Abstract: Compounds of the general formula I: ##STR1## and their pharmaceutically acceptable salts and in vivo hydrolyzable esters, in whichone 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, andR.sup.3 denotes hydrogen or an organic group,are novel compounds with .beta.-lactamase inhibitory and antibacterial properties.

Abstract: A class of 6-alkylidene penem compounds of formula (II) have antibacterial and .beta.-lactamase inhibitory properties: ##STR1## where R.sup.1 and R.sup.2 represent hydrogen or optionally substituted hydrocarbon or heterocyclic, and R.sup.3 represents hydrogen or an organic group.

Abstract: A class of 6-alkylidene penem compounds of formula (II) have antibacterial and .beta.-lactamase inhibitory properties: ##STR1## where R.sup.1 and R.sup.2 represent hydrogen or optionally substituted hydrocarbon or heterocyclic, and R.sup.3 represents hydrogen or an organic group.

Abstract: A process for the preparation of the compounds of the formula (II): ##STR1## and salts and cleavable esters thereof wherein R.sub.1 is a hydrocarbon group of up to 12 carbon atoms and which optionally also contain up to 4 heteratoms selected from oxygen, nitrogen, sulphur or chlorine and bromine which process comprises the reaction of a cleavable ester of a compound of the formula (III): ##STR2## with one equivalent of a strong base of low nucleophilicity and thereafter with a compound of the formula (IV) or (V):R.sup.2 SO.sub.2 SR.sub.1.sup.1 (IV)Cl--S--R.sub.1.sup.1 (V)wherein R.sub.1.sup.1 is a group R.sub.1 in which any amino substituent is protected and R.sup.2 is an alkyl group of 1-4 carbon atoms optionally substituted by a phenyl group or is a phenyl group optionally substituted by an alkyl group of 1-4 carbon atoms and thereafter if desired cleaving the ester moiety to yield the acid of the formula (II) or a salt thereof and removing the protecting group from any protected amino group present in R.