Compounds of formula (I) ##STR1## wherein R.sup.1 is hydrogen, and R.sup.2 is cyano or --C(.dbd.S)--NH--R.sup.3 ; or R.sup.1 and R.sup.2 taken together are .dbd.C(NH.sub.2).sub.2 or .dbd.C(R.sup.5)(NHR.sup.3 '); R.sup.3 and R.sup.3 ' are hydrogen or --COOR.sup.4 ; R.sup.4 is C.sub.1 -C.sub.10 alkyl, substituted C.sub.1 -C.sub.10 alkyl, C.sub.2 -C.sub.10 alkenyl, substituted C.sub.2 -C.sub.10 alkenyl, phenyl, or substituted phenyl; and R.sup.5 is hydrogen or --S--R.sup.6 where R.sup.6 is C.sub.1 -C.sub.4 alkyl, cyano-C.sub.1 -C.sub.4 alkyl or pyridyl-C.sub.1 -C.sub.4 alkyl, and salts thereof, are useful antiviral agents.