Fig. 9: Plausible mechanism for the dearomative annulation of indole.

The plausible mechanism involves anodic oxidation of indole to generate indole radical cation. Two steps of nucleophilic attack by ethylene glycol accompanied with an oxidation process would afford product 3aa. On the other hand, radical–radical cross-coupling with in situ generated sulfur radical and subsequent intramolecular nucleophilic attack would furnish product 3ao.