ALTERNATE PROCESS FOR PREPARING 3,5-DI-OMICRON-ACYL-2-FLUORO-2-C- METHYL-D-RIBONO-GAMMA-LACTONE

The present invention provides a novel process for the preparation of lactone daerivatives of the formula

which are key intermediates in the preparation of l-(2-deoxy-2-fluoro-2-C-methyl- β-D-ribofuranosyl)cytosine

a compound that possesses potent and selective anti-hepatitis C virus activity (PCT Publication WO 2005/003147).

A number of synthetic routes for preparing intermediate 1 have been disclosed in PCT Publication WO 2006/012440, but these synthetic routes have the shortcomings of high manufacturing costs and technical difficulties for commercial scale manufacturing. The use of heavy load of asymmetric dihydroxlyation catalyst (AD-mix-β), fluorinating agent diethylaminosulfur trifluoride, and the Wittig reagent, are major cost drivers. The use of highly toxic reagents, such as AD-mix-β, highly reactive reagent such as diethylaminosulfur trifluoride, and chromatographic isolation of intermediates,contribute to scale up difficulties.

Object of the present invention was to find an alternative synthetic approach which does not suffer from the disadvantages known from the synthesis known in the art.

The object could be reached with the process of the present invention as outlined below.

The process for the preparation of a compound of the formula:

wherein R ¹ independently is Ci- ₆ -alkyl, optionally substituted phenyl-Ci- ₆ -alkyl or optionally substituted phenyl comprises the steps: a) reacting an acetonide compound of the formula

wherein R ² is Ci- ₄ -alkyl, and R ³ stands for

with an acid to form a mixture of the compounds

11 12

b) acylating a mixture of the compounds

11 12

with an acyl halogenide of the formula

R ¹ COX wherein R ¹ is as above and X is a halogen or with an acyl anhydride R ¹ C (O)O(O)C R ¹ wherein R ¹ is as above in the presence of a base to form a mixture of the compounds

wherein R ¹ is as above; c) recyrystallizing a mixture of the compounds

wherein R ¹ is as above in an organic solvent and isolating the compound of formula

1.

As used herein, the following terms have the meanings:

The term "Ph" refers to a phenyl group.

- A -

The term "phenyl-Ci- ₆ -alkyl", as used herein refers to Ci- ₆ -alkyl substituted with a phenyl group, preferably to benzyl.

The term "optionally substituted phenyl-Ci-6-alkyl" or "optionally substituted phenyl" refers to Ci- ₃ -alkyl substituted phenyl-Ci- ₆ -alkyl or Ci- ₃ -alkyl substituted phenyl.

The term "Ci- ₄ -alkyl" or "Ci- ₆ -alkyl" refers to methyl, ethyl, n-propyl, iso- propyl, n-butyl, sec-butyl, t-butyl or in addition to pentyl and its isomers and hexyl and its isomers.

The term "halogen" refers to chloro, bromo, iodo and fluoro, and is preferably chloro.

In a preferred embodiment of the process of the present invention the substituents have the following meaning: R ¹ is phenyl, R ² is methyl or ethyl, preferably methyl and X is chloro.

Step a) Step a) requires reacting an acetonide compound of the formula

wherein R ² is Ci- ₄ -alkyl, and R ³ stands for

with an acid to form a mixture of the compounds

11 12

Access to the acetonide compound of formula 13 is mainly depending on the substituent R ³ .

In case R ³ has the meaning of

the acetonide compound of formula 13 is prepared by converting l-(2-fluoro-l- oxopropyl) pyrrolidine of the formula

with D-glyceraldehyde, 1,2-acetonide of the formula

in the presence of a non-nucleophilic base.

The non nucleophilic base can be selected from lithium diisopropylamide (LDA), lithium 2, 2, 6, 6-tetramethylpiperdine (LTMP) or lithium hexamethyldisilazide (LHMDS). ). Preferred non nucleophilic base is lithium diisopropylamide (LDA).

The reaction of the l-(2-fluoro-l-oxopropyl)pyrrolidine of the formula 4 with the D-glyceraldehyde, 1,2-acetonide of formula 6 is expediently performed in a solvent selected from tetrahydrofuran, 2-methyl tetrahydrofuran, toluene, diethyl ether or tert- butyl methyl ether, preferably in tetrahydrofuran at a temperature below -50 ⁰ C, preferably below -70 ⁰ C.

Isolation of the respective acetonide compound of the formula 13 can happen according to methods known to the skilled in the art, for example by acidifying the reaction mixture, preferably with acetic acid, by a subsequent extraction with a suitable organic solvent e.g. dichloromethane and by concentrating the organic solution.

In case R ³ has the meaning of

' ^S >h

the acetonide compound of formula 13 is prepared by converting S-phenyl-2- fluoropropanethioate of the formula

with D-glyceraldehyde, 1,2-acetonide of the formula

in the presence of a non-nucleophilic base.

The non nucleophilic base can be selected from lithium diisopropylamide (LDA), lithium 2, 2, 6, 6-tetramethylpiperdine (LTMP) or lithium hexamethyldisilazide (LHMDS). Preferred non nucleophilic base is lithium diisopropylamide (LDA).

As a further embodiment of the invention an organic titanium compound, preferably chlorotriisopropoxytitanium (IV) is added.

The conversion of the S-phenyl-2-fluoropropanethioate of the formula 7 with the D-glyceraldehyde, 1,2-acetonide of formula 6 is expediently performed in a solvent selected from dichloromethane, tetrahydrofuran, heptane, toluene, and ethylbenzene, preferably in tetrahydrofuran at a temperature below -50 ⁰ C, preferably below -70 ⁰ C.

Isolation of the respective acetonide compound of the formula 13 can happen according to methods known to the skilled in the art, for example by acidifying the reaction mixture, preferably with acetic acid, by a subsequent extraction with a suitable organic solvent e.g. dichloromethane and by concentrating the organic solution.

In case R ³ has the meaning of

the acetonide compound of formula 13 is prepared by converting 3-(2-fluoro-l- oxopropyl)-2(3H)-benzoxazolone of the formula

with D-glyceraldehyde, 1,2-acetonide of the formula

The process is further characterized in that the reaction of 3-(2-fluoro-l- oxopropyl)-2(3H)-benzoxazolone of the formula 5 with D-glyceraldehyde, 1,2-acetonide of the formula 6 is performed in the presence of a titanium (IV) salt, preferably with titanium (IV) chloride and a tertiary amine., preferably with triethylamine.

Usually the reaction is performed in the presence of a solvent selected from dichloromethane, tetrahydrofuran, heptane, toluene or ethylbenzene, preferably in dichloromethane at a temperature of -10 ⁰ C to 10 ⁰ C, preferably at around 0 ⁰ C.

Isolation of the respective acetonide compound of the formula 13 can happen according to methods known to the skilled in the art, for example by acidifying the reaction mixture, preferably with aqueous hydrochloric acid acetic acid, by a subsequent extraction with a suitable organic solvent e.g. dichloromethane and by concentrating the organic solution.

The acetonide compounds of formula 13 are novel and therefore are a further embodiment of the present invention.

The starting compounds l-(2-fluoro-l-oxopropyl)pyrrolidine of formula 4 and the 3-(2-fluoro-l-oxopropyl)-2(3H)-benzoxazolone of formula 5were prepared according to Scheme 1, set out below.

1 ) (COCl) ₂ , cat DMF o toluene >< "N'

H 2) O ^

The method for preparing l-(2-fluoro-l-oxopropyl) pyrrolidine of formula 4 comprises the conversion of 2-fluoropropionic acid to the 2-fluoropropionyl chloride with an acyl chloride, preferably with oxalyl chloride in a non-reactive solvent, such as in toluene at about 30 ⁰ C. A catalytic amount of dimethylformamide may be added. The addition of pyrrolidine to the reaction mixture may happen at about -70 ⁰ C.

The method for preparing 3-(2-fluoro-l-oxopropyl)-2(3H)-benzoxazolone of formula 5 comprises the reaction of 2-fluoropropionic acid with 2-benzoxazolinone in a non-reactive solvent such as in dichloromethane in the presence of an acylation catalyst, selected from 4-dimethylaminopyridine (DMAP) or dimethylformamide (DMF), preferably 4-dimethylaminopyridine (DMAP) at about 0 ⁰ C followed by the addition of a dehydrating agent selected from the group consisting of N,N'-dicyclohexylcarbodiimide, N,N'-diisopropylcarbodiimide, and l-ethyl-3-(3-dimethylaminopropyl) carbodiimide hydrochloride. Preferably, the dehydrating agent is N,N'-dicyclohexylcarbodiimide. The S-phenyl-2-fluoropropanethioate of formula 7 was prepared according to a literature method (Tetrahedron, 1996, 52 (1), 255).

As outlined above, the acetonide compound of formula 13 is converted into the mixture of the compounds of formula 11 and 12 in the presence of an acid.

As a rule the acetonide compounds of formula 13 do not need to be isolated or further purified before their further acid treatment.

Preferably an aqueous solution of acetic acid is used and the reaction mixture is stirred at a temperature of 60 ⁰ C to 100 ⁰ C, preferably at 85° to 95°C until the conversion is complete.

Isolation of the mixture of the compounds of formula 11 and 12 can happen according to methods known to the skilled in the art, for example by concentrating the reaction mixture, by partitioning the residue between water and a suitable organic

solvent, such as with tert. -butyl methyl ether and finally by concentrating the aqueous phase.

Step b)

Step b) requires acylating a mixture

11 12

with an acyl halogenide of the formula

R ¹ COX wherein R ¹ is as above and X is a halogen or with an acyl anhydride R ¹ C (O)O(O)C R ¹ wherein R ¹ is as above in the presence of a base to form a mixture of the compounds

wherein R ¹ is as above.

Suitable bases are those which do not react with the acyl halogenide and the acyl anhydride reactant. Non-limiting illustrative examples are pyridine, or tertiary amines such as triethylamine, N,N'-diisopropylamine (DIPEA), 4- dimethylaminopyridine (DMAP). Preferably a tertiary amine and more preferably triethylamine is used as base for the acylation.

Preferred acylation agents are the acyl halogenides R ¹ COX with the R and X having the meaning as above. Most preferred acylating agent is benzoyl chloride.

In a preferred embodiment of the acylation step a catalytic amount of 4- dimethylaminopyridine can be added.

Usually the acylation takes place in a suitable solvent such as in acetonitrile, dimethylformamide (DMF), pyridine, and the like.

As a rule the mixture of the compounds 1 and 15 will not be isolated from the reaction mixture but readily be subjected to the recrystallization in step e)

Step c)

Step c) requires recyrystallizing a mixture of the compounds

wherein R ¹ is as above in an organic solvent and isolating the compound of formula 1.

Usually a water soluble organic solvent is that substantially separates compound 1 in the crystalline form from compound 15. Non-limiting illustrative examples may be selected from the group consisting of methanol, ethanol, n- propanol, and isopropanol. A preferred solvent is isopropanol.

The following examples shall illustrate the invention without limiting it.

Examples

Example 1 Preparation of 4 from 2-fluoropropionic acid 1 and Pyrrolidine 2

OH

2)

O 2

To a solution of 4 g of 2-fluoropropionic acid (1) and a catalytic amount of dimethylformamide (DMF) in 50 mL of anhydrous toluene was slowly added 6.1 g of oxalyl chloride. The mixture was stirred at room temperature for 2h and at 30° C for Ih, and then was cooled to -70 ⁰ C. To the mixture was added 10 g of pyrrolidine (2). After the addition, the mixture was slowly warmed to ambient temperature. The mixture was washed consecutively with 5% HCl solution, 5% NaHCO ₃ solution, and brine. The organic solution was dried over MgSCU, filtered, and concentrated to give 2 g of (4). ¹ H- NMR (CDCl ₃ ): δ = 1.5 (dd, 3H, J = 28, 7.0 Hz), 1.75-1.85 (m, 2H), 1.85-2.0 (m, 2H), 3.4-3.6 (m, 4H), 5.1 (dq, IH, J = 51, 7.0 Hz).

Example 2 Preparation of 5 from 2-fluoropropionic acid 1 and 3

A solution of 5 g of 2-fluoropropionic acid (1), 5.6 g of 2-benzoxazolinone (3), and a catalytic amount of 4-dimethylaminopyridine (DMAP) in 40 mL of dichloromethane was cooled to 0 ⁰ C. To this solution was added a solution of 11.2 g of N,N'-dicyclohexylcarbodiimide (DCC) in 20 mL of dichloromethane in one portion. Heavy precipitation formed immediately. The mixture was warmed slowly to ambient temperature and was stirred overnight. The solid was filtered off and the filtrate was concentrated to give a crude product. The crude product was purified with column chromatography (silica gel), eluting with dichloromethane to give 6.4 g of 5. ¹ H-NMR (CDC13): δ = 1.74 (dd, 3H, J = 23.2, 6.8 Hz), 6.10 (dq, IH, J = 48.4, 6.8 Hz), 7.22-7.36 (m, 3H), 8.08-8.14 (m, IH).

Example 3 Preparation of 11 and 12 from 4 and D-glyceraldehyde, 1, 2-acetonide 6

12

A dry, clean, 4-neck round bottom flask, equipped with a mechanical stirrer, a thermo couple, a nitrogen inlet, and an addition funnel, was charged with 5 mL of tetrahydrofuran (THF) and 2.3 mL of 1.8 M solution of lithium diisopropylamide in heptane/THF/ethylbenzene. The solution was cooled to -78° C and to it was slowly charged a solution of 0.6 g of 4 in 3 mL of THF. The mixture was stirred for 30 minutes and a solution of 0.48 g of D-glyceraldehyde, 1, 2-acetonide (6) in 3 mL of THF was slowly added. After the addition, the mixture was stirred at approximately -75° C for 10 minutes and was quenched with a mixture of acetic acid and THF. The mixture was partitioned between dichloromethane and water. The organic layer was separated, dried over MgSCU, filtered and concentrated to give crude intermediate 8, which was not isolated or characterized. Crude intermediate (8) was mixed with 6 mL of acetic acid and 4 mL of water. The mixture was stirred at 90° C overnight. The mixture was concentrated to dryness and the residue was partitioned between water and tert-butyl methyl ether (TBME). The aqueous phase was separated and concentrated to dryness to give 150 mg of a product that contained 63% of 11 and 37% of 12. ¹ H-NMR (DMSO-d6) for 11: δ= 1.46 (d, 3H, J = 24 Hz), 3.55 (dd, IH, J = 12.8, 4.4 Hz), 3.73-3.80 (m, IH), 3.96 (dd, IH, J = 24, 8 Hz), 4.20- 4.28 (m, IH). ¹ H-NMR (DMSO-d6) for 12: δ = 1.48 (d, 3H, J = 24 Hz), 3.40-3.70 (m, IH), 3.75-3.95 (m, IH), 4.05-4.15 (m, IH), 4.30-4.50 (m, IH).

Example 4a Preparation of 11 and 12 from 7 and D-glyceraldehyde, 1,2-acetonide 6

12

A dry, clean, 4-neck round bottom flask, equipped with a mechanical stirrer, a thermo couple, a nitrogen inlet, and an addition funnel, was charged with 5 mL of THF and 2.3 mL of 1.8 M solution of lithium diisopropylamide in heptane/THF/ethylbenzene. The solution was cooled to -78°C and to it was slowly charged a solution of 0.8 g of S- phenyl-2-fluoropropanethioate (7) in 5 mL of THF. The mixture was stirred for Ih and a solution of 0.5 g of D-glyceraldehyde, 1, 2-acetonide (6) in 5 mL THF was slowly added. After the addition, the mixture was stirred at approximately -75°C for 10 minutes and was quenched with a mixture of acetic acid and THF. The mixture was partitioned between dichloromethane and water. The organic layer was separated, dried over MgSCU, filtered and concentrated to give crude intermediate (9), which was not isolated or characterized. Crude intermediate (9) was mixed with 6 mL of acetic acid and 4 mL of water. The mixture was stirred at 90 ⁰ C for 4h. The mixture was concentrated to dryness and the residue was partitioned between water and TBME. The aqueous phase was separated and concentrated to dryness to give 940 mg of a product that contained 28% of ( 11) and 72% of ( 12).

Example 4b

Preparation of 11 and 12 from 7, Chlorotriisopropoxytitanium (IV), and D- Glyceraldehyde, 1,2-acetonide 6

A dry, clean, 4-neck round bottom flask, equipped with a mechanical stirrer, a thermo couple, a nitrogen inlet, and an addition funnel, was charged with 5 mL of THF and 2.3 mL of 1.8 M solution of lithium diisopropylamide in heptane/THF/ethylbenzene. The solution was cooled to -78°C and to it was slowly charged a solution of 0.8 g of 7 in 5 mL of THF. The mixture was stirred for 15 minutes and 2.2 g of chlorotriisopropoxytitanium (IV) was added. The mixture was stirred for Ih and a solution of 0.5 g of D-glyceraldehyde, 1,2-acetonide (6) in 5 mL THF was slowly added. After the addition, the mixture was stirred at approximately -75°C for 10 minutes and

was quenched with a mixture of acetic acid and THF. The mixture was partitioned between dichloromethane and water. The organic layer was separated, dried over MgSCU, filtered and concentrated to give crude intermediate 9, which was not isolated or characterized. Crude intermediate (9) was mixed with 6 mL of acetic acid and 4 mL of water. The mixture was stirred at 90 ⁰ C for 4h. The mixture was concentrated to dryness and the residue was partitioned between water and TBME. The aqueous phase was separated and concentrated to dryness to give 940 mg of a product that contained 45% of ( 11) and 55% of (12). Example 4c

Preparation of 11 and 12 from 5 and D-glyceraldehyde, 1,2-acetonide 6

12

A dry, clean, 4-neck round bottom flask, equipped with a mechanical stirrer, a thermocouple, a nitrogen inlet, and an addition funnel, was charged with 10 mL of dichloromethane and 0.62 g of 5. The mixture was cooled to 0 ⁰ C and to it was added 3.5 mL of 1.0 M solution of titanium (IV) chloride in dichloromethane, followed by 0.36 g of triethylamine. The mixture was stirred for Ih and a solution of 0.5 g of D-glyceraldehyde, 1,2-acetonide (6) in 5 mL dichloromethane was slowly added. After the addition, the mixture was stirred at approximately 0 ⁰ C for 30 minutes and was quenched with 20 mL of 6% HCl. The mixture was transferred to a separatory funnel. The aqueous phase was separated and extracted with dichloromethane. The combined organic solution was washed with saturated NaHCU3 solution, dried over MgSCU, filtered and concentrated to give a crude intermediate (10), which was not isolated or characterized.

Crude intermediate (10) was mixed with 6 mL of acetic acid and 4 mL of water. The mixture was stirred at 90 ⁰ C for 3h. The mixture was concentrated to dryness and the residue was partitioned between water and TBME. The aqueous phase was separated and concentrated to dryness to give a product that contained 78% of ( 11) and 22% of ( 12).

Example 5

Preparation of 2 (R=Ph) from a mixture of 11 and 12 (Ri=ethyl, R ₂ =methyl)

12

A mixture of 11 and 12 was dissolved in 15 g of acetonitrile. To the solution was added a catalytic amount of 4-dimethylaminopyridine (DMAP) and 5.2 g of benzoyl chloride. To this mixture was slowly added 4.1 g of triethylamine while maintaining the batch temperature at <40 ⁰ C. After the addition the mixture was stirred for 1 hour. The mixture was diluted with 36 g of ethyl acetate and was cooled to O ⁰ C, 25 g of water was added. The mixture was transferred to a separatory funnel and the aqueous phase was separated and extracted again with 20 g of ethyl acetate. The combined organic solution was washed with 20 g of saturated NaHCθ3 solution, dried over MgSCU, filtered, and concentrated to give a crude oil. The oil was mixed with 27 g of 2-propanol. The mixture was heated to -6O ⁰ C to become a clear solution. The mixture was then slowly cooled to 1O ⁰ C and held for 1 hour. The solid was filtered and the wet cake was washed with 2- propanol and dried under vacuum at 5O ⁰ C overnight to give 2.0 g of 1 (R=Ph).