Construction of Indole Structure on Pyrroloindolines via AgNTf₂‑Mediated Amination/Cyclization Cascade: Application to Total Synthesis of (+)-Pestalazine B

An N-linked indole structure was constructed on the 3a-position of pyrroloindoline derivatives via a cascade process involving silver-mediated amination of bromopyrroloindolines with 2-ethynylanilines with subsequent 5-endo-dig cyclization. In this reaction, AgNTf₂ was used as a tandem reagent, which activated the bromo group as a σ-Lewis acid and the alkyne moiety as a π-Lewis acid. Switching from the initial step to the second step was conducted by controlling the temperature. This protocol was applied to the synthesis of various pyrroloindolines, α-carboline, and furoindolines and the total synthesis of a dimeric indole alkaloid, (+)-pestalazine B.