The present invention relates to compounds of formula I ##STR00001## wherein one of R.sup.1 and R.sup.2 is hydrogen and the other one is ##STR00002## and wherein A and R.sup.1 to R.sup.4 are as defined in the description and claims, and pharmaceutically acceptable salts thereof. The compounds are useful for the treatment and/or prevention of diseases which are associated with the modulation of H3 receptors.