Abstract: A compound selected from the group consisting of a compound of the formula (I): ##STR1## in which R.sub.1 and R.sub.2 are individually selected from the group consisting of hydrogen, an alkyl of 1 to 5 carbon atoms, --(CH.sub.2).sub.n --OH in which n is between 2 and 5, an aryl or arylalkyl of 7 to 12 carbon atoms, these two radicals being optionally substituted, ##STR2## in which R.sub.3 is an alkyl of 7 to 12 carbon atoms, these two radicals being optionally substituted, with the proviso that R.sub.1 and R.sub.2 are not both aryl simultaneously, or R.sub.1 and R.sub.2 form, together with the nitrogen atom to which they are linked, a saturated or unsaturated heterocyclic which can contain a second heteroatom chosen from oxygen, sulphur and nitrogen optionally substituted by an alkyl of 1 to 5 carbon atoms, aryl, arylalkyl of 7 to 12 carbon atoms, these radicals being optionally substituted, R.sub.4 and R.sub.