A seven-membered carbon-ring-forming annulation occurred, upon treatment of heteroarylindoles with α,β-unsaturated carbonyl compounds in the presence of an indium catalyst. The reductive annulation was observed when octylsilane was used. With 1,3-dibromo-5,5-dimethylhydantoin instead of octylsilane, the oxidative annulation proceeded. These two reactions were completely controlled; this is the first achievement in the biheteroaryl-based seven-membered carbon-ring-forming annulation. Tetracyclic products obtained from the oxidative annulation underwent SEAr-based bromination regioselectively. The resulting Br–C bond was used for the palladium-catalyzed Suzuki–Miyaura cross-coupling reaction to extend the π-conjugated region. Reaction pathways were also discussed based on some mechanistic studies.
