Title of Invention	INDUSTRIAL PROCESS FOR THE MANUFACTURE OF LAMIVUDINE
Abstract	This invention relates to simple, economical, cost-effective method for the preparation of lamivudine of formula(I).

Full Text

1 FORM 2 THE PATENTS ACT, 1970 (39 of 1970) & THE PATENTS RULES, 2003 COMPLETE SPECIFICATION (Se section 10, rule 13) INDUSTRIAL PROCESS FOR THE MANUFACTURE OF LAMIVUDINE" We, Emcure Pharmaceuticals Limited, of 12/2 F-II Block, M.I.D.C. Pimpri, Pune 411018, Maharashtra, India; The following specification particularly describes the invention and the manner in which it is to be performed. 1 INDUSTRIAL PROCESS FOR THE MANUFACTURE OF LAMIVUDINE FIELD OF THE INVENTION This invention relates to simple, economical, cost-effective method for the preparation of lamivudine of formula (I). INTRODUCTION Acquired Immunodeficiency Syndrome (AIDS) is a fatal disease of the immune system transmitted through blood especially by sexual contact or contaminated needles and is presumed to be caused by the HIV virus, which is an RNA genetically unique retrovirus, having a gene not found to date in other retroviruses. The HIV virus appears to preferentially attack receptive T-cells also known as CD4+ cells. These cells are white blood cells that turn the immune system on to fight disease. The virus invades the receptive T-cells and the T-cells become an HIV virus producer. The receptive T-cell is quickly damaged and their number in the human being is depleted to such an extent that the body's beta-cell as well as other T-cells no longer function normally and thereby destroys the body's ability to fight infections (US 4 837 208 and US 4 828 838, Zidovudine). Many of the nucleosides and their analogues belonging to the important class of therapeutic agents, have proved as effective drugs against hepatitis B virus, human immunodeficiency virus (HIV) and human T-lymphotropic virus due to their antiviral activity (Ref: US 5 693 787). One of the many drugs found to be active against HIV virus is Lamivudine of formula (I), chemically known as (-)-l-[(2R,5S)-2-(hydroxymethyl)-l,3, oxathiolan-5-yl]cytosine. 2 Lamivudine is (-) enantiomer of dideoxy analogue of cytidine, marketed under the trade name Epivir. It is an anti-retroviral agent belonging to the class of the drugs called "Nucleoside Reverse Transcriptase Inhibitors (NRTI)." Lamivudine inhibits HIVI, HIV2 reverse transcriptase and also the reverse transcriptase of hepatitis B. The therapeutic action of lamivudine is due to the intracellular conversion of lamivudine to the active metabolite lamivudine-S'-triphosphate, which inhibits the RNA and DNA dependant activities of HIV reverse transcriptase by termination of the viral DNA chain. Lamivudine, exhibits little effect on mammalian cell and mitochondrial DNA content and does not interfere with cellular deoxynucleotide metabolism. (Ref: http://en.wikipedia.ora/wiki/Lamivudine. http://xweb.crha- health.ab.ea/clin/sac/larnivudi.huru http://home.intekom.com/pharm/aspen-p/a-lamiv.html and Drug Label qntp://wwwJ)Ja.aov/cder/foi/label/2004/21003s005.21004s0061bl.pdf) BACKGROUND OF THE INVENTION Lamivudine of formula (I) contains two chiral centers, and exists as four diastereomers (two in the cis-configuration and two in the trans-configuration). However, prior art indicates that only the cis isomer of compound (I) exhibits the desired biological activity (Ref: US 5 693 787). It is well known in the prior art, that optical isomers of pharmaceutical compounds having an asymmetric center, invariably exibit different physiological activities. One optical form may be bioactive, while the other could be inactive or toxic. 3 It is found that among the cis isomers of the compound of formula (I) only (-) enantiomer has considerably lower cytotoxicity and should be provided with the purity of the desired isomer being above 99% (WO 91/17159). US 5 047 407 (assigned to IAF BioChem International) discloses a process for the preparation of of formula (I) according to the method represented in Scheme-1. Scheme-1: Method as disclosed in US 5 047 407 for preparation of compound of formula (I). The method summarized in Scheme-1 discloses the preparation of 2-benzoyloxymethyl-5-(N-acetylcytosin-l-yl)-l,3-oxathiolane (VI) by reaction of 2-benzoyloxymethyl-5-ethoxy-l,3-oxathiolane (II) with silylated cytosine (III) in the presence of a catalyst like 4 trimethylsilyl triflate with three days of refluxing. The racemic product (IV) was then acetylated to give compound of formula (V), which was purified by column chromatography to give the cis isomer of 2-benzoyloxymethyl-5-(N-acetyl-cytosin-l-yl)-1,3-oxathiolane (VI), which was then hydrolyzed to give lamivudine of formula (I). This method suffers from the drawback of longer reaction times, and further the product obtained has to be purified by column chromatography, which is cumbersome and time-consuming on an industrial scale. Also, separation by column chromatography does not give a single enantiomer but gives 60:40 ratio of trans: cis isomers respectively i.e. the cis isomer (desired isomer) is obtained in low yield compare to undesired isomer i.e. trans isomer. Therefore, this method is not suitable for industrial scale. WO 91/17159 discloses another method for preparation of (+)-cis-4-amino-l-(2-benzoyloxymethyl-l,3-oxathiolane-5-yl)-lH-pyrimidin-2-one of formula (IV) and lamivudine of formula (I), according to the method summarised in Scheme-2. The method involves reaction of 5-methoxy-l,3-oxathiolane-2-methanol benzoate (VIII) with silylated cytosine (III) in presence of trimethylsilyl trifluromethane sulfonate to give (±)-cis-4-amino-l-(2-benzoyloxymethyl-l,3-oxathiolane-5-yl)-(lH)-pyrimidin-2-one (IV). Deprotection of hydroxy group on an amberlite resin column gives (±)-cis-4-amino-1 -(2-hydroxymethyl-1,3 -oxathiolane-5-yl)-( 1 H)-pyrimidin-2-one (I-b), which is then separated by chromatography using a chiral column to give lamivudine of formula (I). This method has the following drawbacks: a) utilizing a hazardous chemical like trimethylsilyl trifluromethane sulfonate, b) employs column chromatography twice, firstly by using a resin column for deprotecting hydroxy group and secondly using a chiral column for separation of desired isomer. Utilization of chromatography extensively, increases the cost of 5 the process, since this involves lot of solvent, the method is also quite cumbersome since it involves a lot of time. Scheme-2: Method as disclosed in WO 91/17159 for preparation of compound of formula (I). US 6 329 522 (assigned to Glaxo Group Ltd) discloses preparation of compound of formula (I), according to method summarized in Scheme-3. The method involves preparation of compound of formula (I) from the optically active intermediate (2R,5R)-5-hydroxy-[l,3]-oxathiolane-2-carboxylic acid - 2S-isopropyl-5R-methyl-lR-cyclohexyl ester (IX), which is then converted to its chloro derivative (X) by reaction with a mixture of thionyl chloride / DMF in presence of methanesulphonic acid. 6 Scheme-3: Method as disclosed in US 6 329 522 for preparation of compound of formula (I). The chloro derivative (X) is then reacted with silylated cytosine (III) in presence of triethylamine to give (2R,5R)-5-(4-amino-oxo-2H-pyrimidine-l-yl)-[l,3]oxathiolane-2-carboxylic acid - 2S -isopropyl-5R-methyl-lR-cyclohexyl ester (XI). The menthyl ester group of compound after hydrolysis in alkaline conditions is treated with salicylic acid to give lamivudine monosalicylate (XII), which on treatment with triethyl amine in methylated spirit gives lamivudine of formula (I). This patent further neither mentions optical purity nor the yield of (2R,5R)-5-(4-Amino-oxo-2H-pyrimidine-l-yl)-[l,3]oxathiolane-2-carboxylic acid-2S-isopropyl-5R-methyl-lR-cyclohexyl ester, thus obtained. US 5 204 466 (assigned to Emory University) also describes a method as disclosed in Scheme-4 for preparation of lamivudine, which comprises reaction of compound of 7 formula (XV) with silylated cytosine in the presence of Lewis acid such as SnCl4, to give compound of formula (XVI) in 80 % yield after flash chromatography. Scheme-4: Method as disclosed in US 5 204 466 for preparation of compound of formula (I). However, this method requires flash chromatography to get the desired isomer of the compound, which however, is not feasible on the commercial scale. WO 92/20669 discloses two methods Method-A (Scheme-5A) and Method-B (Scheme- . 5B) for the preparation of lamivudine of formula (I). 8 Scheme-5(A): Method-A as disclosed in WO 92/20669 for the preparation of Lamivudine of formula (I). 9 Scheme-5(B): Method-B as disclosed in WO 92/20669 for the preparation of Lamivudine of formula (I). Both these methods utilize a hazardous reagent like iodotrimethylsilane and t-butyldimethylsilyl triflate for the preparation of menthyl ester derivative of lamivudine. Further, repeated steps of fractional crystallization reduces the yield and increases the cost of utilities and labour required for these repeated steps. Thus the prior art methods suffer from the following drawbacks: a) these methods utilizes a hazardous and a costly reagent like iodotrimethylsilane, which is expensive for utilization on an industrial scale. 10 b) utilization of reagents like trimethylsilyl trifluoromethane sulfonate or Lewis acids does not lead to preferential synthesis of the desired isomer intermediate and requires separation of the desired isomer by chromatographic techniques. c) reaction involving silylated cytosine (III), is quite long and time consuming even after utilization of costly reagents like trimethylsilyl triflate. d) repeated fractional crystallization reduces the yield and consequently increases the cost of utilities, labour etc. The present inventors have therefore developed a method, which overcomes the drawbacks of prior art and provides a improved process for preparation of Lamivudine of formula (I). Futher, it has been reported in literature (US 5 905 082) that lamivudine exists in two polymorphs, known as Polymorphic Form I and Form II. Form I is abtained by crystallization from aqueous solution, in the form of needle shaped crystals. However, this form is not suitable for pharmaceutical formulation in the form of solid dosage forms because of unfavourable properties for preparing dosage form. Form II is bipyramidal and preferred in the manufacture of solid dosage form. This form is obtained by recrystallization from non-aqueous media, like lower alcohols such as ethanol, industrial methylated spirit etc. WO 03/027106 Al discloses that Form II of Lamuvidine of formula (I) is obtained by crystallization of lamivudine from solvents like ethyl acetate, acetonitrile, aliphatic ketones, esters, either straight chain or branched chain ethers containing d-8 atoms. This PCT application discloses that straight chain or branched chain ethers containing C1. 8 atoms can be used for crystallizing lamivudine to give Form-II polymorph. However there are no enabling disclosures for their preparation and neither mentions the ethers that could be employed for preparing Form-II lamivudine. 11 The present inventors have found that Form-II polymorph of lamivudine of formula (I) can be obtained by the utilization of cyclic ethers like tetrahydrofuran. OBJECT OF THE INVENTION An object of the invention is to provide simple and cost effective process, without isolation of any intermediate, for the preparation of (-)-l-[(2R, 5S)-2-(hydroxymethyl)-1,3, oxathiolan-5-yl]cytosine of formula (I). Another object of the invention is to provide a process for the preparation of (-)-l-[(2R, 5S)-2-(hydroxymethyl)-l? 3-oxathiolan-5-yl] cytosine of formula (I) having the desired yield and optical purity. Yet another object of the invention is to make the process commercially viable by avoiding the separation of diastereomers to give lamivudine of formula (I). A still further object of the invention is to provide a process for preparation of a compound of formula (I), which avoids the use of costly reagents like trimethylsilyl iodide and instead employs a mixture of trimethylsilyl chloride and alkali metal iodide, thereby making the process cost-effective. Yet a still further object of the invention is to provide a process for preparation of formula (I), while avoiding the use of high volume of the solvent for getting the desired form of Lamivudine of formula (I). Still, another object of the invention is to utilize cyclic ethers for getting the desired Form-II of Lamivudine of formula (I), without preparation of lamivudine salicylate of formula (XII). 12 SUMMARY OF THE INVENTION: One aspect of the invention is to provide an economical, simple and cost-effective process for the preparation of (-)-l-[(2R,5S)-2-(hydroxymethyl)-l,3-oxathiolan-5-yl] cytosine of formula (I). Another aspect of the invention relates to an industrially feasible and commercially viable process for the preparation of (-)-l-[(2R,5S)-2-(hydroxymethyl)-l;3-oxathiolan-5-yl] cytosine of formula (I), which comprises treatment of (2R,5R)-5-hydroxy-l,3-oxathiolane-2-carboxylic acid, 2S-isopropyl-5R-methyl-R-cyclohexylester of formula (IX) with trimethylsilyl chloride and alkali metal iodide, followed by reaction with silylated cytosine of formula (III) to give (2R,5R)-5-hydroxy-1,3-oxathiolane-2-carboxylic acid, 2S-isopropyl-5R-methyl-R-cyclohexylester of formula (XI) which on reduction in an organic solvent or optionally through the formation of lamivudine salicylate salt of formula (XII) gives lamivudine Form-II of formula (I) having desired optical and chemical purity (Scheme-6). 13 Scheme-6: Method embodied in the present invention for the preparation of lamivudine (I). A further aspect of the invention relates to a method for preparation of Form-II of Lamivudine of formula (I) without preparation of lamivudine salicylate salt of formula (XII), thereby, a) avoiding the steps of preparation of the lamivudine salicylate salt, b) avoiding breaking of the salicylate salt, and c) avoiding the use of salicylic acid, hence making the process cost-effective. 14 Another aspect of the invention relates to the utilization of cyclic ether as an organic solvent for preparation of Form-II of Lamivudine of formula (I) DETAILED DESCRIPTION OF THE INVENTION The process embodied in the present invention relates to a method for the preparation of a lamivudine intermediate of formula (XI) having high enantiomeric purity from compound of formula (IX) by a cost-effective process, avoiding the step of resolution of enantiomers of formula (XI) as normally carried out in prior art, while utilizing relatively less hazardous reagents like trimethylsilyl halide and an alkali iodide. Also, the present invention relates to a method for preparing Form-II of Lamivudine, by employing cyclic ether. The compound of formula (IX) was obtained according to the general methods reported in prior art. According to the method embodied in the present invention, the compound of formula (XI) was prepared by i) insitu preparation of the chloro derivative of formula (X) by reaction of compound of formula (IX) with a mixture of thionyl chloride and dimethyl formamide, ii) reaction of compound (X) with trimethylsilyl chloride in the presence of a alkali metal iodide in an organic solvent, followed by reaction with silylated cytosine of formula (III), to give compound of formula (XI), iii) reduction of compound of formula (XI) with sodium borohydride in an alkaline medium, followed by heating of the resulting compound of formula (I) in an organic solvent to give Lamivudine Form-II or by optional treatment of the resulting compound of formula (I) obtained after reduction of formula (XI), with salicylic acid in an aqueous medium to obtain lamivudine salicylate of formula (XII), followed by treatment with an organic base in an organic solvent at elevated temperature to give Lamivudine Form-II. 15 Step-I: Preparation of (2R,5R)-5-Chloro-l,3-oxathioIane-2-carboxylic acid, 2S-isopropyl-5R-methyl-lR- cyclohexyl menthyl ester of formula (X) In a specific embodiment, (2R,5R)-5-hydroxy-l,3-oxathiolane-2-carboxylic acid, 2S-isopropyl-5R-methyl-lR-cydohexyl menthyl ester of formula (IX) is added to an organic solvent. The organic solvent is selected from the group comprising of chlorinated solvent, ester, and an aliphatic hydrocarbon. The preferred organic solvent is a chlorinated solvent. The chlorinated solvent is selected from the group comprising of chloroform, dichloromethane, ethylene dichloride etc. The preferred solvent is dichloromethane. The volume of dichloromethane is between 1.0 volume and 10.0 volumes of dichloromethane per gram of compound of formula (IX). The preferred volume of dichloromethane is between 2.0 volumes and 5.0 volumes per gram of compound of formula (IX). Dimethyl formamide is added to the reaction mixture. The volume of dimethyl formamide is between 1.0 volume and 5.0 volumes of dimethyl formamide per gram of compound of formula (IX). The preferred volume of dimethyl formamide is between 1.5 volume and 2.5 volumes of dimethyl formamide per gram of compound of formula (IX). Thionyl chloride is added to the reaction mixture. 16 The amount in moles of thionyl chloride added per mole of compound (IX) is between 1.0 mole and 2.0 moles. The preferred amount of thionyl chloride added is between 1.25 moles and 1.75 moles of thionyl chloride per mole of compound (IX). (2R,5R)-5-ChIoro-( 1,3)-oxathiolane-2-carboxylic acid, 2S-isopropyl-5R-methyl-1R-cyclohexyl menthyl ester of formula (X) thus obtained was not isolated but used as such for preparation of (2R,5R)-5-(4-amino-2-oxo-2H-pyrimidin-l-yl)-(l,3)-oxathiolane-2-carboxylic acid, 2S-isopropyl-5R-methyl-lR-cyclohexyl menthyl ester of formula (XI). Step-II: Preparation of (2R,5R)-5-(4-amino-2-oxo-2H-pyrimidin-l-yl)-(l,3)-oxathiolane-2-carboxylic acid, 2S-isopropyl-5R-methyl-lR-cyclohexyl menthyl ester of formula (XI). The organic layer containing the compound of formula (III) was treated with trimethylsilyl chloride and an alkali metal iodide. The organic layer containing the compound of formula (III) was added to a mixture of trimethylsilyl chloride, and alkali metal iodide in an organic solvent. The organic solvent is selected from the group of solvents comprising of nitriles, chlorinated solvents, and ketones or mixtures thereof. The preferred solvent is either a nitrile or a chlorinated solvent. The chlorinated solvent is selected from the group comprising of chloroform, dichloromethane, ethylene dichloride etc. The preferred solvent is dichloromethane. 17 The volume of dichloromethane is between 1 volume and 10 volumes per gram of compound (II). The preferred volume of dichloromethane is between 1.5 volume and 5.0 volumes per gram of compound (II). The nitrile solvent is selected from the group comprising of acetonitrile, benzyl cyanide, etc. The preferred solvent is acetonitrile. The volume of acetonitrile is between 1 volume and 10 volumes per gram of compound en). The preferred volume of acetonitrile is between 1.5 volume and 5.0 volumes per gram of compound (II). The alkali metal of the alkali metal iodide is selected from the group of lithium, sodium, potassium etc. The preferred alkali metal in the alkali metal iodide is either sodium or potassium. The alkali metal iodide is selected from the group comprising of sodium iodide, potassium iodide. The reaction mixture is stirred at ambient or elevated temperature, and utilized for preparing compound of formula (XI). The organic layer containing silylated cytosine of formula (III) dissolved in dichloromethane. 18 A base selected from the group comprising of an inorganic or an organic base is added to the mixture containing silylated cytosine of formula (III). The preferred base is an organic base. The organic base is selected from the group comprising of triethyl amine, pyridine, n-butyl amine is added to the reaction mixture. The preferred organic base is triethyl amine. The organic layer containing the iodo derivative of (2R,5R)-5-chloro-(l,3)-oxathiolane-2-carboxylic acid, 2S-isopropyl-5R-methyl-lR- cyclohexyl menthyl ester of formula (X-a) is added to silylated cytosine of formula (III) thus prepared or vice-versa. The reaction is carried out at elevated temperature and agitated till completion of the reaction and cooled to ambient temperature. The reaction mixture was washed with a dilute aqueous solution of an alkali, followed by optional washing with an aqueous solution of sodium thiosulphate. The organic layer is separated and diluted with an organic solvent. The organic solvent is selected from the group comprising of an aliphatic hydrocarbon, aromatic hydrocarbon and chlorinated hydrocarbon. The preferred organic solvent is aliphatic hydrocarbon. The aliphatic hydrocarbon is selected from the group comprising of hexane, cyclohexane, heptane, and octane. The preferred organic solvent is hexane. 19 The pH of the reaction mass is adjusted with an inorganic base or an organic base. The preferred base is an inorganic base. The inorganic base is selected from the group comprising of bicarbonates, carbonates, hydroxides of an alkali or an alkali earth metal. The inorganic base is preferably the bicarbonate of an alkali metal selected from the group comprising of lithium, sodium and potassium. The preferred inorganic base is sodium bicarbonate or potassium bicarbonate. An aqueous solution of the preferred inorganic base is added to the reaction mixture till the compound of formula (XI) separates out. The mixture is then filtered and the compound of formula (XI) dried. (2R,5R)-5-(4-amino-2-oxo-2H-pyrimidin-I-yl)-(l,3)-oxathiolane-2-carboxylic acid, 2S-isopropyl-5R-methyl-lR-cyclohexyl menthyl ester of formula (XI) thus obtained with a yield of « 90% had optical purity > 99% with specific optical rotation of-116°. The advantages of the present invention lies in a) avoiding utilization of expensive and hazardous iodomethylsilane, b) utilization of reagents like trimethylsilyl chloride and alkali metal iodide, which makes the process environmentally friendly, economical and cost-effective, c) the compound of formula (XI) thus obtained does not require resolution techniques like column chromatography, thereby making the process more suitable for industry, d) the method utilizes solvents very optimally, thereby increasing the batch size, reducing filtration time, cost of utility and making the whole process cost-effective. 20 e) , (2R,5R)-5-(4-amino-2-oxo-2H-pyrimidin-1 -yl)-( 1,3)-oxathiolane-2-carboxylic acid, 2S-isopropyl-5R-methyl-lR-cyclohexyl menthyl ester of formula (XI) is obtained with good yields («90%) and acceptable optical purity of 99% or more. f) (2R,5R)-5-(4-amino-2-oxo-2H-pyrimidinl-yl)-(l,3)-oxathiolane-2-carboxylic acid and 2S-isopropyl-5R-methyl-lR-cyclohexyl ester of formula (XI) are prepared from (2R,5R)-5-hydroxy-(l,3)-oxathiolane-2-carboxylic acid, 2S-isopropyl-5R-methyl-lR-cyclohexyl ester of formula (IX) without any process of isolation of the intermediates, thereby making the process economical and cost-effective by reducing reactor occupancy, labour, time etc. g) obviates the need for preparation of the salicylate salt of lamivudine of formula (XII), before preparation of Form-II of Lamivudine. h) does not involve repeated steps of crystallization for improving the purity, thereby reducing reactor occupancy, utilities and making the process cost-effective. i) provides desired menthyl ester derivative of lamivudine of formula (XI) in high optical purity (99%) and with good yields (» 90%). The compound of formula (XI) is then converted to lamivudine of formula (I) by treating with a reducing agent like sodium borohydride, employing standard procedures reported in prior art. Lamivudine thus prepared was optionally converted to lamivudine salicylate salt of formula (XII). The present invention further relates to a novel method for the preparation of Form-II of lamivudine of formula (I), which can be obtained either directly from impure residue of lamivudine of formula (I) obtained after reduction with sodium borohydride or by converting impure residue of lamivudine of formula (I) obtained after reduction, to the salicylic acid addition salt of lamivudine of formula (XII), which is then converted to Form-II of Lamivudine in an organic solvent, by treatment with a base. 21 Method-I: Preparation of Lamivudine Form-II directly after reduction of compound of formula (XI). Impure lamivudine of formula (I) obtained after reduction of compound (XI) and work up was dissolved in an organic solvent. The organic solvent was selected from the group comprising of cyclic ethers, aliphatic hydrocarbons, and chlorinated hydrocarbons. The preferred solvent was cyclic ether. The cyclic ethers were selected from the group comprising of tetrahydrofuran, 1,4-dioxane, tetrahydropyran etc. The preferred solvent was tetrahydrofuran. The residue containing lamivudine of formula (I) was suspended in tetrahydrofuran. The resulting mixture was heated at elevated temperature, cooled and filtered. Lamivudine of formula (I) thus obtained was confirmed by X-ray diffraction analysis, Diffraction Scanning Colorimetry and Infra-red spectra as Form-II of Lamivudine of formula (I). Lamivudine (Form-II) had melting point of 176-178°C. X-Ray diffraction spectrum showed the values disclosed in Table-I. Table-I: X-Ray Diffraction of Lamivudine (Form-II). Angle (20) d value (angstrom) Angle (26) 13.26 6.67 13.26 17.42 5.08 17.42 20.49 4.33 20.49 21.31 4.16 21.31 24.82 3.58 24.82 22 Method-II: Preparation of Lamivudine Form-II via the preparation of lamivudine # salicylate of formula (XII). The method for the preparation of Form-II of lamivudine of formula (I) comprises preparation of lamivudine salicylate of formula (XII) by prior art methods, followed by isolation of Form-II in an organic solvent in presence of an organic base. The organic solvent was selected from the group comprising of cyclic ethers, aliphatic hydrocarbons, and chlorinated hydrocarbons. The preferred solvent is cyclic ether. The cyclic ether was selected from the group comprising of tetrahydrofuran, dioxane, tetrahydropyran etc. The preferred solvent is tetrahydrofuran. Lamivudine salicylate of formula (XII) was dissolved in tetrahydrofuran. The amount of tetrahydrofuran employed was between 2.0 volumes and 10.0 volumes per gram of compound (XII). The preferred amount of tetrahydrofuran employed was between 3.0 volumes and 7.0 volumes per gram of compound (XII). The mixture was heated at elevated temperature, to get a clear mixture. Triethyl amine was added to the mixture at the same temperature, agitated, cooled and filtered. This invention is illustrated by the following examples but should not be construed to be limited thereto. Example 1 Preparation of (2R,5R)-5-Chloro-(l,3)-oxathiolane-2-carboxylic acid, 2S-isopropyl-5R-methyl-lR-cyclohexyl menthyl ester of formula (III) N,N, Dimethyl formamide (200 ml), dichloromethane (200 ml) and Menthyl ester (2R,5R)-5-Hydroxy-{l,3}-oxathiolane-2-carboxylic acid, 2S-isopropyl-5R-methyl-lR- 23 cyclohexyl ester (100 g; 0.346 moles) were agitated at 30-35°C to get clear solution. Thionyl chloride (61.76 g; 0.52 moles) was added at same temperature under nitrogen. The reaction mass was stirred at 30-35°C till completion of reaction. The reaction mixture was quenched with water (400 ml), organic layer was separated and water (400ml) was added to it. The pH was adjusted between 5.0 and 6.0 by addition of aqueous sodium bicarbonate solution; the resulting organic layer was utilized for the next reaction. Example 2 Preparation of (2R,5R)-5-(4-amino-2-oxo-2H-pyrimidin-l-yl)-(l,3)-oxathiolane-2-carboxylic acid, 2S-isopropyl-5R-methyI-lR-cyclohexyl menthyl ester of formula (V). Trimethylsilyl chloride (56 gms; 0.520 moles), sodium iodide (78 gms; 0.520 moles) were added to acetonitrile (200ml), at 25-30°C under nitrogen and the reaction mixture stirred at 30-35°C for 3 hours. A mixture of chloro menthyl ester (2R)-5-chloro-(l,3)- oxathiolane-2-carboxylic acid, 2S-isopropyl-5R-methyl-lR- cyclohexyl ester) (prepared in Example 1) was added at 25-30°C and stirred further at 30-35°C for 3 hours. The mixture was added slowly to a mixture of silylated cytosine (109 gms; 0.42 moles) dissolved in dichloromethane (50ml) at a temperature of 40-60°C. The mixture was stirred at 60°C till completion of reaction and cooled to 40°C. The reaction mixture was then quenched with 10% sodium bicarbonate solution (100ml) and 10% sodium thiosulphate solution (100ml). The organic layer was separated and diluted with hexane (400 ml). The mixture was then neutralized with 10% sodium bicarbonate solution at 30- 35°C. The solid separating out was filtered and washed with water (100ml) and hexane (100ml). Yield: 118 gms. % Yield: 89%. Melting point: 122°C Chiral HPLC Purity: 99.7 Specific Optical Rotation: -116° 24 Example-3 Preparation of Form-II Lamivudine (Method-I). Menthyl ester of Lamivudine (2R,5R)-5-(4-amino-2-oxo-2H-pyrimidin-1 -yl)- {l,3}oxathiolane-2-carboxylic acid, 2S-isopropyl-5R-methyl-1 R-cyclohexylester (100 gms; 0.262 moles) was added to a mixture of water (200ml), isopropyl alcohol (500ml) & dipotassium hydrogen phosphate (92gms) at 25-30°C. An aqueous solution containing sodium hydroxide (5 mg) and sodium borohydride (22.3gms) in water (100 ml) was gradually added at ambient temperature and stirred at same temperature, till completion of reaction. The mixture was then concentrated and diluted with tetrahydrofuran (600ml). The mixture was refluxed for 6.0 hours and cooled between 40°C and 45°C and filtered. %Yield: 89% Purity: 99% Melting Point: 177.5°C. Example-4 Preparation of Form-II Lamivudine (Method-I). Menthyl ester of Lamivudine (2R,5R)-5-(4-amino-2-oxo-2H-pyrimidin-l-yl)-(l,3)- oxathiolane-2-carboxylic acid, 2S-isopropyl-5R-methyl-lR-cyclohexylester (100 gms; 0.262 moles) was added to a mixture of water (200ml), isopropyl alcohol (500ml) & dipotassium hydrogen phosphate (92gms) at 25-30°C. An aqueous solution containing sodium hydroxide (5 mg) and sodium borohydride (22.3gms) in water (100 ml) was gradually added at ambient temperature and stirred at same temperature, till completion of reaction. The mixture was then concentrated and diluted with 1,4-dioxane (600ml). The mixture was refluxed for 6.0 hours and cooled between 40°C and 45°C and filtered. %Yield: 89% Purity: 98.37% Melting Point: 176.7°C. Example 5 Preparation of Lamivudine salicylate salt (XII) (Method-II). (2R,5R)-5-(4-amino-2-oxo-2H-pyrimidin-l-yl)-(l,3)-oxathiolane-2-carboxylic acid, 2S-isopropyl-5R-methyl-lR-cyclohexyl menthyl ester (l00gms; 0.262 moles) was added to a mixture of isopropyl alcohol (500ml) and water (200ml) at ambient temperature. Dipotassium hydrogen phosphate (92gms; 0.528 moles) followed by an aqueous mixture of sodium hydroxide (5.0mgms) and sodium borohydride (22.3gms; 0.586 moles) in 25 water (100ml) was added to the reaction mixture at 25-30°C. The reaction mixture was then stirred at 30-45°C, till completion of reaction. The reaction mixture was concentrated and the residue was diluted with a mixture of water (100ml) and toluene (200ml). The organic layer was separated and the aqueous layer was heated between 50 and 55°C and salicylic acid (36 gms; 0.261moles) was added. The mixture was heated at same temperature for one hour and cooled to ambient temperature. Lamivudine salicylate of formula (VI) separating out was filtered and dried. Yield: 92.0 gms. % Yield: 95.8%. Example-6 Preparation of Lamivudine Form-II (Method-II). Lamivudine salicylate (XII: l00gms; 0.272moles) was added to tetrahydrofuran (450ml) at ambient temperature. The mixture was heated between 50°C and 55°C. A solution of triethylamine (30 g; 0.296 moles) in tetrahydrofuran (50 ml) was added at 50°C to 55°C and stirred further at 60°C to 65°C. The reaction mass was cooled to 40°C to 45°C gradually, the compound separating out was filtered, washed with tetrahydrofuran (100ml) and dried. Yield: 58.0 gms %Yield: 92.85. Melting point: 177.5°C. Purity: > 99%. Example-7 Preparation of Lamivudine Form-II (Method-II). Lamivudine salicylate (l00gms; 0.272moles) was added to 1,4-dioxane (450ml) at ambient temperature. The mixture was heated between 99 to 101°C. A solution of triethylamine (30 g; 0.296 moles) in tetrahydrofuran (50 ml) was added at 99 to 101°C and stirred further at same temperature. The reaction mass was cooled to 40 to 45°C gradually, the compound separating out was filtered, washed with 1,4-dioxane (100ml) and dried. Yield: 58.5 gms %Yield: 93.15. Melting point: 177.3°C., Purity: > 99%. 26 WE CLAIM: 1. An industrial process for the manufacture of Lamivudine of formula (I), comprising the steps of, a) reacting (2R,5R)-5-hydroxy-{l,3}-oxathiolane-2-carboxylic acid, 2S-isopropyl-5R-methyl-lR-cyclohexyl menthyl ester of formula (IX) with thionyl chloride and dimethyl formamide to give (2R,5S)-5-chloro-(l,3)-oxathiolane-2-carboxylic acid, 2S-isopropyl-5R-methyl-lR- cyclohexyl menthyl ester of formula (X) (2R, 5R)-5-Hydroxy-[l,3]oxathiolane- (2R, 5S)-5-Chloro-[l,3]oxathiolane- 2-carboxylic acid-2S-isopropyl-5R-methyl-l R- 2-carboxylic acid-2S-isopropyl-5R-methyl-1R- cyclohexyl ester (IX) cyclohexyl ester (X) b) reacting the compound of formula (X) with trimethylsilyl chloride and alkali metal iodide, followed by reacting with silylated cytosine (III), in the presence of a base, to give compound of formula (XI), c) reducing compound of formula (XI) and isolating directly Form-II of lamivudine of formula (I) from an organic solvent or by preparing the acid addition salt of formula (XII), and converting to Form-II lamivudine. 27 2. A process according to claim 1(a), wherein the reaction is carried out in presence of an organic solvent. 3. A process according to claim 2, wherein the organic solvent is selected from the group comprising of chlorinated solvent, ester, and aliphatic hydrocarbon. 4. A process according to claim 2 and claim 3, wherein the preferred organic solvent is a chlorinated solvent. 5. A process according to claim 4, wherein the chlorinated solvent is selected from the group comprising of chloroform, dichloromethane, and ethylene dichloride. 6. A process according to claim 5, wherein the preferred solvent is dichloromethane, 7. A process according to claim 1(b), wherein the alkali metal of the alkali metal iodide is selected from the group comprising of lithium, sodium and potassium. 8. A process according to claim 7, wherein the preferred alkali metal is sodium and potassium. 28 9. A process according to claim 7, wherein the preferred alkali metal iodide is selected from the group comprising of lithium iodide, sodium iodide and potassium iodide. 10. A process according to claim 9, wherein the preferred alkali metal iodide is sodium iodide and potassium iodide. 11. A process according to claim 1(b), wherein the reaction of (2R,5S)-5-chloro-(l,3)-oxathiolane-2-carboxylic acid, 2S-isopropyl-5R-methyl-lR- cyclohexyl menthyl ester of formula (X) with trimethylsilyl chloride and alkali metal iodide is carried out using an inorganic or organic base. 12. A process according to claim 11, wherein the preferred base is an organic base. 13. A process according to claim 12, wherein the preferred base is triethyl amine. 14. A process according to claim 1(c), wherein Lamivudine Form-II is obtained directly using an organic solvent after reduction of compound of formula (XI). 15. A process according to claim 14, wherein the organic solvent employed is selected from the group of cyclic ethers, aliphatic hydrocarbons and chlorinated hydrocarbons. 16. A process according to claim 15, wherein the preferred organic solvent is cyclic ether. 17. A process according to claim 16, wherein the cyclic ether is selected from the group comprising of tetrahydrofuran, dioxane, and tetrahydropyran. 18. A process according to claim 17, wherein the preferred cyclic ether is tetrahydrofuran. 19. A process according to claim 1(c), wherein the X-ray diffraction spectrum of Form-II lamivudine of formula (I) has 20 values 13.26, 17.42, 20.49, 21.31 and 24.82. 29 20. A process according to claim 19, wherein the d values corresponding to the 20 values are Angle (2q) d value (angstrom) 13.26 6.67 17.42 5.08 20.49 4.33 21.31 4.16 24.82 3.58 21. A process according to claim 1(c), wherein the compound of formula (XI) is reduced with a reducing agent. 22. A process according to claim 1(c), wherein the reducing agent is sodium borohydride. 23. A process according to claim 1(c), wherein the compound of formula (I) obtained after reduction of compound of formula (XI) is converted to its pharmaceutically acceptable acid addition salt. 24. A process according to claim 23, wherein the acid addition salt is lamivudine salicylate salt of formula (XII). 25. A process according to claim 1(c), wherein the lamivudine salicylate salt of formula (XII) is converted to lamivudine Form-II using organic solvent selected from the group comprising of cyclic ethers, aliphatic hydrocarbons and chlorinated hydrocarbons. 26. A process according to claim 25, wherein the preferred organic solvent is cyclic ether. 27. A process according to claim 26, wherein the cyclic ether is selected from the group comprising of tetrahydrofuran, dioxane, and tetrahydropyran. 28. A process according to claim 27, wherein the preferred cyclic ether is tetrahydrofuran. -30- 29. Lamivudine (Form-II) prepared according to the process of claims 1-28. 30. A process for preparing compound of formula (XI) and lamivudine Form-II of formula (I) substantially as described in foregoing examples. Dated this 14m day of December 20005 31 Abstract; This invention relates to simple, economical, cost-effective method for the preparation of lamivudine of formula (I).

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