Derivatives of clavulanic acid, a process for their preparation and their use

A pharmaceutical composition having enhanced antibacterial activity comprising a synergistically effective amount of a compound of formula (II):

• ##STR12## or a pharmaceutically acceptable salt or pharmaceutically acceptable ester thereof wherein A is a hydrogen atom or an esterifying radical;
• X is an alkylene group of 1-12 carbon atoms optionally substituted by a hydroxy, amino, formyl, C.sub.1-6 alkanoyl, halo (C.sub.1-6) alkanyl, acetyl or trifluoracetyl or C.sub.1-6 alkoxy group, which substituents are not on the carbon atom adjacent the nitrogen atom;
• or a C.sub.5-7 cycloalkyl group;
• or a phenylalkyl group wherein the carbon atom content of the alkyl part is 1-6 and the phenyl part is optionally substituted with a fluorine, bromine, chlorine, C.sub.1-6 alkyl, or C.sub.1-6 alkoxy;
• with the proviso that when X represents an optionally substituted phenylalkyl group and A represents C.sub.1-3 alkyl, then the --CO.sub.2 A group is attached to the alkyl part of the phenylalkyl group, together with an antibacterially effective amount of a penicillin and a pharmaceutically acceptable carrier.