A compound of the formula ##STR31## wherein R.sub.1 represents hydrogen, lower alkanoyl having from 1 to 7 carbon atoms, hydroxy-lower alkanoyl, lower alkoxy-lower alkanoyl, phenoxy-lower alkanoyl, naphthoxy-lower alkanoyl, lower alkanoyloxy-lower alkanoyl, carboxy-lower alkanoyl, lower alkoxycarbonyl-lower alkanoyl, carbamoyl-lower alkanoyl, lower alkylcarbamoyl-lower alkanoyl, di-lower alkylcarbomoyl-lower alkanoyl, alpha-napthoxy-carboxy-lower alkanoyl, alpha-naphthoxy-lower alkoxycarbonyl-lower alkanoyl, alpha-napthoxybenzyloxycarbonyl-lower alkanoyl, alpha-naphthoxy-carbamoyl-lower alkanoyl, alpha-naphthoxy-cyano-lower alkanoyl, alpha-naphthoxy-di-lower alkylamino-lower alkanoyl, alpha-naphthoxyoxo-lower alkanoyl, lower alkenoyl having from 3 to 7 carbon atoms, lower alkynoyl having from 3 to 7 carbon atoms, cycloalkyl-carbonyl having from 4 to 9 carbon atoms, bicycloalkylcarbonyl having from 6 to 11 carbon atoms, tricycloalkylcarbonyl having from 9 to 11 carbon atoms, monocycloalkenylcarbonyl having from 4 to 9 carbon atoms, bicycloalkenylcarbonyl having from 6 to 11 carbon atoms, cycloalkyl-lower alkanoyl having from 5 to 11 carbon atoms, cycloalkyl-lower alkenoyl having from 6 to 11 carbon atoms, or cycloalkenyl-lower alkanoyl having from 5 to 11 carbon atoms;
- benzoyl unsubstituted or mono-or poly-substituted by at least one of lower alkyl, halogen, hydroxy, lower alkoxy, and nitro;
- phenyl-, alpha-naphthyl- or .beta.-naphthyl-lower alkanoyl, in which the phenyl group is unsubstituted or mono-or poly-substituted by at lest one of lower alkyl, halogen, hydroxy, lower alkoxy, and nitro and the lower alkanoyl group is unsubstituted or substituted by hydroxy, lower alkoxy, lower alkanoyloxy, pivaloyloxy, ethylaminocarbonyloxy, 2-benzyloxycarbonylamino-2-methylpropanoyloxy, 2-amino-2-methyl propanoyloxy, acetoacetoxy, carboxy, benzyloxycarbonyl, t-butoxycarbonyl, lower alkoxy carbonyl, carbamoyl, lower alkyl carbamoyl, carboxymethylcarbamoyl, t-butoxycarbonylmethylcarbamoyl, 2-dimethylaminoethylcarbamoyl, 3-hydroxy-2-propylcarbamoyl, 2,2-dimethoxythylcarbamoyl, 5-amino-5-carboxypentylcarbamoyl, cyano, phosphono, di-lower alkoxyphosphoryl, hydroxymethoxyphosphoryl, benzofuranyl or oxo and is optionally branched;
- 2-(O,O-dichloroanilino)-benzylcarbonyl, 2-(O,O-dichloro-N-benzylanilino)-benzylcarbonyl, phnenyl-lower alkenoyl, naphthylcarbonyl, indenylcarbonyl, indanylcarbonyl, phenanthrenylcarbonyl, pyrrolylcarbonyl, furylcarbonyl, thienylcarbonyl, pyridylcarbonyl, indolylcarbonyl, 4,5,6,7-tetrahydroindolyl-2-carbonyl, cyclohepta[b]pyrrolyl-5-carbonyl, quinolylcarbonyl, isoquinolylcarbonyl, 2-quinoxalinylcarbonyl, 2-benzofuranylcarbonyl, benz[e]indolyl-2-carbonyl, .beta.-carbolinyl-3-carbonyl, prrolidinyl-3-carbonyl, hydroxypyrrolidinylcarbonyl, oxopyrrolidinylcarbonyl, piperidinylcarbonyl, indolinylcarbonyl, 1,2,3,4-tetrahydroquinolylcarbonyl, or 1,2,3,4-tetrahydroisoquinolylcarbonyl;
- or substituted pyrrolylcarbonyl, substituted indolylcarbonyl, substituted quinolylcarbonyl, or substituted isoquinolylcarbonyl wherein said substituent is (a) on a nitrogen atom and is lower alkyl, or (b) on one or more carbon atoms and is lower alkyl, phenyl, phenyl-lower alkyl, halogen, hydroxy, lower alkoxy, phenyl-lower alkoxy, or oxo, or (c) both a and b;
- or aryl-lower alkoxycarbonyl, oxamoyl or lower alkyloxamoyl;
- A represents the bivalent residue of leucine, norleucine, phenylalanine, N-methyl-phenylalanine, .beta.-phenylserine, cyclohexylalanine, glutamine, histidine or N-methyl-histidine;
- R.sub.3 represents isobutyl or cyclohexylmethyl;
- R.sub.5 represents isopropyl, cyclohexylmethyl, alpha-decahydronaphthyl or dimethylamino;
- and
- R.sub.6 represents alkylamino having from 1 to 10 carbon atoms, di-lower alkylaminok, hydroxy-lower alkylamino, di-(hydroxy-lower alkyl)-amino, lower alkoxy-lower alkylamino, or lower alkanoyloxy-lower alkylamino;
- phenoxy-lower alkylamino or phenoxy-hydroxy lower alkylamino in which the phenoxy group is unsubstituted or substituted by at least one of lower alkyl, lower alkoxy, hydroxy, carboxy, lower alkoxycarbonyl and carbamoyl;
- carboxy(C.sub.1-10)alkylamino or amino-carboxy(C.sub.1-10)alkylamino in which the carboxy radical is not in the 1-position of the alkyl radical;
- dicarboxymethylamino;
- lower alkoxycarbonyl(C.sub.1-10)alkylamino or butoxycarbonylamino-lower alkoxycarbonyl(C.sub.1-10)alkylamino in which the carbonyl radical is not in the 1-position of the alkyl radical;
- di(lower alkoxycarbonyl)methylamino or physiologically cleavable esterified carboxy(C.sub.1-10)alkylamino in which the ester function is not in the 1-position of the alkyl radical;
- carbamoyl- or hydroxy-lower alkylcarbamoyl(C.sub.1-10)alkylamino in which the carbamoyl radical is not in the 1-position of the alkyl radical, dicarbamoyl-methylamino, di-(lower alkylcarbamoyl)-methylamino, di-(hydroxy-lower alkylcarbamoyl)-methylamino, bis-(di-lower alkylcarbamoyl)-methylamino, amino-lower alkylamino, lower alkylamino-lower alkylamino, di-lower alkylamino-lower alkylamino, lower alkoxycarbonylamino-lower alkylamino, or guanidino-lower alkylamino;
- saturated five- or six-membered heterocyclyl-lower alkylamino that is bonded via a nitrogen atom wherein the heterocyclyl has at least one nitrogen and may have one additional heteroatom selected from nitrogen, oxygen, and sulfur;
- lower alkenylamino, lower alkynylamino, or cyclo-(C.sub.1-10)alkyl-lower alkylamino;
- phenylamino or phenyl-lower alkylamino in which the phenyl group is unsubstituted or mono- or poly-substituted by lower alkyl, hydroxy, lower alkoxy, lower alkanoyloxy, halogen, carboxy, lower alkoxycarbonyl, carbamoyl, amino, lower alkylamino, di-lower alkylamino, t-butoxycarbonylamino or by nitro;
- or pyridyl-lower alkylamino, imidazolyl-lower alkylamino, or indolyl-lower alkylamino;
- or a pharmaceutically acceptable salt of such a compound having a salt forming group.