The oxidative phthalimidoaziridination of conjugated unsaturated N-sulfonylimines leads, in most cases, to di-substituted, tri-substituted and spiro-fused imidoylaziridines in synthetically useful yields typically above 70 %. The latter are thermally transformed into di-substituted, tri-substituted, and condensed N-sulfonylimidazoles. Interestingly, this ring expansion is accompanied by an intramolecular 1,3-sulfonyl shift from one nitrogen atom to another.