Compounds of formula (I): ##STR00001## wherein R.sup.1 is (C.sub.1-8)alkyl, (C.sub.3-7)cycloalkyl, {(C.sub.1-6)alkyl-(C.sub.3-7)cycloalkyl} or Het, which are all optionally substituted from 1 to 3 times with halo, cyano, nitro, O--(C.sub.1-6)alkyl, amido, amino or phenyl, or R.sup.1 is C.sub.6 or C.sub.10 aryl which is optionally substituted from 1 to 3 times with halo, cyano, nitro, (C.sub.1-6)alkyl, O--(C.sub.1-6)alkyl, amido, amino or phenyl; or a pharmaceutically acceptable salt thereof, useful as an inhibitor of the HCV NS3 protease.