Elemental Sulfur‐Promoted [2+3+1] Annulation for Synthesis of Functionalized Thiochromeno[2,3‐b]indoles from Indole Derivatives

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An intermolecular [2+3+1] annulation between indole and 2‐bromobenzylaldehyde derivatives was successfully achieved by utilizing elemental sulfur as the promoter and coupling partner. This direct and operationally simple procedure provided a rapid and reliable approach to synthesize functionalized thiochromeno[2,3‐b]indoles. Preliminary mechanistic studies indicated that elemental sulfur enhanced the nucleophilicity of the 3‐position of indole to attack an aldehyde group, and C−H cleavage of indole was not involved in the rate‐determining step.