Metal-Catalyzed Cyclotrimerization Reactions of Cyanamides: Synthesis of 2-Aryl-α-carbolines.

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The synthesis of annulated 2-aryl-α-carboline heterocycles is described using transition metal catalysis. A linear strategy is described that uses Rh(I) catalysis to form the α-carboline scaffold by [2+2+2] cyclotrimerization. Alternatively, a tandem catalytic approach using a Pd(II) precatalyst afforded the same target molecules by mediating a Sonogashira reaction and a [2+2+2] cyclotrimerization in the same reaction flask. In each case, nine different 2-aryl-α-carbolines have been prepared in high to modest isolated yields.