Carbazole Annulation via Cascade Nucleophilic Addition-Cyclization Involving 2-(Silyloxy)pentadienyl Cation

We report a new strategy toward the synthesis of highly functionalized carbazoles via 2-(silyloxy)pentadienyl cation intermediates, which were generated upon ionization of vinyl-substituted alpha-hydroxy silyl enol ethers under Bronsted acid catalysis. These electrophilic species were found to readily undergo cascade reactions with substituted indoles to generate carbazole molecular scaffolds in good yields via a sequence of regioselective nucleophilic addition, followed by intramolecular dehydrative cyclization.