Enantioselective Synthesis of Spirocyclohexadienones by NHC‐Catalyzed Formal [3+3] Annulation Reaction of Enals

The enantioselective synthesis of pyrazolone‐fused spirocyclohexadienones was demonstrated by the reaction of α,β‐unsaturated aldehydes with α‐arylidene pyrazolinones under oxidative N‐heterocyclic carbene (NHC)catalysis. This atom‐economic and formal [3+3] annulation reaction proceeds through a vinylogous Michael addition/spiroannulation/dehydrogenation cascade to afford spirocyclic compounds with an all‐carbon quaternary stereocenter in moderate to good yields and excellent ee values. Key to the success of the reaction is the cooperative NHC‐catalyzed generation of chiral α,β‐unsaturated acyl azoliums from enals, and base‐mediated tandem generation of dienolate/enolate intermediates from pyrazolinones.