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

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.