Cyclohepta[b]indole Synthesis through [5 + 2] Cycloaddition: Bifunctional Indium(III)-Catalyzed Stereoselective Construction of 7‑Membered Ring Fused Indoles

A new approach for the synthesis of highly functionalized tetrahydrocyclohepta­[b]­indoles through [5 + 2] cycloaddition was developed. Two carbon–carbon bonds were formed by the simple addition of an indium catalyst, which acted as both a π-Lewis acid and σ-Lewis acid to activate the alkyne and unsaturated ester, respectively. The reaction could be applied to various substrates and proceeded regio- and diastereoselectively in all cases.