N‑Heterocyclic Carbene Catalytic [4 + 2] Cyclization of 3‑Alkylenyloxindoles with Enals: γ‑Carbon Activation for Enantioselective Assembly of Spirocarbocyclic Oxindoles

The ubiquitous structure of all-carbospirocyclic oxindoles makes the development of new methods for their enantioselective and stereoselective synthesis a significant ongoing challenge. Herein, we disclose a formal [4 + 2] annulation through N-heterocyclic carbene (NHC) catalysis for highly enantioselective synthesis of intriguing spirocarbocyclic oxindoles in the presence of Lewis acids. This protocol features good substrates tolerance, good yields, and excellent diastereoselectivities and enantioselectivities (up to 97% ee) under mild conditions.