A compound of formula (VII) can be prepared by reacting a compound of formula (VIII): ##STR10## wherein R1 is as defined in formula (I), P1, P2 and P3 are hydrogen or are the same or different protecting groups, L1 is a leaving group, for example a halogen atom, with NH2 R2 or a salt of NH2 R2 wherein R2 is as defined above, in the presence of a base.