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Cyclization of benzylsulphonamides with aldehydes in strong acid media is a synthetically useful route to 3,4‐dihydro‐1H‐2,3‐benzothiazine 2,2‐dioxides III. With insufficient acid strength or reaction time, kinetic products IV and VI are obtained; the latter compounds can be converted into the thermodynamic products III under stronger conditions. The reactions proceed via imine VII or iminium VIII compounds as common intermediates.

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1,2,4,5‐Tetrahydro‐3,2‐benzothiazepine 3,3‐dioxides 2, with a variety of substituents on the nitrogen atom, can be easily obtained by the title reaction. The isomeric compounds 4–6 are also formed from sulphonamides bearing an N‐aralkyl group with a chain of two or more carbon atoms. Activation of the ring closure‐position or deactivation of the aromatic ring in the substituent can direct the reaction to give compounds 2. Cyclization results are influenced by the size of the new heterocycle ring and by the predominant formation of derivatives with the SO2 group outside the ring.