Electrochemically Driven [3+3] Tandem Cyclization of 3-Substituted Indoles with Amidines to Access Polysubstituted Pyrimido[4,5- <i>b</i> ]indoles

A simple and environmentally friendly electrochemical method is reported for accessing polysubstituted pyrimido[4,5- b ]indoles via the cyclization of 3-substituted indoles with amidines. By employing tetrabutylammonium iodide ( n Bu 4 NI) as the electrolyte and 2,2,6,6-tetramethylpiperidinooxy (TEMPO) as a redox mediator, the target heterocycles were efficiently synthesized in moderate to good yields. Mechanistic studies demonstrated that both TEMPO and n Bu 4 NI are indispensable redox mediators and essential for achieving high selectivity in this transformation.