The present invention relates to a process for preparing a compound of formula (Id-1)or a pharmaceutically acceptable salt thereof, which comprisesreacting a compound of formula (35) with amine compound of formula (18)wherein,R1 and R2, which may be same or different, are independently selected from hydrogen and substituted or unsubstituted alkyl;U is substituted or unsubstituted heterocycles selected from thiazole and isoxazole; andV is substituted or unsubstituted aryl.