is a hydrogen atom, for example, by reaction with a compound of formulat S.sup.1-A wherein A is a customaryleaving group, such as a halogen atom (for example, chlorine, bromine or iodine), and S.sup.1 is the hydroxyl protective group.The especially preferred compound of formula II wherein S.sup.1 is t-butyldiphenylsilyl protective group can advantageously be prepared, for example, by reacting with