According to one embodiment of the invention we provide a process for the preparation of a compound of the formula ##STR11## (wherein A and B are as hereinbeforedefined, G is > S or > S.fwdarw.O, J is a group in which 1 or 2 carbon atoms link the nuclear sulphur atom and the carbon atom bearing the carboxyl group) and R is a hydrogen atom or a lower alkylthio or lower alkoxy group having e.g up to