Abstract: The invention concerns compounds of formula ##STR1## or salts thereof; wherein Ar is an optionally substituted aryl radical;R represents hydrogen or an optionally substituted alkyl or aralkyl group;R.sup.1 and R.sup.2 are the same or different and are selected from hydrogen and saturated or unsaturated, cyclic or acyclic aliphatic hydrocarbon residues optionally substituted by one or more groups selected from halogen, OH, carboxy, CN, alkoxy, alkylthio, aryloxy, alkoxycarbonyl, amino, substituted amino, and optionally substituted aryl;A represents a group of formula --XR.sup.3 whereinX is (i) a group of formula --(CHR.sup.6).sub.p --Y--(CHR.sup.7).sub.q --or (ii) a group of formula --(CHR.sup.6).sub.r --O--(CHR.sup.7).sub.qin which formulae: Y represents --S--, NR.sup.8 or a direct bond, p and q each represent 0, 1 or 2 providing that p and q do not both represent 0, 1 or 2 and the other is zero; R.sup.6, R.sup.7 and R.sup.8 independently represent hydrogen or lower alkyl, and R.sup.