Selective Synthesis of Pyrano[3,2-<i>b</i>]indoles or Cyclopenta[<i>b</i>]indoles Tethered with Medium-Sized Rings via Cascade C–C σ-Bond Cleavage and C–H Functionalization

Abstract

Highly atom-economical tandem reactions have been developed for the synthesis of pyrano[3,2-<i>b</i>]indoles or cyclopenta[<i>b</i>]indoles tethered with 7-, 8-, or 9-membered rings. These reactions first undergo a carbon-carbon σ-bond cleavage reaction of cyclic β-ketoesters. Next, in the presence of CuCl₂ and Ag₂CO₃, intramolecular O-H/C-H coupling occurs to give pyrano[3,2-<i>b</i>]indoles. This is the first example for capture of the enoloxyl radical of the intramolecular C-O bond formation reaction, whereas C3 nucleophilic addition afforded cyclopenta[<i>b</i>]indoles using TsOH·H₂O.