Synthesis of Spirocyclic 1‐Pyrrolines from Nitrones and Arynes through a Dearomative [3,3′]‐Sigmatropic Rearrangement

Brønsted Acid Catalyzed Dearomatization by Intramolecular Hydroalkoxylation/Claisen Rearrangement: Diastereo- and Enantioselective Synthesis of Spirolactams

Described herein is a novel Brønsted acid catalyzed intramolecular hydroalkoxylation/Claisen rearrangement, allowing the practical and atom-economic synthesis of a range of valuable spirolactams from readily available ynamides in generally good to excellent yields with excellent diastereoselectivities and broad substrate scope. Importantly, an unexpected dearomatization of nonactivated arenes and heteroaromatic compounds is involved in this tandem sequence. Moreover, an asymmetric version of this tandem cyclization was also achieved by efficient kinetic resolution by chiral phosphoric acid catalysis. In addition, the [3,3]-rearrangement is shown to be kinetically preferred over the related [1,3]-rearrangement by theoretical calculations.

Nitrone and Alkyne Cascade Reactions for Regio- and Diastereoselective 1-Pyrroline Synthesis

The synthesis of 1-pyrrolines from N-alkenylnitrones and alkynes has been explored as a retrosynthetic alternative to traditional approaches. These cascade reactions are formal [4+1] cycloadditions that proceed through a proposed dipolar cycloaddition and N-alkenylisoxazoline [3,3']-sigmatropic rearrangement. A variety of cyclic alkynes and terminal alkynes have been shown to undergo the transformation with N-alkenylnitrones under mild conditions to provide the corresponding spirocyclic and densely substituted 1-pyrrolines with high regio- and diastereoselectivity. Mechanistic studies provide insight into the balance of steric and electronic effects that promote the cascade process and control the diastereo- and regioisomeric preferences of the 1-pyrroline products. Diastereoselective derivatization of the 1-pyrrolines prepared by the cascade reaction demonstrate the divergent synthetic utility of the new method.

Concise Synthesis of Furo[2,3-b]indolines via [3,3]-Sigmatropic Rearrangement of N-Alkenyloxyindoles

A concise new synthetic route to furo[2,3-b]indolines has been developed by taking advantage of the reactivity of N-alkenyloxyindole intermediates. These compounds spontaneously undergo [3,3]-sigmatropic rearrangement followed by cyclization to form hemiaminals as single diastereomers. Tin-promoted N-hydroxyindole formation followed by conjugate addition to activated alkynes provides simple and modular access to a diverse array of N-alkenyloxyindoles and their corresponding furo[2,3-b]indolines. Microscale high-throughput experimentation was used to facilitate investigation of the scope and tolerance of this transformation and related studies on the nucleophilic aromatic substitution and rearrangement of N-hydroxyindoles with halogenated arenes have also been evaluated.