Polycyclic Indolines by Acid-Mediated Intramolecular Dearomative Strategy: Reversing Indole Reactivity in the Pictet‒Spengler-Type Reaction

Indoline-fused polycycles have been synthesized through a TFA-promoted intramolecular dearomative cyclization of indole-tethered pyrroles. Mechanistically, the strategic C-C bond formation is hypothesized to proceed via a Pictet–Spengler-type reaction wherein a reversal of conventional indole reactivity of tryptamine derivatives occurs. The synthetic versatility of this operationally simple, atom-economic approach is demonstrated in the preparation of the pyrido[1,2-a:3,4-b']diindole core of natural product homofascaplysin C.