Compounds of formula (I): wherein: X represents an oxygen atom or a sulphur atom, R.sup.1, R.sup.2, R.sup.3 and R.sup.4 are as defined in the description, A represents an alkylene chain as defined in the description, and B represents an alkyl or alkenyl group substituted by a group or R.sup.7, or B represents a group or R.sup.7.