Three‐Component Thieno[2,3‐b]indole Synthesis from Indoles, Alkenes or Alkynes and Sulfur Powder under Metal‐Free Conditions

A Bronsted acid promoted three‐component synthesis of substituted thieno[2,3‐b]indole has been developed starting from indoles, alkenes or alkynes, and sulfur powder. N,N‐Dimethylformamide plays an important role to convert the starting materials into the fused products. Various functional groups attached to substrates were well tolerated to afford the corresponding products in moderate to good yields under very simple reaction conditions.