dioxane dichloromethane diethylether or chloroform the said aprotic solvent used in step b is selected from acetonitrile tetrahydrofuran dimethylformamide dioxane dichloromethane chloroform or diethylether the said aprotic solvent used in step c is selected from acetonitrile tetrahydrofuran dimethylformamide or dioxane and the base used is selected from tertiaryamine pyridine or alkali carbonates and the said aprotic solvent used in step d is selected from acetonitrile tetrahydrofuran dimethylformamide dioxane dichloromethane diethylether or chloroform in the above process in step a of the process the said aprotic solvent used is preferably tetrahydrofuran or dichloromethane in step b of the process said aprotic solvent used is preferably tetrahydrofuran or acetonitile in step c of the process said aprotic solvent used is preferably tetrahydrofuran or acetonitile and said base used is preferably triethylamine or potassium carbonate and in step d of the process the said aprotic solvent used is preferably acetonitrile r4 has the same meaning as defined above are prepared by a process comprising steps of a reacting tert butyloxycarbonyl protected formula 36 or 37 with trifluoroacetic anhydride in aprotic solvent at 10 40 u00b0 c for 2 4 hrs to give corresponding cyano compound of formula 39 or 40 b deprotecting compounds obtained in step a using trifluoroacetic acid in an aprotic solvent at 5 40 u00b0 c for 0 25 2 hrs to give compound of formula no 41 or 42 as trifluoroacetate salt respectively and c purifying the compound so obtained by neutrializing with an aqueous alkali followed by purification of free base thus obtained by column chromatography and then converting the product thus obtained into a pharmaceutically acceptable hydrochloride salt by treating with methanolic hydrochloride at 0 20 u00b0 c for 0 5.