A compound of the formula (I) or a pharmaceutically acceptable salt thereof: ##STR00001## [wherein X.sup.1 and X.sup.2 each is independently lower alkylene; X.sup.3 is .dbd.CH.sub.2, .dbd.CHF or .dbd.CF.sub.2; R.sup.1 is substituent, R.sup.2 and R.sup.3 each is independently H or lower alkyl; n is 0, 1, 2, 3 or 4.] having the activity inhibiting DPP-IV activity. They are therefore useful in the treatment of conditions mediated by DPP-IV, such as NIDDM.