This application claims priority to this Indian patent application number 1819/CHE/2012 filed on May 09, 2012.

FIELD OF THE INVENTION

The present invention provides a novel process for the preparation of Emtricitabine.

BACKGROUND OF THE INVENTION

Emtricitabine is marketed under the trade name Emtriva. It is a synthetic nucleoside analog of cytidine with activity against human immunodeficiency virus type-1 (HIV-1) reverse transcriptase. Chemically Emtricitabine [commonly known as cis FTC] is 5-Fluoro-l-(2R,5S)-[2-(hydroxymethyl)-l,3-oxathiolan-5-yl]cytosine and is structurally represented in Formula I. Emtricitabine is the (-) enantiomer of thio analogs.

Formula I

WO 199214743 discloses the racemic mixture of cis isomers, which are prepared using standard reactions followed by resolution employing enzymatic methods to yield 2R, 5S enantiomer.

The substituent's on chiral carbons of purine and pyrimidine rings of 1,3-oxathiolane nucleosides can be either in cis or trans forms. Both cis and trans racemates consist of pair of optical isomers and further each compound has four individual optical isomers.

Emtricitabine has two chiral centers and therefore four stereo isomers exist namely two cis (2R,5S and 2S,5R) and two trans (2R,5R and 2S,5S). The two cis enantiomers (2R,5S; 2S;5R) together are referred to as racemic mixture of a-enantiomers. Among the four pairs the cis (2R,5S) is found to possess profound therapeutic activity.

US 5696254 discloses a stereo selective synthesis approach for the synthesis of Emtricitabine by reacting 5-acetoxy-[l,3]-oxathiolane-2S-carboxylic acid-2S-isopropyl methyl-5R-methyl-lR-cyclohexyl ester with silylated cytosine derivative in the presence of silylated Lewis acid followed by reduction.

US 6051709 discloses a process for the preparation of Lamivudine by reacting 5-chloro-[ 1,3]-oxathiolane-2S-carboxylic acid-2S-isopropylmethyl-5R-methyl-1 R-cyclohexyl ester with silylated cytosine without Lewis acid, reduction of the obtained ester, followed by addition of salicylic acid to isolate Lamivudine as a salicylate salt. Further, the obtained salt is converted to Lamivudine.

US 5728575 discloses a method to obtain Lamivudine and Emtricitabine via enzymatic resolution of the 5'-acyl protected racemic nucleoside using pig liver esterase, porcine pancreatic lipase, or subtilisin.

US 5892025 discloses a method for the resolution of the combination of the enantiomers of cis-Emtricitabine by passing the cis-Emtricitabine through an acetylated 0-cyclodextrin chiral column

Still there exists need in the art for the preparation of pure cis-FTC.

OBJECT OF THE INVENTION

The principle object of the present invention is to provide a novel process for preparing Emtricitabine.

Another object of the present invention is to provide a novel intermediate of formula III.

III wherein P is a Silylated protecting group

SUMMARY OF THE INVENTION

In one aspect, the present invention provides a novel process for preparing Emtricitabine comprising the steps of:

a) reducing the compound of formula II,

II wherein R is a chiral auxiliary

b) protecting the compound of step a) to yield formula III, HI wherein P is a Silylated protecting group

c) deprotecting the compound of formula III, optionally in the presence of a base, and

d) isolating Emtricitabine of formula I.

In another aspect, the present invention provides a novel intermediate of formula III.

DETAILED DESCRIPTION OF THE INVENTION

The present invention provides a novel process for preparing Emtricitabine through a novel intermediate of formula III.

The main aspect of the present invention is to provide a novel process for preparing Emtricitabine comprising the steps of:

a) reducing the compound of formula II,

b) protecting the compound of step a) to yield formula III,

c) deprotecting the compound of formula III, optionally in the presence of a base, and

d) isolating Emtricitabine of formula I.

In one embodiment of the present invention, the compound of formula II wherein R is chiral auxiliary such as menthol, is reduced by treating with a reducing agent in an organic solvent in presence of buffer or acid, wherein the reducing agent is selected from sodium borohydride, Potassium borohydride, lithium borohydride, diisobutylaluminum hydride, vitride and the like, preferably sodium borohydride; the organic solvent is selected from polar solvents such as methanol, ethanol, propanol, isopropanol (IPA), butanol, tetrahydrofuran and the like. Preferably isopropanol. The buffer is selected from dipotassium hydrogen phosphate, potassium hydrogen phosphate, disodium hydrogen phosphate, dipotassium hydrogen orthophosphate or mixtures thereof, preferably mixture of dipotassium hydrogen phosphate and potassium hydrogen phosphate.

In another embodiment of the present invention, the -OH of the reduced compound is protected with silyl group such as trimethylsilyl (TMS), triethylsilyl (TES), dimethylisopropylsilyl (IPDMS), diethylisopropylsilyl (DEIPS), t-butyldimethylsilyl (TBS), t-butyldiphenylsilyl (TBDPS), triisopropylsilyl (TIPS), tetraisopropyldisilylene (TIPDS) or di-t-butyldimethylsilylene (DTBS).

In another embodiment of the present invention, the silyl protected compound of formula III is deprotected to yield Emtricitabine, wherein the deprotection is carried out by heating at 30-50 °C, preferably 35-45 °C, more preferably at 40-45 °C in a solvent or mixtures thereof, wherein the solvent is selected from toluene, ethyl acetate, propyl acetate, methyl acetate, tetrahydrofuran, 1,4-dioxane, dimethyl sulfoxide, water and the like. Preferably mixture of toluene and water.

In one embodiment of the present invention, alternatively the deprotection of silyl protected compound of formula III can be achieved by treating with a base, wherein the base is selected from an organic bases such as triethylamine, tributylamine, N- methylmorpholine, N-diisopropylethylamine, N-methylpyrrolidine, pyridine; inorganic bases such as alkali metal hydrides such as sodium hydride, potassium hydride and the like; sodamide; n-butyl lithium; lithium diisopropylamide; alkali metal hydroxides such as lithium hydroxide, sodium hydroxide, potassium hydroxide, cesium hydroxide; alkaline metal hydroxides such as aluminum hydroxide, magnesium hydroxide, calcium hydroxide and the like; alkali metal carbonates such as sodium carbonate, potassium carbonate, lithium carbonate, cesium carbonate and the like, alkaline earth metal carbonates such as magnesium carbonate, calcium carbonate and the like; alkali metal bicarbonates such as sodium bicarbonate. Preferably triethyl amine to isolate Emtricitabine.

Yet another embodiment of the present invention, the obtained Emtricitabine is optionally purified by treating in an organic solvent, wherein the organic solvent is selected from polar solvents such as methanol, ethanol, propanol, isopropanol, acetic acid, butanol and the like. Preferably isopropanol.

In one embodiment of the present invention, the compound resulting from the reduction of formula II is optionally isolated.

As per the present invention, the compound of formula II is reduced by treating with a reducing agent preferably sodium borohydride in a suitable solvent such as isopropanol in presence of a buffer, preferably mixture of dipotassium hydrogen phosphate and potassium hydrogen phosphate to yield reduced compound, in which the -OH group is protected by treating with a hydroxyl protecting group, preferably silylating group such as HMDS. The resulting silyl protected compound of formula III is further deprotected by heating the reaction mixture in a suitable solvent mixture such as Toluene and water at 40-45 °C, optionally in presence of a base. The obtained residue is treated with ethyl acetate and crude Emtricitabine is isolated. The crude Emtricitabine is optionally purified by treating with an activated carbon in a suitable solvent such as isopropanol to yield pure Emtricitabine.

Certain specific aspects and embodiments are further explained in more detailed with the following examples. These examples should not be construed as limiting the scope of the invention in any manner.

Examples:

Example-1: Preparation of Emtricitabine

74.7 g of Dipotassium Hydrogen phosphate and 8.3 g of Potassium dihydrogen phosphate were dissolved in 300 mL of purified water. 800 mL of isopropanol (IPA) and 100 g of [(2R,5S)-5-(4-Amino-5-fluoro-2-oxo-2H-pyrimidin-l-yl)-[l,3]-oxathiolane-2-carboxylicacid,2S-isopropyl-5R-methyl-lR-cyclohexyl ester] were added to the reaction mixture. To the reaction mixture was slowly added 20 g of Sodium borohydride solution (20 g in 95 ml of water containing 0.5 g of Sodium hydroxide) at 17-21°C, reaction mass temperature was maintained till the completion of reaction. Organic and aqueous layers were separated; aqueous layer was back extracted with 100 mL of IPA. Both the organic layers were combined and the pH was adjusted to 6.0 -6.5 using 8 ml of dil. HC1. The pH of the reaction mass was further adjusted to 8.0 - 8.5 using 2N Sodium hydroxide solution. Solvent was distilled out atmospherically till the residual volume becomes 250 ml and the reaction mass was cooled to room temperature and extracted with 150 ml of toluene. Toluene layer was back extracted with 100 mL of water. Both the aqueous layers were combined and treated with 5 g of activated carbon; filtered through hyflo bed.

Aqueous layer was then distilled under vacuum at 55±5°C, stripped off with IPA (2x 100 mL) and then 500 mL of IPA was charged to it and heated to 60±5°C. It was then refluxed for 30 min, and hot filtered at 63±2°C. Filtrate was concentrated to residue and stripped off with 100 mL of Toluene and again 1000 mL of Toluene was added to the reaction mass. To the reaction mixture was added 4 g of ammonium chloride and 40.4 g of HMDS and heated to 115±5°C for 6 hrs, then reaction mass was cooled to 60±5°C and filtered through hyflo-bed and bed washed with 100 mL of Toluene. 400 mL of water and triethylamine were added to the reaction mixture and stirred at 43±3°C for 12 hrs.

Aqueous layer separated and organic layer was washed with 100 mL of water. Mixed aqueous layer was then distilled out under vacuum at 55±5°C, stripped out with IPA (2x100 mL) and then with 100 mL of Ethyl acetate. 400 mL of Ethyl acetate was charged
to the residue and stirred for 4 hrs and then filtered and washed with 100 mL of Ethyl acetate. The solid was then dried to get 53 g of Emtricitabine.

Example-2: Preparation of Emtricitabine

74.7 g of Dipotassium Hydrogen phosphate and 8.3 g of Potassium dihydrogen phosphate
were dissolved in 300 mL of Purified water. 800 mL of IPA and 100 g of (2R,5S)-5-(4-
Amino-5-fluoro-2-oxo-2H-pyrimidin-1 -yl)-[ 1,3]-oxathiolane-2-carboxylic acid,2S-
isopropyl-5R-methyl-lR-cyclohexyl ester were added to the reaction mixture. 20 g of Sodium borohydride solution (20 g in 95 mL of water containing 0.5 g of Sodium hydroxide) was slowly added to the reaction mixture at 17-21°C and the temperature was maintained till the completion of the reaction. After completion of the reaction, organic and aqueous layers were separated and aqueous layer was back extracted with 100 mL of IPA. Both the organic layers were combined and pH was adjusted to 6.0-6.5 using 8 mL of dil. HC1, again the pH of the reaction mixture was further adjusted to 8.0 - 8.5 using 2N Sodium hydroxide solution. Reaction mixture was distilled out atmospherically till the residual volume becomes 250 mL and cooled to room temperature followed by extraction with 150 mL of Toluene. Toluene layer was further back extracted with 100 mL of water. Both the aqueous layers were mixed and treated with 5 g of activated carbon; filtered through hyflo bed. Aqueous layer was then distilled under vacuum at 55±5°C; 600 mL of toluene was charged to it and then refluxed for 30 min. The water was removed azeotropically and the reaction mixture was cooled to 50-60 °C. 4 g of Ammonium chloride and 40.4 g of HMDS were added to the reaction mixture and heated to 115±5°C for 6 hrs, then reaction mass was cooled to 60±5°C, filtered through hyflo-bed and bed washed with 100 mL of Toluene. 250 mL of water and triethylamine were added to it and stirred at 43±3°C for 12 hrs, layers were separated and organic layer was washed with 50 mL of water. Mixed aqueous layer was then distilled out under vacuum at 55±5°C, stripped out with IPA (2x100 mL) and then with 100 mL of ethyl acetate. 400 mL of ethyl acetate was added to the residue and stirred for 4 hrs and then filtered and washed with 100 mL of ethyl acetate. The solid was then dried to get 52 g of Emtricitabine.

Example-3: Preparation of Emtricitabine

74.7 g of Dipotassium Hydrogen phosphate and 8.3g of Potassium dihydrogen phosphate were dissolved in 300 mL of purified water. 800 mL of IPA and 100 g of [(2R,5S)-5-(4-Amino-5-fluoro-2-oxo-2H-pyrimidin-l -yl)-[ 1,3]-oxathiolane-2-carboxylicacid,2S-isopropyl-5R-methyl-lR-cyclohexyl ester] were added to the reaction mixture. To the reaction mixture was slowly added 20 g of Sodium borohydride solution (20 g in 95 ml water containing 0.5g Sodium hydroxide) at 17-21°C, reaction mass temperature was maintained till the completion of reaction. Organic and aqueous layers were separated; aqueous layer was back extracted with 100 mL of IPA. Both the organic layers were combined and the pH was adjusted to 6.0 -6.5 using 8 ml of dil. HC1. The pH of the reaction mass was further adjusted to 8.0 - 8.5 using 2N Sodium hydroxide solution.

Solvent was distilled out atmospherically till the residual volume becomes 250 ml and the reaction mass was cooled to room temperature and extracted with 150 ml of toluene.

Toluene layer was back extracted with 100 mL of water. Both the aqueous layers were combined and treated with 5 g of activated carbon and filtered through hyflo bed.

Aqueous layer was then distilled under vacuum at 55±5°C, stripped off with IPA (2x 100 mL) and then 500 mL of IPA was charged to it and heated to 60±5°C. It was then refluxed for 30 min, and hot filtered at 63±2°C. Filtrate was concentrated to residue and stripped off with 100 mL of Toluene and again 1000 mL of Toluene was added to the reaction mass. To the reaction mixture was added 4 g of ammonium chloride and 40.4 g of HMDS and heated to 115±5°C for 3-5 hrs to get clear solution, then reaction mass was cooled to 60±5°C and filtered through hyflo-bed and bed washed with 100 mL of Toluene. 30 mL of water and 4g of Triethylamine was added to the filtrate and stirred at 43±3°C for 12 hrs. Reaction mass was then distilled out under vacuum at 55±5°C, stripped out with hexane (2x100 mL). 400 mL of hexane was charged to the residue and stirred for 4 hrs and then filtered and washed with 50 mL of hexane. The solid was then dried to get 55 g of Emtricitabine.

Example-4: Preparation of Emtricitabine

74.7 g of Dipotassium Hydrogen phosphate and 8.3g of Potassium dihydrogen phosphate were dissolved in 300 mL of purified water. 800 mL of IPA and 100 g of [(2R,5S)-5-(4-Amino-5-fluoro-2-oxo-2H-pyrimidin-l-yl)-[l,3]-oxathiolane-2-carboxylicacid,2S-isopropyl-5R-methyl-lR-cyclohexyl ester] were added to the reaction mixture. To the reaction mixture was slowly added 20 g of Sodium borohydride solution (20 g in 95 ml of water containing 0.5 g of Sodium hydroxide) at 17-21°C, reaction mass temperature was maintained till the completion of reaction. Organic and aqueous layers were separated; aqueous layer was back extracted with 100 mL of IPA. Both the organic layers were combined and the pH was adjusted to 6.0 -6.5 using 8 ml of dil. HC1. The pH of the reaction mass was further adjusted to 8.0 - 8.5 using 2N Sodium hydroxide solution.

Solvent was distilled out atmospherically till the residual volume becomes 250 ml and the reaction mass was cooled to room temperature and extracted with 150 ml of toluene.

Toluene layer was back extracted with 100 mL of water. Both the aqueous layers were combined and treated with 5 g of activated carbon and filtered through hyflo bed.
Aqueous layer was then distilled under vacuum at 55±5°C, stripped off with IPA (2x 100 mL) and then 500 mL of IPA was charged to it and heated to 60±5°C. It was then refluxed for 30 min and hot filtered at 63±2°C. Filtrate was concentrated to residue and stripped off with 100 mL of Toluene and again 1000 mL of Toluene was added to the reaction mass. To the reaction mixture was added 4 g of ammonium chloride and 40.4 g of HMDS and heated to 115±5°C for 3-5 hrs to get clear solution, then reaction mass was cooled to 60±5°C and filtered through hyflo-bed and bed washed with 100 mL of Toluene. 30 mL of water and 4g of Triethylamine was added to the reaction mixture and stirred at 43±3°C for 12 hrs. Reaction mass was then distilled out under vacuum at 55±5°C, stripped out with IPA (2x100 mL). 400 mL of IPA was charged to the residue and stirred for 4 hrs and then cooled to 10-15°C. Slurry was then filtered and washed with 50 mL of chilled IPA. The solid was then dried to get 50 g of Emtricitabine.

Example-5: Preparation of silyl protected Emtricitabine (Formula III)

74.7 g of Dipotassium Hydrogen phosphate and 8.3 g of Potassium dihydrogen phosphate were dissolved in 300 mL of purified water. 800 mL of IPA and 100 g of [(2R,5S)-5-(4-Amino-5-fluoro-2-oxo-2H-pyrimidin-1 -yl)-[ 1,3]-oxathiolane-2-carboxylicacid,2S-isopropyl-5R-methyl-lR-cyclohexyl ester] were added to the reaction mixture. To the reaction mixture was slowly added 20 g of Sodium borohydride solution (20 g in 95 ml water containing 0.5g Sodium hydroxide) at 17-21°C, reaction mass temperature was maintained till the completion of reaction. Organic and aqueous layers were separated; aqueous layer was back extracted with 100 mL of IPA. Both the organic layers were combined and the pH was adjusted to 6.0 -6.5 using 8 ml of dil. HC1. The pH of the reaction mass was further adjusted to 8.0 - 8.5 using 2N Sodium hydroxide solution.

Solvent was distilled out atmospherically till the residual volume becomes 250 ml and the reaction mass was cooled to room temperature and extracted with 150 ml of toluene. Toluene layer was back extracted with 100 mL of water. Both the aqueous layers were combined and treated with 5 g of activated carbon; filtered through hyflo bed. Aqueous layer was then distilled under vacuum at 55±5°C, stripped off with IPA (2x 100 mL) and then 500 mL of IPA was charged to it and heated to 60±5°C. It was then refluxed for 30 min, and hot filtered at 63±2°C. Filtrate was concentrated to residue and stripped off with 100 mL of Toluene and again 1000 mL of Toluene was added to the reaction mass. To the reaction mixture was added 4 g of ammonium chloride and 40.4 g of HMDS and heated to 115±5°C for 3-5 hrs to get clear solution, then reaction mass was cooled to 60±5°C and filtered through hyflo-bed and bed washed with 100 mL of Toluene. Toluene was distilled out under vacuum at 55±5°C to get solid material of silyl protected Emtricitabine.

1H NMR : 0.0 (s, 9H), 2.99 (dd, 1H), 3.29 (dd, 1H), 3.72-3.89(m, 2H), 5.10 (t, 1H), 6.02(m, 1H), 7.44(bs,lH), 7.69(bs,lH), 8.06 (d, 1H).

Example-6: Purification of Emtricitabine

50 g of Emtricitabine crude was added to 1000 mL of IPA and heated to reflux at 80±3°C, to this was added 5 g of activated carbon and stirred at 70-80°C for 1 hr, then hot filtered through hyflo-bed. The bed was washed with 50 mL of hot IPA and filtrate
was distilled out under vacuum till the residual volume comes to 275 mL. The residue was cooled to 25-30°C and stirred for 6 hrs, and further cooled to 5-10°C and continued stirring for 4 hrs. The slurry was filtered and washed with 50 mL of chilled IPA and the solid was dried to get 38.8g of pure Emtricitabine.

We Claim:

1. A process for preparing Emtricitabine comprising the steps of:

a) reducing the compound of formula II,

b) protecting the compound of step a) to yield formula III,

c) deprotecting the compound of formula III, and

d) isolating Emtricitabine of formula I.

2. The process according to the claim 1, wherein the compound of formula II is reduced by treating with a reducing agent selected from sodium borohydride, Potassium borohydride, lithium borohydride, diisobutylaluminum hydride or vitride.

3. The process according to the claim 1, wherein the silylated protecting group is selected from trimethylsilyl (TMS), triethylsilyl (TES), dimethylisopropylsilyl (IPDMS), diethylisopropylsilyl (DEIPS), t-butyldimethylsilyl (TBS), t-butyldiphenylsilyl (TBDPS), triisopropylsilyl (TIPS), tetraisopropyldisilylene (TIPDS) or di-t-butyldimethylsilylene (DTBS).

4. The process according to the claim 3, wherein the silylated protecting group is trimethylsilyl.

5. A process for preparing Emtricitabine comprising the steps of:

a) deprotecting the compound of formula III; and III wherein P is a Silylated protecting group

b) isolating Emtricitabine.

6. The process according to the claims 1 and 5, wherein the deprotection is carried out by heating compound of formula III at 30-50 °C.

7. The process according to the claims 1 and 5, wherein the deprotection is carried out in presence of a base.

8. A process for preparing Emtricitabine comprising the steps of

a) reducing the compound of formula

b) protecting the compound of step a) to yield formula

c) deprotecting compound of formula step b); and

d) ioslating Emtricitabine.

9. A novel intermediate compound of formula III.

10. Use of compound of formula HI for the preparation of Emtricitabine.