An N-Heterocyclic Carbene-Catalyzed [8 + 3] Annulation of Tropone and Enals via Homoenolate

A novel protocol for the annulation of tropone to enals involving nucleophilic heterocyclic carbene (NHC) catalyzed homoenolate formation has been developed. Interestingly, the reaction led to bicyclic δ-lactones instead of the expected γ-spirolactones, presumably by the uncommon [8 + 3] annulation pathway. The strategy works well with a variety of enals.