Dimethyldioxirane Epoxidation of 3-Phenyl-Substituted Benzofurans: A Reversible Valence Isomerization between Benzofuran Epoxides and Quinone Methides

Low-temperature oxidation of the phenyl-substituted benzofurans 1 by dimethyldioxirane afforded the rather labile benzofuran epoxides 2, which are in equilibrium with their equally labile quinone methides 3 through valence isomerization. This reversible valence isomerization was established on the basis of the chemical transformations displayed by the benzofuran epoxides 2 and quinone methides 3. These include tetraethylammonium bromide catalyzed rearrangement of epoxide 2a to benzofuran-2-one 4a, dimerization of quinone methide 3c to dibenzofurodioxane 4c, nucleophilic trapping by methanol in the form of the adducts 5, the 1,3-dipolar cycloaddition with tetracyanoethylene (TCNE) to afford 6, and the inverse Diels-Alder reaction to the [4+2] cycloadducts 7 with ethyl vinyl ether. Also styrene gave with epoxide 2a the benzodihydropyran 8a through cycloaddition to its quinone methide valence isomer 3a, while with acetic anhydride the diacetylated adduct 9a was obtained. The driving force for these latter transformations derives from strain relief and aromatization. The labile benzofuran epoxides function as effective alkylating agents, which add nucleophiles at subambient temperatures without acid and base activation.