| abstract |
A compound of formula (XI) or (XIa): ** Formula ** wherein R 'is hydrogen and R is selected from benzyl, acetyl, benzoyl, trifluoromethanesulfonyl, benzenesulfonyl, p-toluenesulfonyl, methoxycarbonyl, ethoxycarbonyl, tert-butoxycarbonyl, allyloxycarbonyl and benzyloxycarbonyl optionally substituted on the aromatic ring with up to three substituents selected from halogen, cyano and trifluoromethyl; or R 'is trimethylsilyl and R is tert-butoxycarbonyl; or R and R 'taken together with the nitrogen atom to which they are attached form a phthalimido group. |