N-Heterocyclic carbene-catalyzed formal [3 + 3] annulation of alkynoic acid esters with indolin-3-ones: access to functionalized pyrano[3,2-b]indol-2-ones

Abstract

An N-heterocyclic carbene-catalyzed formal [3 + 3] annulation of alkynoic acid esters with indolin-3-ones has been developed for the rapid and efficient construction of the pyrano[3,2-b]indol-2-one skeleton, which is found as the core structure in numerous biologically active compounds. The catalytically generated alkynyl acylazoliums through the combination of a carbene with alkynoic acid esters proved to be the key for the success of this transformation, which enriches and explores the chemistry of alkynyl acylazolium intermediates.