The N-heterocyclic carbene-catalyzed oxidative [3 + 2] annulation of dioxindole and enals was developed, giving the corresponding spirocyclic oxindole-γ-lactones in good yields with high to excellent diastereo- and enantioselectivities. The challenging aliphatic enals worked effectively using this strategy. The oxidative cross coupling of homoenolate and enolate via single electron transfer was proposed as the key step for the reaction.