Synthesis of annulated γ-carbolines and heteropolycycles by the palladium-catalyzed intramolecular annulation of alkynes

A variety of N-substituted 2-bromo-1H-indole-3-carboxaldehydes incorporating an alkyne-containing tether on the indole nitrogen have been converted to the corresponding tert-butylimines, which have been subjected to palladium-catalyzed intramolecular iminoannulation, affording various gamma-carboline derivatives with an additional ring fused across the 4- and 5-positions in good to excellent yields. When the tethered carbon-carbon triple bond is terminal or substituted with a triethylsilyl group, the iminoannulation generates a tert-butyl-gamma-carbolinium salt as the major product. The palladium-catalyzed intramolecular annulations of N-substituted 2-bromo-1H-indole-3-carboxaldehyde, methyl 2-iodo-1H-indole-3-carboxylate, and 2-iodo-3-phenyl-1H-indole containing a phenylpentynyl tether produce the corresponding heteropolycycles in moderate to good yields.