WO2007027564A2

WO2007027564A2 - Process for preparing gemcitabine and associated intermediates

Info

Publication number: WO2007027564A2
Application number: PCT/US2006/033431
Authority: WO
Inventors: Jingshan Shen; Yafei Li; Joseph Kaspi
Original assignee: Chemagis Ltd.; Perrigo Company
Priority date: 2005-08-29
Filing date: 2006-08-28
Publication date: 2007-03-08
Also published as: WO2007027564A3; CN101296934A; KR20080039502A; EP1931693A2; JP2009506118A

Abstract

Provided is a process for preparing gemcitabine or a salt thereof, which preferably includes removing at least a substantial portion of the α anomer of a N-1-protected-2'-deoxy-2',2'-difluoro-cytidine-3',5'-diester from an anomeric mixture thereof; removing the 3'-ester, the 5'-ester and the N-protecting group; and optionally forming a salt. The 3'-ester and 5'-ester can be the same or different and at least one of the esters preferably is cinnamoyl, benzoyl, 1-naphthoyl, 1-naphthylmethylcarbonyl, 2-methylbenzylcarbonyl, 4-methylbenzylcarbonyl or 9-fluorenylmethyloxycarbonyl ester. Also provided are novel intermediates, including but not limited to 2-deoxy-2,2-difluoro-D-erythro-pentofuranos-1-ulose-3,5-diesters, and methods of producing such intermediates.

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 T- deoxy-2',2'-difluorocytidine (β isomer). Gemcitabine HCl has the following structure:

[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] D-glyceraldehyde ketal 2 is reacted with bromodifluoroacetic acid ethyl ester

(BrCFaCOOEt) in the presence of activated zinc, to obtain ethyl 2,2-difluoro-3-hydroxy-3-

(2,2-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- lactone 4. The hydroxy groups of the lactone are protected with fert-butyldimethylsilyl

(TBDMS) protecting groups to obtain the protected lactone 3,5-bis-(tert- butyldimethylsilyloxy)-2-desoxy-2,2-difluoro-l-oxoribose 5, and the product is reduced to obtain 3,5-bis-(tert-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-(ter?-butyldimethylsilyloxy)-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 -dioxolanyl-2,2-difluoro-3 -hydroxy-propionate with an acidic ion exchange resin. See also, Hertel et al. in J Org. Chem. 53, No. 11, 2406 (1988). [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:

11, 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

10 11 11

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, CN₃ or N₃), in presence of a tertiary amine or a catalyst such as 4-(dimethylamino)pyridine 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-4,5-dihydroxypentanoate 9A. 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 andthreo isomers. The '374 patent describes isolating at least a portion of the erytliro 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 methanesulfonyl chloride to obtain an anomeric mixture of mesylates, 2-deoxy-2,2-difluoro-D-ribofuranosyl-3,5-di-O- benzoyl-1-O-β-methanesulfonate 13, and coupled with N,O-bis(trimethylsilyl)-cytosine 8 to obtain the dibenzoate ester of the silyl-protected nucleoside 14 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 filtration.

[0013] Processes for separating anomeric mixtures of alkylsulfonate intermediates also have been 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-alkylsulfonates, and U.S. Patent No. 5,256,798 describes a process for obtaining α-anomer-enriched ribofuranosyl sulfonates.

[0014] 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- diiluororibosyl azide and corresponding amine intermediates that can be prepared, e.g., by reacting a 2-deoxy-2,2-difluoro-D-ribofuranosyl-3,5-di-O-benzoyl-l-O-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.

[0015] Other known intermediates include, e.g., l-alkylsulfonyl-2,2-difluoro-3- carbamoyl ribose and related nucleoside intermediates (U.S. Patent No. 5,521,294), tritylated intermediates (U.S. Patent No. 5,559,222), 2-deoxy-2,2-diflubro-β-D-ribo-pentopyranose (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).

[0016] Other processes for preparing gemcitabine have been described e.g., in WO

2006/070985 and WO 2006/071090, for stereoselective preparation of 2'-deoxy-2', T- difluorocytidine, and are generally outlined in Scheme 3 below.

Scheme 3

12A P1 is an ester protecting group

gemcitabine

The process comprises activating the 2'-deoxy-2',2'-difiuoro-D-ribofuranosyl-3,5-diester 12A with diphenyl chlorophosphate followed by isomer separation, then obtaining the corresponding 1-bromo-ribofuranose intermediate, which is coupled withN,O-bis- (trimethylsilyl)-cytosine 8, then deprotected to obtain gemcitabine. However, this process requires an additional step as compared to the process outlined in Scheme 2, which makes it less attractive for industrial implementation. [0017] There are inherent problems associated with the production of gemcitabine, particularly for processes that require the production and separation of isomers, which tend to afford 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 process for preparing gemcitabine or a salt thereof, which preferably includes: removing at least a substantial portion of the α anomer from an anomeric mixture of a N-l-protected-2'-deoxy-2',2'-difluorocytidine-3',5'-diester to produce a product, which is at least enriched in the β anomer; removing the 3 '-ester, the 5 '-ester and the N-protecting group; and optionally converting the product into a salt.

[0019] The 3'-ester and 5'-ester can be the same or different and at least one of the esters preferably is cinnamoyl, benzoyl, 1-naphthoyl, 1-naphthylmethylcarbonyl, 2- methylbenzylcarbonyl, 4-methylbenzylcarbonyl or 9-fluorenylmethyloxycarbonyl ester. [0020] The N-l-protected-2'-deoxy-2',2'-difluoro-cytidine-3',5'-diester may be prepared by any suitable process. Preferably, the N-l-protected-2'-deoxy-2',2'-difluoro-cytidine-3',5'- diester is prepared by: reducing a 2-deoxy-2,2-difluoro-D-erythro-pentofuranos-l-ulose-3,5-diester, to produce a 2-deoxy-2,2-difluoro-D-ribofuranos-3,5-diester; reacting the 2-deoxy-2,2-difluoro-D-ribofiiranos-3,5-diester with methanesulfonyl chloride in the presence of a base to afford a 2-deoxy-2,2-difluoro-D-ribofuranos-3,5-diester- 1-methanesulfonate; and coupling a N-1-protected-cytosine to the 2-deoxy-2,2-difiuoro-D-ribofuranos-3,5- diester-1-methanesulfonate to produce the N-l-protected-2'-deoxy-2',2'-difluoro-cytidine, 3',5'-diester.

[0021] The 3-ester and 5-ester are the same or different and at least one of the esters preferably is cinnamoyl, benzoyl, 1-naphthoyl, 1-naphthylmethylcarbonyl, 2- methylbenzylcarbonyl, 4-methylbenzylcarbonyl or 9-fluorenylmethyloxycarbonyl ester. [0022] The 2-deoxy-2,2-difluoro-D-erythro-pentofuranos- 1 -ulose-3 ,5-diester can be obtained by any suitable process. Preferably, the 2-deoxy-2,2-difluoro-D-erythro- pentofuranos-1 -ulose-3, 5-diester is obtained by separating the product from a mixture of erythro (3-R) and threo (3-S) isomers of a 2-deoxy-2,2-difiuoropentofuranos-l-ulose-3,5- diester. The 3 -ester and 5 -ester are the same or different and at least one of the esters preferably is cinnamoyl, benzoyl, 1-naphthoyl, 1-naphthylmethylcarbonyl, 2- methylbenzylcarbonyl, 4-methylbenzylcarbonyl or 9-fluorenyrmethyloxycarbonyl ester. [0023] The protected 2-deoxy-2,2-difluoro-D-erythro-pentofuranos- 1 -ulose-3 ,5 -diesters of the present invention are particularly useful for the commercial scale production of gemcitabine. These D-erythro derivatives are crystalline materials, and can be purified by separation from a mixture of the D-erythro and L-threo isomers, e.g., by precipitation. The intermediates and methods of the present invention facilitate the total synthesis of gemcitabine, and are easier to use and produce higher yields than processes reported in the literature.

DETAILED DESCRIPTION OF THE INVENTION

[0024] The present invention provides a process for preparing gemcitabine or a salt thereof, which preferably includes: removing at least a substantial portion of the α anomer from an anomeric mixture of a N-l-protected-2'-deoxy-2^l,2'-difluorocytidine-3',5'-diester to produce a product, which is at least enriched in the β anomer thereof; removing the 3 '-ester, the 5'-ester and the N-protecting group; and optionally converting the product into a salt.

[0025] The 3 '-ester and 5 '-ester can be the same or different and at least one of the esters preferably is cinnamoyl, benzoyl, 1-naphthoyl, 1-naphthylmethylcarbonyl, 2- methylbenzylcarbonyl, 4-methylbenzylcarbonyl or 9-fluorenylmethyloxycarbonyl ester. [0026] The anomeric mixture of the N-l-protected-2'-deoxy-2',2'-difluoro-cytidine-3',5'- diester can be prepared by any suitable process. Preferably, the anomeric mixture of the N-I- protected-2'-deoxy-2',2'-difluoro-cytidine-3'₅5'-diester is prepared by: reducing a 2-deoxy-2,2-difluoro-D-erythro-pentofuranos-l -ulose-3, 5-diester, to produce an anomeric mixture of a 2-deoxy-2,2-difluoro-D-ribofuranos-3, 5-diester; reacting the 2-deoxy-2,2-difluoro-D-ribofuranos-3,5-diester with methanesulfonyl chloride in the presence of a base to afford a 2-deoxy-2,2-difluoro-D-ribofuranos-3,5-diester- 1-methanesulfonate; and coupling aN-1-protected-cytosine to the 2-deoxy-2,2-difluoro-D-ribofuranos-3,5- diesterl-methanesulfonate to produce an anomeric mixture of the N-l-protected-2'-deoxy- 2',2'-difluoro-cytidine, 3',5'-diester.

[0027] The 3 -ester and 5-ester can be the same or different and at least one of the esters preferably is cinnamoyl, benzoyl, 1-naphthoyl, 1-naphthylmethylcarbonyl, 2- methylbenzylcarbonyl, 4-methylbenzylcarbonyl or 9-fluorenylmethyloxycarbonyl ester. [0028] . The 2-deoxy-2,2-difluoro-D-erythro-pentofuranos-l-ulose-3,5-diesters of the present invention can be obtained by any suitable process. Preferably, the 2-deoxy-2,2- difluoro-D-erythro-pentofuranos-l-ulose-3,5-diester is obtained by separating the product from a mixture of erythro (3-R) and threo (3-S) isomers of a 2-deoxy-2,2- difluoropentofuranos-l-ulose-3,5-diester. The 3-ester and 5-ester can be the same or different and at least one of the esters preferably is cinnamoyl, benzoyl, 1-naphthoyl, 1- naphthylmethylcarbonyl, 2-methylbenzylcarbonyl, 4-methylbenzylcarbonyl or 9- fluorenylmethyloxycarbonyl ester.

[0029] The mixture of erythro (3 -R) and threo (3-S) isomers of a 2-deoxy-2,2- difluoropentofuranos-l-ulose-3,5-diester of the present invention can be obtained by any suitable process. In one embodiment, the mixture of erythro (3-R) and threo (3-S) isomers of 2-deoxy-2,2-difluoropentofuranos-l-ulose-3,5-diester is prepared by esterifying a mixture of erythro (3-R) and threo (3-S) isomers of 2-deoxy-2,2-difluoropentofuranos-l-ulose. When the 3-ester and 5-ester are the same, the 3-ester and 5-ester preferably are cinnamoyl, 1- naphthoyl or 1-naphthylmethylcarbonyl. The mixture of erythro (3-R) and threo (3-S) isomers of 2-deoxy-2,2-difluoropentofuranos-l-ulose can be prepared by any suitable process such as, for example, cyclizing a mixture of 3-R and 3-S isomers of an alkyl 2,2-difiuoro-3- hydroxy-3-(2,2-dimethyldioxolan-4-yl)-propionate. Suitable alkyl 2,2-difluoro-3-hydroxy-3- (2,2-dimethyldioxolan-4-yl)-propionates intermediate can include, for example, C_1-6 alkyl- 2,2-difluoro-3-hydroxy-3-(2,2-dimethyldioxolan-4-yl)-propionates such as, e.g., ethyl-2,2- difluoro-3-hydroxy-3-(2,2-dimethyldioxolan-4-yl)-propionate. [0030] In another embodiment, the mixture of erythro (3 -R) and threo (3 -S) isomers of 2- deoxy-2,2-difluoropentofuranos-l-ulose-3,5-diester of the present invention is prepared by esterifying a mixture of erythro (3 -R) and threo (3 -S) isomers of 2-deoxy-2,2- difiuoropentofuranos-l-ulose-3-ester. The mixture of erythro (3-R) and threo (3-S) isomers of the 2-deoxy-2,2-difluoropentofuranos-l-ulose-3 -ester can be prepared by any suitable process such as, for example, esterifying a mixture of 3-R and 3-S isomers of an alkyl-2,2- difluoro-3-hydroxy-3-(2,2-dimethyldioxolan-4-yl)-propionate, and cyclizing the product. The cyclization reaction can be carried out using any suitable conditions, e.g., by treating the mixture of 3-R and 3-S isomers of an alkyl-2₅2-difluoro-3-hydroxy-3-(2,2-dimethyldioxolan- 4-yl)-propionate with a suitable acid. In this embodiment, when the 3 -ester and 5 -ester are different, either the 3 -ester or the 5-ester preferably is cinnamoyl, benzoyl, or 1- naphthylmethylcarbonyl. Suitable alkyl 2,2-difluoro-3-hydroxy-3-(2,2-dimethyldioxolan-4- yl)-propionates intermediate can include, for example, Cμ₆ alkyl-2,2-difluoro-3-hydroxy-3- (2,2-dimethyl-dioxolan-4-yl)-propionates such as, e.g., ethyl-2,2-difluoro-3-hydroxy-3-(2,2- dimethyl-dioxolan-4-yl)-propionate.

[0031] Surprisingly, in accordance with the present invention, certain ester groups, such as, e.g., cinnamoyl and l-naphthylmethylcarbonyl, not only enable formation of the preferred isomer of the 3,5-ribo diester lactones, but are also stable crystalline materials, which enable the separation of the 3,5-ribo diester lactone isomer from the 3,5-xylo diester lactone isomer by using simple crystallization or precipitation techniques.

[0032] In accordance with the present invention, the ester groups of the ribo protected lactone in the 3- and 5-positions can be the same or different, i.e., the 2-deoxy-2,2-difluoro- D-erythro-pentofuranos-l-ulose-3,5-diester may contain either identical or different ester protecting groups in the 3- and 5-positions.

[0033] A particularly preferred embodiment of the present invention is generally depicted in scheme 4. Scheme 4

mixture of α and β anomers

R and R' are the same or different and at least one of R and R' is selected from phenyl,

2-phenylethenyl (thus foπning a cinnamoyl ester), 1- naphthyl, 1-naphthylmethyl, 2-methylbenzyl and

4-methylbenzyl

gemcitabine

[0034] In accordance with Scheme 4, the α and β anomers of the N-I- trimethylsilylacetyl-2'-deoxy-2',2'-difluorocytidine-3',5'-diester, which is preferably N-I- trimethylsilylacetyl-2'-deoxy-2',2'-difluoro-3',5'-dicinnamoyl-cytidine, can be carried out by selective precipitation of the α-anomer from a solvent mixture, preferably a mixture of 1,2- dichloroethane and methanol. Gemcitabine is obtained by further carrying out the following steps: a) reducing the lactone 15 with a suitable reducing agent in an organic solvent to obtain 2-deoxy-2,2-difluoro-D-riboufuranose-3,5-diester 16 in about 1:1 isomer mixture; b) reacting compound 16 with methanesulfonyl chloride in the presence of a base to obtain 2-deoxy-2,2-difluoro-D-ribofuranose-3,5-diester-l-methanesulfonate 17; c) coupling compound 17 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 ofthe N-l-trimethylsilylacetyl-2'-deoxy-2',2'-difluorocytidine-3,5-diester; d) precipitating the α anomer 18, thus separating the two anomers by filtration; and e) removing the protecting groups by hydrolysis, to obtain gemcitabine.

[0035] The reduction of the lactone 15, e.g., as depicted in Scheme 4, can be carried out using any suitable reducing agent such as, for example, one or more reducing agents selected from lithium aluminium hydride, diisobutyl aluminium hydride, and sodium bis-(2- methoxyethoxy)aluminium hydride, and the like. The reduction, e.g., as illustrated in Scheme 4, is preferably carried out using lithium aluminium hydride, particularly for commercial scale production, because 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 per molecule.

[0036] The coupling reaction, e.g., as depicted in Scheme 4, 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 triflate (Me₃SiOTf).

[0037] Removal of the protecting groups, e.g., as depicted in Scheme 4, can be carried out by using any suitable conditions, which can include, for example, basic hydrolysis, e.g., methanolic ammonia.

[0038] The 2-deoxy-2,2-difluoro-D-erythro-pentofuranos-l-ulose-3,5-diesters of the present invention can be prepared by any suitable method. Schemes 5 and 6 depict two preferred methods, denoted as methods A and B₅ respectively, for obtaining exemplary 2- deoxy-2,2-difluoro-D-erythro-pentofuranos-l-ulose-3,5-diesters of the present invention. [0039] The process depicted in Scheme 5 is particularly useful for preparing 2-deoxy-2,2- difluoro-D-erythro-pentofuranos-l-ulose-3,5-diesters of the present invention, in which 3- ester and 5-ester are the same, e.g., 2-deoxy-2,2-difluoro-D-erythro-pentofuranos-l-ulose- 3,5-dicinnamate, wherein the two isomers are separated by precipitation, to afford the erythro isomer in a purity of 99.9% and ee of at least 99.6%.

Scheme 5 - Method A

R is selected from the group consisting of 2-phenylethenyl, 1-naphthyl, 1-naphthylmethyl, 2-methylbenzyl, 4-methylbenzyl and 9-fluorenylmethoxycarbonyl

21

[0040] In accordance with Scheme 5, the 2-deoxy-2,2-difluoro-D-erythro-pentofuranos- l-ulose-3,5-diesters can be prepared by a process that includes: a) cyclizing an alkyl-2,2-difluoro-3-hydroxy-3-(2,2~dimethyldioxolan-4-yl)- propionate, e.g., 3, to produce 2-deoxy-2,2-difluoropentofuranos-l-ulose 4; b) reacting 2-deoxy-2,2-difluoropentofuranos-l-ulose 4 with the appropriate acid chloride in an organic solvent in presence of a base, to obtain the 2-deoxy-2,2- difiuoropentofuranos-l-ulose-3,5-diester as a mixture of erythro and threo isomers 21; and c) separating the isomers by selective precipitation.

[0041] Preferably, the alkyl-2,2-difluoro-3-hydroxy-3-(2,2-dimethyldioxolan-4~yl)- propionate, e.g., compound 3, is cyclized by refluxing the starting material in a solvent mixture, in presence of an acid, followed by distillation, to produce 2-deoxy-2,2-difluoro-D- erythro-pentofuranos-1-ulose 4. The cyclization can be carried out using an ion exchange resin according to U.S. Patent No. 4,808,614. See also, Scheme 1. [0042] The cyclization, e.g., as depicted in Scheme 5, also can be carried out via intermediate 9A (Scheme 2) by refluxing the alkyl-2,2-difluoro-3-hydroxy-3-(2,2- dimethyldioxolan-4-yl)-propionate, e.g., compound 3, in presence of an acid in a suitable solvent mixture, followed by distillation, as described by Chou et al, Synthesis, 565-570, (1992), although this method adds one additional step to the process. However, the reflux/distillation process can be advantageous from a commercial standpoint in that it is more economical for industrial use. The ion exchange resin route can require a reaction time of up to 4 days, which is not practical for some commercial applications. [0043] An exemplary process for cyclizing the alkyl-2,2-difluoro-3-hydroxy-3-(2.,2- dimethyldioxolan-4-yl)-propionate includes : a) providing a solution of crude ethyl-2,2-difluoro-3-hydroxy-3-(2,2- dimethyldioxolan-4-yl)-propionate in a solvent mixture containing an acid and heating for a time period sufficient to allow substantial completion the reaction; b) reducing the solution volume by distillation; c) adding an organic solvent and further refluxing; d) further distilling off the solvent mixture thus obtaining an oil; e) optionally distilling the obtained oil under reduced pressure, to produce 2-deoxy-2,2-difluoropentofuranos-l-ulose.

[0044] Preferably, the distillation solvent containing an acid is a mixture of acetonitrile, water and trifluoroacetic acid. Preferably, the acetonitrilerwateritrifluoroacetic acid ratio is about 150:12:2.2 v/v/v.

[0045] The 2-deoxy-2,2-difluoropentofuranos-l-ulose can be esterified, e.g., as depicted in Scheme 5, using any suitable esterifying agent, hi one embodiment, 2-deoxy-2,2- difluoropentofuranos-1-ulose is esterified with an acid chloride in an organic solvent in presence of a base, e.g., by a process that includes the steps of: a) providing a solution of 2-deoxy-2,2-difluoropentofuranos-l-ulose in an organic solvent; b) adding a base and an acid chloride dropwise, optionally at ambient temperature; c) refluxing the reaction mixture for sufficient time period to allow substantial completion of the reaction; d) washing the reaction mixture, separating and drying the organic phase; and e) evaporating the solvent to obtain the crude diester.

[0046] In accordance with the present invention, suitable acid chlorides, which are useful for esterifying the hydroxy groups, include one or more acid chlorides selected from cinnamoyl chloride, 1-naphthoyl chloride, 1-naphthyl acetyl chloride, 2-methylphenyacetyl chloride, 4-methylphenyacetyl chloride, and the like.

[0047] In accordance with the present invention, the erythro and threo isomers of the 2- deoxy-2,2-difluoropentofuranos-l-ulose-3,5-diesters (e.g., wherein the ester groups in positions 3 and 5 are the same) can be separated from the crude mixture by a crystallization process. One example of such a crystallization process includes the following steps: a) adding a first organic solvent to the crude mixture and stirring at ambient temperature; b) adding a second organic solvent while maintaining stirring; c) cooling for a time period sufficient to allow precipitation; and d) filtering, to obtain the desired product.

[0048] The first solvent can include, e.g., one or more solvents selected from ethyl acetate, isopropyl acetate, n-butyl acetate, isobutyl acetate, tert-butyl acetate, THF, acetonitrile, and mixtures thereof. The second solvent can include, e.g., one or more solvents selected from pentane, hexane, heptane, octane, and petrol ether, or a mixture thereof. In one embodiment, the first solvent is ethyl acetate and the second solvent is hexane. [0049] The process depicted in Scheme 6 (Method B) is particularly useful for preparing 2-deoxy-2,2-difluoro-D-erythro-pentofuranos-l-ulose-3,5-diesters of the present invention, in which the 3 -ester and the 5 -ester are different.

Scheme 6 - Method B

R and R' are the same or different and at least one of R and R is selected from phenyl, 2-phenylethenyl, 1- naphthyl, 1-naphthylmethyl, 2-methylbenzyl, 4-rnethylbenzyl and 9-fluorenylmethyloxycarbonyl

[0050] In accordance with Scheme 6, 2-deoxy-2,2-difmoro-D-erytlπo-pentofuranos-l- ulose-3,5-diesters (optionally containing non-identical ester groups at positions 3 and 5), can be obtained by a method that includes the steps of: a) esterifying the 3-hydroxyl group of an alkyl-2,2-difluoro~3-hydroxy-3-(2,2- dimethyldioxolan-4-yl)-propionate, e.g., ethyl-2,2-difiuoro-3-hydroxy-3-(2,2- dimethyldioxolan-4-yl)-propionate 3, to produce ethyl-2,2-difluoro~3-acyloxy-3-(2.,2- dimethyldioxolan-4-yl)-propionate 22; b) cyclizing the ethyl 2,2-difluoro-3-acyloxy-3-(2,2-dimethyldioxolan-4-yl)- propionate, e.g., by reflux followed by distillation to obtain the 2-deoxy-2,2- difluoropentofuranos-l-ulose-3 -ester as a mixture of erythro (3-R) and threo (3-S) isomers 23, wherein the major product obtained is the erythro isomer; c) esterifying the 5-hydroxyl group of 23 to obtain the diesters having ester groups in positions 3 and 5, which may be the same or different 24; and d) selectively precipitating and isolating the erythro isomer 24 from the reaction mixture.

[0051] The 2-deoxy-2,2-difluoropentofuranos-l-ulose-3,5-diesters of the present invention (regardless of whether the ester groups at positions 3 and 5 are the same or different) allow for facile separation of the preferred isomer by selective precipitation, thus enabling separation of the two isomers by filtration. In accordance with the present invention, the erythro isomer, e.g., 2-deoxy-2,2-difluoro-D-erythro-pentofuranos-l-ulose-3₅5- dicinamate, can be obtained in at least 99% ee.

[0052] An exemplary method for esterifying the 3-hydroxyl group, e.g., as depicted in Scheme 6, includes: a) dissolving ethyl-2,2-difluoro-3-hydroxy-3 -(2,2-dimethyldioxolan~4-yl)- propionate 3 in an organic solvent; b) adding a base and a suitable acid chloride drop- wise, optionally at ambient temperature; c) refluxing the mixture for sufficient time period to allow completion of the reaction; and d) washing the reaction mixture, separating and drying the organic phase, to obtain ethyl-2,2-difluoro-3-acyloxy-3-(2,2-dimethyldioxolan-4-yl)-propionate as a solid material.

[0053] The esterification of the 3 -hydroxy group can be carried out using any suitable alkyl-2,2-difluoro-3-hydroxy-3-(2,2-dimethyldioxolan-4-yl)-propionate such as those described herein, to produce the corresponding alkyl~2,2-difluoro-3-acyloxy-3-(2,2- dimethyldioxolan-4-yl)-propionate, e.g., alkyl-2,2-difluoro-3-acyloxy-3-(2,2- dimethyldioxolan-4-yl)-propionates wherein the alkyl is Cμβ alkyl (e.g., ethyl) and the 3- acyloxy group is RCO₂-, wherein R is any suitable substituent, including but not limited to 2- phenylethenyl (to form the cinnamoyl ester), phenyl, 1-naphthyl, 1-naphthylmethyl, 2- methylbenzyl, 4-methylbenzyl or the like.

[0054] Cyclization of the ethyl-2,2-difiuoro-3 -acyloxy-3 -(2,2-dimethyldioxolan-4-yl)- propionate, e.g., as depicted in Scheme 6, can be achieved by refluxing followed by distillation. An exemplary cyclization process includes: a) heating a solution of crude ethyl-2,2-difluoro-3-acyloxy-3-(2,2- dimethyldioxolan-4-yl)-propionate in a solvent mixture containing an acid to affect cyclization; b) reducing the solution volume by distillation; c) adding an organic solvent and refluxing; d) distilling off the solvent mixture until the inner temperature of the mixture reaches 90-100°C and maintaining this temperature for about one hour; and e) evaporating the remaining of the solvent mixture, optionally under reduced pressure, to obtain the 2-deoxy-2,2-difmoropentofuranos-l-ulose-3-ester as a mixture of erythro (3-R) and threo (3-S) isomers.

[0055] The reaction solvent containing an acid preferably is a mixture of acetonitrile, water and trifluoroacetic acid. Preferably, the acetonitrile:water:trifluoroacetic acid ratio is about 75:3.7:0.65 v/v/v. The organic solvent added after reducing the solution volume by distillation preferably is toluene. Method B is particularly advantageous in that the product obtained after cyclization. e.g., by refluxing the reaction mixture followed by distillation, surprisingly affords the erythro isomer as the major product, hence, the process is selective for the desired isomer.

[0056] Esterification of the 5-hydroxyl group, e.g., as depicted in Scheme 6, can be carried out using any suitable method, to obtain diesters optionally having non-identical ester groups at positions 3 and 5. In one embodiment, the 5-hydroxyl is esterified by a process that includes the steps of: a) dissolving a 2-deoxy-2,2-difluoropentofuranos-l-ulose-3 -ester 23 in an organic solvent; b) adding a base and a suitable acid chloride dropwise, optionally at ambient temperature; c) refluxing the mixture for sufficient time period to allow substantial completion of the reaction; and d) washing the reaction mixture, separating and drying the organic phase, to obtain 2-deoxy-2₅2-difluoropentofuranos-l-ulose-3,5-diester 24 as a solid material, wherein R and R' can be the same or different.

[0057] Separation of the erythro and threo isomers of the 2-deoxy-2,2- difiuoropentofuranos-l-ulose-3,5-diesters, e.g., as depicted in Scheme 6, can be carried out using any suitable process to obtain, e.g., the 2-deoxy-2,2-difluoro-D-erythro-pentafuranos-l- ulose-3,5-diesters. Preferably, the separation is carried out by a process that includes: a) adding a solvent to the crude mixture and stirring at ambient temperature; b) cooling to allow precipitation; and c) separating the product by filtration. [0058] Exemplary solvents can include, e.g., one or more solvents selected from ethyl acetate, isopropyl acetate, n-butyl acetate, isobutyl acetate, tert-butyl acetate, THF, acetonitrile, pentane, hexane, heptane, octane, petrol ether, and mixtures thereof. Preferably the solvent is a mixture of ethyl acetate and heptane.

[0059] In accordance with the present invention, when the 3 -ester and the 5 -ester are different, then at least one of the esters at the 3 and 5 position (or at least one of the esters at the 3' and 5' position) is cinnamoyl, benzoyl, 1-naphthoyl, 1-naphthylmethylcarbonyl, 2- methylbenzylcarbonyl, 4-methylbenzylcarbonyl or 9-fluorenylmethyloxycarbonyl ester, and the other ester can be any suitable ester, including but not limited to cinnamoyl, naphthoyl, napthylmethylcarbonyl, 2-methylbenzylcarbonyl, 4-methylbenzylcarbonyl or 9- fluorenylmethyloxycarbonyl ester. Other suitable esters can include, e.g., alkyl esters such as, e.g., Ci_-6 alkyl esters (e.g., acetyl) and aromatic esters (e.g., benzoic acid and derivatives thereof). Careful selection of the R substituent should be made when using Method B because in some cases the 9-fluorenylmethyloxycarbonyl, naphthylacetyl and acetyl are hydrolyzed off, producing compound 4, whereas other esters such as, e.g., cinnamoyl, are retained producing compound 23.

[0060] The following examples further illustrate the invention but, of course, should not be construed as in any way limiting its scope.

EXAMPLE 1

[0061] This example demonstrates the preparation of crude 2-deoxy-2,2-difluoro-D- riboufuranose-3 ,5-dicinnamate.

[0062] To a 500 ml round flask was added crude 2-deoxy-2,2-difluoro-D-erythro- pentofuranos-l-ulose-3,5-dicinnamate (1Og, 23.36 mmol), which was dissolved in dry THF (100 ml) and cooled to 0-5⁰C. LiAlH₄ (0.46g, 12.1mmol) was added over a period of 5 minutes, after which time the mixture was stirred for another 1 hour. Water (10 ml) was added with stirring. Most of the solvents were distilled off, and to the residual volume (about 30 ml) IN HCl was added (50 ml) thus forming a clear solution with pH value of about 2. Ethyl acetate (100 ml) was added and the organic phase was washed with 5% NaHCO₃ (50 ml), water (50 ml), and brine (50 ml). The ethyl acetate phase was dried over MgSO₄ and concentrated under reduced pressure to obtain crude 2-deoxy-2,2-difluoro-D-riboufuranose~ 3,5-dicinnamate as an oil. Yield: 8.52g, (19.8 mmol, 84.8%, α/β isomer ratio about 1/1).

EXAMPLE 2

[0063] This example demonstrates the preparation of 2-deoxy-2,2-difluoro-D- ribofuranose-3 ,5-dicinnamate- 1 -methanesulfonate.

[0064] In a 100 ml round flask, crude 2-deoxy-2,2-difluoro-D-riboufuranose-3 ,5- dicinnamate (2.5g, 5.8mmol) was dissolved in dichloromethane (20 ml), and triethylamine (0.7g, 6.9 mmol) was added. Methanesulfonyl chloride (0.79g, 6.9 mmol) was added dropwise while cooling to 0-5⁰C. The mixture was stirred for another 1 hour, and washed with IN HCl (15 ml), 5% NaHCO₃ (15 ml), and dried over MgSO₄. The solvent was distilled off under reduced pressure to obtain crude 2-deoxy-2,2-difluoro-D-ribofuranose-3,5- dicinnamate-1 -methanesulfonate as light oil. Yield: 2.82 g, (5.55 mmol), 95.7%, purity 99.2%. Pure anomers were obtained by using preparative HPLC. ¹H-NMR (CDCl₃, 300MHz) β-anomer: 5 =3.14 (CH₃, 3H), 4.52 (H-4,5, 3H), 5.75 (H-I, IH), 5.98 (H-3, IH), 6.50 (cinnam., 2H), 7.40 (aromatic, 6H), 7.54 (aromatic, 4H), 7.79 (cirrnam., 2H).

EXAMPLE 3

[0065] This example demonstrates the preparation of N- 1 -trimethylsilylacetyl-2'-deoxy- 2',2'-difluoro-3',5'-dicinnamoyl-cytidine.

[0066] 1,2-dichloroethane (50 ml) was added to bis(trimethylsilyl)-N-acetylcytosine (2.2 g), to produce a clear solution, followed by trimethylsilyl triflate (Me₃SiOTf), (2.2 ml, 2.75 g) and stirred for 30 minutes. A solution of 2-deoxy-2,2-difluoro-D-ribofuranose-3,5- dicinnamate-1 -methanesulfonate (2.6 g, 5.1 mmol) in 1,2-dichloroethane (10 ml) was added dropwise, and the mixture was refluxed overnight. After cooling, the mixture was washed with water (30 ml), 5% NaHCO₃ (30 ml), and brine (20 ml), and dried over MgSO₄. The solvent was distilled off to obtain crude N-l-trimethylsilylacetyl-2'-deoxy-2'₅2'-difluoro-3',5'- dicinnarnoyl-cytidine as a light yellow solid (2.4 g, 4.24 mmol, 83%). The crude N-I- trimethylsilylacetyl-2'-deoxy-2'₅2'-difluoro-3'₅5'-dicinnamoyl-cytidine (2 g, 3.5 mmol) was dissolved in 1,2-dichloroethane (4 ml) at ambient temperature and methanol (120 ml) was added dropwise with stirring. [0067] The white solid formed was collected by filtration and dried (containing about 0.5 g ofN-l-trimethylsilylacetyl-2'-deoxy-2',2'-difiuoro-3',5'-dicinnamoyl-cytidine, α-anomer), and the remaining liquor was concentrated to dryness to obtain crude N-I- trimethylsilylacetyl-2'-deoxy-2',2'-difluoro -3',5'-dicinnamoyl-cytidine, β-anomer as a light yellow solid (1.6 g, 2.8 rπmol), 80% yield, ee 90%. N-l-trimethylsilylacetyl-2'-deoxy-2',2'- difluoro-3',5'-dicinnamoyl-cytidine, β -anomer: ¹HNMR CDCl₃: δ=2.23 (CH₃, 3H), 4.52 (H- 4, IH), 4.62 (H-5, 2H), 4.79 (cyt-H, IH), 5.50 (H-3, IH), 6.47-6.53 (cinnam.+ H-l, 3H), 7.43 (aromatic 6H), 7.54 (aromatic, 4H), 7.78 (cyt-H + cinnam. 3H) 9.2-9.35( NH broad IH).

EXAMPLE 4

[0068] This example demonstrates the preparation of gemcitabine hydrochloride. [0069] To a solution of methanol in ammonia (about 20%, 40 ml), crude N-I- trimethylsilylacetyl-2'-deoxy-2',2'-difluoro -3',5'-dicinnamoyl-cytidine β-anomer was added (1.5 g, 2.6 mmol), and stirred at ambient temperature overnight. The mixture was concentrated to light yellow oil and IN HCl (20 ml) was added followed by dichloromethane (20 ml) with stirring. The water phase was separated and active charcoal (0.2 g) was added and the mixture was heated to 75⁰C for 30 minutes. After filtration, the remaining liquor was concentrated to dryness under reduced pressure to obtain gemcitabine hydrochloride as a solid (1 g). The solid was dissolved in water (1 ml) at ambient temperature, and acetone (40 ml) was added dropwise with stirring. After 30 minutes, the precipitate was collected by filtration and dried at 7O⁰C to obtain crude gemcitabine. Yield: 0.3 g, 38.5% yield. Purity: 95%, ee 95

EXAMPLE 5

[0070] This example demonstrates the preparation of 2-deoxy-2,2-difluoropentofuranos- 1-ulose.

[0071] To a 500 ml glass flask equipped with reflux condenser was added crude ethyl- 2,2-difluoro-3-hydroxy-3-(2,2-dimethyldioxolan-4-yl)-propionate (30 g, 0.118 mol) and dissolved in a mixture of acetonirile (150 ml), water (12 ml) and trifluoroacetic acid (2.2 ml). After refluxing for 3 hours, the reflux condenser was adjusted for distillation at atmospheric pressure and a volume of 130-150 ml of solvents was distilled out. Toluene (70 ml) was added and the mixture was refluxed for additional 3 hours, and concentrated to obtain 20.2 g of oil, which was distilled under reduced pressure (140°C/2 mm Hg) to obtain 12.2 g (0.073 mole) of 2-deoxy-2,2-difluoropentofuranos-l-ulose in 61.9% yield.

EXAMPLE 6

[0072] This example demonstrates the preparation of 2-deoxy-2,2-difluoro-D-erythro- pentofuranos-l-ulose-3,5-dicinnamate

[0073] To a 1 liter glass flask equipped with reflux condenser was added freshly distilled 2-deoxy-2,2-difluoropentofuranos-l-ulose (also named 2-deoxy- 2,2-difluoropentanoic acid- γ-lactone) (19.5g, 0.116 mol) and dissolved in ethyl acetate (200 ml). Pyridine (37 ml, 0.458 mol) was added and the mixture was stirred at ambient temperature. A solution of cinnamoyl chloride (56 g, 0.336 mol) in ethyl acetate (100 ml) was added dropwise at ambient temperature and the mixture was refluxed for 4 hours. After cooling, the mixture was washed with water (100 ml), IM HCl (100 ml), 5% NaHCO₃ (100 ml), water (100 ml), and brine (100 ml). The organic phase was dried over Na₂SO₄, and concentrated by rotatory evaporator to obtain 46 g of crude 2-deoxy-2,2-difluoropentofuranos-l-ulose-3,5-dicinnamate as an oil. Ethyl acetate (80 ml) was added to the obtained oil and the mixture was stirred at ambient temperature, then hexane (150 ml) was added while maintaining the stirring. After about 30 minutes, the mixture (two layers) was cooled overnight to a temperature range of (-7 to - 10⁰C). A precipitate was collected by filtration to obtain 26 g (60.7 mmol) of crude solid of 2-deoxy-2,2-difluoro-D-erythro-pentofuranos-l-ulose-3,5-dicinnamate in 52.3% yield, purity 99.9% and ee of 99.6%. ¹H NMR₅ CDCl₃: 8=4.59 (H-5, 2H), 4.88 (H-4, IH), 5.63 (H-3, IH), 6.50 (cinnam., 2H), 7.44 (aromatic 6H), 7.55 (aromatic 4H), 7.75-7.88 (cinnam., 2H).

EXAMPLE 7

[0074] This example demonstrates the preparation of 2-deoxy-2,2-difluoro-D-erythro- pentofuranos-l-ulose-3,5-dinaphthoate

[0075] In a 100 ml glass flask equipped with reflux condenser, 2-deoxy-2,2-difluoro- pentofuranos-1-ulose (2.2 g, 13 mmol) was dissolved in dry THF (15 ml), and dry pyridine (3 ml, 37 mmol) was added. A solution of 2-naphthoyl chloride (obtained by heating 2- naphthoic acid in thionyl chloride) in dried THF (10 ml) was added dropwise. After refluxing for 2 hours, the mixture was cooled and washed with water (10 ml), IM HCl (10 ml), 5% NaHCO₃ (10 ml), water (10 ml), and brine (10 ml). The organic phase was dried over Na₂SO₄, and concentrated by rotatory evaporator to obtain a solid, which was crystallized to obtain white solid of 2-deoxy-2₅2-difluoro-D-erythro-pentofuranos-l-ulose- 3,5-dinaphthoate at purity of 98% and ee of 90%.

EXAMPLE 8

[0076] This example demonstrates the preparation of 2-deoxy-2,2-difluoropentofuranos- 1 -ulose-3 ,5-di-2-naphthylacetate.

[0077] 2-deoxy-2,2-difluoropentofuranos-l-ulose (0.82 g) and pyridine (0.85 ml) were mixed in dichloromethane (10 ml) and a solution of 2-naphthylacetyl chloride (obtained by reacting 2-naphthylacetic acid with oxalyl chloride) in dichloromethane (10 ml) was added dropwise. After refluxing for 2 hours, the mixture was cooled and washed with water (10 ml), IM HCl (10 ml), 5% NaHCO₃ (10 ml), water (10 ml), and brine (10 ml). The organic phase was dried over Na₂SO₄, and concentrated by rotatory evaporator to obtain a residue which was chromatographed to obtain an oil of 2-deoxy-2,2-difluoro-D-erythro- pentofuranos-1 -ulose-3, 5-di-2-naphthylacetate, as a mixture of erythro and threo isomers.

EXAMPLE 9 [0078] This example demonstrates the preparation of 2-deoxy-2,2-difluoropentofuranos-

1 -ulose-3 ,5-di-4-methylphenylacetate.

[0079] 2-deoxy-2,2-difluoropentofuranos-l-ulose (1.61 g), pyridine (1.7 ml), and 4- (dimethylamino)-pyridine (DMAP) (300 mg) were mixed in dichloromethane (15 ml), a solution of 4-methyl-phenylacetyl chloride (obtained from 3 g 4-methylphenylacetic acid and 5 ml of oxalyl chloride) in dichloromethane (10 ml) was added dropwise. After refluxing for

2 hours, the mixture was cooled and washed with water (10 ml), IM HCl (10 ml), 5% NaHCO₃ (10 ml), water (10 ml), and brine (10 ml). The organic phase was dried over Na₂SO₄, and concentrated by rotatory evaporator to obtain a residue which was chromatographed to obtain an oil of 2-deoxy-2,2-difluoropentofuranos-l -ulose-3, 5-di-4- methylphenylacetate as a mixture of erythro and threo isomers. EXAMPLE 10

[0080] This example demonstrates the preparation of 2-deoxy-2,2-difl.uoropentofuranos- 1 -ulose-3 -cinnamate.

[0081] To a 1 liter glass flask equipped with reflux condenser was added crude ethyl 2,2- difluoro-3-ciixnamoyloxy-3-(2,2-dimethyldioxolan-4-yl)-propionate (43 g, 0112 moles) and dissolved in a solvent mixture Of CH₃CN: H₂O: CF₃CO₂H, 25:1.25:0.215 v/v/v, (400 ml), and refluxed for 4 hours. The reflux condenser was adjusted for distillation at atmospheric pressure and about 50 ml of the solvent was distilled out. Toluene (50 ml) was then added and the distillation was continued until the inner temperature of the mixture was 95-100⁰C. After heating at this temperature for another 1 hour, the mixture was concentrated to obtain crude 2,2,-difluoropentofuranos-l -ulose-3 -cinnamate (33 g, 0.111 moles), in 99% yield. ¹H- NMR (CDCl₃): 6=4.0 ( H-5, 2H), 4.6 (H-4, IH), 5.69 (H-3, IH), 6.50 (cinnam., IH), 7.44 (aromatic 3H), 7.56 (aromatic 2H), 7.83 (cinnam., IH).

EXAMPLE Il

[0082] This example demonstrates the preparation of 2-deoxy-2,2,-difluoro-D-erythro- pentofuranos- 1 -ulose-3 ,5 -dicinamate.

[0083] 2-deoxy-2,2-difluoropentofuranos-l -ulose-3 -cinnamate (33g) was dissolved in ethyl acetate (350 ml), pyridine (66 ml, 0.816 mmol) was added, and the mixture was stirred at ambient temperature. A solution of cinnamoyl chloride (40 g, 0.24 mole) in ethyl acetate (150 ml) was added drop wise at ambient temperature and the mixture was refluxed for 4 hours. After cooling, the mixture was washed with water (150 ml), IM HCl (150 ml), 5% NaHCO₃ (150 ml), water (150 ml), and brine (150 ml). The organic phase was dried over Na₂SO₄, and concentrated by rotatory evaporator to obtain crude 2-deoxy-2,2-difluoro- pentofuranos-1 -ulose-3 ,5-dicinnamate as a solid, (11.3 g, 23.5%), which was crystallized from ethyl acetate-hexane and washed with petroleum ether to obtain 8.9 g (0.0208 moles) of 2-deoxy-2,2-difluoro-D-erythro-pentofuranos-l -ulose-3, 5-dicinnamate, in 18.7% yield and ee of 99%.

EXAMPLE 12

[0084] This example demonstrates the preparation of 2-deoxy-2,2-difluoropentofuranos- 1 -ulose-3 -cinnamate-5-benzoate. [0085] To a 500 ml glass flask equipped with reflux condenser was added crude ethyl- 2₅2-difluoro-3-cinnainoyloxy-3-(2,2-dimethyldioxolan-4-yl)-propionate (10 g) and dissolved in a mixture of acetonirile (75 ml), water (3.7 ml) and trifluoroacetic acid (0.65 ml). After heating at reflux for 3 hours, toluene (75 ml) was added and the mixture was distilled until the internal temperature of the mixture was 99-100⁰C. The mixture was concentrated to obtain crude 2-deoxy-2₅2-difluoropentofuranos-l-ulose-3-cinnamate (7.73 g), which was dissolved in a mixture of THF (50 ml), pyridine (4.5 ml) and 4-(dimethylamino)-pyridine (DMAP) (0.317g). Freshly re-distilled benzoyl chloride (4.8 g) was added dropwise at 0- 1O⁰C with stirring. The mixture was stirred at 20-30⁰C for 2 hours, and then concentrated to dryness. Ethyl acetate (30 ml) was added and the solid thus formed was removed off by filtration, and the remaining liquor was concentrated to obtain an oil. A solvent mixture of ethyl acetate and heptane was added, and the solution thus formed was cooled to -7 to -1O⁰C overnight. The precipitate was collected by filtration to obtain the solid of 2-deoxy-2,2- difluoropentofuranos-l-ulose-3-cinnamate-5-benzoate (erythro isomer, 0.5g). ¹H-NMR (CDCl₃): 6=4.66-4.74 (H-5, 2H), 4.92 (H-4, IH), 5.65 (H-3, IH), 6.49 (cinnam., IH), 7.42- 7.48 (aromatic 5H), 7.54-7.62 (aromatic 3H), 7.82 (cinnam., IH), 8.03 (aromatic 2H).

[0086] All references, including publications, patent applications, and patents, cited herein are hereby incorporated by reference to the same extent as if each reference were individually and specifically indicated to be incorporated by reference and were set forth in its entirety herein.

[0087] The use of the terms "a" and "an" and "the" and similar referents in the context of describing the invention (especially in the context of the following claims) are to be construed to cover both the singular and the plural, unless otherwise indicated herein or clearly contradicted by context. The terms "comprising," "having," "including," and "containing" are to be construed as open-ended terms (i.e., meaning "including, but not limited to,") unless otherwise noted. Recitation of ranges of values herein are merely intended to serve as a shorthand method of referring individually to each separate value falling within the range, unless otherwise indicated herein, and each separate value is incorporated into the specification as if it were individually recited herein. All methods described herein can be performed in any suitable order unless otherwise indicated herein or otherwise clearly contradicted by context. The use of any and all examples, or exemplary language (e.g., "such as") provided herein, is intended merely to better illuminate the invention and does not pose a limitation on the scope of the invention unless otherwise claimed. No language in the specification should be construed as indicating any non-claimed element as essential to the practice of the invention.

[0088] Preferred embodiments of this invention are described herein, including the best mode known to the inventors for carrying out the invention. Variations of those preferred embodiments may become apparent to those of ordinary skill in the art upon reading the foregoing description. The inventors expect skilled artisans to employ such variations as appropriate, and the inventors intend for the invention to be practiced otherwise than as specifically described herein. Accordingly, this invention includes all modifications and equivalents of the subject matter recited in the claims appended hereto as permitted by applicable law. Moreover, any combination of the above-described elements in all possible variations thereof is encompassed by the invention unless otherwise indicated herein or otherwise clearly contradicted by context.

Claims

CLAIMS:

1. A process for preparing gemcitabine or a salt thereof, the process comprising: removing at least a substantial portion of the α anomer from an anomeric mixture of a

N-l-protected-2'-deoxy-2',2'-difluorocytidine-3',5'-diester to produce a product, which is at least enriched in the β anomer; removing the 3'-ester, the 5'-ester and the N-protecting group; and optionally converting the product into a salt, wherein the 3'-ester and 5'-ester are the same or different and at least one of the esters is cinnamoyl, benzoyl, 1-naphthoyl, 1- naphthylmethylcarbonyl, 2-methylbenzylcarbonyl, 4-methylbenzylcarbonyl or 9- fluorenylmethyloxycarbonyl ester.

2. The process of claim 1 , wherein the anomeric mixture of the N- 1 -protected-2'- deoxy-2^l,2'-difluoro-cytidine-3',5'-diester is prepared by a process comprising: reducing a 2-deoxy-2,2-difluoro-D-erytlτro-pentofuranos-l-ulose-3,5-diester, to produce a 2-deoxy-2,2-difluoro-D-ribofuranos-3,5-diester; reacting the 2-deoxy-2,2-difluoro-D-ribofuranos-3,5-diester with methanesulfonyl chloride in the presence of a base to afford a 2-deoxy-2,2-difluoro-D-ribofuranos-3,5-diester- 1-methanesulfonate; and coupling a N-I -protected-cyto sine to the 2-deoxy-2,2-difluoro-D-ribofuranos-3,5- diester-1-methanesulfonate to produce the N-l-protected-2'-deoxy-2',2'-difluorocytidine-3'₃5'- diester, wherein the 3-ester and 5-ester are the same or different.

3. The process of claim 2, wherein at least one of the esters is cinnamoyl, benzoyl, 1-naphthoyl, 1-naphthylmethylcarbonyl, 2-methylbenzylcarbonyl, 4- methylbenzylcarbonyl or 9-fluorenylmethyloxycarbonyl ester.

4. The process of claim 2, wherein the 2-deoxy-2,2-difluoro-D-erythro- pentofuranos-l-ulose-3,5-diester is obtained by separation from a mixture of erythro and threo isomers of a 2-deoxy-2,2-difluoropentofuranos-l-ulose-3,5-diester, wherein the 3-ester and 5-ester are as defined in claim 2.

5. The process of claim 4, wherein the mixture of erythro and threo isomers of the 2-deoxy-2,2-difluoropentofuranos-l-ulose-3,5-diester is prepared by esterifying a mixture of erythro and threo isomers of 2-deoxy-2,2-difluoropentofuranos-l-ulose.

6. The process of claim 5, wherein the mixture of erythro and threo isomers of the 2-deoxy-2,2-difluoropentofuranos-l-ulose is prepared by cyclizing a mixture of 3-R and 3-S isomers of an alkyl-2,2-difluoro-3-hydroxy-3-(2,2-dimethyldioxolan-4-yl)-propionate.

7. The process of claim 5, wherein the 3-ester and 5-ester are the same.

8. The process of claim 7, wherein the 3-ester and 5-ester are cinnamoyl, 1- naphthoyl or 1-naphthylmethylcarbonyl ester.

9. The process of claim 4, wherein the mixture of erythro and threo isomers of the 2-deoxy-2,2-difluoropentofuranos-l-ulose-3,5-diester is prepared by esterifying a mixture of erythro and threo isomers of a 2-deoxy-2,2-difluoropentofuranos-l-ulose-3 -ester.

10. The process of claim 9, wherein the mixture of erythro (3-R) and threo (3-S) isomers of the 2-deoxy-2,2-difluoropentofuranos-l-ulose-3 -ester is prepared by a process comprising: esterifying a mixture of 3-R and 3-S isomers of an alkyl-2,2-difluoro-3-hydroxy-3- (2,2-dimethyldioxolan-4-yl)-propionate to produce an alkyl-2,2-difluoro-3-acyloxy-3-(2,2- dimethyldioxolan-4-yl)-propionate, and cyclizing the alkyl-2,2-difluoro-3 -acyloxy-3 -(2,2-dimethyldioxolan-4-yl)-propionate to produce a mixture of erythro and threo isomers of the 2-deoxy-2,2-difluoropentofuranos-l- ulose-3 -ester.

11. The process of claim 9, wherein the 3-ester and 5-ester are different.

12. The process of claim 11, wherein the 3-ester or the 5-ester is cinnamoyl, benzoyl, 1-naphthoyl or 1-naphthylmethylcarbonyl ester.

13. The process of claim 1, comprising: reducing the 2-deoxy-2,2-difluoro-D-erythro-pentofuranos-l-ulose-3,5-diester with a reducing agent in an organic solvent to produce a 2-deoxy-2,2-difluoro-D-ribofuranose-3,5- diester; reacting the 2-deoxy-2,2-difluoro-D-riboufuranose-3,5-diester with methanesulfonyl chloride in the presence of a base to obtain a 2-deoxy-2,2-difmoro-D-ribofuranose-3,5- diester- 1 -methanesulfonate; coupling the 2-deoxy-2,2-difluoro-D-ribofuranose-3 ,5-diester- 1 -methanesulfonate with bis(trimethylsilyl)-N-acetylcytosine in an organic solvent, optionally in the presence of a catalyst, to produce N-l-trimethylsilylacetyl-2'-deoxy-2',2'-difluoro-cytidine-3',5'-diester; precipitating the α isomer of the N-l-trimethylsilylacetyl-2'-deoxy-2',2'-difluoro- cytidine-3',5'-diester and filtering to isolate the β isomer; and removing the N-1-trimethylsilylacetyl, the 3 '-ester and the 5 '-ester by hydrolysis to produce gemcitabine.

14. The process of claim 13, wherein the reducing agent is lithium aluminium hydride, diisobutyl aluminium hydride, or sodium bis-(2-methoxyethoxy)-aluminium hydride.

15. The process of claim 14, wherein the reducing agent is lithium aluminium hydride.

16. The process of claim 13 , wherein the catalyst is trimethylsilyl triflate (Me₃SiOTf).

17. The process of claim 13, wherein the α and β anomers are separated by a process comprising precipitating the α anomer in a solvent mixture comprising 1,2- dichloroethane and methanol.

18. The process of claim 13 , comprising hydrolyzing the N- 1 -trimethylsilylacetyl, the 3 '-ester and the 5 '-ester in the presence of methanolic ammonia.

19. The process of claim 7, comprising: cyclizing ethyl-2,2-difluoro-3 -hydroxy-3 -(2,2-dimethyldioxolan-4-yl)-propionate to produce 2-deoxy-2,2-difluoropentofuranos-l -ulose; esterifying tlie 2-deoxy-2,2-difluoropentofuranos-l-ulose with an acid chloride in an organic solvent in presence of a base to produce a mixture of erythro and threo isomers of the 2-deoxy-2,2-difluoropentofuranos- 1 -ulose-3 ,5-diester; and selectively precipitating one of the isomers.

20. The process of claim 19, wherein the cyclization comprises: dissolving the ethyl-2,2-difluoro-3 -hydroxy-3 -(2,2-dimethyldioxolan-4-yl)-propionate in a solvent mixture containing an acid and heating to produce the 2-deoxy-2,2- difluoropentofuranos- 1 -ulose; reducing the solution volume by distillation; adding an organic solvent and refluxing; distilling off the solvent mixture to afford an oil; and optionally distilling the oil under reduced pressure.

21. The process of claim 20, wherein the reaction solvent comprises acetonitrile, water and trifluoroacetic acid.

22. The process of claim 21, wherein the acetonitrile: water:trifluoroacetic acid ratio is about 150:12:2.2 (v/v/v), respectively.

23. The process of claim 19, wherein the esterifϊcation comprises: dissolving 2-deoxy-2,2-difluoropentofuranos-l -ulose in an organic solvent; adding a base or other acid scavenger and an acid chloride, optionally at ambient temperature; refluxing the reaction mixture to produce the 2-deoxy-2,2-difluoropentofuranos-l- ulose-3 ,5-diester; washing the reaction mixture, separating and drying the organic phase; and evaporating the solvent to isolate the crude 2-deoxy-2₅2-difluoropentofuranos-l- ulose-3,5-diester.

24. The process of claim 23, wherein the acid chloride is cinnamoyl chloride, 1- naphthoyl chloride, 2-methylphenylacetyl chloride, 4-methylphenylacetyl chloride, 1- naphthylacetyl chloride, or 9-fluorenylmethyloxycarbonyl chloride.

25. The process of claim 19, wherein the erythro and threo isomers are separated by a process comprising: adding a first organic solvent to an isomeric mixture of a 2-deoxy-2,2- difluoropentofuranos-l-ulose-3,5-diester and stirring at ambient temperature; adding a second organic solvent with stirring; cooling to selectively precipitate one of the isomers; and filtering.

26. The process of claim 25, wherein the first solvent is ethyl acetate, isopropyl acetate, π-butyl acetate, isobutyl acetate, tert-butyl acetate, THF, acetonitrile, or a mixture thereof.

27. The process of claim 25, wherein the second solvent is pentane, hexane, heptane, octane, petroleum ether, or a mixture thereof.

28. The process of claim 25, wherein the 2-deoxy-2,2-difluoropentofuranos-l- ulose-3,5-diester is 2-deoxy-2,2-difluoro-D-erythro-pentofuranos-l-ulose-3,5-dicinnamate, the first solvent is ethyl acetate, and the second solvent is hexane.

29. The process of claim 28, comprising isolating the erythro isomer of 2-deoxy- 2,2-difluoro-D-erythro-pentofuranos-l-ulose-3,5-dicinnamate in a purity of at least 99.9% and an ee of at least 99.6%.

30. The process of claim 10, comprising: esterifying a mixture of 3-R and 3-S isomers of ethyl-2,2-difluoro-3-hydroxy-3-(2,2- dimethyldioxolan-4-yl)-ρropionate to produce ethyl-2,2-difluoro-3-acyloxy-3-(2,2- dimethyldioxolan-4-yl)-propionate; cyclizing the ethyl-2,2-difluoro-3 -acyloxy-3 -(2,2-dimethyldioxolan-4-yl)-propionate to produce a 2-deoxy-2,2-difmoropentofuranos-l-ulose-3-ester optionally having an erythro:threo isomer ratio of greater than 1:1; esterifying the 5-hydroxy group to produce a 2-deoxy-2,2-difluoropentofuranos-l- ulose-3,5-diester, wherein the 3-ester and 5-ester are the same or different; and separating the erythro and threo isomers of the 2-deoxy-2,2-difluoropentofuranos-l- ulose-3,5-diester by selective precipitation and filtration.

31. The process of claim 30, wherein esterifying the mixture of 3-R and 3-S isomers of ethyl-2,2-difluoro-3 -hydroxy-3 -(2,2-dimethyldioxolan-4-yl)-propionate comprises: dissolving ethyl-2,2-difmoro-3 -hydroxy-3 -(2,2-dimethyldioxolan-4-yl)-propionate in an organic solvent; adding a base and an acid chloride dropwise, optionally at ambient temperature; refluxing the mixture to produce the 2-deoxy-2,2-difluoropentofuranos-l-ulose-3- ester; and washing the reaction mixture, separating and drying the organic phase to obtain a solid material.

32. The process of claim 31 , wherein the acid chloride is cinnamoyl chloride, benzoyl chloride, 2-methylphenylacetyl chloride, 4-methylphenylacetyl chloride or 1- naphthylacetyl chloride.

33. The process of claim 30, wherein the cyclization comprises: dissolving crude ethyl-2,2-difluoro-3 -acyloxy-3 -(2,2-dimethyldioxolan-4-yl)- propionate in a solvent mixture containing an acid and heating to affect cyclization; reducing the solvent volume by distillation; adding an organic solvent and refluxing; distilling off the solvent mixture until the internal temperature of the mixture reaches about 90-100°C and maintaining the temperature for about one hour; and concentrating the cyclization product by evaporating, optionally under reduced pressure.

34. The process of claim 33, wherein the reaction solvent comprises a mixture of acetonitrile, water and trifluoroacetic acid.

35. The process of claim 34, wherein the acetonitrile:water:trirluoroacetic acid ratio is about 75:3.7:0.65 (v/v/v) respectively.

36. The process of claim 33, wherein the solvent added after reducing the solution volume by distillation is ethyl acetate, xylenes, toluene, or a mixture thereof.

37. The process of claim 30, wherein the 5 -hydroxy group is esterified by a process comprising: dissolving ethyl-2,2-difluoro-3~acyloxy-pentofuranos-l-ulose in an organic solvent; adding a base or other acid scavenger and an acid chloride, optionally at ambient temperature; refluxing the mixture to produce the 2-deoxy-2,2-difluoropentofuranos-l-ulose-3,5- diester; washing the reaction mixture, separating and drying the organic layers; distilling off the solvent to obtain a solid, and optionally crystallizing the product.

38. The process of claim 37, wherein the acid chloride is cinnamoyl chloride, benzoyl chloride, 1 -naphthoyl chloride, 2-methylphenylacetyl chloride or 4- methylphenylacetyl chloride.

39. The process of claim 30, wherein the erythro and threo isomers are separated by a process comprising: adding a solvent mixture to a crude mixture of erythro and threo isomers of the 2- deoxy-2,2-difluoropentofuranos-l-ulose-3,5-diester and stirring at ambient temperature; cooling for a time period sufficient to selectively precipitate one of the isomers; and filtering.

40. The process of claim 39, wherein the solvent mixture comprises ethyl acetate, isopropyl acetate, n-butyl acetate, isobutyl acetate, tert-buty\ acetate, THF, acetonitrile, pentane, hexane, heptane, octane, petroleum ether, or a mixture thereof.

41. A process for preparing an isomer enriched 2-deoxy-2,2-difluoropentofuranos- l-ulose-3,5-diester, the process comprising: cyclizing ethyl-2,2-difluoro-3-hydroxy-3-(2,2-dimethyldioxolan-4-yl)-propionate to produce 2-deoxy-2,2-difluoropentofuranos-l -ulose; esterifying the 2-deoxy-2,2-difluoropentofuranos-l-ulose with an acid chloride in an organic solvent in presence of a base to produce a mixture of erythro and threo isomers of the 2-deoxy-2,2-difluoropentofuranos-l -ulose-3,5-diester; and selectively precipitating one of the isomers.

42. The process of claim 41, wherein the cyclization comprises: dissolving the ethyl-2,2-difluoro-3-hydroxy-3-(2,2-dimethyldioxolan-4-yl)-propionate in solvent mixture containing an acid and heating to produce the 2~deoxy-2,2- difluoropentofuranos- 1 -ulose; reducing the solution volume by distillation; adding an organic solvent and refluxing; distilling off the solvent mixture to afford an oil; and optionally distilling the oil under reduced pressure.

43. The process of claim 42, wherein the reaction solvent comprises acetonitrile, water and trifluoroacetic acid.

44. The process of claim 43, wherein the acetonitrile:water:trifluoroacetic acid ratio is about 150:12:2.2 (v/v/v), respectively.

45. The process of claim 41, wherein the esterification comprises: dissolving 2-deoxy-2,2-difluoropentofuranos-l -ulose in an organic solvent; adding a base or other acid scavenger and an acid chloride, optionally at ambient temperature; refluxing the reaction mixture to produce the 2-deoxy-2,2-difTuoropentofuranos-l- ulose-3,5-diester; washing the reaction mixture, separating and drying the organic phase; and evaporating the solvent to isolate the crude 2-deoxy-2,2-difmoropentofuranos-l- ulose-3,5-diester.

46. The process of claim 45, wherein the acid chloride is cinnamoyl chloride, 1- naphthoyl chloride, 2-methylphenylacetyl chloride, 4-methylphenylacetyl chloride, 1- naphthylacetyl chloride, or 9-fluorenyhnethyloxycarbonyl chloride.

47. The process of claim 41 , wherein the erythro and threo isomers are separated by a process comprising: adding a first organic solvent to an isomeric mixture of a 2-deoxy-2,2- difluoropentofuranos-l-ulose-3,5-diester and stirring at ambient temperature; adding a second organic solvent with stirring; cooling to selectively precipitate one of the isomers; and filtering.

48. The process of claim 47, wherein the first solvent is ethyl acetate, isopropyl acetate, rø-butyl acetate, isobutyl acetate, tert-butyl acetate, THF, acetonitrile, or a mixture thereof.

49. The process of claim 47, wherein the second solvent is pentane, hexane, heptane, octane, petroleum ether, or a mixture thereof.

50. A process for preparing an isomer enriched 2-deoxy-2,2-difluoropentofuranos- l-ulose-3,5-diester, the process comprising: esterifying a mixture of 3 -R and 3 -S isomers of emyl-2,2-difluoro-3-hydroxy-3-(2,2- dimethyldioxolan-4-yl)-propionate to produce ethyl-2,2-difluoro-3-acyloxy-3-(2,2- dimethyldioxolan-4-yl)-propionate; cyclizing the ethyl-2,2-difluoro-3-acyloxy-3-(2,2-dimethyldioxolan-4-yl)-propionate to produce a 2-deoxy-2,2-difluoropento:faranos-l-ulose-3 -ester optionally having an erythro:threo isomer ratio of greater than 1:1; esterifying the 5-hydroxy group to produce a 2-deoxy-2,2-difluoropentofuranos-l- ulose-3,5-diester, wherein the 3-ester and 5-ester are the same or different; and separating the erythro and threo isomers of the 2-deoxy-2,2-difmoropentofuranos-l- ulose-3,5-diester by selective precipitation and filtration.

51. The process of claim 50, wherein esterifying the mixture of 3-R and 3-S isomers of ethyl -2,2-difluoro-3 -hydroxy-3 -(2,2-dimethyldioxolan-4-yl)-propionate comprises: dissolving ethyl-2,2-difluoro-3 -hydroxy-3 -(2,2-dimethyldioxolan-4-yl)-propionate in an organic solvent; adding a base and an acid chloride dropwise, optionally at ambient temperature; refluxing the mixture to produce the 2-deoxy-2,2-difluoropentofuranos-l-ulose-3- ester; and washing the reaction mixture, separating and drying the organic phase to obtain a solid material.

52. The process of claim 51, wherein the acid chloride is cinnamoyl chloride, benzoyl chloride, 2-methylphenylacetyl chloride, 4-methylphenylacetyl chloride or 1- naphthylacetyl chloride.

53. The process of claim 50, wherein the cyclization comprises: dissolving crude ethyl-2,2-difluoro-3 -acyloxy-3 -(2,2-dimethyldioxolan-4-yl)- propionate in a solvent mixture containing an acid and heating to affect cyclization; reducing the solvent volume by distillation; adding an organic solvent and refluxing; distilling off the solvent mixture until the internal temperature of the mixture reaches about 90-100⁰C and maintaining the temperature for about one hour; and concentrating the cyclization product by evaporating, optionally under reduced pressure.

54. The process of claim 53, wherein the reaction solvent comprises a mixture of acetonitrile, water and trifluoroacetic acid.

55. The process of claim 54, wherein the acetonitrile:water:trifluoroacetic acid ratio is about 75:3.7:0.65 (v/v/v) respectively.

56. The process of claim 53, wherein the solvent added after reducing the solution volume by distillation is ethyl acetate, xylenes, toluene, or a mixture thereof.

57. The process of claim 50, wherein the 5-hydroxy group is esterified by a process comprising: dissolving ethyl-2,2-difluoro-3-acyloxy-pentofuranos-l-ulose in an organic solvent; adding a base or other acid scavenger and an acid chloride, optionally at ambient temperature; refluxing the mixture to produce the 2-deoxy-2,2-difluoropentofuranos-l-ulose-3,5- diester; washing the reaction mixture, separating and drying the organic layers; distilling off the solvent to obtain a solid, and optionally crystallizing the product.

58. The process of claim 57, wherein the acid chloride is cinnamoyl chloride, benzoyl chloride, 1-naphthoyl chloride, 2-methylphenylacetyl chloride or 4- metliylphenylacetyl chloride.

59. The process of claim 50, wherein the erythro and threo isomers are separated by a process comprising: adding a solvent mixture to a crude mixture of erythro and, threo isomers of the 2- deoxy-2,2-difluoropentofuranos-l-ulose-3,5-diester and stirring at ambient temperature; cooling for a time period sufficient to selectively precipitate one of the isomers; and filtering.

60. The process of claim 59, wherein the solvent mixture comprises ethyl acetate, isopropyl acetate, n-butyl acetate, isobutyl acetate, tert-bntyl acetate, THF, acetonitrile, pentane, hexane, heptane, octane, petroleum ether, or a mixture thereof.

61. A compound of the formula:

wherein R and R' are the same or different and each is 2-phenylethenyl, phenyl, 2- methylbenzyl, 4-methylbenzyl, 1-naphthyl, 1-naphthylmethyl or 9-fluorenylmethyl.

62. A compound of the formula:

as the erythro isomer, the threo isomer or a mixture thereof, wherein R is, 2-phenylethenyl, phenyl, 2-methylbenzyl, 4-methylbenzyl or 1-naphthylmethyl, and AIk is C_1-6 alkyl.

63. A compound the formula:

as the α anomer, the β anomer or a mixture thereof, wherein R and R' are the same or different and at least one of R and R¹ is 2-phenylethenyl, phenyl, 2-methylbenzyl, 4- methylbenzyl, 1-naphthyl, 1-naphthylmethyl or 9-fluorenylmethyl, and the other represents the residue of a carboxylate ester.

64. A process for producing gemcitabine, the process comprising converting the β anomer of the compound of claim 63 into gemcitabine.