In some instances, in order to facilitate purification, a derivative of formula (XX) wherein R9 is a hydrogen atom can be further converted into a derivative of formula (XX) wherein R9 is an alkoxycarbonyl or aralkyloxycarbonyl group, preferably a tert-butoxycarbonyl group, for example by treatment with di-tert-butyl dicarbonate in methylene chloride followed by chromatography.