A compound of the formula: ##STR1## wherein R.sup.1 is (A) a group of the formula, ##STR2## wherein R.sup.4 is H, a halogen or trifluoromethyl group, or (B) a group of the formula, ##STR3## wherein n is 0 or an integer of 1 to 3; R.sup.5 is H or an alkyl group of 1 to 6 carbon atoms; respective R.sup.6 and R.sup.7 are an alkyl group of 1 to 6 carbon atoms, an alkenyl group of 2 to 6 carbon atoms, a cycloalkyl group of 3 to 7 carbon atoms, or a cycloalkenyl group of 5 to 7 carbon atoms; R.sup.6 and R.sup.7 may be combined with the neighboring carbon atom to represent a cycloalkyl group of 3 to 7 carbon atoms; a cycloalkenyl group of 5 to 7 carbon atoms, a bicycloalkyl group of 7 to 10 carbon atoms, or a bicycloalkenyl group of 7 to 10 carbon atoms; R.sup.2 is an alkyl group of 1 to 6 carbon atoms; and R.sup.3 is H, an alkyl group of 1 to 6 carbon atoms or an aralkyl group, or a pharmaceutically acceptable salt thereof. The compound has hypoglycemic, glucose tolerance improving and insulin sensitivity potentiating activities.