Ring contraction of 7-chloro-3-phenyl -2H-1,2,4-benzothiadiazine

Abstract

7-Chloro-3-phenyl-2H-1,2,4-benzothiadiazine (2) undergoes ring contraction to 6-chloro-2-phenylbenzothiazole (3) when it is heated in inert solvents above 180°. The reaction is markedly promoted by tervalent phosphorus compounds and in the presence of triphenylphosphine or triethyl phosphite, the ring contraction occurs even at room temperature. Photochemical ring contraction has also been observed. The benzothiadiazine (2) can act as a chlorinating agent at high temperatures: this remarkable property is ascribed to its reaction through the tautomeric structure (5a).