The present invention relates to compounds comprising the general formula (I), in which R.sub.1 and R.sub.2, which may be identical or different, are selected from the group comprising --H or a linear or branched alkyl group having from 1 to 6 carbon atoms or together form an aromatic or aliphatic ring with 5 or 6 atoms; Y and Z, which may be identical or different, are selected from the group comprising --H, --OH, --COOH, --OR.sub.3, --CH(OR.sub.3)COOH, in which R.sub.3 is selected from H, phenyl, benzyl, --CF.sub.3 or --CF.sub.2CF.sub.3, vinyl, allyl and a linear or branched alkyl group having from 1 to 6 carbon atoms.