In the Scheme below R20 represents an ester or nitrile and LG represents a leaving group such as a halogen atom, e.g. chlorine or bromine, or a sulfonyloxy group such as an alkylsulfonyloxy group, e.g. trifluoromethylsulfonyloxy, or an arylsulfonyloxy group, e.g. p-toluenesulfonyloxy.