Abstract: A process for the preparation of a pharmaceutically active compound in a stereospecific form of the formula ##STR1## or a pharmaceutically acceptable salt or ester thereof, like an alkali metal salt or an alkaline earth metal salt or a pivaloyl ester, wherein R.sub.1 represents an optionally substituted aryl group such as a phenyl or naphthyl group optionally included in a heterocyclic ring system, which is optionally substituted, or represents a heteroaromatic ring system containing in addition to carbon atoms one or more atoms selected from nitrogen, sulphur and oxygen, this ring system being optionally substituted, which comprises subjecting a compound of the formula ##STR2## wherein R.sub.