Asymmetric Construction of Spiropyrazolone Skeletons via Amine-Catalyzed [3+3] Annulation

Here we report a secondary amine-catalyzed, highly enantioselective [3+3] cyclization reaction of alpha-alkylidene pyrazolinones and alpha,beta-unsaturated aldehydes, which facilely delivers a variety of enantioenriched spiropyrazolones showing structural and stereochemical complexity. The reaction uses an iminium ion activation strategy to activate both the beta- and ipso-positions of the enals as dielectrophilic sites. A broad range of substrates are compatible with this mild reaction system.