Abstract: Compounds corresponding to the general formulae: ##EQU1## in which R represents hydrogen, an alkyl radical, a halogenated alkyl raal, a cycloalkyl radical, an optionally substituted aryl radical, an optionally substituted aralkyl radical, an acyl radical, aroyl radical or an optionally substituted heterocycle;R.sub.1 and R.sub.3, which may be the same or different, represent hydrogen, aralkyl radical containing 1 to 5 carbon atoms;R.sub.2 represents hydrogen, an alkyl radical (optionally halogenated or substituted by a hydroxyl), a formyl radical, an acyl radical, a carbamoyl radical monosubstituted or disubstituted on the nitrogen;R and R.sub.2 cannot both represent hydrogen;R.sub.4 is the carboxylic acid radical or one of its ester, amide, nitrile derivatives or a salt of an alkaline metal, alkaline-earth metal or heavier metal, in which case several molecules (1) can be associated with the metal atom;X represents oxygen or sulphur, and their salts, especially their sulphonium salts.