Y is preferably an aliphatic group having 1 to 10 carbon atoms, an aromatic group or cycloaliphatic group having 6 to 10 carbon atoms, a ???CH2CH2N(R1)SO2??? group (R1 is an alkyl group having 1 to 4 carbon atoms.) or a ???CH2CH(OY1)CH2??? group (Y1 is a hydrogen atom or an acetyl group.).