7-Oxo-1-azabicyclo[3.2.0]hept-2-ene-2-carboxylic acid derivatives of formula (I): ##STR1## (in which:) R.sup.1 represents a hydrogen atom or an optionally substituted alkyl group;X represents an oxy group, a thio group, a sulphinyl group or a sulphonyl group;A represents a bivalent for trivalent saturated aliphatic hydrocarbon group;p is 1 (when A is bivalent) or 2 (when A is trivalent);R.sup.2 represents an azido, nitro or cyano group or one of the groups of formula ##STR2## m and n are the same or different and each is 0 or 1; R represents an amidino group or a group of formula R.sup.4, --N(R.sup.4).sub.2 or --YR.sup.4,B represents a group of formula ##STR3## D represents a group of formula ##STR4## (provided that, where m and n are both 1, the groups represented by B and D must be different);R.sup.4 represents a hydrogen atom or an alkyl group and, where there are 2 or more groups represented by R.sup.4, they may be the same or different, andY represents an oxygen atom or a sulphur atom; andR.sup.3 represents a carboxy group or a protected carboxy group)and pharmaceutically acceptable salts thereof are valuable antibiotics and may be prepared by a process which includes heating a corresponding phosphorus-ylide compound.