Abstract: The compounds of the formula (I): ##STR1## wherein X represents a moiety of the sub-formula (a) or (b): ##STR2## HAVE BEEN FOUND TO BE .beta.-LACTAMASE INHIBITORS WHICH MAY BE USED IN PHARMACEUTICAL COMPOSITIONS TO ENHANCE THE EFFECTIVENESS OF PENICILLINS OR CEPHALOSPORINS. The diene of the formula (I) may be produced by the decarboxylation of clavulanic acid or a derivative thereof and the monoene of the formula (I) may be produced by the 1,4 addition of hydrogen to the diene of the formula (I). The diene is thus also useful as a chemical intermediate and other such uses are demonstrated.

Abstract: Compounds of the formula ##STR1## wherein R is hydrogen, alkyl of 1 to 4 carbon atoms, alkenyl of 2 to 4 carbon atoms, epoxide of 2 to 4 carbon atoms or alkyl of 1 to 4 carbon atoms substituted by 1 or 2 halogen atoms, hydroxyl, etherified or acylated hydroxyl of 1 to 7 carbon atoms, azido, phthaliamido, SR.sup.1, SOR.sup.1 or SO.sub.2 R.sup.1 wherein R.sup.1 is aryl, are useful for their .beta.-lactamase inhibitory activity and their antibacterial activity.