The invention relates to compounds of the formula I ##STR1## in which R.sup.1, R.sup.2 and R.sup.4 represent optionally substituted aliphatic, alicyclic, alicyclic-aliphatic, aromatic, araliphatic, heteroaromatic or heteroaromatic-aliphatic radicals, with R.sup.2, if not already covered by the above-mentioned definitions, denoting the side-chain, which is protected if necessary, of a naturally occurring .alpha.-amino acid, R.sup.4 can additionally represent ##STR2## R.sup.3 denotes an optionally substituted aliphatic, alicyclic, aromatic or araliphatic radical, or R.sup.3 and R.sup.4 form, together with the atoms carrying them, a mono-, bi- or tricyclic heterocyclic ring system,agents containing these, and the use thereof as pharmaceuticals having a psychotropic action for the prophylaxis of disorders of the central nervous system, especially having a nootropic action for the treatment of cognitive dysfunctions.