Compounds of the general formula I: ##STR1## and their pharmaceutically acceptable salts and in vivo hydrolyzable esters, in which one of R.sup.1 and R.sup.2 denotes hydrogen, the other of R.sup.1 and R.sup.2 denotes an unsubstituted or substituted five-membered hetero-aromatic ring bonded through a carbon atom thereof and having one hetero-atom selected from nitrogen, oxygen and sulphur and additionally having from one to three nitrogen atoms, and R.sup.3 denotes hydrogen or an organic group, are novel compounds with .beta.-lactamase inhibitory and antibacterial properties.