Compounds of formula (I) can be prepared by reaction of a compound of formula (II): in which X, Y and Z are as defined in formula (I) or are protected derivatives thereof, with a compound of formula (III): L-CR1R2CO2R12???(III) Where R1 and R2 are as defined in formula (I) or are protected derivatives thereof, R12 is H is or C1-C10 alkyl group and L is a leaving group, and optionally thereafter in