MONO- AND DIORGANYLBOROHYDRIDES AND BORANES

AND PROCESS FOR PREPARING AND USING SAME

BACKGROUND

(I) Field of Invention

This invention relates to a simple procedure for synthesizing novel optically active alkali metal mono- and diorganylborohydrides, MR*BH ₃ , MR*RBH ₂ , MR*R*BH ₂ and

MR*RBH ₂ , where M is an alkali metal, B is boron, R* is an optically active organyl group with boron directly attached to the asymmetric center of the organic group or groups, and R is an organyl group which does not have a resolved asymmetric center.

Et ₂ 0, pentane R*B(OR ^» ) ₂ + MAIH4 -} MR*BH ₃ + (R'0) ₂ A1H4-

Et ₂ 0, pentane R*RB0R'+ MAl-^OR' MR*RBH ₂ + (R'0) ₂ A1HI

Et ₂ 0, pentane R*R*BOR ^! + MA1H ₃ 0R' —> MR*R*BH ₂ + (R'0) ₂ A1H<V

I previously reported that lithium borohydride reacts readily with boronic esters in an ethyl ether- pentane solution to give the lithium onoorganylborohydride

[B. Singara , T.E. Cole, H.C. Brown, Orαano etallics. 3,

774 (1984)].

Et ₂ 0, pentane RB(OMe) ₂ + LiAlH ₄ -→ LiRBH ₃ + ( '0) ₂ A1H1

The by-product, (R'0) ₂ A1H, precipitates to give a solution of the pure lithium monoorganylborohydride.

Similarly, borinic esters can be converted into the corresponding lithium diorganylborohydr ide

[B. Singaram, T.E. Cole, H.C. Brown, Orqanometallics , 3,

1520 (1984)].

Et ₂ 0, pentane R ₂ B(OR') + LiAlH ₃ (0R') > LiR ₂ BH ₂ + (R'0) ₂ A1H4-

I have recently developed procedures for the synthesis of optically pure monoorganylboronic acid esters, R*B(0R') ₂ [H.C. Brown, B. Singaram, J. Am. Che . Soc.. 106

I have also shown that I can produce diorganyl- boriniσ acid esters of high optical purity, R*RBOR' and R*R*B0R'. [H.C. Brown, P.K. Jadhav, M.C. Desai,

It is possible for these diorganyl derivatives to be cyclic derivatives, such as that obtained by the cyclic hydroboration of optically active limonene.

These boronic and borinic esters contain the boron atom attached directly to an asymmetric center. It is known that such boron compounds are very labile, readily undergoing isomerization [H.C. Brown, "Hydroboration, " Benjamin, New York, 1962, Chapter 9].

I have now discovered that such optically active boronic and borinic esters containing the boron atom attached directly to one or more asymmetric centers can be converted into the corresponding alkali metal mono- and diorganylborohydrides without any observable racemization of the asymmetric center or centers by treatment in an appropriate solvent with alkali metal aluminum hydrides.

II. Description of Prior Art

My earlier discovery of the conversion of boronic esters into lithium monoorganylborohydrides by treatment with lithium aluminum hydride and the corresponding conversion of borinic esters into lithium diorganylboro¬ hydrides by treatment with lithium ethoxyaluminu hydride was described in two publications: B Singaram, T.E. Cole, H.C. Brown, Orqanometallics . 3, 774 (1984) and B. Singaram, T.E. Cole, H.C. Brown, Orqanometallics , 3, 1520 (1984). The synthesis of boronic esters of high optical purity was described in a third publication: H.C. Brown, B. Singaram, J. Am. Che . Soc. , 106, 1797 (1984) . The synthesis of borinic esters of somewhat lower optical purity was described in a fourth publication: H.C. Brown, P.K. Jadhav, M.C. Desai, Tetrahedron , 40, 1325 (1984) .

The essence of the present discovery was reported in an article: H.C. Brown, B. Singaram, T.E. Cole, J . A . Che . Soc .. 107, 460 (1985) , publication date: January 23, 1985.

At the time of this publication, there was no prior art for the conversion of optically active boronic esters, R*B(0R') ₂ , or optically active borinic esters, R*RB0-R', R*R*B0R' and R*RB0R' where R* and R may be the same or different, or portions of a cyclic structure, as in

li onyl boronic ester) , with boron attached directly to the optically active center.

Recently, S. Masa une and his coworkers employed this procedure to convert an optically active cyclic borinic ester, methoxy-2 , 5-dimethylborolane, into the corresponding borohydride: S. Masumune, B. Kim, J.S. Petersen, T. Sato, S.J. Veenstra, J. Am. Chem. Soc _τ . 107, 459 (1985). However, this publication follows mine by nearly nine months and is based on my earlier work.

Consequently, there appears to be no prior art which discloses or suggests the possibility of proceeding from optically active monoorganylboronic esters and diorganylborinic esters containing boron attached directly to the asymmetric center or centers, prepared by the asymmetric hydroboration of prochiral olefins, by treatment with alkali metal aluminum hydrides to give the correspond¬ ing mono- and diorganylborohydrides without detectable racemization of the asymmetric center or centers.

SUMMARY OF THE DISCLOSURE

It has been discovered that optically active boronic and borinic esters containing boron directly attached to the asymmetric center or centers, are essentially quantitatively converted into the correspond¬ ing, novel alkali metal monoorganyl- and diorganylboro¬ hydrides without measurable loss of optical purity. The novel compounds have essentially 100% optical purity and are represented by the formulae:

I MR*BH ₃

II MR*RBH ₂ ;

III MR*R*BH ₂ ; and

IV MR*RBH wherein M is an alkali metal, B is boron, H is hydrogen, R* is an optically active organyl group with boron directly attached to the asymmetric center of said organyl group; R is an organyl group which does not have a resolved asym-

metric center; and the symbol ^ shows that R* and R form part of a ring system.

The novel compounds of Formulae I-IV are new asymmetric reducing agents and provide a valuable source for the corresponding optically active boranes, R*BH ₂ , R*RBH, R*R*BH and R*RBH.

These borohydrides are readily converted' into optically active onoorganylboranes and optically active diorganylboranes containing boron directly attached to the asymmetric center or center of the organyl group or groups, by treatment with hydrogen chloride, methyl iodide or trimethylsilicon chloride.

DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS

No limitation has been observed due to the nature of the R* group. All optically active boronic esters, R*B(OR ^» ) ₂ , examined, containing boron attached directly to the asymmetric center have undergone essentially quantitative conversion into the corresponding alkali metal monoorganylborohydride, MR*BH ₃ , with complete retention of the optical activity of the R* group. Similarly, all optically active borinic esters, R*RB(OR'), R*R*B(OR') and cyclic derivatives R*RB(0R') are converted into the corresponding alkali metal diorganylborohydrides, MR*RBH ₂ , MR*R*BH ₂ and MR*RBH ₂ with complete retention of the optical activity of the R* group or groups.

While any of the various alkali metal aluminohydrides can be used, i.e., LiAlH4 , NaAlH4 and KAIH4, the preferred aluminohydride is LiAlH ₄ as it is currently more economical and is easily soluble in ethyl ether, an economical solvent.

The preferred solvent is a mixture of ethyl ether and pentane. However, other ethers, such as diisopropyl ether, di-n-propyl ether and di-n-butyl ether, can be used. Similarly, other hydrocarbons, such as hexane, heptane, cyσlohexane and toluene can be used. Similarly, cyclic ethers, such as dioxane, tetrahydrofuran and

tetrahydropyran, can be used. However, there is an advantage in having a volatile solvent which can easily be removed from the product by volatilization. Therefore, ethyl ether-pentane is the solvent of choice.

The alkoxy groups on the boronic ester can be greatly varied. It is preferred to use the methoxy and ethoxy groups as being the most economical. However, n-propoxy, isopropoxy and n-butoxy, phenoxy, benzyloxy and higher groups can be used with no. apparent disadvantage. It is also possible to use cyclic esters, such as ethylene glycol, propylene glycol, trimethylene glycol -and catechol, for example.

Of all these choices, it is preferred to employ the methoxy, ethoxy and the tri ethylenedioxy esters,

R*B(0Me) ₂ , R*B(0Et) ₂ and in the synthesis of the novel compounds of this invention.

In the case of the borinic esters, it is preferred to use the methoxy and ethoxy esters represented by the formulae:

R*RB0Me, R*ROEt, R*R*B0Me, R*R*B0Et, R*RBOMe and R^OE However, it is also possible to use glycol esters of the formulae:

R*RBO(CH ₂ ) _n OBRR*, R*R*BO(CH ₂ ) _n OBR*R* and R*RBO(CH ₂ ) _n OBRR* where n is preferably 2 or 3.

The parent organoboranes, R*BH ₂ , R*RBH, R*R*BH and R*RBH, are relatively unstable materials, undergoing rapid redistribution and isomerization. Consequently, they must be freshly prepared and used directly. However, the novel alkali metal monoorganyl- and diorganylborohydrides, MR*BH ₃ , MR*RBH ₂ , MR*R*BH ₂ and MR*RBH, are very stable materials. They show no detectable change over periods of months. Consequently, they are readily stored and shipped. The desired organoboranes, R*BH , R*RBH, R*R*BH, and R*RBH, are readily generated from the corresponding alkali metal

borohydride by treatment with a reactive alkyl halide, such as methyl iodide or benzyl bromide, a strong mineral acid, such as hydrogen chloride or hydrogen bromide, or reactive silicon halides, such as trimethylsilicon chloride.

As will be readily seen by the following examples, the present invention also provides a simple method for preparing essentially 100% optically pure alkali metal monoorganyl- and diorganylborohydrides, such as lithium onoalkyl- and dialkylborohydrides under mild conditions. Generally speaking, boronic esters of very high optical purity are reacted with the appropriate alkali metal aluminum hydride in diethyl ether-pentane, preferably at O'C, to form the corresponding alkali metal monoorganylborohydrides of very high optical purity and dialkoxyalane . Under these reaction conditions, the dialkoxyalane generally precipitates quantitatively from the solution. The reaction is essentially quantitative and is broadly applicable to any available essentially optically pure boronic ester. The addition of 1 ol equivalent of very high optically pure borinic esters to, for example, alkali metal monoorganylaluminohydride in diethyl ether at O'C results in a facile and rapid precipitation of the dialkoxyalane as a solid, producing the corresponding alkali metal diorganylborohydride of very high optical purity in quantitative yield. The reaction is quite general, proceeds without detectable racemization and is applicable to essentially optically pure borinic esters of widely varied structural requirements.

As mentioned above, the alkali metal monoorganyl- and diorganylborohydrides of the present invention are very stable, and can be stored under nitrogen at 25 ^* C without hydride loss, redistribution, isomerization or racemization of the organyl groups. Methyl iodide or acids react readily and quantitatively remove metal hydride from these derivatives, generating the corresponding optically pure monoorganyl- and diorganylboranes for further use.

As used herein, the term "essentially 100% optical purity" means an enantiomeric excess of at least

80% of one of the members of an enantiomeric pair. The term

"a high degree of optical purity" is synonymous. The term

"ee" is an abbreviation for "enantiomeric excess." The term

"enantiomeric pair" refers to a pair of substances whose molecules are nonidentical mirror images.

The term "R*" refers to an optically active [ (+) or (-) ] substituted or unsubstituted, cyclic or acyclic (straight or branched chain) organyl group containing boron attached directly to the asymmetric center of the organyl group .

The following examples further illustrate the preferred embodiments of the present invention, and for illustrative purposes, are broken into four groups: A. The preparation of novel optically active alkali metal mono¬ organylborohydrides; B. The preparation of novel optically active alkali metal diorganylborohydrides; C. The prepar¬ ation of optically active monoorganylboranes from the corresponding borohydrides; and D. The preparation of optically active diorganylboranes from the corresponding borohydrides .

A. The Preparation of Alkali Metal Monoorganylborohydrides of High Optical Purity

Example I ^'

Preparation of Lithium [lj5,2S]-trans- ( 2 -me th lcyclohexyl) borohydride

Lithium [IS ,2S] -trans- (2-methylcyclohexyl) - borohydride was prepared by the following procedure. A 50-mL centrifuge vial fitted with a rubber septum and magnetic stirring bar was charged with 20 L of a 0.5 M solution of dimethyl [ IS , 2S] - (+) -trans- ( 2-methylcyσlo- hexyl) boronate (10 mmol) in n-pentane and cooled to 0*C. A 1.0 M solution of LiAlH4 in ethyl ether (10 mL, 10 mmol) was added with vigorous stirring. A voluminous precipitate of (MeO) ₂ A1H was separated. The reaction mixture was

stirred as efficiently as possible for 0.25h at O'C. The reaction mixture was then centrifuged and the clear supernatant liquid was transferred via a double-ended needle to another vial. The solid (MeO) ₂ A1H was washed with n-pentane (2 X 10 mL) , and the washings were combined with the supernatant solution. ^• The solvent was evaporated at 25*C under reduced pressure (12 m Hg) . The residue (1.6 g) was dissolved in ethyl ether (18.4 L) and estimated by hydride analysis: 0.43 M, 8.6 mmole, ^' 86% yield; ^l B NMR δ-25.5(q, BH ^{=74 Hz} ) ' ^IR 2 2180. No signal attributable to the presence of aluminum compounds could be detected in the solution either in ⁷ A1 NMR or in the IR spectrum. The borohydride solution was quenched with methanol and then oxidized with alkaline hydrogen peroxide. The product alcohol, [lS.2S]-(+)-trans-2-methyl- cyclohexanol, exhibited [ _α ] ²³ D+42.8±0.1' (c 1, MeOH) , establishing an optical purity ee < 99.7% for the boro¬ hydride.

Example II

Preparation of lithium [2S-(+) -3-methyl-2- butylborohydride

Methyl [2≤] - ( + ) -3-methyl-2-butylboronate was converted by the method of Example I into lithium [2S_]-( + )- 3 -methyl-2-butylborohydride in 92% yield. Following hydrolysis, oxidation by alkaline hydrogen peroxide established the purity to be<^99.6% ee.

Example III Preparation of lithium [ IS .2S] - (+ ) -trans- ( 2-methyl- cyclopentyl ) borohydride

Methyl [IS., 2S.] - ( + ) -trans- (2-methylcyσlopentyl) - boronate was converted by the method of Example I into lithium [ IS, 2S] - (+) -trans- (2 -me thy lcyclopentyl) borohydride in a yield of 85%. Following hydrolysis, oxidation by alkaline hydrogen peroxide established the purity to be <99.8% ee.

Example IV

Preparation of lithium [2S,3R]-(-)-erythro-3-phenyl-2- pentylborohydride

Methyl[2S_.3R]-(-)-erythro-3-phenyl-2-pentylbor- onate was converted by the method of Example I into lithium [2S.,3E]-(-) -erythro-3-phenyl-2-pentvlborohydride in a yield of 86%. Following hydrolysis, oxidation by alkaline hydrogen peroxide established the purity to be 199.8% ee.

B". The Preparation of Optically Pure Alkali Metal Di¬ organylborohydrides

: Example V

Preparation of lithium [2S]-3-methyl-2-butyl- H-pentylborohydride

Lithium [ 2 S.] -3 -methyl-2 -butyl-n-pentylboro- hydride was prepared as follows. A 50-mL centifuge vial fitted with a rubber septum and a magnetic stirring bar was charged with 10 L of a 1.0 M ethyl ether solution of LiAlH ₄ . To this , a 1.0 M ethyl ether solution of ethyl acetate (5 mL, 5 mmol) was added at 25 'C with' stirring. The resulting lithium monoethoxyaluminum was cooled to O' C, and 0.5 M ethyl ether solution of ethyl [2S] - (+) -3- methyl-2-butyl-n-pentylborinate of ee ^ 99. 6% (2 mL, 10 mmol ) was added with constant stirring . After the addition, the reaction mixture was mixed well and then centr i fuged . The c l ear sup ernatant solution was transferred via a double-ended needle_ to another vial . The solid dialkoxyalane was washed with ethyl ether (2 X 5 mL) , and the washings were combined with the supernatant solution . The solvent ethyl ether was evaporated under reduced pressure ( 12 mmHg) . The residue ( 2 . 0 g) was dissolved in tetrahydrofuran (THF) (18 mL) and the resulting solution was estimated by hydride analysis : 0 .4 M, 8.0 mole , 80% yield; ^l B NMR 6-14.5 (t, J _BH = 68 Hz) ; IR v 2093. No signal attributable to the presence of aluminum compounds in the solution could be detected either in ²⁷ A1 NMR or the IR spectrum. The borohydride solution was

quenched with ethanol and then oxidized with alkaline hydrogen peroxide. The product alcohol, [2£3]-(+) -3-methyl- 2-butanol, exhibited a] ²³ ' D+4.94 (neat), establishing an optical purity ee< 99.6% for the borohydride.

Example VI Preparation of lithium f IS .2S -trans- (2-methvlcyclo- pentyl) -n-pentylborohydride Ethyl [IS, 2S_] -trans- (2 -methyl cyclopentyl) -n- pentylborinate was prepared from the title compound following the procedure of Example V. . Following hydrolysis, oxidation by alkaline hydrogen peroxide produced the optically active alcohol in an optical purity of £99.8% ee.

Example VII Preparation of lithium [IS , 2S] -trans- (2-methylcyclo- hexyl) -n-pentylborohydride Ethyl [ IS , 2S ] -trans- ( 2 -methylcyclohexyl ) -n- pentylborinate was converted into lithium [ IS. 2S.] -trans-2-methylcyclopentyl) -n-pentylborohydride using the procedure of Example V. Following hydrolysis, oxidation by alkaline hydrogen peroxide produced the optically active alcohol in an optical purity of < 99.7% ee.

Example VIII Preparation of lithium [2S_,3R]-erγthro-3-phenyl-2- penty 1 -n-pentylborohydride Ethyl [2S, 3R]-erythro-3-phenyl-2-pentyl-n- pentylborinate was converted into the title compound following the procedure of Example V in 72% yield. Following hydrolysis, oxidation by alkaline hydrogen peroxide produced the alcohol in an optical purity of 99.8%ee.

Example IX . Preparation of lithium [ IR , 4RS , 5R , 8R] -2-borato- , 8-dimethylbicyclo [ 3.3.1] nonane A 0.5 M ethyl ether solution of β-methoxy-2- bora- , 8-dimethylbicyclo [ 3.3.1] nonane (25 mmol) was reacted with an ethyl ether solution of lithium monoethoxyaluminohydride (25 mmol) at O'C according to the method of Example V. The crude borohydride (4 g) was dissolved in tetrahydrofuran (45 mL) and the resulting solution was estimated by hydride analysis: 0..45 M, 22.5 mmole, 90% yield; ^X1 B NMR δ -14.3 (t, J _BH =70 Hz); IR v 2100 cm" ¹ . No signal attributable to the presence of aluminum compounds in the solution could be detected either in ²⁷ A1 NMR or the IR spectrum.

C. Preparation of Optically Active Monoorganylboranes from the Corresponding Borohydrides.

Example X Preparation of [2S]-3-methyl-2-butylborane The following procedure for the preparation of optically pure monoisoa ylborane dimers is representative.

A 50 L centrifuge vial with a rubber septum and a magnetic stirring bar was charged with a 0.5 M ethyl ether solution of lithium [2S]-3-methyl-2-butylborohydride ( 20 mL, 10 mmol) and cooled to O'C. A 3.0 M solution of HCl in ethyl ether (3.3 mL, 10 mmol-) was added slowly with vigorous stirring. Hydrogen gas evolved with the concurrent precipitation of lithium chloride. The reaction mixture was then centrifuged, and the clear supernatant solution (21 mL) containing the free [2S.]-3-methyl-2- butylborane was transferred to another vial and estimated by hydrolysis: 85% yield; ^1X B NMR & +23.8 (t, J _BH =131 Hz). The monoisoamylborane (8 mmol) in ethyl ether was reacted with 4 mmol of N,N,N ^! ,N' ,tetramethylethylenediamine (TMED) at O'C with stirring. The ethyl ether was evaporated and the bis-adduct was washed with cold (O'C) n-pentane (2 x 3 mL) and dried at 25'C under reduced pressure (12 m Hg) :

1.07 g (94% yield); p 46-48'C; ^X1 B NMR 5 -0.5 (d,J _BH =90

Hz); [ _α ] ²³ D +26.87'± 0.05' (c 4,THF). Oxidation of the TMED adduct gave [2S]-(+) -3-methyl-2-butanol which exhibit¬ ed [ ] ²³ D +4.94* (neat), establishing an optical purity ee <99.6% for the TMED adduct.

Example XI Preparation of [ IS .2S ] -trans-2-methylcyclo- pentylborane

[ IS., 2S_] -Trans-2-methylcyclopentylborane was prepared by the method of Example X from lithium [1S,2S]- trans-2-methylcyclopentylborohydride in 82% yield. ¹¹ B NMR δ+23.3 (d,J _B H=130 Hz). Oxidation of the TMED adduct gave riS.2S]-trans-2-methylcyclopentanol which exhibited [α] ²³ D+84.1' (c, THF) , establishing an optical purity ee<99.7% for the TMED adduct.

Example XII Preparation of [lS,2S -trans-2-methylcyclohexylborane

[ IS., 2S -Trans-2-methylcyclohexylborane was prepared by the method of Example X ^' from lithium [1S,2S]- trans-2-methylcyclohexylborohydride in 85% yield. ¹¹ B NMR <5f23.8 (d, J _BH =130 Hz). Oxidation of the TMED adduct gave riS.2S .-trans-2 -methylcylohexanol which exhibited [ _α ] ²³ D+72.2* (c, THF), establishing an optical purity ee<99.7% for the TMED adduct.

Example XIII

Preparation of [ 2S , 3R] -ervthro-3-phenyl-

2-pentylborane

[2S, 3R] -erythro-3-phenyl-2-pentylborane was prepared by the method of Example X from lithium [2S.,3R]- erythro-3-phenyl-2-pentylborohvdride in 82% yield. ¹¹ B NMR δ+23.6 (br, s) . Oxidation of the TMED adduct gave [2Si,3R]- erythro-2-phenyl-2-pentanol in 82% yield which exhibited

[e _t ] ²³ D+8.2* (c, 4, THF), establishing an optical purity ee

199.8% for the TMED adduct.

Example XIV

Application of lithium monoisopinoca pheyl- borohydride. Asymmetric hydroboration of

1 -methyl cy cl opent ene

Free monoisopinocampheylborane (25 mmol) was generated from lithium monoisopinocampheylborohydride ( 25 mmol) by treating it with HCl (25 mmol) in ethyl ether at O'C. Hydroboration of 1-methylcyclopentene (25 mmol) was carried out at -35 'C using this reagent as recommended in the literature. Optically pure isopinocampheyl-[lS,2S]- trans- (2-methylcyclopentyl) borane was isolated at .60% yield. This dialkylborane was reacted with acetaldehyde to remove the isopinocampheyl group. The [l_5,2 ]-(+) - trans- (2-methylcyclopentyl) borinic acid, thus obtained on oxidation, afforded [lj>,2S]-(+)-trans-2-methylcyclopentan- ol, which exhibited [α] ²³ D +46.8' (c, 1, MeOH) , ee 99.9%.

D. Preparation of Optically Active Diorganylboranes from the Corresponding Borohydrides.

Example XV

Application of lithium diisopinocampheylborohydride.

Asymmetric hydroboration of cis-2-butene

Lithium diisopinocampheylborohydride (30 mmole) and (+)- -pinene (ee 92%, 9 mmole) in tetrahydrofuran

(THF) was cooled to O'C. Free diisopinoca pheylborane was generated by adding methyl iodide ( 40 mmole) with stirring. The resulting slurry was cooled to -25 *C and utilized for the hydroboration of cis-2-butene (30 mmole) according to the literature procedure. The reaction

mixture was warmed to 25'C, washed with water (2 X 10 L) to remove lithium iodide and then oxidized. Distillation provided [2R]-(-)-2-butanol in 65% yield, which exhibited

[ _α ] ²³ D-10.368* (neat, _£ l.0), eel97%.

Example XVI

Application of lithium [lR,4RS,5R,8R]-2-borata-4,8- dimethylbicyclo[3.3.1]nonane. Asymmetric hydroboration of

2-methyl-2-butene

Lithium 2-borata-4,8-dimethylbicyclo[3.3.1]- ^' nonane (50 mmol) in THF (100 L) was cooled to O'C. Free dialkylborane, 2-bora-4,8-dimethylbicyclo[3.3.l]nonane, was generated by adding methyl iodide (60 mmol) with stirring. The reaction mixture was stirred for 1 hour at O'C and then cooled to -25'C. It was then reacted with 2-methyl-2-butene (50 mmol, 3.5 g) . The hydroboration was complete after 48 hours at -25*C. The reaction mixture was warmed to 25*C, washed with water (2 X 10 mL) to remove lithium iodide and then oxidized. Distillation provided [2R]-(-)-3-methyl-2-butanol in 70 % yield, which exhibited [ _α ] ²³ D-3.3* (neat,£ 1.0), 67% ee.

The above description has been given by way of illustration. It will be understood by those skilled in the art that modifications may be made without departing from the spirit and scope of the claimed invention.