The present invention provides compounds represented by general formula (I): a prodrug thereof, or pharmaceutical acceptable salts thereof, wherein R.sup.1 is hydrogen or lower alkyl; each of R2 and R3 is independently hydrogen or lower alkyl; each of R.sup.4, R.sup.5 and R.sup.6 is independently hydrogen, halogen, lower alkyl or lower alkoxy; R.sup.7 is hydrogen or lower alkyl; R.sup.8 is hydrogen, halogen, lower alkyl, lower alkoxy, etc; R.sup.9 is --COR.sup.10, -A.sup.1-COR.sup.10, --O-A.sup.2-COR.sup.10, etc; Ar is optionally substituted phenyl or heteroaryl; and A is a bond, --OCH.sub.2--, etc, which exhibit potent and selective .beta.3-adrenoceptor stimulating activities. The present invention also provides pharmaceutical compositions containing said compound, and uses thereof.