Compounds having coronary and vasodilator effects, a hypotensive effect, and useful as antianginal drugs and drugs for treating hypertension are represented by the following general formula: ##STR1## wherein R.sub.1 and R.sub.2 are the same or different groups and each represents a lower alkyl group, one of R.sub.3 and R.sub.4 represents a lower alkyl group and the other represents a group of the general formula: ##STR2## wherein R.sub.5 represents an aralkyl group or an acyl group and n represents 0 or an integer of 1-3; the pharmaceutically acceptable acid addition salts thereof are also effective therapeutic compounds.