Ring contraction of 1,2,4-benzoxadiazines to benzoxazoles

Abstract

1,2,4-Benzoxadiazines (1) undergo a ring contraction to benzoxazoles (2) on heating. Other products of the reaction have been identified as nitrogen and ammonia. A mechanism is suggested for this reaction which involves (i) electrocyclic ring opening to an o-benzoquinone imine, (ii) recyclisation to a diaziridine, and (iii) extrusion of a nitrene fragment. Evidence is presented which is consistent with this proposal: thus, the benzoxazole (7) is produced on heating 3,5-di-t-butyl-o-benzoquinone with benzamidine and 4-nitrobenzoylnitrene is intercepted as the ylide (11) from the thermolysis of the substituted benzoxadiazine (10).