Compounds of general formula 1, or a pharmaceutically acceptable salt thereof, wherein R1 is selected from H, lower alkyl, R4—CO, R4—O2CCH2, R5—OCO and R5—SO2; R2 is selected from lower alkyl, cycloalkyl optionally substituted with an alkyl or alkyloxy group, (C5-C12)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, NH2, NO2, NH-acyl, CN and CF3, and aralkyloxymethyl optionally substituted with up to three groups chosen from F, Cl, Br, OH, lower alkyl and O-(lower alkyl); or R1 and R2 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); R3 is selected from H, OH and O-lower alkyl; R4 is selected from H, lower alkyl and phenyl; and R5 is selected from lower alkyl, phenyl and benzyl. The compounds are useful as pharmaceutical compositions.