Adducts obtained by 1,3-dipolar cycloadditions to 2,3-dihydroisothiazol-3-one 1,1-dioxides are inclined to undergo a cheletropic process, by which the newly formed heterocyclic part undergoes aromatization, while SO2 is extruded and an isocyanate is generated. The isocyanates are stable under the conditions of their formation, and are subject to standard isocyanate reactions. The process might be employed to synthesize isocyanates and derivatives of urethanes, ureas etc., whenever the use of mild and neutral reaction conditions is dictated by the sensitivity of the intermediates and products involved.