Abstract:
The present invention relates to a process for the preparation of substituted 1,3-oxathiolanes. The present invention specifically relates to a process for the preparation of lamivudine.

Description:
FIELD OF THE INVENTION 
       [0001]    The present invention relates to a process for the preparation of substituted 1,3-oxathiolanes. The present invention specifically relates to a process for the preparation of lamivudine. 
       BACKGROUND OF THE INVENTION 
       [0002]    Substituted 1,3-oxathiolanes of Formula I and stereoisomers thereof, 
         [0000]    
       
                 
         
             
             
         
       
     
         [0000]    wherein R 1  is hydrogen, alkyl or aryl, and R 2  is a substituted or unsubstituted purine or pyrimidine base or an analogue or derivative thereof, are an important class of therapeutic agents and they have shown antiviral activity against retroviruses such as human immunodeficiency virus (HIV), hepatitis B virus (HBV) and human T-lymphotropic virus (HTLV). Lamivudine is a substituted 1,3-oxathiolane and it is presently available in the market as an antiretroviral agent. Lamivudine is the cis-(−)-isomer and it is chemically (2R,cis)-4-amino-1-(2-hydroxymethyl-1,3-oxathiolan-5-yl)-(1H)-pyrimidin-2-one as represented by Formula I (A). 
         [0000]    
       
                 
         
             
             
         
       
     
         [0003]    Two different approaches have been reported for preparing substituted 1,3-oxathiolanes by using specific leaving groups and Lewis acid catalysts. 
         [0004]    The first approach involves condensing an intermediate of Formula II, or its stereoisomers thereof 
         [0000]    
       
                 
         
             
             
         
       
     
         [0000]    wherein P 1  is a protecting group and L 1  is OCH 3 , OC 2 H 5  or OCOCH 3 , with a silyl and/or acetyl protected pyrimidine or purine base. The condensed product is finally deprotected to obtain desired substituted 1,3-oxathiolanes. This approach is provided in U.S. Pat. Nos. 5,047,407 and 5,905,082 , J. Org. Chem ., (1992), 57:2217-2219 , J. Med. Chem ., (1993), 36:181-195, and  J. Org. Chem ., (1991), 56:6503-6505. These references describe processes wherein the condensation is carried out in the presence of silyl Lewis acids such as trimethylsilyl triflate. However, this approach does not provide optically pure 1,3-oxathiolanes and preparation of lamivudine by this approach results in a mixture of at least two of the following isomers. 
         [0000]    
       
                 
         
             
             
         
       
     
         [0005]    The prior art references mentioned above employ chiral chromatography, or enzymatic resolution to isolate lamivudine of Formula I (A) from said mixture.  Synthetic Communications , (2002), 32:2355-2359 provides a separation method for lamivudine using a chiral auxiliary from its mixture with the compound of Formula I (B). U.S. Pat. No. 5,204,466 employs stannic chloride instead of silyl Lewis acids in the condensation of the intermediate of Formula II with silylated cytosine. However,  J. Org. Chem ., (1992), 57:2217-2219 says that the use of stannic chloride as a catalyst also results in a racemic mixture based on optical rotation and chiral HPLC analysis of the product obtained. 
         [0006]    The second approach involves condensing an intermediate of Formula III, or its stereoisomers thereof 
         [0000]    
       
                 
         
             
             
         
       
     
         [0000]    wherein P 1  is a protecting group, L 2  is OCOCH 3 , L 3  is halo, with a silyl and/or acetyl protected pyrimidine or purine base. The condensed product is finally reduced and deprotected to obtain desired substituted 1,3-oxathiolanes. U.S. Pat. No. 5,663,320 provides a method to carry out the condensation in the presence of silyl Lewis acids such as iodotrimethyl silane. U.S. &#39;320 patent employs the compound of Formula III with OCOCH 3  group at 5-position (L 2 ) as an intermediate. U.S. &#39;320 patent further says that the choice of silyl Lewis acids is the key feature of the above process. However, it has been reported in  Tetrahedron Letters , (2005), 46:8535-8538 that the approach involving iodotrimethyl silane is proved to be inefficient for preparing lamivudine as it is low yielding and requires selective crystallization of the intermediates to obtain desired optical purity and so is low yielding. To overcome such purity and yield issues related to Lewis acid catalysts,  Tetrahedron Letters , (2005), 46:8535-8538 and U.S. Pat. No. 6,329,522 provide a method to carry out the condensation in the absence of any Lewis acid catalyst by selectively using the compound of Formula III with a halo group at 5-position (L 3 ) as an intermediate. However, this method requires stereoselective synthesis of the compound of Formula III with halo group at 5-position and the condensation step requires reflux temperature conditions in a high boiling solvent such as toluene. 
         [0007]    U.S. Pat. No. 6,329,522 also provides a method to purify lamivudine by preparing the salicylate salt of lamivudine. The process involves dissolving a mixture of lamivudine with excess of salicylic acid in water by heating at 71° C., cooling to 58° C. and seeding with standard lamivudine salicylate. The solution of lamivudine salicylate seeded with authentic lamivudine salicylate is cooled to 5° to 10° C. and stirred at this temperature for 4 hours to obtain solid lamivudine salicylate. The lamivudine salicylate is isolated by filtration and treated with triethylamine in the presence of industrial methylated spirit. The solution is concentrated and the concentrated solution is seeded. The seeded mixture is treated with isopropyl acetate to obtain lamivudine. However, this method requires heating the reaction mixture to 70° C. to 75° C. while treating with triethylamine and while seeding. Further, US &#39;522 does not disclose or suggest any method to obtain the seed of lamivudine salicylate or lamivudine base. U.S. &#39;522 patent also does not disclose the quantities of seeds to be used in the process. Further, U.S. &#39;522 patent does not provide any method for efficient removal of the residual salicylic acid from lamivudine. 
       SUMMARY OF THE INVENTION 
       [0008]    We have surprisingly found that substituted 1,3-oxathiolanes, such as lamivudine, can be prepared with high optical and chemical purity by using non-silyl Lewis acids in the condensation step. This process also provides a way to obtain substituted 1,3-oxathiolanes with better yield. A chiral auxiliary may need to be used in executing the process but the use of optically pure intermediates and high temperature conditions in the condensation step are not required. The present process can be carried out at a temperature of about 50° C. or less. We have also developed an advantageous method for purifying lamivudine by preparing lamivudine salicylate without seeding at any stage. Further, the salt purification step of the present invention can be carried out at temperature conditions of about 55° C. or below. The process reported herein also provides pure lamivudine, which is substantially free of salicylic acid. The present process is also suitable to prepare lamivudine at industrial scale. 
         [0009]    The term “purine or pyrimidine base or an analogue or derivative thereof” in the present invention refers to a purine or pyrimidine base, which may be found in native nucleosides or a synthetic analogue or derivative thereof that mimics or are derived from such bases in their structures, which may either possess certain additional functional properties of native bases or lack certain functional properties of native bases. The analogues or derivatives include but are not limited to those compounds derived by replacement of a CH 2  moiety by a nitrogen atom or replacement of a nitrogen atom by a CH 2  moiety, or both, or those compounds wherein the ring substituents are either incorporated, removed or modified by conventional substituents known in the art. The functional groups of purine or pyrimidine base or an analogue or derivative thereof may also be protected with hydroxy, amino or carboxyl protecting groups. 
         [0010]    The term “protecting group” in the present invention refers to those known and used in the art and serve the function of blocking the carboxyl, amino or hydroxyl groups while the reactions are carried out at other sites of the molecule. Examples of a carboxyl protecting group include, but are not limited to, substituted or unsubstituted alkyl, substituted or unsubstituted alkenyl, substituted or unsubstituted aralkyl, substituted or unsubstituted aryl, substituted or unsubstituted amino, hydrocarbonated silyl, hydrocarbonated stannyl, and a pharmaceutically active ester forming group. Examples of hydroxyl and amino protecting groups include, but are not limited to, lower alkylsilyl groups, lower alkoxymethyl groups, aralkyl groups, acyl groups, lower alkoxycarbonyl groups, alkenyloxycarbonyl groups and aralkyloxycarbonyl groups. Examples of amino protecting groups include but are not limited to alkylidene groups substituted with optionally protected hydroxy groups. The hydroxyl or carboxyl protecting groups can also be chiral auxiliaries, which may possess one or more chiral centers. 
         [0011]    The term “leaving group” in the present invention refers to an atom or a group which is displaceable upon reaction with a purine or pyrimidine base or an analogue or derivative thereof. Examples of leaving groups include but are not limited to substituted or unsubstituted, saturated or unsaturated acyloxy groups, alkoxy groups, alkoxy carbonyl groups, halogens, amido, azido, cyano, isocyanato, substituted or unsubstituted, saturated or unsaturated thiolates, and substituted or unsubstituted seleno, seleninyl or selenonyl. Examples of leaving groups also include but are not limited to —OX, wherein X is substituted or unsubstituted aryl, heteroaryl, phosphonate, sulfinyl or sulfonoyl group. 
     
    
     DETAILED DESCRIPTION OF THE INVENTION 
       [0012]    A first aspect of the present invention provides a process for the preparation of a substituted 1,3-oxathiolane of Formula I or its stereoisomers, and salts thereof, 
         [0000]    
       
                 
         
             
             
         
       
     
         [0000]    wherein R 1  is hydrogen, alkyl or aryl, and R 2  is a substituted or unsubstituted purine or pyrimidine base or an analogue or derivative thereof,
       wherein the process comprises,   a) reacting a compound of Formula III or its stereoisomers thereof,       
 
         [0000]    
       
                 
         
             
             
         
       
       
         
           
             wherein P 1  is hydrogen or a protecting group and L is a leaving group, with a substituted or unsubstituted purine or pyrimidine base or an analogue or derivative thereof, in the presence of a Lewis acid with the proviso that the Lewis acid does not contain any silyl group, to obtain a compound of Formula IV or its stereoisomers thereof, 
           
         
       
     
         [0000]    
       
                 
         
             
             
         
       
       
         
           
             wherein P 1  is hydrogen or a protecting group and R 2  is a substituted or unsubstituted purine or pyrimidine base or an analogue or derivative thereof, 
             b) educing the compound of Formula IV or its stereoisomers thereof to obtain the compound of Formula I or stereoisomers thereof, and 
             c) isolating the compound of Formula I or its stereoisomers, and salts thereof, from the reaction mixture thereof. 
           
         
       
     
         [0019]    The compound of Formula III or its stereoisomers thereof can be prepared according to the methods provided in U.S. Pat. No. 5,663,320 or  Tetrahedron Letters, ( 2005), 46:8535-8538. The compound of Formula III may be used as a single isomer or as a mixture of two or more isomers. The leaving group L of the compound of Formula III is preferably selected from the group consisting of acyloxy groups, alkoxy groups, and alkoxy carbonyl groups. The leaving group is more preferably acetyloxy group. The compound of Formula III is reacted with a substituted or unsubstituted purine or pyrimidine base or an analogue or derivative thereof, in the presence of a Lewis acid with the proviso that the Lewis acid does not contain any silyl group. The purine or pyrimidine base or an analogue or derivative thereof is preferably selected from the group consisting of: 
         [0000]    
       
                 
         
             
             
         
       
     
         [0000]    wherein P 1  is a protecting group, R 3  and R 4  are independently selected from the group consisting of hydrogen, hydroxyl, amino, and substituted or unsubstituted C 1-6  alkyl, C 1-6  alkenyl, C 1-6  alkynyl, C 1-6  acyl or aracyl; R 5  and R 6  are independently selected from the group consisting of hydrogen, halogen, hydroxyl, amino, cyano, carboxy, carbamoyl, alkoxycarbonyl, hydroxymethyl, trifluoromethyl, thioaryl, and substituted or unsubstituted C 1-6  alkyl, C 1-6  alkenyl, C 1-6  alkynyl, or C 1-10  acyloxy; R 7  is C 1-6  alkyl, C 1-6  alkenyl, or C 1-6  alkynyl; R 8  is selected from the group consisting of hydrogen, hydroxy, alkoxy, thiol, thioalkyl, substituted or unsubstituted amino, halogen, cyano, carboxy, alkoxycarbonyl, carbamoyl, and substituted or unsubstituted C 1-6  alkyl, C 1-6  alkenyl, C 1-6  alkynyl, or C 1-10  acyloxy; and R 9  and R 10  is selected from the group consisting of hydrogen, hydroxy, alkoxy, substituted or unsubstituted amino, halogen, azido, and substituted or unsubstituted C 1-6  alkyl, C 1-6  alkenyl, C 1-6  alkynyl, or C 1-10  acyloxy. 
         [0020]    The Lewis acid is preferably stannic chloride or titanium tetrachloride. The Lewis acid is used in about 0.5 to about 1.5 molar equivalents to the quantity of the compound of Formula III. The Lewis acid is preferably used in about 0.8 to about 1.1 molar equivalents. The reaction is carried out in the presence of an organic solvent selected from the group consisting of aliphatic hydrocarbons, aromatic hydrocarbons, halogenated hydrocarbons, nitriles, amides, esters, and ketones. The reaction is preferably carried out at a temperature of about 50° C. or below. The reaction is carried out for about 10 minutes to about 100 hours. The compound of Formula IV or its stereoisomers can be isolated from the reaction mixture or directly used in the subsequent step without isolation. The compound of Formula IV or its stereoisomers are preferably isolated from the reaction mixture. The compound of Formula IV or its stereoisomers may be subjected to purification to remove chemical impurities and/or undesired isomers. The protecting groups, if any, present in the compound of Formula IV can be removed and the deprotected compound reduced to obtain the compound of Formula I or its stereoisomers. The reduction is carried out by using a reducing agent. The reducing agent can be, for example, sodium borohydride, lithium aluminium hydride or lithium borohydride. The compound of Formula I or its stereoisomers can be further purified by salt formation, crystallization, isomer separation or chromatographic methods or a combination thereof. 
         [0021]    A second aspect of the present invention provides a process for the preparation of lamivudine of Formula I (A) or a compound of Formula I (C), or mixtures thereof, 
         [0000]    
       
                 
         
             
             
         
       
     
         [0000]    wherein the process comprises,
       a) reacting a compound of Formula III (A) or Formula III (B), or mixtures thereof,       
 
         [0000]    
       
                 
         
             
             
         
       
       
         
           
             wherein P 1  is a chiral auxiliary and L is a leaving group, with cytosine, wherein the amino or hydroxy, or both the groups of said cytosine are optionally protected with protecting groups, in the presence of a Lewis acid with the proviso that the Lewis acid does not contain any silyl group, to obtain a compound of Formula IV (A) or Formula IV (B), or mixtures thereof 
           
         
       
     
         [0000]    
       
                 
         
             
             
         
       
       
         
           
             wherein P 1  is a chiral auxiliary and R 2  is cytosine, wherein the amino or hydroxy, or both the groups of said cytosine are optionally protected with protecting groups, 
             b) reducing the compound of Formula IV (A) or Formula IV (B), or mixtures thereof, to obtain lamivudine of Formula I (A) or the compound of Formula I (C), or mixtures thereof, and 
             c) isolating lamivudine of Formula I (A) or the compound of Formula I (C), or mixtures thereof, from the reaction mixture thereof. 
           
         
       
     
         [0027]    The compound of Formula III (A) or Formula III (B), or mixtures thereof, can be prepared according to the methods provided in U.S. Pat. No. 5,663,320 or  Tetrahedron Letters , (2005), 46:8535-8538. The leaving group L of the compound of Formula III (A) or Formula III (B), or mixtures thereof, is preferably selected from the group consisting of acyloxy groups, alkoxy groups, and alkoxy carbonyl groups. The leaving group is more preferably acetyloxy group. The chiral auxiliary P 1  of the compound of Formula III (A) or Formula III (B), or mixtures thereof, is preferably an L-menthyl group. The compound of Formula III (A) or Formula III (B), or mixtures thereof is reacted with cytosine, wherein the amino or hydroxy, or both the groups of cytosine are optionally protected with protecting groups. The cytosine is preferably protected with acetyl and/or silyl protecting groups. The reaction is carried out in the presence of a Lewis acid with the proviso that the Lewis acid does not contain any silyl group. The Lewis acid is preferably stannic chloride or titanium tetrachloride. The Lewis acid is used in about 0.5 to about 1.5 molar equivalents to the quantity of the compound of Formula III. The Lewis acid is preferably used in about 0.8 to about 1.1 molar equivalents. The reaction is carried out in the presence of an organic solvent selected from the group consisting of aliphatic hydrocarbons, aromatic hydrocarbons, halogenated hydrocarbon, nitriles, amides, esters, and ketones, The reaction is preferably carried out at a temperature of about 50° C. or below. The reaction is carried out for about 10 minutes to about 100 hours. 
         [0028]    The compound of Formula IV (A) or Formula IV (B), or mixtures thereof can be isolated from the reaction mixture or directly used in the subsequent step without isolation. The compound of Formula IV (A) or Formula IV (B), or mixtures thereof are preferably isolated from the reaction mixture. A deprotection step may be carried out to remove the protecting groups, if any, present in R 2  of the compound of Formula IV (A) or Formula IV (B), or mixtures thereof. The compound of Formula IV (A) or Formula IV (B), or mixtures thereof may be subjected to purification to remove chemical impurities and/or undesired isomers. The compound of Formula IV (A) or Formula IV (B), or mixtures thereof are reduced to obtain lamivudine of Formula I (A) or the compound of Formula I (C), or mixtures thereof. The reduction is carried out by using a reducing agent. The reducing agent can be, for example, sodium borohydride, lithium aluminium hydride, lithium borohydride, lithium-tri-ethyl borohydride or lithium-tri-sec-butyl borohydride. Lamivudine of Formula I (A) or the compound of Formula I (C), or mixtures thereof can be further purified by salt formation, crystallization, isomer separation or chromatographic methods or a combination thereof. 
         [0029]    A third aspect of the present invention provides a process for the preparation of lamivudine of Formula I (A), 
         [0000]    
       
                 
         
             
             
         
       
     
         [0000]    wherein the process comprises,
       a) reacting a compound of Formula III (C),       
 
         [0000]    
       
                 
         
             
             
         
       
       
         
           
             wherein P 1  is a chiral auxiliary and L is a leaving group, with cytosine, wherein the amino or hydroxy, or both the groups of said cytosine are optionally protected with protecting groups, in the presence of a Lewis acid with the provisothat the Lewis acid does not contain any silyl group, to obtain a compound of Formula IV (C), 
           
         
       
     
         [0000]    
       
                 
         
             
             
         
       
       
         
           
             wherein P 1  is a chiral auxiliary and R 2  is cytosine, wherein the amino or hydroxy, or both the groups of said cytosine are optionally protected with protecting groups, 
             b) separating a compound of Formula IV (A) from the reaction mixture thereof, 
           
         
       
     
         [0000]    
       
                 
         
             
             
         
       
       
         
           
             wherein P 1  is a chiral auxiliary and R 2  is cytosine, wherein the amino or hydroxy, or both the groups of said cytosine are optionally protected with protecting groups, 
             c) reducing the compound of Formula IV (A), and 
             d) isolating lamivudine of Formula I (A) from the reaction mixture thereof. 
           
         
       
     
         [0037]    The compound of Formula III (C) can be prepared according to the methods provided in U.S. Pat. No. 5,663,320 or  Tetrahedron Letters , (2005), 46:8535-8538. The leaving group L of the compound of Formula III (C) is preferably selected from the group consisting of acyloxy groups, alkoxy groups, and alkoxy carbonyl groups. The leaving group is more preferably acetyloxy group. The chiral auxiliary P 1  of the compound of Formula III (C) is preferably an L-menthyl group. The compound of Formula III (C) is reacted with cytosine, wherein the amino or hydroxy, or both the groups of said cytosine are optionally protected with protecting groups. The cytosine is preferably protected with acetyl and/or silyl protecting groups. The reaction is carried out in the presence of a Lewis acid with the proviso that the Lewis acid does not contain any silyl group. The Lewis acid is preferably stannic chloride or titanium tetrachloride. The Lewis acid is used in about 0.5 to about 1.5 molar equivalents to the quantity of the compound of Formula III. The Lewis acid is preferably used in about 0.8 to about 1.1 molar equivalents. The reaction is carried out in the presence of an organic solvent selected from the group consisting of aliphatic hydrocarbons, aromatic hydrocarbons, halogenated hydrocarbon, nitriles, amides, esters, and ketones. The reaction is preferably carried out at a temperature of about 50° C. or below. The reaction is carried out for about 10 minutes to about 100 hours. 
         [0038]    The compound of Formula IV (C) so obtained may be subjected to deprotection to remove the silyl protecting groups, if any, present in the cytosine group. The compound of Formula IV (C) is isolated from the reaction mixture by concentrating the reaction mixture. The compound of Formula IV (A) is separated from the compound of Formula IV (C) by selective crystallization methods, chiral chromatographic methods or by chiral salt formation, or a combination thereof. The compound of Formula IV (A) is preferably separated by treating the compound of Formula IV (C) with a solvent, which selectively dissolves the undesired isomers while the compound of Formula IV (A) is partially or completely insoluble in said solvent. The treatment with the solvent may be carried out once or more than once to achieve desired optical purity. The solvent is preferably a C 1-3  alkanol or an aliphatic ester, or a mixture thereof, more preferably methanol or isopropyl acetate, or a mixture thereof. The compound of Formula IV (A) may be isolated from the reaction mixture by filtration after treating with the solvent. 
         [0039]    The compound of Formula IV (A) is deprotected to remove the acetyl protecting groups, if any, present in R 2  of the compound of Formula IV (A), and reduced to obtain lamivudine of Formula I (A). The reduction is carried out by using a reducing agent. The reducing agent can be, for example, sodium borohydride, lithium aluminium hydride, lithium borohydride, lithium-tri-ethyl borohydride or lithium-tri-sec-butyl borohydride. The reducing agent is preferably sodium borohydride. The reduction is carried out in the presence of a phosphate or borate buffer. The buffer is preferably dipotassium hydrogen phosphate. Lamivudine of Formula I (A) can be further purified by salt formation, crystallization, or chromatographic methods, or a combination thereof. 
         [0040]    A fourth aspect of the present invention provides a process for the isolation of lamivudine, wherein the process comprises,
       a) reducing a compound of Formula IV (A),       
 
         [0000]    
       
                 
         
             
             
         
       
       
         
           
             wherein P 1  is a chiral auxiliary and R 2  is cytosine, in the presence of water or an organic solvent or a mixture thereof, to obtain lamivudine, 
             b) treating lamivudine with salicylic acid in the presence of water or an organic solvent or a mixture thereof, 
             c) isolating lamivudine salicylate from the reaction mixture thereof without seeding of lamivudine salicylate, 
             d) treating lamivudine salicylate with a base in the presence of water or an organic solvent or a mixture thereof, and 
             e) isolating lamivudine from the reaction mixture thereof, 
           
         
       
     
         [0047]    A fifth aspect of the present invention provides a process for the isolation of lamivudine, wherein the process comprises,
       a) reducing a compound of Formula IV (A),       
 
         [0000]    
       
                 
         
             
             
         
       
       
         
           
             wherein P 1  is a chiral auxiliary and R 2  is cytosine, in the presence of water or an organic solvent or a mixture thereof, to obtain lamivudine, 
             b) treating lamivudine with salicylic acid in the presence of water or an organic solvent or a mixture thereof, 
             c) isolating lamivudine salicylate from the reaction mixture thereof, 
             d) treating lamivudine salicylate with a base in the presence of an organic solvent, or a mixture of water and an organic solvent, and 
             e) isolating lamivudine from the reaction mixture thereof without seeding of lamivudine. 
           
         
       
     
         [0054]    The compound of Formula IV (A), which is the starting material, can be obtained by following the methods provided in U.S. Pat. No. 5,663,320 or  Tetrahedron Letters , (2005), 46:8535-8538, or the methods disclosed in the previous aspects of the present invention. The compound of Formula IV (A) is reduced in the presence of water or an organic solvent or a mixture thereof, to obtain lamivudine. The organic solvent is preferably selected from the group consisting of alkanols, ethers and esters. The organic solvent is more preferably selected from the group consisting of methanol, ethanol, tetrahydrofuran, dioxane, isopropyl acetate and ethyl acetate. The reduction is carried out by using a reducing agent. The reducing agent is preferably sodium borohydride, lithium aluminium hydride, lithium borohydride, lithium-tri-ethyl borohydride or lithium-tri-sec-butyl borohydride. The reducing agent is more preferably sodium borohydride. The reduction is preferably carried out in the presence of a phosphate or borate buffer. The buffer is preferably dipotassium hydrogen phosphate. The lamivudine so obtained need not be isolated from the reaction mixture and it is treated with salicylic acid. The lamivudine salicylate is isolated from the reaction mixture without the addition of seed lamivudine salicylate. The isolation of lamivudine salicylate is carried out by stirring the reaction mixture in a temperature range from about 10° C. to about 25° C. The stiffing is preferably carried out initially at about 25° C. to about 30° C. and subsequently at about 10° C. to about 15° C. The stiffing can be carried out from about 10 minutes to about 100 hours. The lamivudine salicylate so obtained is treated with a base in the presence of an organic solvent, or a mixture of water and an organic solvent. Preferably a mixture of water and an organic solvent is used as a solvent medium while treating lamivudine salicylate with a base. The organic solvent is preferably selected from the group consisting of alkanols, ethers and esters. The organic solvent is more preferably selected from the group consisting of methanol, ethanol, tetrahydrofuran, dioxane, isopropyl acetate and ethyl acetate. The base is preferably an amine, more preferably a tertiary amine. The treatment of lamivudine salicylate with the base is carried out at a temperature of about 55° C. or below, preferably at about 40° C. to about 50° C. The process is accompanied by stiffing to facilitate the liberation of lamivudine as a free base. The lamivudine is isolated from the reaction mixture without adding any seed. The isolation is carried out by stirring the reaction mixture at a temperature of about 0° C. to about 35° C., preferably at about 15° C. to about 30° C., followed by filtration, distillation and/or concentration. A washing of lamivudine with an organic solvent is optionally employed after isolation. 
         [0055]    The lamivudine so obtained may be purified further to remove any impurities including any residual salicylic acid. The purification process can be carried out by dissolving lamivudine in a C 1-3  alkanol at reflux temperature and treating the solution with activated charcoal. After removal of the charcoal, lamivudine is obtained as a solid by stiffing the solution at about 0° C. to about 15° C., and the solid can be isolated by filtration. The lamivudine so obtained is pure and it is substantially free of salicylic acid. The lamivudine so obtained has salicylic acid content of about 0.1% w/w or less, preferably of about 0.05% w/w or less, more preferably of about 0.01% w/w or less. The lamivudine so obtained has a chemical purity of about 99% or above and a chiral purity of about 99.5% or above, preferably of about 99.8% or above. 
         [0056]    A sixth aspect of the present invention provides a pharmaceutical composition comprising lamivudine substantially free of salicylic acid and an excipient/carrier. The lamivudine substantially free of salicylic acid has salicylic acid content of about 0.1% w/w or less, preferably of about 0.05% w/w or less, more preferably of about 0.01% w/w or less. 
         [0057]    A seventh aspect of the present invention provides a pharmaceutical composition comprising lamivudine having a chemical purity of about 99% or above and a chiral purity of about 99.5% or above, preferably of about 99.8% or above, and an excipient/carrier. 
         [0058]    While the present invention has been described in terms of its specific embodiments, certain modifications and equivalents will be apparent to those skilled in the art and are intended to be included within the scope of the present invention. 
       EXAMPLES 
     Example 1 
     Preparation of (1R,2S,5R)-2-isopropyl-5-methylcyclohexyl (2R,5S)-5-[4-(acetylamino)-2-oxopyrimidin-1(2H)-yl]-1,3-oxathiolane-2-carboxylate 
       [0059]    N-Acetyl cytosine (100 g) was added to xylene (200 mL), followed by the addition of hexamethyldisilazane (200 mL) and trimethylchlorosilane (5.0 mL). The reaction mixture was heated in at 140° to 145° C. for 3 to 4 hours. The reaction mixture was cooled to 100° C. and xylene (200 mL) was added to the reaction mixture. The reaction mixture was distilled under vacuum at 120° to 130° C. Xylene (200 mL) was added to the reaction mixture and recovered under vacuum to obtain a residue. Dichloroethane (2 L) was added to the residue, followed by the addition of (2S,5R)-2-isopropyl-5-methylcyclohexyl (2R)-5-(acetyloxy)-1,3-oxathiolane-2-carboxylate (194 g). Stannic chloride (152 g) was added drop-wise to the reaction mixture at 40° to 45° C. and stirred for 5 hours at 40° to 45° C. The reaction mixture was cooled to about 25° C., followed by the addition of a mixture of methanol (1 L) and water (1 L). The reaction mixture was allowed to settle and the organic layer was added to a mixture of water (1 L) and methanol (1 L). The reaction mixture was stirred for 5 to 10 minutes and allowed to settle. The organic layer was concentrated under vacuum at 40° to 50° C. Isopropyl acetate (1.5 L) was added to the resultant mass and stirred overnight at about 25° C. The reaction mixture was filtered, washed with isopropyl acetate (2×100 mL) and dried at 45° to 50° C. for 5 hours. The solid so obtained was suspended in methanol (500 mL) at about 25° C. and stirred for 3 hours at the same temperature. The mixture was filtered, washed with methanol (100 mL) and dried at 45° to 50° C. for 5 hours to obtain the title compound. 
         [0060]    Yield: 28 g 
       Example 2 
     Preparation of (1R,2S,5R)-2-isopropyl-5-methylcyclohexyl (2R,5S)-5-(4-amino-2-oxopyrimidin-1(2H)-yl)-1,3-oxathiolane-2-carboxylate 
       [0061]    (1R,2S,5R)-2-isopropyl-5-methylcyclohexyl (2R,5S)-5-[4-(acetylamino)-2-oxopyrimidin-1(2H)-yl]-1,3-oxathiolane-2-carboxylate (100 g) obtained from Example 1 was suspended in methanol (600 mL) at about 25° C. Methane sulphonic acid (29.4 g) was added drop-wise to the suspension in 15 to 20 minutes at 25° to 30° C. and stirred for 4 hours at about 25° C. The reaction mixture was added slowly to a mixture of dichloromethane (1 L) and aqueous sodium bicarbonate solution (28 g of sodium bicarbonate dissolved in 1.2 L of water.) The reaction mixture was stirred for 5 to 10 minutes and allowed to settle. The layers were separated and the aqueous layer was re-extracted with dichloromethane (250 mL). The organic layers were combined and concentrated, followed by the addition of hexane (500 mL). The reaction mixture was stirred for about 2 hours and filtered. The solid was washed with hexane (100 mL) and subsequently with isopropyl acetate (200 mL), and dried at 45° to 50° C. to obtain the title compound. 
         [0062]    Yield: 82 g 
       Example 3 
     Preparation of Lamivudine Salicylate 
       [0063]    Dipotassium hydrogen orthophosphate (205.5 g) was added to deionised water (423 mL) and stirred at 25° to 30° C. to obtain a solution. The solution was cooled to 17° to 22° C., followed by the addition of denaturated spirit (900 mL) at the same temperature and stirred for 5 minutes. (1R,2S,5R)-2-isopropyl-5-methylcyclohexyl (2R,5S)-5-(4-amino-2-oxopyrimidin-1(2H)-yl)-1,3-oxathiolane-2-carboxylate (150 g) obtained from Example 2 was added to the mixture at 17° to 22° C. and stirred for 30 minutes at 18° to 20° C. Sodium borohydride solution was added slowly to the reaction mixture over a period of 2 to 3 hours at 18° to 20° C. (Preparation of sodium borohydride solution: Sodium hydroxide (0.75 g) was dissolved in deionised water (143 mL). Sodium borohydride (30 g) was added to the solution at 20° to 35° C., stirred at 20° to 35° C. to obtain a solution and cooled to 17° to 22° C.). The reaction mixture was stirred for 6 hours at 18° to 22° C. and the reaction mixture was allowed to settle at 18° to 25° C. The organic layer was separated and denaturated spirit (150 mL) was added to the aqueous layer at 18° to 25° C. The reaction mixture was stirred for 15 minutes at the same temperature and allowed to settle. The organic layer was separated and combined with the previously obtained organic layer. The pH of the combined organic layer was adjusted to 6.0 to 6.5 with dilute hydrochloric acid (20 mL; prepared by mixing 10 mL of concentrated hydrochloric acid with 10 mL of deionised water) at 18° to 25° C., followed by stirring for 10 minutes at the same temperature. The pH of the reaction mixture was adjusted to 8.0 to 8.5 with aqueous sodium hydroxide solution (28 mL; prepared by dissolving 2.1 g of sodium hydroxide in 27 mL of deionised water) at 18° to 25° C. The reaction mixture was concentrated under vacuum at about 55° C. till the residual volume was about 375 mL. Deionised water (300 mL) was added to the concentrated reaction mixture at 25° to 30° C. and stirred for 10 minutes. The reaction mixture was washed with toluene (2×150 mL) at 25° to 30° C. and the toluene layer was extracted with deionised water (150 mL) at 25° to 30° C. The aqueous layers were combined and salicylic acid (57 g) was added at 25° to 30° C. Deionised water (150 mL) was added to the reaction mixture and heated to 78° to 82° C. to get a clear solution. The reaction mixture was cooled to 25° to 30° C. over a period of 2 hours and stirred at the same temperature for 4 hours. The reaction mixture was further cooled to 10° to 15° C. and stirred for 2 hours at 10° to 15° C. The solid was filtered, washed with deionised water (150 mL) and dried by suction. The solid so obtained was washed with methanol (90 mL, pre-cooled to 5° to 10° C.) and dried at 45° to 50° C. in hot air oven to obtain the title compound. 
         [0064]    Yield: 132 g 
       Example 4 
     Preparation of Lamivudine 
       [0065]    Lamivudine salicylate (120 g) obtained from Example 3 was added to a mixture of ethyl acetate (720 mL) and water (6 mL) at 25° to 35° C. The reaction mixture was heated to 45° to 50° C., followed by the addition of triethylamine (104.76 g) over 30 minutes at 45° to 50° C. The reaction mixture was stirred for 4 hours at the same temperature and cooled to 25° to 30° C. The reaction mixture was stirred for further 30 minutes at 25° to 30° C., filtered and dried by suction. The solid obtained was washed with ethyl acetate. Ethyl acetate (600 mL) was added to the washed solid and heated to 50° to 55° C. The mixture was stirred at 50° to 55° C. for 15 minutes, cooled to 25° to 30° C. and stirred for further 30 minutes. The solid was filtered at 25° to 30° C., washed with ethyl acetate (60 mL) and dried under vacuum at 45° to 50° C. to obtain the title compound. 
         [0066]    Yield: 68.5 g 
       Example 5 
     Purification of Lamivudine 
       [0067]    Lamivudine (60 g) obtained from Example 4 was added to absolute alcohol (1.2 L) at 25° to 35° C. The reaction mixture was heated to 75° to 78° C. and stirred to obtain a solution. Activated carbon (6 g) was added to the solution so obtained at 75° to 78° C., stirred for 30 minutes at the same temperature and filtered through Celite bed at the same temperature. The carbon bed was washed with absolute alcohol (60 mL; preheated to 75° to 76° C.) and the reaction mixture was concentrated under vacuum to obtain a volume of about 300 mL. The concentrated reaction mixture was heated to 74° to 76° C., stirred for 15 minutes and cooled to 20° to 25° C. in 1 hour time. The reaction mixture was further cooled to 5° to 10° C. in 1 hour time and stirred for 30 minutes. The solid was filtered, washed with absolute alcohol (30 mL, pre-cooled to 5° to 10° C.) and dried under vacuum at 50° to 55° C. to obtain the title compound. 
         [0068]    Yield: 53 g 
         [0069]    HPLC Purity: 99.0% 
         [0070]    Chiral Purity: 99.8% 
         [0071]    Salicylic acid content (HPLC): Not detectable