Compounds of the formula ##STR1## wherein R.sub.1 and R.sub.2 are the same or different and represent a straight-chain or branched alkyl group of 1 to 15 carbon atoms, a cycloalkyl group of 3 to 6 carbon atoms, a 2-furyl, 3-furyl, 3-pyridyl, 4-pyridyl or 2-thienyl group or an aromatic group of the formula ##STR2## wherein X, Y and Z are the same or different and represent hydrogen, alkyl of 1 to 3 carbon atoms, trifluoromethyl, alkoxy with 1 to 4 carbon atoms, halogen, nitro or hydroxy groups are useful as pharmaceutical, veterinary and cosmetic agents.