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

The structurally interesting pyrano[3,2-b]indol-2-ones were synthesized by an N-heterocyclic carbene-catalyzed formal [3 + 3] annulation of alkynoic acid esters.