Benzofurans 3,3 -sigmatropic rearrangements

The [3,3]-sigmatropic rearrangement of O-aryl oximes was first described by Sheradsky and it has been used as an adequate route to benzofuran ring, a structural unit largely present in natural products. 

One potential approach for the preparation of benzofuran derivatives involves the acid-catalyzed cyclization of O-aryl oximes 1193. 39 (equation 36). The transformation is analogous to well-known Fisher indole synthesis. The reaction proceeds through a [3,3]-sigmatropic rearrangement of enehydroxylamine 120 to compound 121 followed... 

A new synthetic method of benzofuran was reported (equation 39). The [3,31-sigma-tropic rearrangement of Af-trifluoroacetyl enehydroxylamines 136 obtained in situ by acylation of oxime ethers 135 in the presence of trifluoroacetic anhydride lead to the synthesis of cyclic or acyclic dihydrobenzofurans 138. The effects of base and temperature on the reaction products were studied. A similar pathway to that of Fisher indolization was proposed. The acylimine formed by the [3,3]-sigmatropic rearrangement of the V-trifluoroacetyl enehydroxylamine 136 gave the dihydrobenzofuran 137 by an intramolecular cyclization or the benzofuran 138 after elimination. 

However, 3,5-dimethylpyridine 1-oxide (244) and benzyne (from decomposition of 4 below 45°C) afforded 250 (24%) and isomeric adducts 247 and 248 (25 and 23%, respectively) via 1,3- and 1,5-sigmatropic rearrangement of the primary 1,3-cycloadduct (Scheme 23).121 Both 247 and 248 also rearranged at 100°C to form 250. The rearranged adduct corresponding to 248 from 3,5-dichloropyridine 1-oxide spontaneously lost hydrogen chloride at 0°C to give 3-chloropyrido[3,2-b]benzofuran (251).121 Acridine N-oxide (120)... 

Decomposition of 4 in the presence of l,3,4-oxadiazin-6-one 4-oxides, 267 (R = Me, Et, Ph), leads to the formation of benzofurans 272, benzofuranones 273, and the cyclic anhydrides 271, all of which can be rationalized in terms of an initial 1,3-cycloaddition of benzyne to 267, 1,3-sigmatropic rearrangement of 268 - 269, and subsequent decomposition steps as outlined in Scheme 26.120 Formation of 271 requires a cycloaddition of the benzyne precursor 5 to the ketene intermediate 270, which is supported by the observation of a similar capture of 5 by diphenylketene. 

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