Compounds of the formula: ##STR1## wherein R.sup.1 and R.sup.2 are independently fluorine, chlorine or bromine atom, and R.sup.3 is a C.sub.4 -C.sub.7 alkyl having at least a branch at the C.sub.1 position, exhibit K.sup.+ channel opening activity and are useful as hypotensive agents and coronary vasodilators.