Ring Cleavage of Benzofurans and Tetrahydrobenzofurans by m‐Chloroperbenzoic Acid Epoxidation

Abstract

AbstractThe 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O 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 1g, 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.