A straightforward synthesis of 3,4,5-trisubstituted isoxazole derivatives via 1,3-dipolar cycloaddtion/SO₂ extrusion of benzoisothiazole-2,2-dioxide-3-ylidenes with nitrile oxides is reported using 4 Å molecular sieves as a dehydrochlorinating agent. This tandem ring closing/ring opening synthesis of 3,4,5-trisubstituted isoxazoles is the first of its kind. This methodology would enrich the synthetic chemistry of isoxazoles from easily available starting materials and benefit the discovery of novel isoxazoles with potential biological activities.