wherein R17 is as defined above, R18 and R19 each is an electron-withdrawing group capable of stabilizing the carbanion C??? or one of them is such an electron-withdrawing group and the other represents a hydrogen atom, an alkyl group containing 1 to 10 carbon atoms or a phenyl group, R20 represents a direct bond or a bivalent organic group containing 1 to 10 carbon atoms, which may contain one or