Counter-Ion-Dependent Alkyne Iminium Ion Cyclization for Divergent Synthesis of N-Fused Indolylidine, Indole, and Indoline Derivatives Promoted by the Lewis/Bronsted Acid

Divergent synthesis of N-fused indolylidine, indole, and indoline derivatives using alkyne iminium ion cyclization is described. Trapping of vinyl cation intermediate generated after alkyne iminium ion cyclization was found to be dependent on the Lewis/Bronsted acid and solvent used. N-Fused indolylidine triflate could be used in the divergent synthesis of N-fused indole derivatives.