3-(Aryliminophosphoranyl)-5-phenyl-4-arylsulfoxy-isoxazoles undergo ring closure via an intramolecular aza-Wittig type reaction of an iminophosphorane onto a non-cumulated sulfoxide. The products obtained are isoxazolo[4,3-c]-2,1- benzothiazines, a hitherto unreported heterocyclic system. This is the first example of the construction of the sulfimide linkage (S=N) via a Wittig type process involving non-cumulated sulfoxides. The requisite iminophosphoranes were synthesised using the Staudinger reaction of an azide with triphenylphosphine. Other key steps include the regioselective addition of a carbanion to a nitrile oxide in the presence of an azide, and the synthesis and use of the little reported o-azidobenzonitrile oxide.