WO2007070804A2

WO2007070804A2 - Process for preparing gemcitabine and associated intermediates

Info

Publication number: WO2007070804A2
Application number: PCT/US2006/061940
Authority: WO
Inventors: Vladimir Naddaka; Eyal Klopfer; Shady Saeed; Dionne Montvilisky; Oded Arad; Joseph Kaspi
Original assignee: Chemagis Ltd.; Perrigo Company
Priority date: 2005-12-13
Filing date: 2006-12-12
Publication date: 2007-06-21
Also published as: KR20080094890A; WO2007070804A3; CN101356165A

Abstract

Provided is a process for preparing gemcitabine and intermediates useful in the production of gemcitabine. Exemplary intermediates include mixtures of D-erythro and D-threo diastereomers of ethyl 3-(N-carbamoyloxy)-2,2-difluoro-3-(2,2-dimethyldioxolan-4-yl)propionates, from which a preferred isomer can be separated, e.g., by selective crystallization, in accordance with the invention. Also provided are novel alkyl (D-erythro)-3-hydroxy-2,2-difluoro-3-(2,2-dimethyldioxolan-4-yl)propionate intermediates, and novel ribo lactones, which can be used for producing gemcitabine.

Description

PROCESS FOR PREPARING GEMCITABINE AND ASSOCIATED INTERMEDIATES

BACKGROUND OF THE INVENTION

[0001] Gemcitabine HCl, marketed by Eli Lilly under the trademark Gemzar^®, is a nucleoside analogue that exhibits antitumor activity and belongs to a general group of chemotherapy drugs known as antimetabolites. Gemcitabine prevents cells from producing DNA and RNA by interfering with the synthesis of nucleic acids, thus stopping the growth of cancer cells and causing them to die.

[0002] Gemcitabine is a synthetic glucoside analog of cytosine, which is chemically described as 4-amino-l-(2-deoxy-2,2~difluoro-β-D-ribofuranosyl)-pyrimidin-2(lH)-one or 2'-deoxy-2^I,2'~difluorocytidine (β isomer). Gemcitabine HCl has the following structure:

gemcitabine hydrochloride 1

[0003] Gemzar^® is supplied in vials as the hydrochloride salt in sterile form, only for intravenous use, containing either 200 mg or 1 g of gemcitabine HCl (as free base) formulated with mannitol (200 mg or 1 g, respectively) and sodium acetate (12.5 mg or 62.5 mg, respectively) as a sterile lyophilized powder. Hydrochloric acid and/or sodium hydroxide may have been added for pH adjustment.

[0004] U.S. Patent No. 4,808,614 ("the '614 patent") describes a process for synthetically producing gemcitabine, which process is generally illustrated in Scheme 1. Scheme 1

[0005] The D-glyceraldehyde ketal 2 is reacted with bromodiflυoroacetic acid ethyl ester (BrCF₂COOEt) in the presence of activated zinc, to obtain ethyl 2,2-difluoro-3- hydroxy-3-(2j2-dimethyldioxolan-4-yl)-propionate 3 as a mixture of 3-R and 3-S isomers. The 3-R to 3-S isomer ratio is about 3:1. The 3-R isomer has the stereochemistry required for producing the desired erythro (3-R) ribose structure, and can be separated from the 3-S isomer by chromatography.

[0006] The resulting product is cyclized by treatment with an acidic ion exchange resin, such as Dowex 50W-X12, to produce 2-deoxy-2,2-difluoro-D-erythro-pentanoic acid-γ-Iactone 4. The hydroxy groups of the lactone are protected with tert- butyldimethylsilyl (TBDMS) protecting groups to obtain the protected lactone 3,5-bis- (ter/-butyldimethylsilyloxy)-2-desoxy-2₅2-difluoro-l-oxoribose 5, and the product is reduced to obtain 3,5-bis-(terf-butyldimethylsilyl)-2-desoxy-2,2-difluororibose 6. [0007] The 1 -position of the carbohydrate is activated by the introduction of a leaving group, e.g., methanesulfonyloxy (mesylate), formed by reacting compound 6 with methanesulfonyl chloride to obtain 3,5-bis-(fer/-butyldimethyϊsilyloxy)-l- methanesulfonyloxy-2-desoxy-2,2~difluororibose 7. The base ring is coupled to the carbohydrate by reacting compound 7 with N,O-bis-(trimethylsilyl)-cytosine 8 in the presence of a reaction initiator, such as trifluoromethanesulfonyloxy trimethylsilane

(trimethylsilyl triflate). Removal of the protecting groups and chromatographic purification affords gemcitabine free base.

[0008] U.S. Patent No. 4,526,988 describes a similar process in which the cyclization is carried out by hydrolyzing an alkyl 3 -dioxolany 1-2, 2-difiuoro-3 -hydroxy-propionate with a mildly acidic ion exchange resin. See also, Hertel et al. in J Org. Chem. 53, 2406

(1998).

[0009] U.S. Patent No. 4,965,374 ("the '374 patent") describes a process for producing gemcitabine from an intermediate 3,5-dibenzoyl ribo protected lactone of the formula:

where the desired erythro isomer can be isolated in a crystalline form from a mixture of erythro and threo isomers. The process described in the '374 patent is generally outlined in Scheme 2.

Scheme 2

mixture of α and β anomers

[0010] The 3 -hydroxy group of compound 3 is esterified with a benzoyl protecting group by reaction with benzoyl chloride, benzoyl bromide, benzoyl cyanide, benzoyl azide, etc. (e.g., PhCOX, wherein X = Cl, Br, CN, or N3), in presence of a tertiary amine or a catalyst such as 4-dimethylaminopyridine or 4-pyrrolidinopyridine, to obtain ethyl 2,2~difluoro-3-benzoyloxy-3-(2,2-dimethyldioxolan-4-yl)-propionate 9. [0011] The isoalkylidene protecting group of 9 is selectively removed, e.g., by using a strong acid such as concentrated sulfuric acid in ethanol, to produce ethyl-2,2-difluoro-3- benzoyloxy-4₅5-dihydroxypentanoate 9 A. The product is cyclized to lactone 10 and converted to the dibenzoate ester to produce the lactone 2-deoxy-2,2~ difluoropentofuranos-l-ulose-3,5-dibenzoate 11 as a mixture of erythro and threo isomers. The '374 patent describes isolating at least a portion of the erythro isomer from the mixture by selective precipitation. See also, Chou et al, Synthesis, 565-570, (1992). [0012] Compound 11 is then reduced to obtain a mixture of α and β anomers of 2- desoxy-2,2-difluorpentofuranose-dibenzoate 12, which is activated with methane-sulfonyl chloride to obtain an anomeric mixture of mesylates, 2-deoxy-2,2-difluoro-D- ribofuranosyl-3,5-di-O-benzoyl-l-O-β-methanesulfonate 13, and coupled with N₅O- bis(trimethylsilyl)-cytosine 8 to obtain silyl-protected nucleoside 14 as the dibenzoate ester as a mixture of the α- and β-anomers (about a 1 :1 α/β anomer ratio). Removal of the esters and silyl protecting group provides a mixture of the β-anomer (gemcitabine) and the α-anomer (about a 1:1 α/β anomer ratio). The '374 patent describes selectively isolating the β-anomer (gemcitabine) by forming a salt of the anomeric mixture, e.g., the hydrochloride or hydrobromide salt, and selectively precipitating to obtain 2'-deoxy-2',2^!- difluorocytidine as the salt in 1:4 α/β ratio. The '374 patent also describes selectively precipitating the β-anomer in free base form in a slightly basic aqueous solution. One such process involves dissolving the 1 :1 α/β anomeric mixture in hot acidic water (pH adjusted to 2.5-5.0) and, once the mixture is substantially dissolved, increasing the pH to 7.0-9.0 and allowing the solution to cool, to produce crystals, which are isolated by flltration.

[0013] U.S. Patent No. 5,521,294 ("the '294 patent") is directed to 1-alkylsulfonyl- 2,2-difluoro-3 -carbamoyl ribose intermediates and nucleosides derived therefrom. The compounds are particularly useful in the preparation of 2'-deoxy-2',2'-difluoro-β-cytidine and other β-anomer nucleosides. The '294 patent teaches using, inter alia, the 3 -hydroxy carbamoyl group on the difluororibose intermediate to enhance formation of the desired β anomer nucleoside derivative. The process that uses the 3 -hydroxy carbamoyl group on the difluororibose intermediate includes converting the lactone 11 (in a two-step reaction) to form the dibenzoyl mesylate 13, followed by deprotection at the 3-ρosition to obtain the 5-monobenzoyl mesylate intermediate 15, which is reacted with various isocyanates to obtain the compounds of formula 16. The next steps are coupling and deprotection as described in the previous patents. The process and the intermediates 15 and 16 are generally illustrated in scheme 3 below: Scheme 3

11 13 15

PhNCθπ^"EA

16

1 coupling

2 deprotectson

16 gerrrøtabme

[0014] Processes for separating anomeric mixtures of alkylsulfonate intermediates also have heen described. U.S. Patent Nos. 5,256,797 and 4,526,988 describe processes for separating anomers of 2-deoxy-2,2-difluoro-D-ribofuranosyl-l-alkyl sulfonates, and U.S. Patent No. 5,256,798 describes a process for obtaining α-anomer-enriched ribofuranosyl sulfonates.

[0015] Other intermediates that may be useful for preparing gemcitabine have been disclosed. For instance, U.S. Patent No. 5,480,992 describes anomeric mixtures of 2_}2- difluororibosyl azide and corresponding amine intermediates that can be prepared, e.g., by reacting a 2-deoxy-2,2-difluoro-D-ribofuranosyl-3,5-di-0-benzoyl-l-0-β- methanesulfonate with an azide nucleophile, such as lithium azide, to obtain the azide. Reduction of the azide produces the corresponding amine, which can be synthetically converted into a nucleoside. See also U.S. Patent Nos. 5,541,345 and 5,594,155. [0016] Other known intermediates include, e.g., tritylated intermediates (U.S. Patent No. 5,559,222), 2-deoxy-2,2-difluoro-β-D-ribo-pentoρyranose (U.S. Patent No. 5,602,262), 2-substituted-3,3"difluorofuran intermediates (U.S. Patent No. 5,633,367), and α,α-difluoro-β-hydroxy thiol esters (U.S. Patent Nos. 5,756,775 and 5,912,366). [0017] There are inherent problems associated with the production of gemcitabine, particularly for processes that require the production and separation of isomers, and tend to produce poor yields on a commercial scale. Accordingly, there is a need for improved methods of preparing gemcitabine and intermediates thereof, which facilitate the production of gemcitabine, particularly on a commercial scale. The present invention provides such methods and intermediates, as will be apparent from the description of the invention provided herein.

BRIEF SUMMARY OF THE INVENTION

[0018] The present invention provides a method for preparing gemcitabine, which preferably includes converting a compound of the formula:

1OA, into gemcitabine, and optionally converting the gemcitabine into a salt. Preferably, X is O or S, and R is selected from the group consisting of unsaturated and saturated CrCs alkyl, unsubstituted and substituted aryl, unsaturated and saturated C₁-Cg alkylsulfonyl, and unsubstituted and substituted arylsulfonyl.

[0019] The compound of formula 1 OA can be converted into gemcitabine using any suitable method including, e.g., one or more methods described in PCT/US2006/033431, which claims priority from U.S. Patent Application No. 60/712,217, filed on August 29, 2005, which are incorporated herein by reference. Preferably, the present invention includes protecting the 5-hydroxyl of a compound of formula 1OA, reducing the lactone carbonyl to produce a 3-carbamoyl-5-hydroxyl-protected-2-deoxy-2,2-difluoro-D- ribofuranose, coupling with a protected N-I -protected cytosine to produce a N-I- protected-3^!-carbamoyl-5'-hydroxyl-protected-2'-deoxy-2,2-difiuoro~cytidine, removing the S'-carbamoyl, N-I protecting group and 5'-tiydroxyl protecting group to produce gemcitabine, and optionally converting the gemcitabine into a salt, wherein X, AIk and R are as defined herein. [0020] Any suitable protecting group can be used for protecting the 5-hydroxyl. Preferably, protecting group is an ester, e.g., benzoyl The compound of formula 1 OA can be obtained by any suitable method, and is preferably obtained by cyclizing a compound of the formula:

to produce the compound of formula 1OA, wherein X and R are as defined herein, and AIk is C i-₆ alkyl. In one embodiment, the compound of formula 1OA is obtained by separating the Ω-erythro isomer from a mixture of V>-erythro and O-ihreo isomers of the formula:

and cyclizing the O-erythro isomer to obtain the compound of formula 1OA, wherein X, AIk and R are as defined herein. Such mixture of O-erythro and D-threo isomers is preferably obtained by reacting a compound of the formula:

with a compound of the formula R — N=C=X, to produce the corresponding carbamate as a mixture of O-erythro and O-threo isomers, wherein X, AIk and R are as herein. [0021] In another embodiment, the compound of formula 1OA is obtained by reacting a compound of the formula:

with a compound of the formula R — N=C=X to produce a compound of the formula:

and cyclizing to produce the compound of formula 1OA, wherein X, AIk and R are as defined herein.

[0022] The present invention further provides intermediates that are useful in the preparation of gemcitabine, which intermediates can include mixtures of O-erythro and D- threo (3R and 3S) diastereomers of ethyl 3-(N-carbamoyloxy)-2,2-difluoro-3-(2,2- dimethyldioxolan-4-yl)propionates, of the general formula 9B:

9B wherein X is O or S, and R is selected from the group consisting of unsaturated and saturated Cj-Cg alkyl, unsubstiluted and substituted phenyl, unsaturated and saturated C₁- Cg alkylsulfonyl, and unsubstituted and substituted phenylsulfonyl. [0023] The present invention also provides a process for preparing a compound of formula 9B, which process preferably includes reacting a mixture of the D-erylhro and D- threo diastereomers of ethyl 2,2-difluoro-3-hydroxy-3-(2,2-dimethyldioxolan-4- yl)propionate, e.g., of formula 3, with isocyanates or isothiocyanates of formula RNCX₅ wherein X and R are as defined herein. [0024] In accordance with the present invention, intermediates of the formula 9B can be isolated in a yield of at least about 95.3% and preferably in a yield of at least about

97.4%.

[0025] The intermediates of the present invention also can include D-erythro diastereomers of ethyl (3R)-3-(N-carbamoyloxy)-2,2~difiuoro-3-(2,2-dimethyldioxolan-4~ yl)propionates, having the general formula 9C:

9C wherein X and R are as defined herein.

[0026] The present invention further provides purification processes for selectively isolating the D-erythro diastereomers ethyl (3R)-3-(N-carbamoyloxy)-2,2-difluoro-3-(2,2- dimethyldioxolan-4-yl)propionates, having the general formula 9C. The purification process of the invention can be carried out by dissolving the mixture of D-erythro and D- threo (3 R and 3S) diastereomers of a compound of formula 9C, preferably in a non-polar solvent, cooling the solution to a temperature in the range of about -20° C to about 10° C, and collecting the D- erythro isomer in the form of crystals.

[0027] The purification process of the present invention can provide intermediates of the general formula 9C in high purity, e.g., in a purity of at least about 96.5%, more preferably in a purity of at least about 98.5%.

[0028] The present invention also provides a process for preparing ethyl (p-erythro)~ 3-hydroxy-2,2-di0uoro-3-(2,2-dimethyldioxolan-4-yl)propionate having the formula 3C:

3C from a starting material, which is preferably a compound of the general formula 9C. The process preferably comprises reacting a compound of the general formula 9C with a primary or a secondary amine in a dry solvent in the presence of catalytic amount of a Lewis acid. After reaction completion, the by-product can be separated by filtration, to obtain the by-product as the filtrate. The solvent can be removed under reduced pressure to produce an oil, which can be precipitated from an organic solvent. [0029 J The present invention also provides a process for preparing ribo lactones 3-(N- carbamoyloxy)-2-deoxy-2_j2-difluoro-D-er>'^ro-pentofuranose-l-ulose having the general formula 1OA:

1OA wherein X and R are as defined herein. In one embodiment, the process includes refluxing the D-erythro isomers having the general formula 9C in a solvent mixture containing an acid (e.g., a mixture of acetonitrile, water and trifluoroacetic acid) followed by azeotropic distillation of water.

[0030] According to the present invention, gemcitabine can be obtained from the lactones of the formulae 10A₅ e.g., as depicted in Scheme 6 below, and as further described in detail herein.

DETAILED DESCRIPTION OF THE INVENTION [0031] In accordance with the present invention, the preparation of 3 -(N- carbamoyloxy)-2-deoxy-2_?2~difluoro~D-erμf/zro-pentofuranose- 1 -ulose intermediates can be achieved in high yield and high purity. The intermediates of the present invention are particularly useful for producing gemicitabine.

[0032] The intermediates can include mixtures of erythro and threo (3R and 3S) isomers of ethyl 3-(N-carbamoyloxy)-2,2-difluoro-3-(2,2-dimethyldioxolan-4- yl)propionates having the general formula 9B, wherein R is selected from the group consisting of unsaturated and saturated Ci-C₈ alky I, unsubstituted and substituted aryl, unsaturated and saturated Cj-Cg alkylsulfonyl, and unsubstituted and substituted arylsulfonyl, and X is O or S:

9B,

[0033] In a preferred embodiment, the present invention provides a process for preparing intermediates useful in the synthesis of gemcitabine, which process includes: a) providing a mixture of O-erythro and O-threo diastereomers of ethyl 2,2- difluoro-3-hydroxy-3-(2,2-dimethyldioxolan-4-yl)ρroρionate of formula 3 in toluene; b) optionally heating the mixture with azeotropic distillation of water to dry the toluene; and c) reacting the mixture of D-erythro and D-lhreo diastereomers of ethyl 2,2- difluoro-3-riydroxy-3-(2,2-dimethyldioxolan-4-yl)-propionate of formula 3 in toluene with an isocyanate or isothiocyanate of the formula RNCX₅ wherein X and Rare as defined herein, in the presence of a base, to obtain a mixture of D-erythro and O-threo ethyl 3-(N- carbamoyloxy)-2,2-difluoro-3-(2,2-dimethyldioxolan~4-yl)propionate.

[0034] According to one aspect of the present invention the azeotropic distillation of water is carried out using a Dean-Stark trap to dry the toluene by heating the mixture at reflux for about 2 hours.

[0035] In accordance with the present invention, the isocyanate or isothiocyanate used in the reaction can include, e.g., 2-chloroethyl isothiocyanate, 5-chloro-2-methylphenyl isothiocyanate, 2-chloro~4-nitrophenyl isothiocyanate, 2-chlorophenyl isothiocyanate, 3- chlorophenyl isothiocyanate, 4-chlorophenyl isothiocyanate, 3-acetylphenyl isothiocyanate, 4-acelylphenyl isothiocyanate, 2-(chloromethyl)phenyl isocyanate, 2- chloro-5-methyl-phenyl isocyanate, 2-chloro-6-methylphenyl isocyanate, 3-chloro-2- methylphenyl isocyanate, 3-chloro-4-methylphenyl isocyanate, 4-(chloromethyl)-phenyl isocyanate, 4-cbloro-2-methylphenyl isocyanate, 5-chloro-2-methylphenyl isocyanate, 2- chloro-4-nitrophenyl isocyanate, 2-chloro-5-nitrophenyl isocyanate, 4-chloro-2- nitrophenyl isocyanate, 4-chloro-3-nitrophenyl isocyanate, 2~chloro-2~mtrophenyl isocyanate, 2-chloroρhenyl isocyanate, 3-chlorophenyl isocyanate, 4-chlorophenyl isocyanate, 3-acetylphenyl isocyanate, phenyl isocyanate, N-benzenesulfonyl isocyanate, p-toluenesulfonyl isocyanate, or o-toluenesulfonyl isocyanate.

[0036] Preferably, the isocyanate is 4-nitrophenyl isocyanate, 4-chlorophenyl isocyanate, 3-acetylphenyl isocyanate, or phenyl isocyanate.

[0037] The base is preferably an organic base selected from triethyl amine, lutidines, morpholine, diisopropylethylamϊne, pyridine, 2-(dimethylamino)-pyridme, 4-

(dimethylamino)pyridine, and the like, and combinations thereof. In a preferred embodiment, the base is 4-(dimethylamino)ρyridine.

[0038] According to a preferred embodiment of present invention, the process for preparing the isomers of ethyl 3-(N-carbamoyloxy)-2,2-difluoro-3-(2,2-dimethyldioxolan-

4~yl)propionates of the general formula 9B comprises providing a mixture of erythro and threo (3 R and 3S) isomers of ethyl 2,2-difluoro-3~hydroxy-3-(2,2-dimethyldioxolan-4- yl)propionate of compound 3 in toluene and refluxing the mixture at the temperature in which water is azeotropically removed therefrom.

[0039] Preferably, the mixture contains at least about 1 ml of toluene, and more preferably from about 5 ml to about 10 ml of toluene, per gram of ethyl 2,2-difluoro-3- hydroxy-3-(2_s2-dimethyldioxolan-4-yl)propionate. When the mixture is sufficiently dry, it is allowed to cool and a base is added, followed by addition of the isocyanate or isothiocyanate. The mixture preferably is heated to a suitable temperature for adequate time period to allow completion of the reaction.

[0040] Once the reaction is completed, the reaction mixture is separated from the crystalline residue of the by-product and subsequently washed with an acidic solution and water followed by separating the layers and drying the organic layer over magnesium sulfate. The product is typically obtained as a solid by evaporating the solvent and is preferably crystallized.

[0041] According to yet another preferred embodiment of the present invention the mixtures of D-erythro and D-threo isomers of ethyl 3-(N-carbamoyloxy)-2,2-difluoro-3-

(2,2-dimethyldioxolan-4-yl)propionates of the general formula 9B are obtained in high yields in the range of at least about 95.3, preferably at least about 97.4%. [0042] The present invention also provides D-erythro diastereomers of ethyl (3R)-3- (N-carbamoyloxy)- 2,2-difluoro-3-(2,2-dimethyldioxolan-4~yl)propionates, having the general formula 9C:

9C

[0043] The present invention also provides a process for separating the D-erythro isomer from the mixture of D-erythro and D-threo isomers of ethyl 3-(N-carbamoyloxy)- 2_;2-difluoro-3-(2,2-dimethyldioxolan-4-yl)propionates of the general formula 9B by selective crystallization. Preferably, the process comprises: a) dissolving the crude mixture of O-erythro and D-threo ethyl 3-(N- carbamoyloxy)-2,2-difluoro-3-(2,2-dimethyldioxolan-4-yl)ρropionate of formula 9B in an organic solvent, optionally at elevated temperature; b) allowing the solution to cool sufficiently to a temperature in the range of about -20° C to about 10° C, e.g., overnight, to produce crystals; and c) collecting the crystals by filtration, washing the crystals with an organic solvent and drying, optionally at elevated temperature.

[0044] An exemplary process for preparing the O-erythro isomers of ethyl 3-(N- carbamoyloxy)-2,2-difluoro-3-(2,2-dimethyldioxolan-4-yl)propionates of the general formula 9C is depicted in Scheme 4 below:

Scheme 4

9B

In the process depicted in Scheme 4, R is preferably an unsaturated or saturated Ci-Cg alkyl, an unsubstituted or substituted aryl, an unsaturated or saturated Cj-Cg alkylsulfonyl, or an unsubstituted or substituted arylsulfonyl.

[0045] Suitable solvents that can be used for crystallizing the compound of formula

9C include organic solvents such as, for example, dichloromethane, chloroform, ethyl acetate, 1 -propyl acetate, 2-propyl acetate, butyl acetate, tert-butyl acetate, o-xylene, m- xylene, o-dichlorobenzene, toluene, and the like, or a mixture thereof. A preferred solvent for crystallizing the compound of formula 9C is toluene.

[0046] Suitable organic solvents, which can be used for washing the obtained crystals, include, for example, toluene, pentane, hexane, heptane, octane, petroleum ether, cyclohexane, or a mixture thereof. Preferred solvents for washing the obtained crystals include toluene and toluene/hexane mixtures, e.g., a 1:1 (v/v) mixture of toluene and hexane.

[0047] In accordance with the present invention, the O-erythro diastereomers ethyl

(3 R)-3-(N-carbamoyloxy)-2,2-difiuoro-3-(2,2-dimethyldioxolan-4-yl)propionates having the general formula 9C can be isolated having purities in the range of at least about 96.5%, preferably at least about 98.5%, [0048] The present invention additionally provides a process for preparing ethyl (D- erythro)~3 ~hydroxy-2,2-difluoro-3 -(2,2-dimethyldioxolan-4-yl)propionate, having the formula 3C:

3C from a starting material, which is preferably selected from O-erythro diastereomers of ethyl (3R)-3-(N-carbamoyloxy)-2_J2-difiuoro-3~(2,2-dimethyl-dioxolan-4-yl)propionates_;, having the general formula 9C. Exemplary starting materials can include the following: ethyl (D-ery^ro)-3-(4-nitrophenylcarbamoyl)-3-(2,2-dimethyldioxolan-4-yl)propionate of formula 9E, ethyl (D-ery^ro)-3-(4-chlorophenylcarbamoyl)-2, ,2-difluoro-3-(2,2- dimethyldioxolan-4-yl)propionate of formula 9F, ethyl (D-eryrtrc>)-3-(3- acetyIphenylcarbamoyl)-2,2-difiuoro-3~(2,2~dimethyldioxolan-4-yl)proρionate of formula 9G, and ethyl (D-ery^ro)-3-(ρhenylcarbamoyl)-2_!2-difluoro-3-(2,2-dimethyl-dioxolan-4- yl)proρionate of formula 9H, preferably ethyl (D-erμ^ro)-3-(4-chloropheny- lcarbamoyloxy)-2,2-difluoro-3-(2,2-dimethyl- dioxolan-4-yl)propionates of formula 9E. In one embodiment, the process includes: a) healing a mixture of toluene and a catalyst such as, e.g., a Lewis acid, with azeotropic distillation of water to dry the toluene; b) adding the O-erythro isomer of ethyl 3-(N-carbamoyloxy)-2,2-difluoro-3- (2,2-dimethyldioxolan-4-yl)proρionate of the general formula 9C and refluxing the mixture; c) adding a base, preferably a primary or a secondary amine, and maintaining the reflux until completion of the reaction; d) separating the by-product as crystals by filtration to obtain a filtrate; e) washing the filtrate and evaporating the solvent optionally under reduced pressure to obtain an oil; and f) dissolving the oil in an organic solvent and cooling, to produce crystals of the desired product. [0049] Suitable catalysts can include, e.g., cupric acetate, stannous chloride, stannous oxalate, aluminium alkoxide and the like, or a combination thereof. A preferred catalyst is cupric acetate.

[0050] Suitable bases can include organic bases such as, for example, primary and secondary amines such as n-propyl amine, n-butyl amine, n-hexylamine, dipropylamine, dibutylamine, dicyclohexylamine, morpholine, piperidine, 2,6-dimethylpiperidine, and the like, or a combination thereof. A preferred organic base is morpholine. [0051] Suitable solvents for dissolving the obtained oil can include, e.g., dichloromethane, chloroform, ethyl acetate, 1 -propyl acetate, 2-propyl acetate, butyl acetate, tert-butyl acetate, diethyl ether, diisopropyl ether, tert-butyl methyl ether, or a mixture thereof. A preferred solvent for dissolving the oil is tert-butyl methyl ether. [0052] The present invention also provides a process for cyclizing the O-erythro diastereomers of ethyl (3R)-3-(N-carbamoyloxy)-2,2-difluoro~3-(2,2-dimethyldioxolan-4- yl)propionate of the general formula 9C, to obtain a 3-(N-carbamoyloxy)-2~deoxy-2,2- difluoiO-D-er^Aro-pentofui-anose-l-ulose having the general formula 1OA:

1OA wherein X is O or S, and R is preferably an unsaturated or saturated Ci-C₈ alkyl, or an unsubstituted or substituted phenyl. The process preferably comprises: a) providing a solution of a D-erythro diastereoisomer of ethyl (3R)-3- (carbamoyloxy)-2,2-difluoro-3-(2,2-dimethyldioxolan-4-yl)propionate of the general formula 9C in a solvent mixture containing an acid and heating for a sufficient time period to allow substantial completion of the reaction; b) reducing the solution volume by distillation; c) adding toluene and drying the mixture by azeotropic distillation; and d) further distilling off the solvent mixture, to obtain the product as a solid. [0053] The solvent mixture containing an acid preferably includes a mixture of acetonitrile, water and trifluoroacetic acid. Exemplary acetomtrile:water:trifluoroacetic acid ratios can include 30:0.64:0.14 v/v/v, 40:1.0:0.21 v/v/v, and 30:1.5:0.15 v/v/v. [0054] The azeotropic distillation of water can be carried out using a Dean-Stark trap to dry the toluene e.g., by heating the mixture at reflux for about 5.5-7 hours. [0055] In accordance with the present invention, 3-(N-carbamoyloxy)-2-deoxy-2,2- difluoro-D-ery#zrø-pentofuranose-l-uiose having the general formula 1OA can be obtained in high yields, e.g., in yields of at least about 95.5%, preferably in yields of at least about 98.4%.

[0056] An exemplary process for preparing the 3-(carbamoyloxy)-2-deoxy-2,2- difluoro-D-erj^rø-pentofuranose-1-ulose having the general formula 1OA is depicted in Scheme 5 below:

Scheme 5

9C₅ X-O₅S 1OA

In the process depicted in Scheme 5, R is preferably selected from the group consisting of unsaturated and saturated Ci-Cg alkyl, unsubstituted and substituted aryl, unsaturated and saturated Ci-Cg alkylsulfonyl, and unsubstituted and substituted arylsulfonyl. [0057] In accordance with Scheme 6 below, gemcitabine can be obtained from the lactones of the formulae 1OA, in a process comprising fewer steps than the process described in the '294 patent, e.g., by further carrying out the following steps: a) reacting the lactone 1OA with an acid chloride or an acid bromide to obtain the 3 -carbamoyl lactone intermediate having the formula 17, wherein X is O or S and R is selected from the group consisting of unsaturated and saturated Ci-Cg alkyl, unsubstituted and substituted phenyl; b) reducing the 3-carbamoyl lactone intermediate of the formula 17 with a suitable reducing agent in an organic solvent to obtain a lactol intermediate, which is reacted with methanesulfonyl chloride in the presence of a base to obtain the sulfonate intermediate of the formula 16 A; c) coupling the compound 16 A with bis(trimethylsilyl)-N-acetylcytosine, preferably at ambient temperatures using a catalyst in an organic solvent to obtain a mixture of α and β anomers of the 3,5-diρrolected-N-l-trimethylsilylacetyl-2'-deoxy-2',2¹- difluorocytidine; d) precipitating the β isomer of the 3,5-diprotected -N-I- trirnethylsilylacetyl-2'-deoxy-2',2'-difϊuorocytϊdine, thus separating the two isomers by filtration; and e) removing the protecting groups by hydrolysis, to obtain gemcitabine. [0058] Thus j in accordance with the present invention, the process using the starting materials 3-(carbamoyloxy)-2-deoxy-2,2-difluoro~D-eryt/^:ZTO"pentofuranose-l-ulose having the general formulae 1OA is advantageous over conventional processes for obtaining gemcitabine, e.g., as depicted in Schemes 2 and 3, as the present invention provides a process that requires fewer synthetic steps, and in addition the erythro isomer 1 OA is obtained in high purity and yield.

Scheme 6

PhCOOCI or 1 reduction PhCOOBr 2 methanesulfonyl chloride OMs

1OA 17 16A

gemcitabine

In the process depicted in Scheme 6, X is O or S and R is preferably an unsaturated or saturated CrC₈ alkyl, or an unsubstituted or substituted phenyl.

[0059] The reduction of the lactone 10A₅ e.g., as depicted in Scheme 6, can be carried out using any suitable reducing agent such as, for example, lithium aluminium hydride, diisobutyl aluminium hydride, and sodium bis-(2-methoxyethoxy)-aluminium hydride, and the like, or a combination thereof. The reduction, e.g., as illustrated in Scheme 6, is preferably carried out using lithium aluminium hydride, particularly for commercial scale production in view of its low molecular weight and relatively high reduction capacity (4 available H atoms per molecule). The reduction also can be carried out using diisobutyl aluminium hydride (e.g., as taught in U.S. Patent No. 4,808,614 and Chou et al, Synthesis, 565-570 (1992)), although diisobutyl aluminium hydride is less preferred in view of its molecular weight and the fact that it has only 1 H atom available for reduction. [0060] The coupling reaction, e.g., as depicted in Scheme 6, can be carried out in any suitable solvent, which can include, for example, one or more organic solvents selected from acetonitrile, dichloromethane, chloroform, 1,2-dichloroethane, toluene and xylenes. In one embodiment, the coupling reaction is carried out in 1 ,2-dichloroethane. Optionally, the coupling reaction can be facilitated by using a suitable catalytic reagent such as, for example, trimethylsilyl trifiate (Me₃SiOTf).

[0061] Removal of the protecting groups, e.g., as depicted in Scheme 6, can be carried out by using any suitable conditions, which can include, for example, basic hydrolysis, e.g., 20% ammonia in methanol.

EXAMPLES

[0062] Although, the following examples illustrate the practice of the present invention in some of its embodiments, the examples should not be construed as limiting the scope of the invention. Other embodiments will be apparent to one skilled in the art from consideration of the specification and examples.

EXAMPLE 1

[0063] This example demonstrates the preparation of ethyl (D~erythro)~3~(4~ nitrophenylcarbamoyloxy)-3-(2,2-dimethyldioxolan-4-yl)propionate (9E). [0064] A solution of ethyl (a 2.6 :1 mixture of D-erythro and O-threo)-3-(h.y droxy)- 2,2-difluoro-3-(2,2-dimethyldioxalane-4-yl)propionate (5.5 g) in toluene (50 ml) was heated under reflux using a Dean-Stark trap for 2 hours to dry the mixture. The solution was cooled under nitrogen to ambient temperature, and 4-(dimethylamino)ρyridine (0.05 g) and 4-nitrophenyl isocyanate (3.8 g) were then added. This mixture was heated at 80- 90° C for 6 hours, cooled to ambient temperature, and the yellow crystals of l,3-di(4- nitrophenyl)urea were collected by filtration. The filtrate was washed with water, 1 N HCl, and water (50 ml each) and dried over MgSO₄ The solvent was removed under reduced pressure to give a solid ethyl (D-erythro, D-f/zr<?o)-3-(4-nilrophenyl-carbamoyl)- 2,2-difluoro-3-(2,2-dimethyl-dioxolan-4-yl)propionate, yield: 7.2 g (96%). The crude product was dissolved under heating in toluene (15 ml) and the solution was cooled at 5° C overnight. The crystals were collected by filtration, washed with n-hexane and dried at 50° C overnight to yield ethyl (D-ery/Aro)-3-(4-nilroρhenyl-carbamoyl)-2,2-difluoro-3- (2,2-dimethyldioxolan-4-yl)propionate (9E); yield: 2.1 g (28%); [α]_D ²⁵~- 17,5° (c 1, acetonitrile); nip. 126-128° C. HPLC assay indicated a purity of 96.5% with 1.3% of (D- threo)-isomev.

¹H NMR (CDCl₃): δ - 1.36 (t, 3 H, CH₂CHs)₅ 1-35, 1.39[d, 6 H₅ C(CH₃)₂], 4.09 (m, 2 H, CH₂) , 4.34 (m, 2 H, CH₁CH₃ and 1 H, H-4), 5.60 (m, 1 H, H-3), 7.65 (d, 2 H _arom), 7.88

(S₅ I H, NHCO), 8.21 (d, 2 H arom ).

¹³C NMR (CDCl₃): δ = 13.8 (CH₂CH₃), 24.9, 26.1 [C(CH₃)₂], 63.7 (CH₂CH₃), 65.6 (C-5), 72.0 (C-3, J c-F = 22.O₅ 26.0 Hz), 72.2 (C~4), 110.4 [C(CH₃)J, 112.7 (C-2, J _C-F = 252, 252 Hz), 118.3, 125.2, 143.42, 143.46 (C arom), 151.2 (OCONHAr), 162.1 (C-I, J _C-_F = 31 ,31 Hz). ESI (negative)/MS: m/z = 417.1 [M-H⁺].

EXAMPLE 2

[0065] This example demonstrates the preparation of ethyl (D-erγthro)-3-(4- chlorophenylcarbamoyloxy)-2,2-difluoro-3-(2,2-dimethyldioxolan-4-yl)propionate (9F). [0066] A solution of ethyl (a 2.85:1 mixture of O-erythro andO-threo) -3 -(hydroxy )- 2,2-difluoro-3-(2,2-dimethyldioxalane-4-yl)propionate (3) (60.0 g) in toluene (300 ml) was heated under reflux with Dean- Stark trap for 2 hours to dry the mixture. The mixture was cooled under nitrogen to ambient temperature, and 4-(dimethyl- amino)pyridine (0.50 g) and 4-chlorophenyl isocyanate (35.0 g) were then added. The reaction mixture was heated at 80-90° C for 6 hours, and then the crystals of l,3-di(4-chlorophenyl)urea were collected by hot filtration. The filtrate was washed with water, 1 N HCl, and again with water (50 ml each), and dried over MgSO₄ and treated with activated carbon The solvent was removed under reduced pressure to give 77.8 g of solid ethyl (D-erythro, D-threo)-3- (4-chlorophenylcarbamoyloxy)-2,2-difluoro-3-(2₅2-dimethyldioxolan-4-yl)propionate in 97.4% yield. The crude product was dissolved under heating in toluene (100 ml), and the solution was filtered. The filtrate was cooled to 5° C overnight. The colorless crystals were collected by filtration, washed with toluene-n-hexane (1 :1) mixture and dried at 50° C overnight to yield 37, 7g of ethyl (D-ery?fe"o)-3-(4-chlorophenylcarbamoyloxy)~2,2- difluoro-3-(2,2-dimethyldioxolan-4-yl)propk>nate (9F) in 47.2% yield, having a purity of 98.5% by HPLC: (0.09% of D-threo isomer); [α]_D ²⁵- 10.2° (c LO₅ acetonitrile); mp 120- 122° C.

¹H NMR (CDCl₃ ): δ - 1.33 (t, 3 H, CH₂CH₁), 1.33, 1.37 [d, 6 H₅ C(CHs)₂], 4.07 (m, 2 H, CH₂) _, 4.32 (m, 2 H, CH₅CH₃ and 1 H, H-4), 5.56 (m, 1 H, H-3), 7.27 (m, 2 H atom and 1 H₅ NHCO)₅ 7.37 (4 2 H ^_n1 ).

¹³C NMR (CDCl₃): δ = 13.8 (CH₂CH₃), 25.0, 26.0 [C(CHs)₂], 63.6 (CH₂CH₃), 65.6 (C-5), 71.6 (C-3, J c-F = 22.0, 26.0 Hz), 72.3 (C-4), 110.3[C(CHs)₂], 1 12.8 (C-2, J _C-F = 256, 256 Hz), 120.1, 129.2, 135.8 (C ^ ), 151.4 (OCONHAr)₅ 162.2 (C-I, J _C-F - 31 ,31 HZ). ¹⁹F NMR indicated that mainly the one fluorine containing product was present. APCI (negative)MS: m/z = 406.2 [M-H⁺].

EXAMPLE 3

[0067] This example demonstrates the preparation of ethyl (D-erμt/zro)-3-(3- acetylphenylcarbamoyloxy)-2_J2-difluoro~3-(2,2-dimethyldioxolan-4-yl)propionate (9G). [0068] A solution of ethyl (a 2.6:1 mixture of D-erylhro, D-//*π?ø)-3-(hydroxy)-2,2- difluoro-3-(2,2-dimethyldioxalane-4-yl)propionate (3) (5.0 g) in toluene (50 ml) was heated under reflux with Dean-Stark trap for 2 hours to dry the mixture. The mixture was cooled under nitrogen to ambient temperature, and 4-(dimethyl- amino)pyridine (0.025 g) and 3-acetylphenyl isocyanate (3.05 g) were then added. The reaction mixture was heated at 80-90° C for 6 hours, cooled to ambient temperature, and the crystals of l,3-di(3- acetylphenyl)urea were collected by filtration. The filtrate was washed with water, 1 N HCl, and again with water (50 ml each), and dried over MgSO₄ The solvent was removed under reduced pressure to give 6.3 g of ethyl {D-erythro, O-threo)-Z-Q- acetylphenylcarbamoyl)-2,2-difluoro-3-(2,2-dimethyldioxolan-4-yl)propionate as a oil in 96.5% yield. The crude product was dissolved under heating in toluene (7 ml) and the solution was cooled at 5° C overnight. The crystals were collected by filtration, washed with a pre-cooled toluene-n-hexane (1 :1, v/v) mixture and dried at 50° C overnight to yield 2.0 g of ethyl (D-ery^/zro)-3-(3-acetylphenylcarbamoyloxy)-2,2-difluoro-3-(2_J2- dimethyldioxolan-4-yl)proρionate (9G) in 30.6% yield; [O]_D ²⁵ -14.5° (c 1, acetonitrile); mp 92-93° C. APCI (negative)MS: m/z - 414.2 [M-H⁺].

¹H NMR (CDCl₃): δ = 1.34 (t, 3 H, CH₂CH₃), 1.34, 1.38 [d, 6 H, C(CH₃)₂], 2.62 [s, 3 H, C(O)CH₃], 4.03, 4.12 (2m, 2 H₃ CH₂) , 4,33 (m, 2 H, CH₂CH₃ and 1 H, H-4), 5.60 (m, 1 H, H-3), 7.44 (t), 7.69 (d), 7.84 (d), 8.00 (s) (4 H _arom ), 8.05 (s, 1 H, NHCO).

¹³C-NMR (CDCl₃): δ = 13.8 (CH₂CH₃), 25.0, 26.0 [C(CH₃).], 26.7 [C(O)CH₃], 63.5(CH₂CH₃), 65.6 (C-5), 71.4 (C-3, J _C-_F = 20.0, 22.0 Hz), 72.3 (C-4), 110.2 [C(CH₃)₂], 112.9 (C-2, J c-F = 250, 250 Hz)₅ 118.3, 123.3, 123.9, 129.5, 137.8, 138.2 (C a_101n ), 151.6 (OCONHAr), 162.2 (C-I, J _C-F = 30,30 HZ), 198.4 [C(O)CH₃].

¹⁹F NMR indicated that mainly the one fluorine containing product was present

EXAMPLE 4

[0069] This example demonstrates the preparation of ethyl (D-erythro)-3~ (phenylcarbamoyloxy)-2,2-difluoro-3-(2,2-dimethyldioxolan-4-yl)propionate (9H). [0070] A solution of ethyl (a 2.6: 1 mixture of Ω~erythro, D-//?reo)-3-(hydroxy)-2,2- difluoro-3-(2,2-dimethyldioxalane-4-yl)propionate (3) (10.0 g) in toluene (100 ml) was heated under reflux with Dean-Stark trap for 2 hours to dry the mixture. The mixture was cooled under nitrogen to ambient temperature, and 4-(dimethyl- amino)pyridine (0.1 g) and phenyl isocyanate (4.59 g) were then added. The reaction mixture was heated at 80° C for 6 hours, cooled to ambient temperature, and the crystals of 1, 3-diphenylurea were collected by filtration. The filtrate was washed with water, 1 N HCl, and again with water (50 ml each), and dried over MgSO₄, and treated with activated carbon. The solvent was removed under reduced pressure to give 11.2 g of crude ethyl (O-erythro, D-?Areo)-3- (phenylcarbamoyloxy)-2_s2-difluoro-3-(2,2-diraelhyldioxolaii-4-yl)propionate as a oil in 95.3%) yield. The crude product was dissolved under heating in toluene (7 ml) and the solution was cooled at -20° C for 5 days. The crystals were collected by filtration, washed with a solution of pre-cooled toluene-n-hexane (1 :1, v/v) mixture and dried at 50° C overnight to yield 3.0 g of ethyl (D-eryt/jro)-3-(phenylcarbamoyloxy)~2,2-difluoro-3-(2,2- dimethyldioxolan-4-yl)propionate (9H) in 25.5% yield; mp 75-77° C. ¹HNMR (CDCl₃): δ - 133 (t, 3 H, CH₂CH₃), 1.34, 1.38 [d, 6 H₅ C(CH₃)J, 4.08 (m, 2 H, CH₂) ,4.35 (m, 2 H, CH₂CH₃ and 1 H, H-4), 5.58 (m, 1 H, H-3), 7.09 (t), 7.31 (t), 7.41 (d)

(5 H arom), 7.40 (s, 1 H, NHCO).

¹³C NMR (CDCI₃): δ = 13.9 (CH₂CH₃), 25.1, 26.1 [C(CHa)₂], 63.5 (CH₂CH₃), 65.7

(C-5), 71.5 (C-3, J c-F = 22-O₅ 25.0 Hz), 72.4 (C-4), 110.2 [C(CHs)₂], 112.9 (C-2, J _C-_F = 248, 248 Hz), 118.8, 124.2, 129.2, 137.2 (C _arom), 151.4 (OCONHPh), 162.2 (C-I₅ J _C~_F = 31, 31 Hz). ¹⁹F NMR indicated that mainly the one fluorine containing product was present. ESI (negative)MS: m/z = 372.1 [M-H⁺].

EXAMPLE 5

[0071] This example demonstrates the preparation of ethyl (D-erythro)-3-(hydroxy)- 2,2-difluoro-3-(2,2~dimethyldioxolan-4-yl)propionate (3B).

[0072] A mixture of toluene (120 ml) and cupric acetate monohydrate (0.17 g) was heated under reflux with Dean-Stark trap for 2.5 hours to dry the mixture. Ethyl (D- ery^ro)-3-(4-chlorophenylcarbarnoyloxy)-2,2~difluoro-3-(2,2-dimethyl- dioxolan-4~ yl)propionate (9E) (10.0 g) was added to this mixture at 80° C. The reaction mixture was then heated under reflux and moηpholine (2.8 ml) was added drop- wise during 3 hours. The reflux was continued for 3 hours, and the mixture was cooled to ambient temperature and kept at 5° C overnight. The crystals of 1,1-diethyloxy 3-(4-chlorophenyl)urea were collected by filtration and washed with toluene. The filtrate was washed with water (3 x 30 ml) and dried over MgSO₄. The solvent was removed under reduced pressure to dryness to give an oil. The oil was dissolved in tert-butyl methyl ether (15 ml) and the solution was kept at 5° C overnight An additional amount of the by-product 1,1- diethyloxy 3-(4-chloroρhenyl)urea was collected by filtration, and the solvent was removed from the filtrate to dryness to yield 6.0 g of φ-erythrό)-3 -(hydroxy )-2, 2- difluoro-3 -(2, 2-dimethyldioxolan-4-yl propionate (3B) as oil (97.6%). GC analysis showed a purity of the compound: 81.7%.

¹HNMR (CDCl₃): δ - 1.36 (t₅ 3 H, CH₂CH₂), 1.34, 1.39 [d, 6 H₃ C(CH₃J₂], 3.57 (bs, 1 H) _, 4.04-4.40 (series of m, 6 H). ¹³C-NMR (CDCI₃): δ = 13.8 (CH₂CH₃) 25.0, 26.0 [C(CJHb)₂], 63.1(CH₂CH₃), 65.8 (C-S)₅ 71.7 (C-3, J c-F = 20.0, 22.0 Hz), 73.5 (C-4), 109.8 [C(CH₃)₂], 1 14.1 (C-2, J _C-F = 250, 250 Hz), 163.3 (C-I, J _C-_F - 30,30 Hz).

EXAMPLE 6

[0073] This example demonstrates the preparation of 3-(4-nitrophenylcarbamoyloxy)- 2-deoxy-2,2-difluoro-D-erythro-pentofuranose-l -ulose ( 1 OB).

[0074] A mixture of (D-erythro)-3 -(4-nitrophenylcarbamoyloxy)-2,2-difluoro-3 -(2,2- dimethyldioxolan-4-yl)propionate (9E) (1.4 g), acetonitrile (30 ml), trifluoroacetic acid (0.14 ml) and water (0.64 ml) was heated under reflux for 5.5 hours. The reaction mixture was then concentrated and the acetonitrile was replaced with toluene. The reaction mixture was dried by azeotropic distillation using a Dean- Stark water separator until the pot temperature reached 95-100° C. The solvent was evaporated in vacuum to give 1.05 g of solid 3-(4-nitrophenylcarbamoyloxy)-2~deoxy-2,2-difluoro-D-ery/ⁱ/zr(>-ρentofuranose- 1 - ulose (10B) in 95.5% yield. The crude product was recrystallized from ethyl acetate to yield the colorless crystals; [α]_D ²⁵ + 22.8 ° (c 1, acetonitrile), mp. 155-156.5 ° C.

¹U NMR (DMSCMø: δ = 3.87 (m, 2 H), 4.89 (m, 1 H), 5.59 (s_s 1 H)₅ 5.70 (m, 1 H) 7.74, 7.77 (d, 2 H arom), 8.24, 8.27 (d, 2 H _arom), 10.97 (s, 1 H, NHCO). ¹³C NMR (DMSO-J₆): δ = 59.6 (C-5), 69.3 (C-3, J _OF = 21.8, 25.8 Hz)₅ 81.6 (C-4, J_C._F- 7.0 Hz), 112.2 (C-2, J _OF = 252, 252 Hz), 1 18.3, 125.1, 142.4, 144.6 (C _mom), 151.4 (OCONHAr), 163.6 (C-I. J c_-F = 31, 31 Hz).

EXAMPLE 7

[0075] This example demonstrates the preparation of 3-(4- chlorophenylcarbamoyloxy)-2-deoxy-2,2-difiuoro-D-erythro-pentofaranose-l -ulose (10C).

[0076] A mixture of (D-ery/^ήro)-3-(4-chlorophenylcarbamoyloxy)-2_J2-difluoro-3- (2,2-dimethyldioxolan-4-yl)propionate (9F) (4.0 g), acetonitrile (40 ml), trifluoroacetic acid (0.21 ml) and water (1.0 ml) was heated under reflux for 7 hours. The reaction mixture was then concentrated and the acetonitrile was replaced with toluene. The reaction mixture was dried by azeotropic distillation using a Dean-Stark water separator until the pot temperature reached 95-100° C. The solvent was evaporated in vacuo to give 3.1 g of solid 3-(4-chlorophenylcarbamoyloxy)-2-deoxy-2,2-difluoro-D-erμ/Λrθ" pentofuranose-1-ulose (10C) in 98.4% yield. The crude product was re-crystallized from toluene to yield the colorless crystals; [α]_D ²⁵ +33.4° (c 1, acetonitrile), mp 129-131° C.

¹H NMR (DMSO-J₆): δ = 3.86 (s, 2 H), 4.89 (m, 1 H), 5.57 (s, 1 H), 5.68 (m, 1 H) 7.38, 7.41 (d, 2 H arom ), 7.55, 7.57 (d, 2 H ^m), 10.42 (s, 1 H, NHCO). ¹³C NMR (DMSO-J₆): δ = 59.6 (C-5), 68.9 (C-3, J _C-F = 21.8, 25.8 Hz), 81.6 (C-4, J_C-F = 7.0 Hz), 112.3 (C-2, J c-F = 251, 251 Hz), 120.2, 127.2, 128.9, 137.3 (C ^_01n ), 151.5 (OCONHAr), 163.8 (C-I₅ J

C-F = 31,31 HZ).

EXAMPLE 8

[0077] This example demonstrates the preparation of 3 -(phenyl carbamoyloxy)-2- deoxy-2_?2-difiuoro-D-erythro-pentofuranose-l -ulose (1 OD). [0078] A mixture of (D-erj^o)-3-φhenylcarbamoyloxy)-2,2-difluoro-3-(2,2- dimethyldioxolan-4-yl)propionate (9H) (2.9 g), acetonitrile (30 ml), trifiuoroacetic acid (0.015 ml) and water (1.5 ml) was heated under reflux for 5.5 hours. The reaction mixture was then concentrated and the acetonitrile was replaced with toluene. The reaction mixture was dried by azeotropic distillation using a Dean- Stark water separator until the pot temperature reached 95-100° C. The solvent was evaporated in vacuum to give 2.2 g of solid 3-(phenylcarbamoyloxy)-2-deoxy-2,2-difluoro-D-ery//zro-pentofuranose-l -ulose (10D) in 98.2% yield. The crude product was re-crystallized from toluene to yield the colorless crystals; [α]_D ²⁵ + 36.1 ° (c 1, acetonitrile), mp 119-121° C.

¹H NMR (DMSO-J₆): δ - 3.82 (s, 2 H), 4.86 (m, 1 H), 5.54 (s, 1 H)₅ 5.65 (m, 1 H) 7.06 (/, 1 H aromX 7.33 (r, 2 H _arom), 7.51 (d, 2 H aro_m), 10.24 (s, 1 H, NHCO). ¹³C NMR (DMSO- J₆): δ = 59.6 (C-5), 68.7 (C-3, J _C-_F = 21.8, 25.8 Hz), 81.6 (C-4, J_C-_F = 7.0 Hz), 112.3 (C- 2, J c-F = 251, 251 Hz), 118.6, 123.3, 128.9, 138.2 (C ^_01n ), 151.5 (OCONHAr), 163.8 (C-

U C-_F = 31,31 HZ).

Claims

CLAIMS:

1. A compound having the general fomiula 1OA:

1OA wherein X is O or S, and R is an unsaturated or saturated Ci-C₈ alkyl, an unsubstituted or substituted aryl, an unsaturated or saturated C₁-Cs alkylsulfonyl, or an unsubstituted or substituted arylsulfonyl.

2. A process for preparing the compound of claim 1 , the process comprising: a) providing a solution of a D-erythro diastereoϊsomer of ethyl (3R)-3- N-(carbamoyIoxy)-2,2-diiluoro-3-(2,2-dimethyldioxolan-4-yl)propionate of the general formula 9C

9C in a solvent mixture containing an acid and heating for a sufficient time period to allow substantial completion of the reaction; b) reducing the solution volume by distillation; c) adding toluene and drying the mixture by azeotropic distillation; and d) further distilling off the solvent mixture, to obtain a solid.

3. The process of claim 2, wherein the solvent mixture containing an acid is a mixture of acetonitrile, water and trifluoroacetic acid.

4. The process of claim 3, wherein the aelonitrile:water:trifluoroacetic acid ratio is selected from the group consisting of 30:0.64:0.14 v/v/v, 40:1.0:0.21 v/v/v, and 30:1.5:0.15 v/v/v.

5. The process of claim 2, wherein the azeotropic distillation of water is carried out at reflux for about 5.5-7 hours.

6. The process of claim 2, wherein the 3-(N-carbamoyloxy)-2-deoxy-2,2- difluoro-D-ery/βro-pentofuranose-1-ulose having the general formula 1OA is obtained in a yield of at least about 98.4%.

7. A process for producing the compound of the formulalOA of claim 1 comprising cyclizing a compound of the formula:

wherein X is O or S, AIk is

alkyl, and R is an unsaturated or saturated C₁-Cg alkyl, an unsubstituted or substituted aryl, an unsaturated or saturated C₁-Cg alkylsulfonyl, or an unsubstituted or substituted arylsulfonyl.

8. The process of claim 7, comprising separating the O-erythro isomer from a compound of the formula:

and cyclizing the O-erythro isomer to obtain the compound of formula 1OA, wherein X, AIk and R are as defined in claim 7.

9. The process of claim 8, comprising reacting a compound of the formula:

with a compound of the formula R — N=C=X, to produce a compound of the formula:

separating the D-erythro isomer, and cyclizing the D-erythro isomer to produce the compound of formula 1OA, wherein X, AIk and R are as defined in claim 8.

10. The process of claim 9, comprising reacting a compound of the formula:

with a compound of the formula R — N=C=X, and cyclizing to produce the compound of formula 1OA, wherein X, AIk and R are as defined in claim 9.

11. A compound of the formula:

9B wherein X is O or S, and R is an unsaturated or saturated CrCg alkyl, an unsubstituted or substituted aryl, an unsaturated or saturated Ci-Cg alkylsulfonyl, or an unsubstituted or substituted arylsulfonyl.

12. The process of claim 9 comprising: a) preparing a mixture of D-erythro and D-threo diastereomers of ethyl 2,2-difluoro-3-hydroxy-3-(2,2-dimethyldioxolan-4-yl)proρionate of formula 3 in toluene; b) optionally heating the mixture with azeotropic distillation of water to dry the toluene; c) reacting the mixture of O-erythro and Ω~threo diastereomers of ethyl 2,2-difluoro-3-hydroxy-3-(2_!,2-dimethyldioxolan-4-yl)propionate of formula 3 in toluene with an isocyanate or isothiocyanate of the formula RNCX, wherein X and Rare as defined in claim 9, in the presence of a base to obtain a mixture of D-erythro and D- threo ethyl 3-(N-carbamoyloxy)-2,2-difluoro-3-(2,2-dimethyldioxolan-4-yl)propionate.

13. The process of claim 12, wherein the isocyanate or isothiocyanate is 2- chloroethyl isothiocyanate, 5-chloro-2-methylphenyl isothiocyanate, 2-chloro-4- nitrophenyl isothiocyanate, 2-chlorophenyl isothiocyanate, 3-chlorophenyl isothiocyanate, 4-chlorophenyl isothiocyanate, 3-acetylphenyl isothiocyanate, 4-acetylphenyl isothiocyanate, 2-(chloromelhyl)phenyl isocyanate, 2-chioro-5-methyl-phenyl isocyanate, 2-chloro-6-methylphenyl isocyanate, 3-chloro-2-methylphenyl isocyanate, 3-chloro-4- methylphenyl isocyanate, 4-(chloromethyl)-phenyl isocyanate, 4-chloro-2-methylphenyl isocyanate, 5-chloro-2-methylphenyl isocyanate, 2-chloro-4-nitrophenyl isocyanate, 2- chloro-5-nitrophenyl isocyanate, 4-chloro-2-nitrophenyl isocyanate, 4-chloro-3- nitrophenyl isocyanate, 2-chloro-2-nitrophenyl isocyanate, 2-chlorophenyl isocyanate, 3- chlorophenyl isocyanate, 4-chlorophenyl isocyanate, 3-acetylphenyl isocyanate, phenyl isocyanate, N-benzenesulfonyl isocyanate, p-toluenesulfonyl isocyanate, or o- toluenesulfonyl isocyanate.

14. The process of claim 13, wherein the isocyanate or isothiocyanate is 4- nitrophenyl isocyanate, 4-chlorophenyl isocyanate, 3-acetylphenyl isocyanate, or phenyl isocyanate.

15. The process of claim 12, wherein the base is selected from triethyl amine, one or more lutidines, morpholine, diisopropylethylamine, pyridine, 2-(dimethylamino)~ pyridine, 4-(dimethylamino)pyridine, and combinations thereof.

16. The process of claim 15, wherein the base is 4-(dimethylamino)-pyridine.

17. The process of claim 12, wherein the crude mixture of O-erythro and D- threo ethyl 3-(N-arylcarbamoyl)-2_f2-difluoro-3-(2₅2-dimethyldioxolan-4-yl)propionate is obtained in a crude yield of at least about 97.4%.

18. A compound of the formula :

9C wherein X is O or S, and R is an unsaturated or saturated Cj-Cg alkyl, an unsubstituted or substituted aryl, an unsaturated or saturated CrCg alkylsulfonyl, or an unsubstituted or substituted arylsulfonyl.

19. A process for preparing the D-eryihro diastereomer of ethyl 3-(N- carbamoyloxy)-2,2-difluoro-3-(2,2-dimethyldioxolan-4-yl)propionate of the general formula 9C, the method comprising: a) dissolving a mixture of O-erythro and D-lhreo ethyl 3-(N- carbamoyloxy)-2,2-difluoro-3-(2,2-dimethyldioxolan-4-yl)propionate of the general formula 9B in an organic solvent, optionally at elevated temperature; b) allowing the solution to cool sufficiently to a temperature in the range of about -20° C to about 10° C, to produce crystals; and c) collecting the crystals by filtration, washing the crystals with an organic solvent and drying, optionally at elevated temperature.

20. The process of claim 19, wherein the crystallization solvent is dichloromethane, chloroform, ethyl acetate, 1 -propyl acetate, 2-propyl acetate, butyl acetate, tert-bulyl acetate, o-xylene, m-xylene, o-dichlorobenzene, toluene, or a mixture thereof.

21. The process of claim 20, wherein the crystallization solvent is toluene.

22. The process of claim 19, wherein the organic solvent used for washing the obtained crystals is toluene, pentane, hexane, heptane, octane, petroleum ether, cyclohexane, or a mixture thereof.

23. The process of claim 22, wherein the organic solvent used for washing the crystals is toluene or a mixture of toluene and hexane.

24. The process of claim 19, wherein the D-erythro diastereomer ethyl (3R)-3- (carbamoyloxy)-2_s2-difluoro-3-(2,2-dimethyldioxolan-4-yl)propionate is isolated in a purity of at least about 98.5%.

25. A process for preparing ethyl (D~er>>//zro)-3-hydroxy~2,2-difluoro-3~(2_s2- dimethyldioxolan~4-yl)ρropionate, having the formula 3C:

3C from a starting material of the general formula 9C, the process comprising: a) heating a mixture of toluene and a catalyst with azeotropic distillation of water to dry the toluene; b) adding a D-erythro isomer of ethyl 3-(N-carbamoyloxy)-2,2- difluoro-3-(2,2-dimethyldioxolan-4-yl)propionate, which is ethyl (D-erythro)-3-(4- nitrophenylcarbamoyl)-3-(2,2-dimethyldioxolan-4-yl)propionate of formula 9E, ethyl (D- erμώro)-3-(4-chlorophenylcarbamoyl)-2,2-difluoro-3-(2,2-dimethyldioxolan-4- yl)propionate of formula 9F, ethyl (D-er_iy^ro)-3-(3-acetylphenylcarbanioyl)-2,2-difluoro- 3-(2,2-dimethyldioxolan-4~yl)ρropionate of formula 9G, or ethyϊ (O-erythro)-3- (phenylcarbamoyl)-2,2-difluoro-3-(2,2-dimetliyl-dioxolan-4-yl)propionate of formula 9H, and refluxing the mixture; c) adding a base selected from one or more primary and secondary amines, and maintaining the reflux until the reaction is substantially complete; d) separating the crystals of the by-product by filtration; e) washing the filtrate and evaporating the solvent, optionally under reduced pressure to obtain an oil; and f) dissolving the oil in an organic solvent and cooling, discarding the thus formed by-product crystals and isolating the product.

26. The process of claim 25, wherein the said catalyst is cupric acetate, stannous chloride, stannous oxalate, aluminium alkoxide or a combination thereof.

27. The process of claim 26, wherein the catalyst is cupric acetate.

28. The process of claim 25, wherein the base is selected from n-butyl amine, n-hexylamine, dipropylamine, dibutylamine, dicyclohexylamine, morpholine, piperidine, 2,6-dimethylpiperidine, and combinations thereof.

29. The process of claim 28, wherein the base is morpholine.

30. The process of claim 25, wherein the solvent used for dissolving the obtained oil is dichloromethane, chloroform, ethyl acetate, 1 -propyl acetate, 2-propyl acetate, butyl acetate, tert-butyl acetate, diethyl ether, diisopropyl ether, tert-butyl methyl ether, or a mixture thereof.

31. The process of claim 30, wherein the solvent is tert-butyl methyl ether.

32. The process of claim 2, further comprising: a) reacting the lactone 1 OA with an acid chloride or an acid bromide to obtain the 3 -carbamoyl lactone intermediate having the formula 17, wherein X is O or S and R is an unsaturated or saturated C]-Cg alkyl, or an unsubstituted or substituted phenyl; b) reducing the 3-carbamoyl lactone intermediate of the formula 17 with a reducing agent in an organic solvent to obtain a lactol intermediate; c) reacting the lactol intermediate with methanesulfonyl chloride in the presence of a base to obtain the sulfonate intermediate of the formula 16 A; d) coupling the compound 16A with bis(trimethylsiIyJ)-N- acetylcytosine, in the presence of a catalyst in an organic solvent, to obtain a mixture of α and β anomers of a 3₅5-diprotected-N-l-trimethylsilylacetyl-2'-deoxy-2'₅2^!- difluorocytidine; e) precipitating the β isomer of the 3,5-diprotected -N-I- trimethylsilylacetyl-2'-deoxy~2'_s2'-difluorocytidine, and separating the isomers; and f) removing the protecting groups to obtain gemcitabine.