Oxidative Asymmetric [2 + 3] Annulation of Aldehydes with Azomethine Imines Enabled by N‑Heterocyclic Carbene Catalysis

An efficient asymmetric [2 + 3] annulation of simple aldehydes with N,N′-cyclic azomethine imines via oxidative N-heterocyclic carbene (NHC) catalysis is disclosed, affording bicyclic pyrazolidinones bearing two vicinal tertiary stereogenic centers with moderate to good yields (56–84% for 19 examples), good to excellent diastereoselectivities (>20:1 for 17 examples), and good to excellent enantioselectivities (66–98% for 19 examples). This direct α-carbon functionalization reaction of aldehyde can be readily performed on gram scale under mild conditions, and a five-membered transition state is proposed to rationalize the stereochemical outcome.