Brønsted Acid‐Catalyzed Tandem Cyclizations of Tryptamine‐Ynamides Yielding 1 H ‐Pyrrolo[2,3‐ d ]carbazole Derivatives

Ynamides, as versatile synthetic precursors, have attracted much attention from synthetic chemists and sparked the development of a number of methodologies for the construction of various structures. 1 H ‐Pyrrolo[2,3‐ d ]carbazole is a core scaffold of a series of monoterpene indole alkaloids found in Kopsia , Strychnos , and Aspidosperma , for example. In this study, 1 H ‐pyrrolo[2,3‐ d ]carbazole derivatives were synthesized by a Brønsted acid‐catalyzed tandem cyclization starting from tryptamine‐based ynamides. This strategy prevented Wagner–Meerwein rearrangement by instantaneous intramolecular nucleophilic trapping of the indoleninium to afford a tetracyclic indoline via an in situ‐formed enol species induced by the formation of a more stable conjugate diene moiety. The functional group tolerances were investigated by using a series of readily available substrates. A plausible mechanism has been proposed based on the evidence of the capture of the hemiaminal intermediate. Lastly, a Büchi ketone, which is the pivotal intermediate in the synthesis of the indole alkaloid vindorosine, was synthesized by utilizing our newly developed methodology.