Oxidative N -Heterocyclic Carbene-Catalyzed Intramolecular Friedel-Crafts Alkylation of Indoles for the Synthesis of Spirocyclic Indolenines.

The stereoselective intramolecular dearomatizing spirocyclization of indoles via oxidative N -heterocyclic carbene (NHC) catalysis to afford indolenines bearing an all-carbon quaternary center at the 3-position is reported. The reaction proceeds via the intramolecular nucleophilic addition of the indole to an in situ generated α,β-unsaturated acyl azolium. The cyclized indolenine bearing an acyl azolium functionality is trapped by a suitable external nucleophile that does not efficiently react with the α,β-unsaturated acyl azolium via direct acylation.