The reaction is conducted at a temperature of about 0??? to about 80??? C. Compound (62) is obtained by reacting compound (2A) ##STR41## with P(C6 H5)3 in an organic solvent, such as methylene chloride, CH3 CN, tetrahydrofuran and the like, using a temperature of about 25??? to about 50??? C. In compounds (62) and (2A), Z represents trityl or SEM.