Bismuth(III) Triflate Catalyzed Three-Component Reactions of Indoles, Ketones, and α-Bromoacetaldehyde Acetals Enable Indole-to-Carbazole Transformation

A three-component reaction of indoles, α-bromoacetaldehyde acetals, and ketones was developed by using bismuth(III) triflate as the catalyst to realize a straightforward approach for synthesizing carbazole derivatives. The reaction was established mechanistically through the autotandem catalysis of Bi(OTf)₃ in the following two steps: (i) Friedel-Crafts-type alkylation of indole with α-bromoacetaldehyde acetal, which produced a tryptaldehyde-type intermediate and (ii) [4 + 2] annulation of this intermediate with the ketone component.