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Synthesis of Benzofurans and Benzoxazoles through a [3,3]-Sigmatropic Rearrangement: O–NHAc as a Multitasking Functional Group

The synthesis of heterocycles relies heavily on diverse sigmatropic rearrangements triggered by the cleavage of X–Y (X, Y = C, O, N, S, I) bonds. However, a unified rearrangement approach for constructing heterocyclic libraries is highly desirable. Encouraged by computational analysis of [3,3]-sigmatropic rearrangements, we can now rapidly synthesize oxa-heterocycles by treating N-phenoxyacetamides (Ph–ONHAc) with compounds containing an sp-hybridized carbon. The generality of the process is illustrated by the late-stage diversification of natural products, including estrone and an approved drug. A combination of experimental and computational studies revealed that the reactions proceed through a facile Claisen-like [3,3]-sigmatropic rearrangement/annulation process.