Compounds of formula (I): ##STR1## wherein: R.sup.1 is hydrogen or a hydroxy-protecting group; R.sup.2 is alkyl, alkoxy, halogen, optionally substituted phenyl or optionally substituted phenoxy; R.sup.3 is optionally substituted pyridyl, optionally substituted quinolyl or phenyl group which has a substituent of formula --CYNR.sup.5 R.sup.6, where Y is oxygen or sulfur, and R.sup.5 and R.sup.6 are each alkyl, aryl or aralkyl, or R.sup.5 and R.sup.6 and the nitrogen to which they are attached together form a heterocyclic group; is R.sup.4 hydrogen or an amino-protecting group; and Z is sulfur or oxygen; are valuable intermediates in the preparation of carbapenem compounds and retain a desirable configuration during conversion to such carbapenem compounds. Penem and carbapenem compounds having a group of formula --SA' are prepared from a corresponding compound having a substituted thio, sulfinyl or sulfonyl group at this position by reaction with a compound A'SH (where A' is an organic group) in the presence of a salt of a metal of Group II or III of the Periodic Table.