s a hydrogen atom or a protective group; Xa is a leaving group. [0358] As examples of the ???protective group??? represented by Pa, there may be mentioned alkyls (e.g., C1-6 alkyls such as methyl and ethyl), phenyl-C1-6 alkyls. (e.g., benzyl), C1-6alkyl-carbonyl, alkyl-substituted silyl (e.g., trimethylsilyl, tert-butyldimethylsilyl). [0359] As examples of the ???leaving group??? represented by Xa