Palladium-Catalyzed Synthesis of Fused Heterocycles

Significance

The synthesis of fused polycyclic indole derivatives under palladium catalysis is described. Tetracyclic indenones 2 (R^¹-R^² = benzo) were prepared from diaryl ketones 1 in high yield via an intramolecular arylation process. Unprotected (X = NH) and protected (X = NMe, NPh) indoles were found to participate equally well in the reaction. Examples using furan and thiophene substrates were also presented. Performing the same reaction in the presence of an alkyne 4 gave the tetracyclic products 5 incorporating a fused seven-membered ring. A variety of symmetrical and unsymmetrical alkynes bearing EDGs and EWGs were well tolerated in the reaction. In all cases only a single regioisomer was observed. A plausible mechanism is presented by analogy to that proposed by Larock for similar processes (R. C. Larock et al. J. Org. Chem. 1997, 62, 7536).