"Compounds of formula XIIIA may be prepared by reaction of a corresponding compound of formula XI, as hereinbefore defined, with a suitable source of the vinyl anion (e.g. tributyl(vinyl)tin) under conditions known to those skilled in the art, for example at between room and reflux temperature (e.g. 50??? C.) in the presence of an appropriate solvent (e.g. toluene), a suitable coupling agent (e.g." . . . .