Compounds having the formula ##STR1## and pharmaceutically acceptable salts thereof wherein a, b and d are all carbon atoms or one of a, b and d is a nitrogen atom or --N(O)-- and the others are carbon atoms; Y is a single bond, --CH.sub.2 --, --C(O)--, --O--, --S-- or --N(R.sup.8)--; and R.sup.1 to R.sup.7 are as defined herein. These compounds have potassium channel activating activity and are useful, therefore for example, as cardiovascular agents.