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.