Acid-Promoted Competing Pathways in the Oxidative Polymerization of 5,6-Dihydroxyindoles and Related Compounds:  Straightforward Cyclotrimerization Routes to Diindolocarbazole Derivatives

Oxidation of 5,6-dihydroxyindole (1a) in acidic aqueous media led to isomeric hexahydroxydiindolocarbazoles, isolated as the acetyl derivatives 5a (29%) and 6a (19%). When the reaction is stopped in the very early stages, small amounts of the indolylindoline 17 and the open trimer 18 can be isolated. Similar oxidation of the N-methyl (1b) and O,O-dimethyl (1c) derivatives of 1a, as well as of 5-methoxyindole (9b), 6-hydroxyindole (14a), and 6-benzyloxyindole (14b), afforded the corresponding diindolocarbazoles 5b and 6b, 5c and 6c, 10, 16, and the related tetramer 15 in up to 70% overall yield, whereas 5,6-diacetoxyindole (1d), 5-hydroxyindole (9a), and indole failed to give cyclotrimerization products. Formation of diindolocarbazoles could be explained by a mechanism in which the electron-donating substituents propitiate an array of acid-induced couplings and subsequent dehydrogenation steps driven by the energetically favorable closure of the fused aromatic framework.