Cyclopropanes represent a class of versatile building blocks in modern organic synthesis. While the release of ring strain offers a thermodynamic driving force, the control of selectivity for C–C bond cleavage and the subsequent regiochemistry of the functionalization remains difficult, especially for unactivated cyclopropanes. Here we report a photoredox-coupled ring-opening oxo-amination of electronically unbiased cyclopropanes, which enables the expedient construction of a host of structurally diverse β-amino ketone derivatives. Through one electron oxidation, the relatively inert aryl cyclopropanes are readily converted into reactive radical cation intermediates, which in turn participate in the ensuing ring-opening functionalizations. Based on mechanistic studies, the present oxo-amination is proposed to proceed through an SN2-like nucleophilic attack/ring-opening manifold. This protocol features wide substrate scope, mild reaction conditions, and use of dioxygen as an oxidant both for catalyst regeneration and oxygen-incorporation. Moreover, a one-pot formal aminoacylation of olefins is described through a sequential cyclopropanation/oxo-amination.
Herein, we report a strategically novel method for the efficient construction of indole skeletons using 2-phenylisoxazol-5-ones as the starting material. This reaction proceeds via Brønsted acid-promoted α-iminyl cation generation by N–O bond cleavage and a subsequent intramolecular cyclization to afford 1H-indole-3-carboxylic acid, which further undergoes decarboxylation to yield the final product. Control experiments show that N–O bond cleavage and intramolecular cyclization proceed so fast that the 1H-indole-3-carboxylic acid could be isolated in high yields even after 5–10 min. The substrate scope of this transformation is broad, and the desired products are obtained in moderate to good yields. The transition-metal-free reaction condition, CO2 as the sole byproduct, and good practicability add to the synthetic potential of this transformation in the pharmaceutical and flavor industries.
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