Metal-Free Triple Annulation of Ene-Yne-Ketones with Isocyanides: Domino Access to Furan-Fused Heterocycles via Furoketenimine.

A new furoketenimine intermediate from the coupling of ene-yne-ketones and o-alkenyl arylisocyanides, which enables the efficient synthesis of a wide range of tetracyclic and pentacyclic furan-fused heterocycles in a one-pot domino process under catalyst-free conditions, is disclosed. Based on the control experiments, a cascade of 1,6-addition, cyclization, intramolecular Diels-Alder reaction, and oxidative aromatization was proposed for the mechanism.