There are described compounds of formula I ##STR1## wherein R.sub.1 is hydrogen or lower alkyl,R.sub.2 is hydrogen, lower alkyl, lower alkoxycarbonyl, phenyl- or naphthyl-lower alkoxycarbonyl, heterocyclylcarbonyl wherein heterocyclyl contains 5 or 6 ring atoms, is saturated and is bonded to the carbonyl group via a ring nitrogen atom and, in addition to the bonding nitrogen atom, contains as ring member one or more further hetero atoms selected from unsubstituted or C.sub.1 -C.sub.4 alkyl-substituted NH, O, S, S.dbd.O and SO.sub.2, or is lower alkanoyl, phenyl- or naphthyl-lower alkanoyl or lower alkanesulfonyl,R.sub.3 is morpholino, thiomorpholino, S-oxothiomorpholino or S,S-dioxothiomorpholino, the heterocyclyl radical in heterocyclylcarbonyl R.sub.2 having the same definition as R.sub.3 or having a definition other than R.sub.3, andR.sub.4 is hydrogen, hydroxy, C.sub.1 -C.sub.4 alkoxy, cyano, trifluoromethyl or fluorine, and salts of those compounds where salt-forming groups are present. These compounds are inhibitors of HIV-protease and are used in the treatment of ADS.