There are described compounds of formula I*, ##STR1## wherein R.sub.1 is lower alkoxycarbonyl,R.sub.2 is secondary or tertiary lower alkyl or lower alkylthio-lower alkyl,R.sub.3 is phenyl that is unsubstituted or substituted by one or more lower alkoxy radicals, or C.sub.4 -C.sub.8 cycloalkyl,R.sub.4 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, sulfinyl (--SO--) and sulfonyl (--SO.sub.2 --) and is unsubstituted or substituted by lower alkyl or by phenyl-lower alkyl,R.sub.5, independently of R.sub.2, has one of the meanings mentioned for R.sub.2, andR.sub.6, independently of R.sub.1, 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.