-NH2, and when A1 is --NH2, A2 is --NCO, and 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), or R2' and R3' may form together with the adjacent nitrogen atom a heterocyclic ring.