A process for preparing a compound of formula (I'): ##STR1## comprising reacting a compound of formula (II'): ##STR2## with a compound of formula (III'): A'SH (III') wherein the reaction of the compound of formula (II') and the compound of formula (III') are carried out in the presence of a salt of a metal of Group II or III of the Periodic Table of Elements, wherein A' is an alkyl, aryl, aralkyl or heterocyclic, R.sup.1' is hydrogen or a carboxy protecting group, R.sup.2' and R.sup.3' can be hydrogen or alkyl, X' can be sulfur, k' is 1 or 2 and R.sup.4' is alkyl, alkenyl, aryl, aralkyl, cycloalkyl or heterocyclic.