Amino acid derivatives

A compound of the formula ##STR23## wherein R.sup.1 signifies hydrogen or methyl, R.sup.2 signifies ethyl, propyl, isopropyl, imidazol-2-yl, pyrazol-3-yl, thiazol-4-yl, thien-2-yl or t-butoxy, R.sup.3 signifies isobutyl, cyclohexylmethyl or benzyl, R.sup.4 and R.sup.5 each independently signify hydrogen, alkanoyl, which is optionally mono- or multiply-substituted by amino, monoalkylamino, dialkylamino, alkanoylamino, alkanoyloxyamino, carboxy, alkoxy or hydroxy, or an O-protecting group or together signify a cyclic O-protecting group, R.sup.6 signifies one of the groups ##STR24## and A signifies one of the groups ##STR25## wherein D signifies a methyne group or a nitrogen atom, R.sup.7 signifies alkyl, aryl or arylalkyl and R.sup.8 signifies hydrogen, alkyl, aryl or arylalkyl or R.sup.7 and R.sup.8 together with the two atoms to which they are attached signify aryl, heteroaryl, cycloalkenyl or heterocycloalkenyl, R.sup.9 signifies hydrogen or alkyl and R.sup.10 and R.sup.11 each independently signify alkyl, aryl, arylalkyl, cycloalkyl or the group

• --CH.sub.2 --X--R.sup.14 (e)
• or, together with the carbon atom to which they are attached, cycloalkyl or heterocycloalkyl, with the proviso that, where R.sup.9 signifies alkyl, R.sup.10 and R.sup.11 also signify alkyl, the dotted line can signify an additional bond, R.sup.12 signifies phenyl, substituted phenyl, benzyl or naphthyl and R.sup.13 signifies hydrogen, alkoxycarbonyalkyl, alkylcarbonylalkyl, cycloalkylcarbonylalkyl, heterocycloalkylcarbonylalkyl, arylcarbonylalkyl, aminocarbonylalkyl, substituted aminocarbonylalkyl, alkoxycarbonylhydroxyalkyl, alkylcarbonylhydroxyalkyl, cycloalkylcarbonylhydroxyalkyl, heterocycloalkylcarbonylhydroxyalkyl, arylcarbonylhydroxyalkyl, aminocarbonylhydroxyalkyl, substituted aminocarbonylhydroxyalkyl, dialkoxyphosphoroxyalkyl, diphenyloxyphosphoroxyalkyl, arylalkyl, alkoxycarbonylamino, arylalkoxycarbonylamino, alkylthioalkyl, alkylsulphinylalkyl or alkylsulphonylalkyl, with the proviso that R.sup.13 can not signify alkoxycarbonylamino or arylalkoxycarbonylamino when R.sup.12 signifies phenyl, benzyl or .alpha.-naphthyl, Y signifies the bivalent residue of optionally N- and/or .alpha.-methylated phenylalanine, cyclohexylalanine, tyrosine, O-methyltyrosine, .alpha.-naphthylalanine or homophenylalanine linked with Z at the N-terminal, Z signifies acyl, X signifies an oxygen or sulphur atom or the group --NH-- and R.sup.14 signifies hydrogen, alkyl, cycloalkyl, arylalkyl, cycloalkylalkyl, alkylcarbonyl, arylcarbonyl or arylalkylcarbonyl, in the form of an optically pure diastereomer, a diastereomeric racemate, or mixtures thereof as well as pharmaceutically usable salts thereof.