In one aspect of the invention, a compound of formula (IIA) may be prepared by reaction of a compound of formula (II): wherein R1 is as hereinbefore defined for a compound of formula (I) and R3 is as hereinbefore defined for a compound of formula (IIA), with a compound of formula (IIB): R2-L (IIB) wherein R2 is as hereinbefore defined for a compound of formula (I) and L is a suitable leaving group