Compounds of the general formula (I): ##STR1##

or a salt thereof, wherein R¹ is hydrogen, hydroxyl or halogen; R² is NHSO₂CH₃, SO₂NHCH₃ or the like; R⁵ and R⁶ each independently is hydrogen, C_1-6 alkyl, optionally substituted phenyl or optionally substituted benzyl; X is NH, sulfur, oxygen or methylene; Y is oxygen, NR⁷, sulfur, methylene or a bond; and * represents an asymmetric carbon atom. The compounds are useful as a medicine for treating or preventing diabetes, obesity, hyperlipidemia, digestive diseases, depression or urinary disturbances.