The present invention relates to compounds of formula I: ##STR00001## wherein X, R.sup.1, R.sup.2, R.sup.3, R.sup.4 and R.sup.5 are as defined in the description and claims, and pharmaceutically acceptable salts thereof, to the preparation of such compounds and pharmaceutical compositions containing them. The compounds are useful for the treatment and/or prevention of diseases which are associated with the modulation of H3 receptors.