-yl)ethy l)-4-methylenetetrahydrofuran-2- yl)methanol

((2R,5S)-5-(2-((2S,4R,6R)-6-(((2S,3R,4S,4aS,6R,7R,8aS)-7- ((tert-butyldiphenylsilyl)oxy)-6-(2-((tert- butyldiphenylsilyl)oxy)ethyl)-4-methyl-3-((triethylsilyl)oxy )octahydropyrano[3,2-b]pyran-^

methyl-5-methylenetetrahydro-2H-pyran-2-yl)ethyl)-4-methy ^ pivalate (compound 28; 94 mg, 0.08 mmol) was dissolved in dichloromethane (3.0 mL) and cooled to -78 °C. DIBAL in toluene (1 .0 M, 0.24 mL) was charged at this temperature. After being stirred for 1 h between -65 to -78 °C, the reaction mixture was treated with MeOH (0.5 mL) and a Rochelle's salt solution (5.0 mL). The resulting mixture was warmed to ambient temperature and extracted with MTBE (15 mL). The organic layers were washed 30% aqueous NaCI (5.0 mL) and dried over MgSC . Filtration and concentration in vacuo provided the target product (86mg), which was used in next step without further purification. ¹ H NMR (400 MHz, CDC ) δ ppm 0.65 (q, J=8.1 Hz, 6 H) 0.97 (t, J=7.8 Hz, 9 H) 1 .02 (s, 9 H) 1 .04 (s, 9 H) 1 .06 - 1 .10 (m, 6 H) 1 .45 - 1 .52 (m, 2 H) 1 .58 - 1 .69 (m, 4 H) 1 .72 - 1 .83 (m, 3 H) 1 .83 - 1 .98 (m, 3 H) 2.09 - 2.25 (m, 2 H) 2.37 - 2.46 (m, 1 H) 2.56 - 2.65 (m, 1 H) 3.18 - 3.27 (m, 2 H) 3.43 - 3.53 (m, 3 H) 3.55 - 3.64 (m, 3 H) 3.65 - 3.72 (m, 1 H) 3.74 - 3.85 (m, 1 H) 4.10 - 4.21 (m, 1 H) 4.37 - 4.44 (m, 1 H) 4.78 (s, 1 H) 4.84 (d, J=2.0 Hz, 1 H) 4.86 (s, 1 H) 4.97 (d, J=2.3 Hz, 1 H) 7.30 - 7.42 (m, 12 H) 7.61 - 7.71 (m, 8 H)

(E)-ethyl 3-((2R,5S)-5-(2-((2S,4R,6R)-6-(((2S,3R,4S,4aS,6R,7R,8aS)-7-( (tert-butyldiphenylsilyl)oxy)-6- (2-((tert-butyldiphenylsilyl)oxy)ethyl)-4-methyl-3-((triethy lsilyl)oxy)octahydropyrano[3,2-b]pyran-2- yl)methyl)-4-methyl-5-methylenetetrahydro-2H-pyran-2-yl)ethy l)-4-methylenetetrahydrofuran-2- yl)acrylate

To a solution of ((2R,5S)-5-(2-((2S,4R,6R)-6-(((2S,3R,4S,4aS,6R,7R,8aS)-7-((t ert-butyldiphenylsilyl)oxy)- 6-(2-((tert-butyldiphenylsilyl)oxy)ethyl)-4-methyl-3-((triet hylsilyl)oxy)octahydropyrano[3,2-b]pyran-2- yl)methyl)-4-methyl-5-methylenetetrahydro-2H-pyran-2-yl)ethy l)-4-methylenetetrahydrofuran-2- yl)methanol (compound 35; 86 mg, 0.079 mmol) in dichloromethane (3.0 mL) at ambient temperature was added sodium bicarbonate (26.6 mg, 0.316 mmol) and Dess-Martin periodinane (50.3 mg, 0.1 19 mmol). After being stirred for 2 h, the reaction mixture was diluted with MTBE (20 ml) and treated with a saturated aqueous Na2S203 solution (10 mL) and water (5.0 mL). The resulting mixture was stirred for 0.5 h at ambient temperature, and the layers were separated. The organic layer was washed with a saturated aqueous NaHCCb solution (10 mL) twice and 30% aqueous NaCI (5.0 mL) and dried over MgS04. Filtration followed by concentration in vacuo gave the target product ((2R,5S)-5-(2-((2S,4R,6R)- 6-(((2S,3R,4S,4aS,6R,7R,8aS)-7-((tert-butyldiphenylsilyl)oxy )-6-(2-((tert-butyldiphenylsilyl)oxy)ethyl)-4- methyl-3-((triethylsilyl)oxy)octahydropyrano[3,2-b]pyran-2-y l)methyl)-4-methyl-5-methylenetetrahydro-2H- pyran-2-yl)ethyl)-4-methylenetetrahydrofuran-2-carbaldehyde, 80 mg) as an oil. Without further purification, the crude product was dissolved in THF (4.0 mL) and cooled to 0 °C. In an another reaction flask, ethyl 2-(diethoxyphosphoryl)acetate (73.7 μί, 0.368 mmol) was dissolved in THF (2.7 mL), cooled to 0 °C, and treated with 60% sodium hydride (13.26 mg, 0.332 mmol). After 20 min at 0 °C, the resulting solution was added to the starting material reactor. After 30 min, saturated aqueous NhUCI (27% (w/v)) (8.0 mL), water (2.8 mL), and MTBE (20 mL) were added, and the resulting mixture was warmed to ambient temperature. The layers were separated, and the aqueous layer was extracted with MTBE (12 mL). The combined organic layers were washed with 30% aqueous NaCI (1 0 mL) and dried over MgSC . Filtration, concentration in vacuo, and purification by silica gel column chromatography using a 20 - 50% gradient of ethyl acetate in n-heptane as eluent provided 71 mg of the target product (compound 29). ¹ H NMR (400 MHz, CDC ) δ ppm 0.65 (q, J=8.1 Hz, 6 H) 0.97 (t, J=8.2 Hz, 9 H) 1 .02 (s, 9 H) 1 .04 (s, 9 H) 1 .06 - 1 .10 (m, 6 H) 1 .45 - 1 .52 (m, 2 H) 1 .57 - 1 .70 (m, 5 H) 1 .72 - 1 .83 (m, 3 H) 1 .83 - 1 .98 (m, 2 H) 2.08 - 2.23 (m, 2 H) 2.38 - 2.47 (m, 1 H) 2.76 - 2.86 (m, 1 H) 3.18 - 3.28 (m, 2 H) 3.43 - 3.51 (m, 2 H) 3.56 - 3.62 (m, 2 H) 3.64 - 3.71 (m, 1 H) 3.73 - 3.85 (m, 3 H) 4.19 (q, J=7.3 Hz, 2 H) 4.40 - 4.47 (m, 1 H) 4.60 - 4.69 (m, 1 H) 4.78 (s, 1 H) 4.86 (s, 2 H) 4.98 (d, J=2.0 Hz, 1 H) 6.00 (dd, J=15.6, 1 .6 Hz, 1 H) 6.87 (dd, J=15.6, 5.1 Hz, 1 H) 7.30 - 7.42 (m, 12 H) 7.61 - 7.71 (m, 8 H) (E)-3-((2R,5S)-5-(2-((2S,4R,6R)-6-(((2S,3R,4S,4aS,6R,7R,8aS) -7-((tert-butyldiphenylsilyl)oxy)-6-(2- ((tert-butyldiphenylsilyl)oxy)ethyl)-4-methyl-3-((triethylsi lyl)oxy)octahydropyrano[3,2-b]pyran-2- yl)methyl)-4-methyl-5-methylenetetrahydro-2H-pyran-2-yl)ethy l)-4-methylenetetrahydrofuran-2- yl)prop-2-en-1-ol

To a solution of (E)-ethyl 3-((2R,5S)-5-(2-((2S,4R,6R)-6-(((2S,3R,4S,4aS,6R,7R,8aS)-7-( (tert- butyldiphenylsilyl)oxy)-6-(2-((tert-butyldiphenylsilyl)oxy)e thyl)-4-methyl-3- ((triethylsilyl)oxy)octahydropyrano[3,2-b]pyran-2-yl)methyl) -4-methyl-5-methylenetetrahydro-2H yl)ethyl)-4-methylenetetrahydrofuran-2-yl)acrylate (compound 29; 71 mg, 0.061 mmol) in dichloromethane (4m L), at -78 °C, was added a solution of DIBAL-H in toluene (1 .0 M, 0.246 mL, 0.246 mmol). After being stirred for 1 h between -65 to -78 °C, the reaction mixture was treated with MeOH (0.5 mL) and a Rochelle's salt solution (5.0 mL). The resulting mixture was warmed to ambient temperature and extracted twice with MTBE (15 mL). The combined organic layers were washed 30% aqueous NaCI (5.0 mL) and dried over MgSC . Filtration and concentration in vacuo provided the target product (compound 36, 68mg), which was used in the next step without further purification. ¹ H NMR (400 MHz, CDC ) δ ppm 0.65 (q, J=8.1 Hz, 6 H) 0.97 (t, J=8.2 Hz, 9 H) 1 .02 (s, 9 H) 1 .04 (s, 9 H) 1 .06 - 1 .10 (m, 6 H) 1 .45 - 1 .52 (m, 2 H) 1 .57 - 1 .70 (m, 4 H) 1 .72 - 1 .83 (m, 3 H) 1 .84 - 1 .98 (m, 2 H) 2.08 - 2.22 (m, 2 H) 2.34 - 2.42 (m, 1 H) 2.68 - 2.76 (m, 1 H) 3.20- 3.28 (m, 2 H) 3.43 - 3.50 (m, 2 H) 3.56 - 3.62 (m, 2 H) 3.64 - 3.71 (m, 1 H) 3.73 - 3.85 (m, 3 H) 4.14 (br s, 2 H) 4.40 - 4.43 (m, 1 H) 4.49 (q, J=6.6 Hz, 1 H) 4.78 (s, 1 H) 4.83 (d, J=1 .9 Hz, 1 H) 4.86 (s, 1 H) 4.97 (d, J=1 .9 Hz, 1 H) 5.70-5.76 (m, 1 H) 5.83-5.89 (m, 1 H) 7.30 - 7.43 (m, 12 H) 7.61 - 7.72 (m, 8 H)

(((2R,3R,4aS,6S,7R,8S,8aS)-6-(((2R,4R,6S)-6-(2-((2S,5R)-5 -((E)-3-(allyloxy)prop-1-en-1-yl)-3- methylenetetrahydrofuran-2-yl)ethyl)-4-methyl-3-methylenetet rahydro-2H-pyran-2-yl)methyl)-2-(2- ((tert-butyldiphenylsilyl)oxy)ethyl)-8-methyl-7-((triethylsi lyl)oxy)octahydropyrano[3,2-b]pyran-3- yl)oxy)(tert-butyl)diphenylsilane

To a solution of (E)-3-((2R,5S)-5-(2-((2S,4R,6R)-6-(((2S,3R,4S,4aS,6R,7R,8aS) -7-((tert- butyldiphenylsilyl)oxy)-6-(2-((tert-butyldiphenylsilyl)oxy)e thyl)-4-methyl-3-

((triethylsilyl)oxy)octahydropyrano[3,2-b]pyran-2-yl)meth yl)-4-methyl-5-methylenetetrahydro-2H-pyran-2- yl)ethyl)-4-methylenetetrahydrofuran-2-yl)prop-2-en-1 -ol (compound 36, 68 mg, 0.061 mmol) in THF (2.0 mL) at 0 °C was added sodium hydride (60% in oil, 6.1 mg, 0.15 mmol). The resulting slurry was stirred at ambient temperature for 20 min. Allyl bromide (0.032 mL, 0.37 mmol) and DMF (0.5 mL, 6.5 mmol) were then added. The resulting mixture was stirred for 16 h at ambient temperature. Saturated aqueous NH4CI (27% (w/v)) (5.0 mL), water (1 .0 mL), and MTBE (10 mL) were added. The layers were separated, and the aqueous layer was extracted with MTBE (10 mL). The combined organic layers were washed with 30% aqueous NaCI (3.0 mL) and dried over MgSC . Filtration and concentration in vacuo provided 71 mg of the target product. Without further purification, the crude product ((((2R,3R,4aS,6S,7R,8S,8aS)- 6-(((2R,4R,6S)-6-(2-((2S,5R)-5-((E)-3-(allyloxy)prop-1 -en-1 -yl)-3-methylenetetrahydrofuran-2-yl)ethyl)-4- methyl-3-methylenetetrahydro-2H-pyran-2-yl)methyl)-2-(2-((te rt-butyldiphenylsi

((triethylsilyl)oxy)octahydropyrano[3,2-b]pyran-3-yl)oxy)(te rt-butyl)diphenylsilane) (72 mg) was dissolved in acetic acid (4.0 mL) and treated with water (1 .0 mL) at ambient temperature. After 5 h, the reaction mixture was concentrated in vacuo. The resulting residue was purified by silica gel column

chromatography using a 20 - 33% gradient of ethyl acetate in n-heptane as eluent to give 54 mg of the target product (compound 30). ¹ H NMR (400 MHz, CDC ) δ ppm 1 .00 (s, 9 H) 1 .04 - 1 .06 (m, 9 H) 1 .08 - 1 .15 (m, 6 H) 1 .45 - 1 .76 (m, 1 1 H) 1 .80 (br. s., 1 H) 1 .91 - 2.15 (m, 5 H) 2.24 - 2.34 (m, 1 H) 2.36 - 2.45 (m, 1 H) 2.67 - 2.76 (m, 1 H) 3.22 (br. s., 1 H) 3.33 (br. s., 1 H) 3.49 (d, J=7.8 Hz, 1 H) 3.55 - 3.65 (m, 2 H) 3.68 (br. s., 1 H) 3.72 - 3.83 (m, 3 H) 3.91 (t, J=5.9 Hz, 1 H) 3.95 - 3.99 (m, 4 H) 4.07 (q, J=5.6 Hz, 1 H) 4.41 - 4.53 (m, 2 H) 4.81 (br. s., 1 H) 4.81 - 4.84 (m, 1 H) 4.88 (d, J=2.0 Hz, 1 H) 4.98 (d, J=2.0 Hz, 1 H) 5.18 (d, J=10.2 Hz, 1 H) 5.27 (dd, J=17.2, 1 .6 Hz, 1 H) 5.73 (dd, J=16.0, 5.1 Hz, 1 H) 5.79 (dd, J=15.2, 5.1 Hz, 1 H) 5.85 - 5.97 (m, 1 H) 7.30 - 7.43 (m, 12 H) 7.57 - 7.79 (m, 8 H)

(2S,3R,4S,4aS,6R,7R,8aS)-2-(((2R,4R,6S)-6-(2-((2S,5R)-5-( (E)-3-(allyloxy)prop-1-en-1-yl)-3- methylenetetrahydrofuran-2-yl)ethyl)-4-methyl-3-methylenetet rahydro-2H-pyran-2-yl)methyl)-7- ((tert-butyldiphenylsilyl)oxy)-6-(2-((tert-butyldiphenylsily l)oxy)ethyl)-4-methyloctahydropyran b]pyran-3-yl 2-((2R,3S,3aR,4aS,7R,8aS,9S,9aR)-3,9-bis((tert-butyldiphenyl silyl)oxy)-2-(2- ((triethylsilyl)oxy)but-3-en-1-yl)decahydrofuro[3,2-b]pyrano [2,3-e]pyran-7-yl)acetate

To a solution of 2-((2R,3S,3aR,4aS,7R,8aS,9S,9aR)-3,9-bis((tert-butyldiphenyl silyl)oxy)-2-(2- ((triethylsilyl)oxy)but-3-en-1 -yl)decahydrofuro[3,2-b]pyrano[2,3-e]pyran-7-yl)acetic acid (compound 24, 66 mg, 0.071 mmol) in dichloromethane (2 mL) at ambient temperature were added TEA (0.022 mL, 0.156 mmol), 2-methyl-6-nitrobenzoic anhydride (26.8 mg, 0.078 mmol) and 4-dimethylaminopyridine (3.17 mg, 0.026 mmol). After 10 min, the resulting solution was added to a solution of (2S,3R,4R,4aS,6R,7R,8aS)- 2-(((2R,4R,6S)-6-(2-((2S,5R)-5-((E)-3-(allyloxy)prop-1 -en-1 -yl)-3-methylenetetrahydrofuran-2-yl)ethyl)-4- methyl-3-methylenetetrahydro-2H-pyran-2-yl)methyl)-7-((tert- butyldiphenylsilyl)oxy)-6-(2-((tert- butyldiphenylsilyl)oxy)ethyl)-4-methyloctahydropyrano[3,2-b] pyran-3-ol (compound 30, 54 mg, 0.052 mmol) in dichloromethane (1 .0 ml). After being stirred overnight, the reaction mixture was treated with a saturated aqueous NH4CI solution (5.0 mL). The resulting mixture was stirred for 30 min and extracted twice with MTBE (10 mL each). The combined organic layers were washed with 30% aqueous NaCI (5.0 mL) and dried over MgSC . Filtration, concentration in vacuo, and purification by silica gel column chromatography using a 10 - 33% gradient of ethyl acetate in n-heptane as eluent provided 87 mg of the target product (compound 31 ). ¹ H NMR (1 :1 diastereomeric mixture, 400 MHz, CDCb) δ ppm 0.46 - 0.62 (m, 6 H) 0.83 - 0.98 (m, 12 H) 1 .00 (s, 9 H) 1 .06 (s, 9 H) 1 .07 - 1 .1 5 (m, 6 H) 1 .25 - 2.04 (m, 19 H) 2.08- 2.24 (m., 2.5 H) 2.32 - 2.44 (m, 1 .5 H) 2.66 - 2.76 (m, 1 H) 2.86 - 2.96 (m, 1 H) 3.22 - 3.28 (m, 1 H) 3.37 - 3.42 (m, 0.5 H) 3.46 - 3.71 (m, 6.5 H) 3.73 - 3.86 (m, 4 H) 3.93 - 4.00 (m, 4 H) 4.05 - 4.23 (m., 3 H) 4.27 - 4.34 (m, 1 H) 4.40 - 4.46 (m, 1 H) 4.49 (q, J=6.4 Hz, 1 H) 4.58 - 4.68 (m, 1 H) 4.70 (s, 1 H) 4.76 (s, 1 H) 4.84 (br. s., 1 H) 4.90 - 5.14 (m, 2 H) 4.95 - 4.97 (m, 1 H) 5.17 (d, J=10.6 Hz, 1 H) 5.27 (dd, J=17.4, 1 .8 Hz, 1 H) 5.59 - 5.70 (m, 1 H) 5.73 (dd, J=15.6, 5.1 Hz, 1 H) 5.80 (dd, J=15.6, 5.1 Hz, 1 H) 5.84 - 5.97 (m, 1 H) 7.27 - 7.43 (m, 24 H) 7.55 - 7.79 (m, 16H) d 3 according to the following sequence:

1 ) Hoveyda-Grubbs

2nd gen. toluene, 80 °C

2) 80% aq AcOH

To a solution of (2S,3R,4S,4aS,6R,7R,8aS)-2-(((2R,4R,6S)-6-(2-((2S,5R)-5-((E) -3-(allyloxy)prop-1 -en-1 - yl)-3-methylenetetrahydrofuran-2-yl)ethyl)-4-methyl-3-methyl enetetrahydro-2H-pyran-2-yl)m

butyldiphenylsilyl)oxy)-6-(2-((tert-butyldiphenylsilyl)ox y)ethyl)-4-methyloctahydropyrano[3,2-b]pyran 2-((2R,3S,3aR,4aSJR,8aS,9S,9aR)-3,9-bis((tert-butyldiphenyls ilyl)oxy)-2-(2-((triethylsilyl)oxy)but-3-en-1 - yl)decahydrofuro[3,2-b]pyrano[2,3-e]pyran-7-yl)acetate (compound 31 , 87 mg, 0.044 mmol) in toluene (61 mL) at 80 °C was added quinone (4.8 mg, 0.044 mmol) and a solution of Hoveyda-Grubbs 2nd generation catalyst (5.59 mg, 8.891 μιηοΙ) in toluene (1 0 mL). After being stirred at 80 °C for 8h, the reaction mixture was cooled to ambient temperature and treated with DMSO (0.063 mL, 0.889 mmol). The resulting solution was stirred overnight and concentrated in vacuo. Silica gel column

chromatography of the residue using a 20 - 33% gradient of ethyl acetate in n-heptane as eluent afforded 71 mg of a mixture of the target product (compound 33a) and compound 32a: MS m/z 1908.0 and 1936.0 [M+Na] ⁺ . The mixture was dissolved in THF (2.0 mL) at ambient temperature. To the solution was added acetic acid (3.2 mL) and water (0.8 mL). After being stirred for 3 h at ambient temperature, the resulting mixture was concentrated in vacuo. The residue was purified by silica gel column chromatography using a 10 - 33% gradient of ethyl acetate in n-heptane as eluent to give 46 mg of a mixture of compound 33 and compound 32. The mixture was dissolved in dichloromethane (45 mL) and treated with Hoveyda- Grubbs 2nd generation catalyst (2.91 mg, 4.63 μιηοΙ) at reflux (45°C oil bath) over 15 h. The reaction mixture was cooled to ambient temperature and concentrated in vacuo. Purification by silica gel column chromatography of the residue using a 1 5 - 33% gradient of ethyl acetate in n-heptane as eluent afforded 15 mg of the target product (compound 33). ¹ H NMR (major isomer, 400 MHz, CDC ) δ ppm 0.99 - 1 .10 (m, 42 H) 1 .25-2.27 (m, 19 H) 2.32-2.42 (m, J=7.4 Hz, 3 H) 2.45 - 2.51 (m, 1 H) 2.55 (dd, J=14.9, 3.1 Hz,

I H) 2.69 - 2.79 (m, 1 H) 2.85 (t, J=9.0 Hz, 1 H) 3.17 - 3.27 (m, 1 H) 3.33 - 3.55 (m, 4 H) 3.59 - 3.81 (m,

I I H) 3.87 - 3.95 (m, 1 H) 4.15 (dd, J=8.6, 3.9 Hz, 1 H) 4.29 - 4.39 (m, 2 H) 4.41 - 4.48 (m, 1 H) 4.49 - 4.55 (m, 1 H) 4.58 - 4.67 (m, 2 H) 4.76 (s, 1 H) 4.83 (s, 1 H) 4.92 (br. s., 2 H) 5.77 (ddd, J=1 5.6, 7.6, 1 .4 Hz, 1 H) 6.31 (dd, J=15.4, 4.5 Hz, 1 H) 7.30 - 7.43 (m, 24 H) 7.58 - 7.77 (m, 16 H)

(2S,3R,4S,4aS,6R,7R,8aS)-7-((tert-butyldiph

methyl-2-(((2R,4R,6S)-4-methyl-3-methylene-6-(2-((2S,5R)- 3-methylene-5-vinyltetrahydrofuran-2- yl)ethyl)tetrahydro-2H-pyran-2-yl)methyl)octahydropyrano[3,2 -b]pyran-3-yl 2- ((2R,3S,3aR,4aS,7R,8aS,9S,9aR)-3,9-bis((tert-butyldiphenylsi lyl)oxy)-2-(2-hydroxybut-3-en-1- yl)decahydrofuro[3,2-b]pyrano[2,3-e]pyran-7-yl)acetate

¹ H NMR (compound 32, 400 MHz, CDCb) δ ppm 1 .00-1 .1 1 (m, 42 H) 1 .25 - 2.03 (m, 19 H) 2.12 - 2.22 (m, 2 H) 2.33 - 2.42 (m, 2 H) 2.72 (ddd, J=15.3, 6.7, 1 .8 Hz, 1 H) 2.95 (dd, J=9.8, 4.3 Hz, 1 H) 3.24 (dd, J=5.7, 3.7 Hz, 1 H) 3.47 - 3.56 (m, 3 H) 3.59 - 3.69 (m, 5 H) 3.72 - 3.80 (m, 3 H) 3.81 - 3.88 (m, 1 H) 3.99 - 4.08 (m, 1 H) 4.10 - 4.20 (m, 2 H) 4.25 (dd, J=6.3, 4.3 Hz, 1 H) 4.41 - 4.50 (m, 2 H) 4.67 (t, J=8.0 Hz, 1 H) 4.72 (s, 1 H) 4.78 (s, 1 H) 4.85 (d, J=2.0 Hz, 1 H) 4.96 (d, J=2.0 Hz, 1 H) 5.04 (d, J=10.2 Hz, 1 H) 5.10 (d, J=10.2 Hz, 1 H) 5.16 (d, J=16.8 Hz, 1 H) 5.23 (d, J=17.2 Hz, 1 H) 5.72 (ddd, J=16.9, 10.6, 6.1 Hz, 1 H) 5.84 (ddd, J=17.0, 10.4, 6.3 Hz, 1 H) 7.27 - 7.45 (m, 24 H) 7.56 - 7.78 (m, 16 H) Comp

Compound 33 (25.5 mg, 0.015 mmol) was dissolved in dichloromethane (1 .0 ml) at ambient temperature. Sodium bicarbonate (12.3 mg, 0.146 mmol) and Dess-Martin periodinane (1 8.6 mg, 0.044 mmol) were added, and the resulting mixture was stirred at ambient temperature overnight. MTBE (10.0 ml), water (2.0 mL) and a saturated aqueous sodium thiosulfate solution (5 mL) were added. The resulting mixture was stirred at ambient temperature over 30 min. The organic layer was separated out, washed with 30% aqueous NaCI (3.0 mL), and dried over MgSC . Filtration, concentration in vacuo, and purification by silica gel column chromatography using a 10 - 33% gradient of ethyl acetate in n-heptane as eluent provided 1 7 mg of the target product (compound 37). ¹ H NMR (400 MHz, CDCb) δ ppm 0.97 - 1 .17 (m, 42 H) 1 .28 - 2.25 (m, 20 H) 2.49 (dd, J=16.0, 7.0 Hz, 1 H) 2.56 (dd, J=15.0, 10.0 Hz, 1 H) 2.72 (dd, J=15.2, 3.5 Hz, 1 H) 2.75 - 2.82 (m, 1 H) 2.98 (t, J=8.8 Hz, 1 H) 3.19 - 3.25 (m, 1 H) 3.36 (d, J=12.5 Hz, 1 H) 3.42 (t, J=4.3 Hz, 1 H) 3.48 - 3.57 (m, 4 H) 3.60 - 3.70 (m, 5 H) 3.71 - 3.82 (m, 3 H) 3.90 (td, J=10.4, 3.9 Hz, 1 H) 4.01 (ddd, J=10.6, 7.4, 3.1 Hz, 1 H) 4.08 (dd, J=8.0, 4.5 Hz, 1 H) 4.24 - 4.35 (m, 2 H) 4.55 - 4.59 (m, 1 H) 4.62 (t, J=7.4 Hz, 1 H) 4.66 - 4.73 (m, 1 H) 4.76 (s, 1 H) 4.85 (s, 1 H) 4.91 (s, 1 H) 5.00 (d, J=1 .6 Hz, 1 H) 6.41 (d, J=1 6.0 Hz, 1 H) 6.86 (dd, J=16.2, 5.7 Hz, 1 H) 7.28 - 7.43 (m, 24 H) 7.57 - 7.77 (m, 16 H) Comp

Compound 37 (4.0 mg, 2.3 μιηοΙ) was dissolved in deoxygenated toluene (0.6 mL) at ambient temperature. Deoxygenated (purged with nitrogen for 40 min) water (2.4 μΙ_, 0.13 mmol) was added followed by hydrido(triphenylphosphine)copper(l) hexamer (9.0 mg, 4.6 μιηοΙ). After being stirred for 1 h, the reaction mixture was treated with air. Copper-containing decomposition products precipitated.

Concentration in vacuo followed by purification by column chromatography using a 10 - 33% gradient of ethyl acetate in n-heptane as eluent afforded 3.0 mg of the target product (compound 34). ¹ H NMR (400 MHz, CDC ) δ ppm 0.98 - 1 .14 (m, 42 H) 1 .28 - 2.40 (m, 22 H) 2.55 - 2.77 (m, 4 H) 2.81 - 2.90 (m, 1 H) 2.91 (t, J=9.2 Hz, 1 H) 3.06 (t, J=1 1 .7 Hz, 1 H) 3.20 (dd, J=6.3, 3.9 Hz, 1 H) 3.39 - 3.47 (m, 2 H) 3.50 - 3.57 (m, 2 H) 3.62 - 3.69 (m, 3 H) 3.72 (dd, J=6.1 , 4.5 Hz, 1 H) 3.75 - 3.85 (m, 3 H) 3.94 - 4.02 (m, 1 H) 4.09 (dd, J=8.4, 3.7 Hz, 2 H) 4.26 - 4.32 (m, 1 H) 4.33 - 4.39 (m, 1 H) 4.43 - 4.48 (m, 1 H) 4.64 (t, J=7.0 Hz, 1 H) 4.77 (s, 1 H) 4.84 (s, 1 H) 4.86 (s, 1 H) 4.93 (s, 1 H) 7.28 - 7.44 (m, 24 H) 7.57 - 7.77 (m, 16 H)

To compound 34 (3.0 mg, 1 .72 μιτιοΙ) in a vial was added THF (0.6 mL) and N,N-dimethylacetamide (0.21 mL) at ambient temperature. A mixture of TBAF (1 .0 M in THF, 52 μί, 0.052 mmol) and imidazole hydrochloride (2.7 mg, 0.026 mmol) was added, and the resulting mixture was stirred over 3 days at ambient temperature. 30% aqueous NaCI (2.0 mL) was added, and the resulting mixture was extracted twice with a mixture of THF (5.0 mL) and toluene (5.0 mL). The combined organic layers were concentrated with a stream of nitrogen. The residue was dissolved in dichloromethane (1 .0 mL) at ambient temperature, and PPTS (18 mg, 72 μιτιοΙ) was added. Once all starting material was consumed (2 h), the reaction mixture was purified by silica gel column chromatography using a 0 - 10% gradient of methanol in ethyl acetate as eluent to give 0.9 mg of the target product (compound 3). The structure was confirmed by ¹ H NMR comparison with the reported spectrum. ¹ H NMR (400 MHz, CDC ) δ ppm 1 .10 (d, J=6.3 Hz, 3 H) 1 .18 (d, J=7.8 Hz, 3 H) 1 .27 - 2.40 (m, 25 H) 2.41 (dd, J=16.8, 2.7 Hz, 1 H) 2.53 (dd, J=17.2, 1 0.2 Hz, 1 H) 2.76 - 2.86 (m, 1 H) 2.90 (dd, J=9.8, 2.3 Hz, 1 H) 3.29 (s, 1 H) 3.52 - 3.57 (m, 2 H) 3.64 (d, J=10.9 Hz, 1 H) 3.69 - 3.89 (m, 5 H) 4.04 (dd, J=6.3, 4.3 Hz, 1 H) 4.10 - 4.15 (m, 1 H) 4.18 (dd, J=6.4, 4.5 Hz, 1 H) 4.21 - 4.28 (m, 1 H) 4.36 (d, J=1 1 .7 Hz, 1 H) 4.38 - 4.44 (m, 2 H) 4.50 (t, J=2.3 Hz, 1 H) 4.58 - 4.62 (m, 1 H) 4.68 (t, J=4.5 Hz, 1 H) 4.80 (br. s., 2 H) 4.98 (br. s., 1 H) 5.08 (s, 1 H)

Example 6 - C. 19-C.20 Macrocyclization through Nozaki-Hiyama-Kishi Reaction

A halichondrin macrolide 3 can be prepared according to the below sequence.

Compound 38 can be converted to compound 39 through the Nozaki-Hiyama-Kishi reaction (e.g., with Cr(ll) and Ni(ll) salts). Compound 39 can give compound 40 through nucleophilic ring-closing substitution reaction. Compound 40 can be converted to compound 3 after global deprotection (e.g., removal of silyl groups with a fluoride source, such as TBAF). Example 7- C.26-C.27 Macrocyclization through Nozaki-Hiyama-Kishi Reaction

A halichondrin macrolide 3 can be prepared according to the below sequence.

Compound 41 can be converted to compound 42 through the Nozaki-Hiyama-Kishi reaction (e.g., with Cr(ll) and Ni(ll) salts) with subsequent silylation of the formed hydroxyl by chlorotriethylsilane. Compound 42 can be subjected to Vasella fragmentation conditions to provide 43. DDQ deprotection of the MPM- group and selective oxidation of the resulting allylic alcohol by manganese (II) oxide can provide 44. The remaining hydroxyl group can be reacted with methanesulfonyl chloride to give compound 45. Global desilylation of 45 with tetrabutylammonium fluoride (TBAF) can form the compound 46 by nucleophilic displacement of the mesylate and conjugate addition of a deprotected alcohol to an enone. Compound 46 can be reacted with a Bronsted acid (e.g., PPTs) to afford compound 3. Example 8 - C. 14-C.38 Halichondrin Intermediate through Allene-Prins reaction.

A halichondrin C.14-C.38 intermediate 53 can be prepared according to the below sequence.

Compound 47 (Aicher et al. Tetrahedron Letters, 33:1549-1552, 1992) can be converted to 48 in a three step process [(1 ) diisobutylaluminum hydride reduction to the aldehyde, (2) Acid catalyzed acetonide deprotection and dimethylacetal formation and (3) benzoylation]. The allene-Prins reaction between 48 and 49 can be achieved by treatment with boron trifluoride diethyl etherate and methoxyacetic acid to provide 50. Allylic reduction of the methoxyacetate can be accomplished using

tetrakis(triphenylphosphine)palladium(0) with formic acid and triethylamine resulting in 51 . Deprotection of the benzoate groups can be achieved by treatment with magnesium methoxide to provide 52. Finally, treatment of 52 with tert-butyldimethylsilyl chloride can provide the known C.14-C.38 halichondrin intermediate 53 (Aicher et al. J. Am. Chem. Soc, 1 14:3162-3164, 1992).

Other Embodiments

Various modifications and variations of the described compositions and methods of the invention will be apparent to those skilled in the art without departing from the scope and spirit of the invention. Although the invention has been described in connection with specific embodiments, it should be understood that the invention as claimed should not be unduly limited to such specific embodiments. Indeed, various modifications of the described modes for carrying out the invention that are obvious to those skilled in the art are intended to be within the scope of the invention.

Other embodiments are in the claims.