Cu₂Cl₂-pyridine-O₂ system oxidize 2-and 3-substituted indole derivatives to form C₂-C₃ double bond cleaved products. By this reaction, 2-substituted indole (6a), 3-substituted indoles (1a-5a) and 2, 3-disubstituted indole (7a) give N-acetylanthranic acid (6b), 2-acylformanilides (1b-5b) and quinolone derivative (7b), respectively. The reaction could represent a mimic of tryptophan 2, 3-dioxygenase reaction, in which tryptophan undergoes oxidative cleavage to give N-formylkynurenin.