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.