There are described compounds of formula I*, wherein R1 is lower alkoxycarbonyl, R2 is secondary or tertiary lower alkyl or lower alkylthio-lower alkyl, R3 is phenyl that is unsubstituted or substituted by one or more lower alkoxy radicals, or C4-C8cycloalkyl, R4 is phenyl or cyclohexyl each substituted in the 4-position by unsaturated heterocyclyl that is bonded by way of a ring carbon atom, has from 5 to 8 ring atoms, contains from 1 to 4 hetero atoms selected from nitrogen, oxygen, sulfur, suffinyl (—SO—) and sulfonyl (—SO2—) and is unsubstituted or substituted by lower alkyl or by phenyl-lower alkyl, R5, independently of R2, has one of the meanings mentioned for R2, and R6, independently of R1, is lower alkoxycarbonyl, or salts thereof, provided that at least one salt-forming group is present. The compounds are inhibitors of retroviral aspartate protease and can be used, for example, in the treatment of AIDS. They exhibit outstanding pharmacodynamic properties.