Compounds of formula (I): ##STR00001## in which: Ar represents an aromatic or a heteroaromatic radical optionally substituted, in particular, with an alkyl or a carboxyl group, R.sub.2 and R.sub.3 represent, in particular, H or alkyl, or R.sub.2 and R.sub.3, taken together, form a 5- or 6-membered ring, R.sub.4 and R.sub.5 represent, in particular, H or halogen, R.sub.6 represents, in particular, H or lower alkyl, and the salts of the compounds of formula (I). These compounds can be used in particular in the treatment of dermatological complaints associated with a keratinization disorder, and for combating ageing of the skin.