Photochemical and Thermal Rearrangement of Acyl‐Substituted Quinone Methides, Accessible by Dimethyldioxirane Oxidation of Benzofurans and Subsequent Valence Isomerization of the Resulting Epoxides

The photoisomerization of quinone methides 3 to benzocycloheptafuran 4 and the thermolysis of the latter to xanthenes 5 are reported. The quinone methides 3 are accessible by dimethyldioxirane oxidation and subsequent valence isomerization of the resulting benzofuran epoxides 2 . On irradiation (λ > 400 nm), the quinone methides 3 rearrange by cyclization to the corresponding norcaradiene, and ring enlargement affords the benzocycloheptafurans 4 . Thermolysis of the cycloheptatrienes 4 leads to the xanthenes 5 , first by cycloreversion to the norcaradienes, followed by electrocyclization to the chromenes and tautomerization of the latter. The new cycloheptatrienes 4 and xanthenes 5 were fully characterized.