A process for preparing a compound of the formula (I): ##STR46## wherein Ar is a group of the following formula (Ar-1) or (Ar-2): ##STR47## (in which R1 is a halogen atom, R2 is a hydrogen atom or a lower alkyl group, R3 is a hydrogen atom, a lower alkyl group, or a lower alkanoyl group, R4 is a hydrogen atom or a lower alkyl group, R5 and R6 are the same or different and each a hydrogen atom or a