FeCl₃‑Promoted Annulation of 2‑Haloindoles: Switchable Synthesis of Spirooxindole-chromeno[2,3‑<i>b</i>]indoles and Spirooxindole-chromeno[3,2‑<i>b</i>]indoles

Electrophilic indoles bearing a leaving group at C2 undergo C3-regioselective dearomative hydroaryloxylation and subsequent 1,2-tertiary alkyl migration/aromatization. This is the first ring-opening migration of the spiroindolenine intermediate formed by the C3 nucleophilic addition reaction. Various spiro-oxindole-chromeno­[3,2-b]/[2,3-b]­indoles were successfully synthesized in excellent yields (up to 98%). This reaction features selective ring-opening migration (C–C/C–O) of the tertiary alkyl group from the indole C3 position to the C2 position stereoselectively, providing a unique synthetic method for constructing novel polycyclic indole skeletons.