[4+2] Annulation Reaction of Indole‐2‐Carboxamides With Vinylsulfonium Salts: Synthesis of Pyrazinoindolones

A base‐promoted [4+2] annulation reaction between indole‐2‐carboxamides and vinylsulfonium salts has been developed, affording the pyrazinoindolones in 39%–96% yields with high chemoselectivity. The synthesis strategy of vinylsulfonium salts can also approach to benzimidazole‐fused azabicyclo[3.1.0]hexanes via a [3+2]/[2+1] annulation process with benzimidazole‐derived acrylonitriles as the reaction partners.