The Acid and/or Thermal Mediated Ring Contraction of 4H‑1,2,6-Thiadiazines To Afford 1,2,5-Thiadiazoles

A near-quantitative acid and/or thermal mediated ring contraction of 3′,5′-diarylspiro­(benzo­[d]­[1,3]­dioxole-2,4′-[1,2,6]­thiadiazines) affords 3-aryl-4-(2-arylbenzo­[d]­[1,3]­dioxol-2-yl)-1,2,5-thiadiazoles. The reaction scope was studied providing 11 examples of this ring contraction. A double Wagner–Meerwein reaction mechanism is proposed.