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.