A new process for producing compounds of general formula (I), wherein R1 is carboxyl or protected carboxyl; R2 is lower alkoxyl or higher alkoxyl; and A?1 and A2¿ are each an aromatic divalent group, a heterocyclic divalent group or a cyclo(lower)alkylene group or salts of them, characterized by producing compounds of general formula (II), wherein R?1, R2, A1 and A2¿ are eachas defined above or salts thereof from compounds of general formula (III), wherein R?1, R2, A1 and A2¿ are each as defined above or salts thereof and producing compounds of general formula (I) from those of formula (II) or salts thereof.