Search

A novel Brønsted acid-catalyzed reaction of alkynyl thioethers with o -hydroxybenzyl alcohols via an unexpected formal [4 + 2] annulation has been developed. This metal-free protocol leads to the facile and practical synthesis of valuable polysubstituted 2 H -chromenes in mostly good to excellent yields under mild reaction conditions and features a wide substrate scope and excellent functional group tolerance.

Catalytic cycloisomerization-initiated cascade cyclizations of terminal alkynes have received tremendous interest, and been widely used in the facile synthesis of a diverse array of valuable complex heterocycles. However, these tandem reactions have been mostly limited to noble-metal catalysis, and are initiated by an exo-cyclization pathway. Reported herein is an unprecedented copper-catalyzed endo-cyclization-initiated tandem reaction of indolyl homopropargyl amides, where copper catalyzes both the hydroamination and Friedel-Crafts alkylation process. This method allows the practical and atom-economical synthesis of valuable bridged aza-[n.2.1] skeletons (n=3-6) with wide substrate scope, and excellent diastereoselectivity and enantioselectivity by a chirality-transfer strategy. Moreover, the mechanistic rationale for this novel cascade cyclization is also strongly supported by control experiments, and is distinctively different from the related gold catalysis., (© 2019 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim.)