the present invention provides a process for the synthesis of montelukast and its salts involving novel compounds of formula ( 3 ) with a group l where ( l = oac ots and otf ) as shown in scheme 5 . the process of the present invention comprises reacting compound a with a chiral reducing agent like (−) dip chloride ( diisopinocampheylchloroborane ) or with ( borane dimethyl sulfide ) bdms and cat . ( r )- cbs reagent to obtain the ( s )- alcohol ( 1 ). compound ( 1 ) is then reacted with lithium hexamethyldisilazide ( lihmds ) or n , o - dimethylhydroxylamine hydrochloride followed by reaction with methyl magnesium halide ( memgx where x = cl , br ) to give compound ( 2 ). compound ( 2 ) is converted to compound ( 3 ) having group l ( where l = otf , ots , oac , oms ) by reaction with trifluoromethane sulfonic anhydride ( triflic anhydride ) p - toluene sulfonyl chloride ( tosyl chloride ), acetic anhydride , methane sulphonyl chloride to obtain the triflate ( i . e ., trifluoromethanesulfonate , ‘ otf ’), tosylate ( ots ) and acetate ( oac ) and mesylate ( oms ), to obtain the compounds ( 3 ) respectively . in the process of the present invention the trifluoromesylation , tosylation and acetylation can be performed on compound ( 2 ) with the above mentioned reagents at temperatures ranging from − 10 to 50 ° c ., more preferably − 5 to 10 ° c . for addition and warming to room temperature in a water - free inert solvent . the solvents may be chosen from thf , diethyl ether , 1 , 4 dioxane , toluene , benzene , methylene dichloride , chloroform and the like . a tertiary base like trimethyl amine , triethyl amine , pyridine , n , n , diisopropyl ethyl amine ( dipea ) and the like can be added to assist the reaction . a catalytic amount of 4 , 4 dimethyl amino pyridine ( dmap ) can also be added to enhance the reaction . the reaction is completed within 1 to 24 hrs after the addition , preferably after overnight stirring . in another aspect the compound ( 3 ) is further converted into compound ( 4 ) by reaction with 1 -( mercaptomethyl ) cyclopropane acetic acid in presence of a base like sodium hydride , sodamide , cesium carbonate , sodium methoxide , potassium tert - butoxide and the like in an inert solvent like thf , dmf , dioxane , n - methylpyrrolidone ( nmp ) and the like at temperatures ranging from − 10 to 50 ° c . in yet another aspect compound ( 4 ) on treatment with memgx in presence of an inert solvent like toluene , thf , dioxane , dichloromethane , chloroform etc . gives the title compound montelukast ( i ). the reaction can be performed at temperatures ranging from − 10 to 50 ° c . more preferably 0 - 10 ° c . the reaction time may vary from 1 to 24 hrs , preferably in 5 - 10 hrs . after completion of the reaction , montelukast ( i ) is isolated from the reaction mass in a conventional manner . another aspect of the present invention involves preparation of novel compounds of formula ( 7 ) where x =( cl , br , i ) by reacting the compound ( a ) with a halogen donating substituent to obtain compound ( 2 ) and its isomer ( 6 ) as shown in scheme 6 . compound ( 2 ) can be made to undergo with retention of configuration to obtain compound ( 7 )— this can be achieved by carrying out the reaction in the presence of a solvent . similarly , compound ( 6 ) undergoes inversion of configuration to produce compound ( 7 )— this can be achieved by carrying out the reaction in the absence of a solvent . the alternative synthesis for the preparation of montelukast as described in scheme 6 , wherein compound a is chirally reduced to obtain the ( s )— alcohol ( 1 ) and ( r )- alcohol ( 5 ) by using (−) and (+) diisopinocamphenyl borane chloride ( dip chloride ) respectively or borane dimethyl sulfide ( bdms ) with catalytic ( r )- methyl cbs reagent and ( s )- methyl cbs reagent respectively . the ( s ) alcohol ( 1 ) and ( r ) alcohol ( 5 ) are then treated with lithium hexamethyldisilazide ( lihmds ) or n , o - dimethylhydroxylamine hydrochloride followed by reaction with methyl magnesium halide to obtain the ketones ( 2 ) and ( 6 ) respectively . ketones ( 2 ) and ( 6 ) are treated with halogen donating compound to obtain the desired ( s )- halo compound ( 7 ). chlorination can be performed on compound ( 2 ) with retention of configuration with thionyl chloride , phosphorus pentachloride , phosphorus oxychloride , most preferably thionyl chloride in presence of an inert solvent at 0 - 50 ° c . this reaction can also be carried out optionally in the presence of an organic base like pyridine , trimethyl amine , triethyl amine . bromination can be performed on compound ( 2 ) with any of the following : n - bromo succinimide , bromine , phosphorus oxybromide , phosphorus tribromide and the like , most preferably n - bromo succinimide can be used in an inert solvent or without solvent at 0 - 50 ° c . the reaction requires 1 to 24 hrs for completion or more preferably 5 to 10 hrs . the addition of the reagent can be done at 0 to 10 ° c . and warmed to the requisite temperature for completion . iodination can be done with conversion of the hydroxyl group on compound ( 2 ) to trialkylsilyloxy group most conveniently done with trimethylsilyl halide and reacting the same with an alkali metal iodide like potassium iodide , sodium iodide or cesium iodide or can be reacted with tetrabutyl ammonium iodide and the like . the silylation of the hydroxyl group can be done at − 10 ° c . to 25 ° c . in presence of a tertiary base like , triethyl amine , pyridine , dipea and the like . after completion of silylation , the compound is reacted with the alkali metal iodide most preferably sodium or potassium iodide at temperatures ranging from 0 to 100 ° c . polar aprotic solvents like thf , dmf . dma , dmso and the like can be used for performing the reaction . chlorinated and aromatic solvents can however be also used for the reaction . compound ( 6 ) is treated with a halide donating compound so as to obtain compound ( 7 ) thus leading to inversion of configuration as shown in scheme 6 . thus chlorination with inversion of configuration can be done for e . g . with thionyl chloride in absence of any solvent , the reaction is carried out neat without using any solvent . the reaction can be carried out optionally in the presence of a organic base like pyridine , trimethyl amine , triethyl amine and the like to obtain the ( r )- alcohol ( 6 ). the time and temperature for the reaction are similar to as mentioned above . bromination can be done with thionyl bromide in presence of an organic tertiary base or with bromine and a tertiary base . iodination can be performed on compound ( 6 ) under mitsunobu conditions i . e . with triphenylphosphine and diethyl azadicarboxylate ( dead ) and reacting with alkali metal iodide to obtain compound 7 once compound ( 7 ) is prepared , it can be made to react similarly as compound ( 3 ) in the earlier aspect of the present invention to obtain compound ( 4 ) which is then converted to the title compound montelukast ( i ). the invention will now be further described with reference to the following examples . methylene chloride ( 200 ml ) and compound ( a ) ( 40 gm ) was charged , cooled to − 5 ° c ., diisopropylethylamine ( 28 ml ) was added followed by (−) dip chloride ( 120 ml ) at − 5 to 0 ° c . over a period of 45 minutes . the temperature of the reaction mass was maintained at − 5 to 0 ° c . for 3 - 4 hours , after completion of the reaction , triethanolamine ( 14 . 4 gm ) was added at less than 20 ° c ., stirred at 25 - 30 ° c . for 2 hours . separated methylene chloride layer , extracted the aqueous layer with methylene chloride ( 100 ml ), combined methylene chloride layers and washed with 10 % of sodium chloride solution ( 100 ml ). distilled off methylene chloride completely under vacuum at less than 35 ° c . to residue . the residue was dissolved in methanol ( 480 ml ), stirred for 1 hour , clarified , added water ( 200 ml ) slowly over a period of 1 hour , stirred at room temperature for 2 hours , filtered , washed with methanol - water mixture ( 20 ml each ). the material so obtained was stirred with heptane ( 160 ml ) for 45 minutes , filtered , washed with heptane ( 40 ml ) and dried the material under vacuum at 45 - 50 ° c . to give compound ( 1 ) ( 39 gm , 97 % yield , 98 % hplc purity ). lithium hexamethyldisilazide ( 314 ml - 28 % solution in hexane ) was charged under nitrogen atmosphere , chilled to − 5 to 0 ° c ., 3 m solution of methyl magnesium chloride ( 87 . 3 ml ) was added slowly over a period of 30 minutes under nitrogen atmosphere at − 5 to 0 ° c . the contents were stirred for 15 minutes at − 5 to 0 ° c . under nitrogen atmosphere and a solution of compound ( 1 ) in thf ( 20 gm in 200 ml ) was slowly added over a period of 30 minutes , stirred at − 5 to 0 ° c . for 5 hours , adjusted the ph of the reaction mass to neutral using 10 % acetic acid below 10 ° c ., extracted using ethyl acetate ( 200 ml × 2 ), washed ethyl acetate layer with water ( 100 ml ), dried over sodium sulphate ( 5 gm ) and distilled out ethyl acetate under vacuum at 40 - 45 ° c . to give compound ( 2 ) as residue ( 15 . 4 gm , 80 % yield , 80 % hplc purity ). acetic anhydride ( 22 . 3 gm ) and glacial acetic acid ( 34 . 3 gm ) was added followed by compound ( 2 ) ( 25 gm ) at room temperature . to this perchloric acid ( 2 ml ) was added at 25 - 30 ° c ., the contents were stirred at 30 - 35 ° c . for 2 hours , quenched the reaction mass with ice - water mixture ( 250 ml ). extracted the aqueous layer with ethyl acetate ( 250 ml ), washed with 5 % of sodium bicarbonate solution ( 75 ml ) followed by water ( 75 ml ), dried over sodium sulphate ( 2 . 5 gm ) and distilled off ethyl acetate completely under vacuum at less than 40 ° c . to give acetyl derivative of compound ( 2 ) ( 24 gm , 87 . 9 % yield , 83 % hplc purity ). compound ( 2 ) ( 25 gm ) and toluene ( 250 ml ) was charged under nitrogen atmosphere , pyridine ( 25 ml ) was added , cooled to 5 - 10 ° c ., a solution of para toluene sulphonyl chloride in toluene ( 13 gm in 125 ml ) was added dropwise at 5 - 10 ° c ., maintained for 30 minutes then heated to 55 - 60 ° c ., maintained for 3 hours and distilled out toluene completely under vacuum at 45 - 50 ° c . to this ice - water mixture ( 250 ml ) was added , ph of the reaction mass was adjusted to neutral using 1 % acetic acid , extracted with ethyl acetate ( 250 ml ), ethyl acetate layer was washed with water ( 75 ml ), dried over sodium sulphate ( 2 . 5 gm ) and distilled out ethyl acetate completely under vacuum to give tosyl derivative of compound ( 2 ) ( 26 gm , 77 % yield , 78 % hplc purity ). compound ( 2 ) ( 25 gm ) and methylene chloride ( 250 ml ) was charged under nitrogen atmosphere , pyridine ( 5 ml ) was added , triflic anhydride ( 32 gm ) was added at 0 - 5 ° c . and the contents were stirred at 25 - 30 ° c . for 2 hours . water ( 50 ml ) was added , stirred for 30 minutes , separated the layers , the aqueous layer was extracted with methylene chloride ( 25 ml ), the combined methylene chloride layer was washed with water ( 25 ml ), dried over sodium sulphate ( 2 . 5 gm ) and distilled off methylene chloride completely under vacuum at less than 35 ° c . to give triflic derivative of compound ( 2 ) ( 30 gm , 93 % yield , 87 % hplc purity ). preparation of keto - montelukast - dicyclohexylamine salt [ compound ( 4 )- dcha salt ] from acetyl derivative of compound ( 2 ) dimethyl formamide ( 100 ml ) and cesium carbonate ( 70 . 83 gm ) was charged at room temperature under nitrogen atmosphere , heated to 60 - 62 ° c . and maintained for 15 minutes . the contents were chilled to 15 - 20 ° c ., a solution of 1 -( mercaptomethyl ) cyclopropane acetic acid in dmf ( 9 . 04 gm in 50 ml ) was added slowly to the reaction mass at 15 - 20 ° c . over a period of 30 minutes and maintained for 1 hour . the temperature of the reaction mass was raised to room temperature and a solution of acetyl derivative of compound ( 2 ) in dmf ( 25 gm in 100 ml ) was added slowly over a period of 15 - 30 minutes , the reaction mass was heated to 35 - 40 ° c . and maintained for 2 hours . the reaction mass was filtered , washed with methanol ( 12 . 5 ml ), water ( 300 ml ) was charged to the clear filtrate and washed with toluene ( 125 ml × 2 ). the ph of the aqueous layer was adjusted to 5 . 0 - 6 . 0 using 10 % acetic acid solution and extracted with ethyl acetate ( 125 ml × 2 ). the ethyl acetate layer was washed with water ( 125 ml ) followed by 10 % of sodium chloride solution ( 125 ml ), dried over sodium sulphate ( 5 gm ) and distilled out ethyl acetate completely under vacuum at 40 - 45 ° c . to residue . ethyl acetate ( 100 ml ) and dicyclohexylamine ( 10 . 5 gm ) was charged to residue at room temperature , stirred for 2 hours , chilled the contents to 0 - 5 ° c ., filtered , washed with chilled ethyl acetate ( 25 ml ) and dried under vacuum at 40 - 45 ° c . to give keto - montelukast - dicyclohexylamine salt ( 12 . 0 gm , 32 % yield , 97 % hplc purity ). preparation of keto - montelukast - dicyclohexylamine salt [ compound ( 4 )- dcha salt ] from tosyl derivative of compound ( 2 ) dimethyl formamide ( 200 ml ) and cesium carbonate ( 141 . 66 gm ) was charged at room temperature under nitrogen atmosphere , heated to 60 - 62 ° c . and maintained for 15 minutes . the contents were chilled to 15 - 20 ° c ., a solution of 1 -( mercaptomethyl ) cyclopropane acetic acid in dmf ( 17 . 4 gm in 100 ml ) was added slowly to the reaction mass at 15 - 20 ° c . over a period of 30 minutes and maintained for 1 hour . the temperature of the reaction mass was raised to room temperature and a solution of tosyl derivative of compound ( 2 ) in dmf ( 50 gm in 200 ml ) was added slowly over a period of 15 - 30 minutes , the reaction mass was heated to 35 - 40 ° c . and maintained for 2 hours . the reaction mass was filtered , washed with methanol ( 25 ml ), water ( 600 ml ) was charged to the clear filtrate and washed with toluene ( 250 ml × 2 ). the ph of the aqueous layer was adjusted to 5 . 0 - 6 . 0 using 10 % acetic acid solution and extracted with ethyl acetate ( 250 ml × 2 ). the ethyl acetate layer was washed with water ( 250 ml ) followed by 10 % of sodium chloride solution ( 250 ml ), dried over sodium sulphate ( 10 gm ) and distilled out ethyl acetate completely under vacuum at 40 - 45 ° c . to residue . ethyl acetate ( 200 ml ) and dicyclohexylamine ( 18 . 4 gm ) was charged to residue at room temperature , stirred for 2 hours , chilled the contents to 0 - 5 ° c ., filtered , washed with chilled ethyl acetate ( 50 ml ) and dried under vacuum at 40 - 45 ° c . to give keto - montelukast - dicyclohexylamine salt ( 17 . 4 gm , 30 % yield , 95 % hplc purity ). preparation of keto - montelukast - dicyclohexylamine salt [ compound ( 4 )- dcha salt ] from triflic derivative of compound ( 2 ) dimethyl formamide ( 40 ml ) and cesium carbonate ( 28 . 4 gm ) was charged at room temperature under nitrogen atmosphere , heated to 60 - 62 ° c . and maintained for 15 minutes . the contents were chilled to 15 - 20 ° c ., a solution of 1 -( mercaptomethyl ) cyclopropane acetic acid in dmf ( 3 . 04 gm in 20 ml ) was added slowly to the reaction mass at 15 - 20 ° c . over a period of 30 minutes and maintained for 1 hour . the temperature of the reaction mass was raised to room temperature and a solution of triflic derivative of compound ( 2 ) in dmf ( 10 gm in 40 ml ) was added slowly over a period of 15 - 30 minutes , the reaction mass was heated to 35 - 40 ° c . and maintained for 2 hours . the reaction mass was filtered , washed with methanol ( 5 ml ), water ( 120 ml ) was charged to the clear filtrate and washed with toluene ( 50 ml × 2 ). the ph of the aqueous layer was adjusted to 5 . 0 - 6 . 0 using 10 % acetic acid solution and extracted with ethyl acetate ( 50 ml × 2 ). the ethyl acetate layer was washed with water ( 50 ml ) followed by 10 % of sodium chloride solution ( 50 ml ), dried over sodium sulphate ( 2 . 5 gm ) and distilled out ethyl acetate completely under vacuum at 40 - 45 ° c . to residue . ethyl acetate ( 40 ml ) and dicyclohexylamine ( 3 . 5 gm ) was charged to residue at room temperature , stirred for 2 hours , chilled the contents to 0 - 5 ° c ., filtered , washed with chilled ethyl acetate ( 10 ml ) and dried under vacuum at 40 - 45 ° c . to give keto - montelukast - dicyclohexylamine salt ( 4 . 5 gm , 35 % yield , 96 . 5 % hplc purity ). methylene chloride ( 200 ml ) and compound ( 2 ) ( 20 gm ) was charged at room temperature under nitrogen atmosphere , chilled to 0 - 5 ° c . and thionyl chloride ( 16 . 2 gm ) was added slowly to the reaction mass at 0 - 5 ° c . under nitrogen atmosphere over a period of 1 hour . the contents were stirred at 0 - 5 ° c . for 3 - 4 hours and distilled out methylene chloride completely under vacuum at less than 35 ° c . to residue . the residue was dissolved in methylene chloride ( 200 ml ) was quenched into ice - water mixture ( 300 ml ) below 15 ° c . over a period of 30 minutes . the ph of the reaction mass was adjusted to neutral using 10 % sodium bicarbonate solution , separated methylene chloride layer , the aqueous layer was extracted with methylene chloride ( 100 ml ), washed the combined methylene chloride layer with water ( 100 ml ), dried over sodium sulphate ( 5 gm ) and distilled out methylene chloride completely under vacuum at 35 - 40 ° c . to give compound ( 7 ) ( 20 gm , 95 . 7 % yield , 85 % hplc purity ). methylene chloride ( 100 ml ) and compound ( a ) ( 20 gm ) was charged , cooled to − 5 ° c ., diisopropylethylamine ( 14 ml ) was added followed by (+) dip chloride ( 60 ml ) at − 5 to 0 ° c . over a period of 45 minutes . the temperature of the reaction mass was maintained at − 5 to 0 ° c . for 3 - 4 hours , after completion of the reaction , triethanolamine ( 7 . 2 gm ) was added at less than 20 ° c ., stirred at 25 - 30 ° c . for 2 hours . separated methylene chloride layer , extracted the aqueous layer with methylene chloride ( 50 ml ), combined methylene chloride layers and washed with 10 % of sodium chloride solution ( 50 ml ). distilled off methylene chloride completely under vacuum at less than 35 ° c . to residue . the residue was dissolved in methanol ( 240 ml ), stirred for 1 hour , clarified , added water ( 100 ml ) slowly over a period of 1 hour , stirred at room temperature for 2 hours , filtered , washed with methanol - water mixture ( 10 ml each ). the material so obtained was stirred with heptane ( 80 ml ) for 45 minutes , filtered , washed with heptane ( 20 ml ) and dried the material under vacuum at 45 - 50 ° c . to give compound ( 5 ) ( 18 . 5 gm , 92 % yield , 97 . 5 % hplc purity ). lithium hexamethyldisilazide ( 157 ml - 28 % solution in hexane ) was charged under nitrogen atmosphere , chilled to − 5 to 0 ° c ., 3 m solution of methyl magnesium chloride ( 44 ml ) was added slowly over a period of 30 minutes under nitrogen atmosphere at − 5 to 0 ° c . the contents were stirred for 15 minutes at − 5 to 0 ° c . under nitrogen atmosphere and a solution of compound ( 5 ) in thf ( 10 gm in 100 ml ) was slowly added over a period of 30 minutes , stirred at − 5 to 0 ° c . for 5 hours , adjusted the ph of the reaction mass to neutral using 10 % acetic acid below 10 ° c ., extracted using ethyl acetate ( 100 ml × 2 ), washed ethyl acetate layer with water ( 50 ml ), dried over sodium sulphate ( 2 . 5 gm ) and distilled out ethyl acetate under vacuum at 40 - 45 ° c . to give compound ( 6 ) ( 7 . 5 gm , 78 % yield , 79 . 9 % hplc purity ). to 20 gms of compound ( 6 ) phosphorus oxychloride ( 50 ml ) was added slowly under nitrogen atmosphere and stirred at room temperature for 2 hours . after completion of the reaction , methylene chloride ( 100 ml ) was added and stirred to get clear solution . the organic layer was washed with 5 % of sodium bicarbonate ( 20 ml ), dried over sodium sulphate and distilled off methylene chloride to give compound ( 7 ) [ chloro compound ] ( 18 . 7 gm , 90 % yield , 78 . 5 % hplc purity ). dimethyl formamide ( 100 ml ) and cesium carbonate ( 70 . 83 gm ) was charged at room temperature under nitrogen atmosphere , heated to 60 - 62 ° c . and maintained for 15 minutes . the contents were chilled to 15 - 20 ° c ., a solution of 1 -( mercaptomethyl ) cyclopropane acetic acid in dmf ( 9 . 52 gm in 50 ml ) was added slowly to the reaction mass at 15 - 20 ° c . over a period of 30 minutes and maintained for 1 hour . the temperature of the reaction mass was raised to room temperature and a solution of compound ( 7 ) in dmf ( 25 gm in 100 ml ) was added slowly over a period of 15 - 30 minutes , the reaction mass was heated to 35 - 40 ° c . and maintained for 2 hours . the reaction mass was filtered , washed with methanol ( 12 . 5 ml ), water ( 300 ml ) was charged to the clear filtrate and washed with toluene ( 125 ml × 2 ). the ph of the aqueous layer was adjusted to 5 . 0 - 6 . 0 using 10 % acetic acid solution and extracted with ethyl acetate ( 125 ml × 2 ). the ethyl acetate layer was washed with water ( 125 ml ) followed by 10 % of sodium chloride solution ( 125 ml ), dried over sodium sulphate ( 5 gm ) and distilled out ethyl acetate completely under vacuum at 40 - 45 ° c . to residue . ethyl acetate ( 100 ml ) and dicyclohexylamine ( 10 . 8 gm ) was charged to residue at room temperature , stirred for 2 hours , chilled the contents to 0 - 5 ° c ., filtered , washed with chilled ethyl acetate ( 25 ml ) and dried under vacuum at 40 - 45 ° c . to give keto - montelukast - dicyclohexylamine salt ( 12 . 5 gm , 34 . 4 % yield , 98 % hplc purity ). dimethyl formamide ( 200 ml ) and cesium carbonate ( 141 . 66 gm ) was charged at room temperature under nitrogen atmosphere , heated to 60 - 62 ° c . and maintained for 15 minutes . the contents were chilled to 15 - 20 ° c ., a solution of 1 -( mercaptomethyl ) cyclopropane acetic acid in dmf ( 19 . 04 gm in 100 ml ) was added slowly to the reaction mass at 15 - 20 ° c . over a period of 30 minutes and maintained for 1 hour . the temperature of the reaction mass was raised to room temperature and a solution of compound ( 7 ) in dmf ( 50 gm in 200 ml ) was added slowly over a period of 15 - 30 minutes , the reaction mass was heated to 35 - 40 ° c . and maintained for 2 hours . the reaction mass was filtered , washed with methanol ( 25 ml ), water ( 600 ml ) was charged to the clear filtrate and washed with toluene ( 250 ml × 2 ). the ph of the aqueous layer was adjusted to 5 . 0 - 6 . 0 using 10 % acetic acid solution and extracted with ethyl acetate ( 250 ml × 2 ). the ethyl acetate layer was washed with water ( 250 ml ) followed by 10 % of sodium chloride solution ( 250 ml ), dried over sodium sulphate ( 10 gm ) and distilled out ethyl acetate completely under vacuum at 40 - 45 ° c . to residue . ethyl acetate ( 200 ml ) and isopropylamine ( 7 . 04 gm ) was charged to residue at room temperature , stirred for 2 hours , chilled the contents to 0 - 5 ° c ., filtered , washed with chilled ethyl acetate ( 50 ml ) and dried under vacuum at 40 - 45 ° c . to give keto - montelukast - isopropylamine salt ( 22 . 0 gm , 31 . 4 % yield , 96 . 9 % hplc purity ). dimethyl formamide ( 100 ml ) and cesium carbonate ( 70 . 83 gm ) was charged at room temperature under nitrogen atmosphere , heated to 60 - 62 ° c . and maintained for 15 minutes . the contents were chilled to 15 - 20 ° c ., a solution of 1 -( mercaptomethyl ) cyclopropane acetic acid in dmf ( 9 . 52 gm in 50 ml ) was added slowly to the reaction mass at 15 - 20 ° c . over a period of 30 minutes and maintained for 1 hour . the temperature of the reaction mass was raised to room temperature and a solution of compound ( 7 ) in dmf ( 25 gm in 100 ml ) was added slowly over a period of 15 - 30 minutes , the reaction mass was heated to 35 - 40 ° c . and maintained for 2 hours . the reaction mass was filtered , washed with methanol ( 12 . 5 ml ), water ( 300 ml ) was charged to the clear filtrate and washed with toluene ( 125 ml × 2 ). the ph of the aqueous layer was adjusted to 5 . 0 - 6 . 0 using 10 % acetic acid solution and extracted with ethyl acetate ( 125 ml × 2 ). the ethyl acetate layer was washed with water ( 125 ml ) followed by 10 % of sodium chloride solution ( 125 ml ), dried over sodium sulphate ( 5 gm ) and distilled out ethyl acetate completely under vacuum at 40 - 45 ° c . to residue . ethyl acetate ( 100 ml ) and cyclohexylamine ( 5 . 9 gm ) was charged to residue at room temperature , stirred for 2 hours , chilled the contents to 0 - 5 ° c ., filtered , washed with chilled ethyl acetate ( 25 ml ) and dried under vacuum at 40 - 45 ° c . to give keto - montelukast - cyclohexylamine salt ( 12 gm , 33 . 1 % yield , 97 . 4 % hplc purity ). part 1 : keto - montelukast - dicyclohexylamine salt ( 10 gm ), water ( 100 ml ) and methylene chloride ( 100 ml ) was charged under nitrogen atmosphere , adjusted the ph of the reaction mass to 5 . 0 - 6 . 0 using 10 % acetic acid solution , stirred for 10 minutes and separated the methylene chloride layer . aqueous layer was extracted with methylene chloride ( 50 ml ), combined methylene chloride layer was dried over sodium sulphate ( 2 . 5 gm ), distilled off methylene chloride completely under vacuum below 40 ° c . and stripped off methylene chloride with toluene ( 50 ml × 2 ) to residue . the residue keto - montelukast was dissolved in toluene ( 20 ml ) and used in part 2 . part 2 : thf ( 100 ml ) and cerium chloride ( 3 . 6 gm ) was charged under nitrogen atmosphere , heated the contents to reflux and maintained for 3 hours . it was then , chilled to − 5 ° c . and 3m methyl magnesium chloride ( 36 ml ) was added dropwise over a period of 45 minutes and maintained at 0 - 5 ° c . for 2 hours . to this part 1 solution was added at − 5 to 0 ° c . over a period of 1 hour and maintained for 1 hours . after completion of reaction , the mass was quenched into ice - water mixture ( 100 ml ), ph of the reaction mass was adjusted to 6 . 0 - 6 . 5 using 10 % acetic acid solution and extracted with ethyl acetate ( 50 ml × 2 ). the ethyl acetate layer was dried over sodium sulphate ( 5 gm ) and distilled off ethyl acetate completely under vacuum at less than 45 ° c . to residue . the residue was dissolved in ethyl acetate ( 40 ml ), isopropylamine ( 0 . 86 gm ) was added , stirred for 1 hour , cooled to 0 - 5 ° c . and maintained for 1 hour . the material was filtered , washed with chilled ethyl acetate ( 5 ml ) and dried under vacuum at 40 - 45 ° c . to give montelukast - isopropylamine salt ( 6 . 0 gm , 70 % yield , 99 % hplc purity ). part 1 : keto - montelukast - dicyclohexylamine salt ( 5 gm ), water ( 50 ml ) and methylene chloride ( 50 ml ) was charged under nitrogen atmosphere , adjusted the ph of the reaction mass to 5 . 0 - 6 . 0 using 10 % acetic acid solution , stirred for 10 minutes and separated the methylene chloride layer . aqueous layer was extracted with methylene chloride ( 25 ml ), combined methylene chloride layer was dried over sodium sulphate ( 1 . 25 gm ), distilled off methylene chloride completely under vacuum below 40 ° c . and stripped off methylene chloride with toluene ( 25 ml × 2 ) to residue . the residue keto - montelukast was dissolved in toluene ( 10 ml ) and used in part 2 . part 2 : thf ( 50 ml ) and cerium chloride ( 1 . 8 gm ) was charged under nitrogen atmosphere , heated the contents to reflux and maintained for 3 hours . it was then , chilled to − 5 ° c . and 3m methyl magnesium chloride ( 18 ml ) was added dropwise over a period of 45 minutes and maintained at 0 - 5 ° c . for 2 hours . to this part 1 solution was added at − 5 to 0 ° c . over a period of 1 hour and maintained for 1 hours . after completion of reaction , the mass was quenched into ice - water mixture ( 50 ml ), ph of the reaction mass was adjusted to 6 . 0 - 6 . 5 using 10 % acetic acid solution and extracted with ethyl acetate ( 25 ml × 2 ). the ethyl acetate layer was dried over sodium sulphate ( 2 . 5 gm ) and distilled off ethyl acetate completely under vacuum at less than 45 ° c . to residue . the residue was dissolved in ethyl acetate ( 20 ml ), morpholine ( 0 . 63 gm ) was added , stirred for 1 hour , cooled to 0 - 5 ° c . and maintained for 1 hour . the material was filtered , washed with chilled ethyl acetate ( 2 . 5 ml ) and dried under vacuum at 40 - 45 ° c . to give montelukast - morpholine salt ( 3 . 0 gm , 67 . 4 % yield , 98 . 6 % hplc purity ). part 1 : keto - montelukast - dicyclohexylamine salt ( 20 gm ), water ( 200 ml ) and methylene chloride ( 200 ml ) was charged under nitrogen atmosphere , adjusted the ph of the reaction mass to 5 . 0 - 6 . 0 using 10 % acetic acid solution , stirred for 10 minutes and separated the methylene chloride layer . aqueous layer was extracted with methylene chloride ( 100 ml ), combined methylene chloride layer was dried over sodium sulphate ( 5 gm ), distilled off methylene chloride completely under vacuum below 40 ° c . and stripped off methylene chloride with toluene ( 100 ml × 2 ) to residue . the residue keto - montelukast was dissolved in toluene ( 40 ml ) and used in part 2 . part 2 : thf ( 200 ml ) and cerium chloride ( 7 . 2 gm ) was charged under nitrogen atmosphere , heated the contents to reflux and maintained for 3 hours . it was then , chilled to − 5 ° c . and 3m methyl magnesium chloride ( 72 ml ) was added dropwise over a period of 45 minutes and maintained at 0 - 5 ° c . for 2 hours . to this part 1 solution was added at − 5 to 0 ° c . over a period of 1 hour and maintained for 1 hours . after completion of reaction , the mass was quenched into ice - water mixture ( 200 ml ), ph of the reaction mass was adjusted to 6 . 0 - 6 . 5 using 10 % acetic acid solution and extracted with ethyl acetate ( 100 ml × 2 ). the ethyl acetate layer was dried over sodium sulphate ( 10 gm ) and distilled off ethyl acetate completely under vacuum at less than 45 ° c . to residue . the residue was dissolved in ethyl acetate ( 80 ml ), cyclohexylamine ( 2 . 9 gm ) was added , stirred for 1 hour , cooled to 0 - 5 ° c . and maintained for 1 hour . the material was filtered , washed with chilled ethyl acetate ( 10 ml ) and dried under vacuum at 40 - 45 ° c . to give montelukast - cyclohexylamine salt ( 12 . 4 gm , 68 % yield , 98 . 4 % hplc purity ). keto - montelukast - isopropylamine salt and keto - montelukast - cyclohexylamine salt can be converted to montelukast - isopropylamine salt , montelukast - morpholine salt and montelukast - cyclohexylamine salt by following the process as described in example 14 . montelukast - isopropylamine salt ( 10 gm ), methylene chloride ( 100 ml ) and water ( 100 ml ) was charged under nitrogen atmosphere and stirred for 10 minutes . the ph of the reaction mass was adjusted to 4 . 0 - 4 . 5 with 10 % acetic acid solution , separated methylene chloride layer and extracted the aqueous layer with methylene chloride ( 50 ml ), the combined methylene chloride layer was dried over sodium sulphate ( 5 gm ) and distilled off methylene chloride completely under vacuum at 25 - 30 ° c . to residue . the residue was dissolved in methanol ( 60 ml ), a solution of sodium hydroxide in methanol ( 0 . 68 gm in 30 ml of methanol ) was added and stirred for 1 hour at 25 - 30 ° c ., clarified , distilled off methanol completely under vacuum at less than 35 ° c ., stripped off methanol with ethyl acetate ( 50 ml × 3 ) to residue , the residue was dissolved in toluene ( 30 ml ), n - heptane ( 100 ml ) was added dropwise under nitrogen atmosphere at 25 - 30 ° c . and stirred for 30 minutes , filtered the material so obtained was dried under vacuum at 70 - 80 ° c . for 24 hours to give montelukast sodium ( 8 . 5 gm , 90 % yield , 99 % hplc purity ). montelukast - morpholine salt and montelukast - cyclohexylamine salt can be converted to montelukast sodium by adopting the process as described in example 16 . it will be appreciated that the invention may be modified within the scope of the claims .