The conversion from the compound [VI] (Z2=alkyl) to the compound [VI] (Z2 is hydrogen atom) can be carried out in any conventional manner reducing ketone to alcohol, for example, by treating with a reducing agent such as zinc borohydride, sodium triacetoxyborohydride, sodium borohydride or the like in an appropriate solvent (water, methanol, ethanol, chloroform, methylene chloride or the like, or