RING-CLEAVAGE OF BENZOFURANS AND TETRAHYDROBENZOFURANS BY M-CHLOROPERBENZOIC ACID EPOXIDATION

The oxidation of the benzofurans 1a-f (tetrahydrobenzofurans 1g, h) with excess m-CPBA is reported. The in situ generated, highly reactive benzofuran epoxides 2a-f and their quinone methides 3a-f (cis-ene diones 3g, h) afford the labile tautomeric peroxy esters 5 and 5' by nucleophilic addition of the peroxy acid. On elimination of m-chlorobenzoic acid, the peroxy esters 5/5' of the benzofuran derivatives 1a-f rearrange thermally to the keto esters 6 by C-C cleavage or to the spiro epoxides 7 by C-0 cleavage. The latter undergo thermal isomerization to the 1,3-benzodioxoles 8 and Diels-Alder cycloaddition to the corresponding dimers 9. Independently, the keto esters 6 and the 1,3-dioxoles 8 were synthesized by thermolysis of the dioxetanes 11. The tautomeric m-CPBA adducts 5/5' of the persistent ene diones 3g, h, derived from the tetrahydrobenzofuran derivatives 2g, h, rearrange as well to the spiro epoxides 7g, h. In contrast to the benzofuran derivatives 6a-f, the keto enol ester 6h suffers Baeyer-Villiger rearrangement with another molecule of m-CPBA to form the ene diester 10h.