Compounds of the formula I ##STR1## wherein R is a hydrogen atom or an alkyl radical, R.sub.1 is a hydrogen atom or an alkyl, alkenyl or a cycloalkyl radical, R.sub.2 is a hydrogen atom or an alkyl, alkenyl, alkylcarbonyl, alkoxycarbonyl, aminocarbonyl or hydrazinocarbonyl radical or R.sub.1 and R.sub.2, together with the carbon atom to which they are attached, form a cycloalkyl ring, X is a valency bond or an alkylene or vinylene radical, R.sub.3 is an aromatic heterocyclic five-membered ring containing 1 to 4 heteroatoms or pyridinyl which the five- and six-membered rings are optionaly substituted one or more times by alkyl, alkoxy, alkoxycarbonyl, carboxyl, alkylthio, hydroxyl, nitro, amino, halogen or cyano and the tautomers, optically-active forms and physiologically acceptable salts thereof with inorganic and organic acids.These compounds are useful for treatment of immunological disorders or autoimmune diseases such as aids/ARC, rheumatoid arthritis, lupus erythematosus and to suppress rejection reactions after organ/tissue transplants.