Conversion of the 4-position acetyl group into COR2 can be effected, for example, in accordance with a method in which a reaction with a compound represented by R21???X (R21 represents an alkyl group, an alkenyl group or an aryl group and X represents a halogen atom such as iodine atom, bromine atom or the like or a leaving group such as methanesulfonyl group, p-toluenesulfonyl group or the like)