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 qu inone methides 3 are accessible by dimethyldioxirane oxidation and sub sequent valence isomerization of the resulting benzofuran epoxides 2. On irradiation (lambda > 400 nm), the quinone methides 3 rearrange by cyclization to the corresponding norcaradiene, and ring enlargement af fords the benzocycloheptafurans 4. Thermolysis of the cycloheptatriene s 4 leads to the xanthenes 5, first by cycloreversion to the norcaradi enes, followed by electrocyclization to the chromenes and tautomerizat ion of the latter. The new cycloheptatrienes 4 and xanthenes 5 were fu lly characterized.