Compounds of the formula ##STR1## wherein R.sub.1 is methyl, ethyl or propyl, R.sub.2 is hydrogen, methyl or ethyl, R.sub.3 is hydrogen or methyl, and R.sub.4 is hydrogen, alkyl of 1 to 4 carbon atoms or alkanoyl of 2 to 5 carbon atoms, And non-toxic, pharmacologically acceptable acid addition salts thereof; the compounds as well as the salts are useful as analgesics.