nsubstituted or substituted by 13 alkyl groups having a carbon number of 1-4 together with the bonded carbon atoms; R22 is a hydrogen atom or an alkyl group having a carbon number of 1-4; R23 is a hydrogen atom or an alkanoyl group having a carbon number of 1-12 or a benzoyl group.A compound represented by general formula (I), in which R7, R8, R9, R10 and R11 each independently are a hydrogen atom