A compound represented by the formula wherein ring A is an optionally substituted 5- to 10-membered aromatic ring; R.sub.1 and R.sub.2 are the same or different and each is an optionally substituted hydrocarbon group or an optionally substituted heterocyclic group; X and Y are the same or different and each is a bound, --O--, --S--, --SO--, --SO?2#191- or --NR.sub.3-- (R.sub.3 is a hydrogen atom or an optionally substituted hydrocarbon group); and L is a divalent hydrocarbon group, or a salt thereof shows a superior peptidase-inhibitory activity and is useful as a prophylactic or therapeutic agent of diabetes and the like. ##STR00001##