. "This process is to provide a free carboxy compound (Ik) or its salt, especially the compound (Ik) wherein R1 is thiadiazolyl or thiazolyl of the formula: ##STR31## in which R6 is as defined above and A is a group of the formula: ##STR32## in which R2 is as defined above, which generally exhibits higher antimicrobial activities as compared with the corresponding functionally modified carboxy compou" . . .