The present invention relates to a process for preparing a compound of formula (Id-1) or a pharmaceutically acceptable salt thereof, which comprises reacting a compound of formula (35) with amine compound of formula (18) wherein, R 1 and R 2 , 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; and V is substituted or unsubstituted aryl.