The present invention provides a compound represented by the formula (I): wherein R.sub.1 is a C?1-6#191 alkyl group optionally substituted by a C?3-10#191 cycloalkyl group, R.sub.2 is a C?2-6#191 alkyl group, R.sub.3 is a hydrogen atom, a C?1-6#191 alkyl group or a halogen atom, and X is --OR.sub.6 or --NR.sub.4R.sub.5 wherein R.sub.4 and R.sub.6 are each independently a hydrogen atom, an optionally substituted hydrocarbon group or an optionally substituted heterocyclic group, R.sub.5 is an optionally substituted hydrocarbon group, an optionally substituted heterocyclic group or an optionally substituted hydroxy group, or R.sub.4 and R.sub.5 optionally form, together with the adjacent nitrogen atom, an optionally substituted nitrogen-containing heterocycle, or a salt thereof. The compound of the present invention has a superior peptidase inhibitory action and is useful as an agent for the prophylaxis or treatment of diabetes and the like. ##STR00001##