Compounds of general formula 1, or a pharmaceutically acceptable salt thereof, ##STR00001## wherein R.sup.1 is selected from H, lower alkyl, R.sup.4--CO, R.sup.4--O.sub.2CCH.sub.2, R.sup.5--OCO and R.sup.5--SO.sub.2; R.sup.2 is selected from lower alkyl, cycloalkyl optionally substituted with an alkyl or alkyloxy group, (C.sub.5-C.sub.12)cycloalkylalkyl optionally substituted with an alkyl or alkyloxy group, aralkyl optionally substituted with up to three groups chosen from F, Cl, Br, I, OH, lower alkyl, O-(lower alkyl), O-benzyl, NH.sub.2, NO.sub.2, NH-acyl, CN and CF.sub.3, and aralkyloxymethyl optionally substituted with up to three groups chosen from F, Cl, Br, OH, lower alkyl and O-(lower alkyl); or R.sup.1 and R.sup.2 together are an o-xylylene group optionally substituted on the aromatic ring with a group selected from F, Cl, Br, OH, lower alkyl and O-(lower alkyl); R.sup.3 is selected from H, OH and O-lower alkyl; R.sup.4 is selected from H, lower alkyl and phenyl; and R.sup.5 is selected from lower alkyl, phenyl and benzyl.The compounds are useful as pharmaceutical compositions.