Compounds of the invention in which n is 1, X is sulphur and the other groups have the meanings given except that when R1 and R2 are both hydrogen, Y is other than ???CHR7 may also be prepared from compounds of formulae (VI) or (VII) using a thiol of formula (VIII): ##STR16## In the above formula (VI) L represents a leaving group e.g. halogen (e.g. bromine) or an acyloxy (e.g. acetoxy) group.