The present invention relates to compounds of formula I ##STR00001## wherein R.sup.1, R.sup.2, R.sup.3, R.sup.4 and m are as defined in the description and claims, and pharmaceutically acceptable salts thereof as well as to pharmaceutical compositions comprising these compounds and to methods for their preparation. The compounds are useful for the treatment and/or prevention of diseases which are associated with the modulation of H3 receptors.