Lewis Acid-catalyzed Regioselective Addition of Allenoates to Indoles for Synthesis of Bisindolylesters

Indole derivatives, especially bisindolylesters, have attracted intense attention due to their important applications in medicinal chemistry and organic synthesis. Here we develop a Lewis acid-catalyzed efficient and regioselective strategy to prepare a series of symmetric and unsymmetric bisindolylesters in high to excellent yields under mild conditions. The systematic investigations, which include stoichiometric nuclear magnetic resonance (NMR) experiments and structural characterization of intermediates, have provided insights into the possible reaction mechanism for this B(C₆F₅)₃-catalyzed addition reaction. Moreover, the sequential employment of Al(C₆F₅)₃ and B(C₆F₅)₃ as catalysts enabled us to successfully prepare the unsymmetric bisindolyl-compounds in one-pot two-step manner without the separation step.