Penem-3-carboxylic acid derivatives of formula (I): ##STR1## (wherein: R.sup.1 represents a hydrogen atom, an alkyl group, an alkoxy group, a hydroxyalkyl group, an acyloxyalkyl group, an alkylsulphonyloxyalkyl group, an arylsulphonyloxyalkyl group or a trialkylsilyloxyalkyl group; R.sup.2 represents a hydrogen atom or an alkyl group; R.sup.3 represents a hydrogen atom, an amino-protecting group or a group of formula ##STR2## in which R.sup.5 and R.sup.6 are the same or different and each represents a hydrogen atom or an alkyl group; A represents a branched-chain alkylene group; and R.sup.4 represents a carboxy group or a protected carboxy group) and pharmaceutically acceptable salts thereof, may be prepared by heating a corresponding phosphorus-ylide compound or by reacting a corresponding azetidin-2-one with a suitable phosphorous acid triester or triamide, and have been found to have excellent antibacterial activity accompanied by a very low acute toxicity.
6-, 1- and 2-substituted-1-carbapen-2-em-3-carboxylic acids, processes for the preparation of such compounds and pharmaceutical composition comprising such compounds