These substituent groups may be further substituted.R.sup.21 and R.sup.22 are each preferably an aryl group, a heteroaryl group, an alkenyl group or an alkynyl group, more preferably an aryl group or a heteroaryl group, still more preferably an aryl group, and particularly preferably unsubstitutedphenyl.R.sup.23, R.sup.24, R.sup.25 and R.sup.26 each represents a hydrogen atom or a substituent grou