Substrate-Assisted Redox-Neutral Formal [4 + 3] Annulation of Alkynyl Aldehydes with 9,10-Phenanthrenequinones via N-Heterocyclic Carbene Catalysis.

Medium-sized oxygen-containing heterocycles are important structural motifs in natural products and functional molecules, yet their efficient synthesis remains challenging. Herein, we disclose a N-heterocyclic carbene (NHC)-catalyzed redox-neutral formal [4 + 3] annulation of alkynyl aldehydes with 9,10-phenanthrenequinone that enables the efficient construction of fused [1,4] dioxepin-2-one derivatives without the need for external oxidants. Distinct from conventional oxidative NHC catalysis, this transformation exploits the dual role of 9,10-phenanthrenequinone as both an annulation partner and an internal oxidizing mediator, enabling catalyst turnover through a substrate-assisted redox relay process. The protocol features good functional group tolerance and high atom economy, providing a practical strategy for the construction of structurally complex seven-membered O-heterocycles.