DIMETHYLDIOXIRANE EPOXIDATION OF BENZOFURANS - REVERSIBLE THERMAL ANDPHOTOCHEMICAL VALENCE ISOMERIZATION BETWEEN BENZOFURAN EPOXIDES, QUINONE METHIDES, AND BENZOXETENES

Low-temperature oxidation of the four possible regioisomeric methoxy-s ubstituted benzofurans 1 by dimethyldioxirane afforded the rather labi le epoxides 2, which are in equilibrium with their equally labile quin one methides 3 through reversible valence isomerization. Photochemical cyclization of the latter afforded the hitherto unknown benzoxetenes 4, which are sufficiently persistent at subambient temperatures to per mit spectral characterization. The labile oxetenes 4 slowly revert to the same equilibrium mixtures of the epoxides 2 and quinone methides 3 as are obtained in the dioxirane epoxidation of the benzofurans 1. Th e mutual transformations of the benzofuran epoxides 2, quinone methide s 3, and benzoxetenes 4 include thermal isomerization to the allylic a lcohols 5 and their ring-opened tautomers 5', nucleophilic trapping in the form of the methanol adducts 6 and their ring-opened tautomers 6' , inverse Diels-Alder reaction to the [4 + 2] cycloadducts 7, and the 1,3-dipolar cycloaddition products 8 with tetracyanoethylene (TCNE).