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N-Heterocyclic Carbene-Catalyzed Stereoselective [3 + 2] Cycloaddition of N,N'-Cyclic Azomethine Imines with Aryl Acetaldehydes.

A highly stereoselective [3 + 2] cycloaddition reaction of N,N'-cyclic azomethine imines with aryl acetaldehydes enabled by a chiral N-heterocyclic carbene catalyst is accomplished, giving efficient access to a plethora of enantioenriched N,N'-bicyclic pyrazolidinones featuring aromatic substituents at the C2 position. The current strategy can be directly conducted on a gram scale, and the product could be further reduced to bicyclic pyrazolidine without loss of enantiopurity.