A compound as claimed in claim 1 wherein R9 is a hydrogen atom or a tert-butoxycarbonyl group, Z??? is a hydroxy group, a tert-butyldimethylsilyloxy or methylthio group, and R6 is a benzyloxymethyl, allyl, tert-butyldimethylsilyl, 2-(trimethylsilylethoxy)methyl or benzyl group, B??? is a methoxy, tert-butoxy or benzyloxy group and D??? is a hydrogen or fluorine atom, a dibenzylamino group or a bis