Among the compounds of the formula (XII), those wherein A1 is --NH2 are preferred. ##STR10## wherein each of R2' and R3' is an alkyl group, COR6 (wherein R6 is an alkyl group or an alkoxy group) or --SO2 R6 (wherein R6 is as defined above), provided that either one of R2' and R3' is --COR6, --SO2 R6, and R1 and A1 are as defined above.