The compounds of the formula ##STR1## in which one of the groups R.sup.1, R.sup.2 and R.sup.3 represents hydrogen, C.sub.1-8 -alkyl, C.sub.3-6 -cycloalkyl, phenyl optionally substituted by halogeno, C.sub.1-4 -alkyl, C.sub.1-4 -alkoxy and/or trifluoromethyl, or C.sub.7-10 -phenylalkyl optionally substituted in the phenyl moiety by halogeno, C.sub.1-4 -alkyl, C.sub.1-4 -alkoxy and/or trifluormethyl, and the other two are hydrogen, or salts thereof.These compounds have valuable pharmaceutical properties.