The first 5,6-dihydroxyindole tetramer by oxidation of 5,5',6,6'-tetrahydroxy-2,4'-biindolyl and an unexpected issue of positional reactivity en route to eumelanin-related polymers

The first tetramer of the eumelanin precursor 5,6-dihydroxyindole has been obtained, as the acetyl derivative, by peroxidase/H2O2-induced oxidative coupling of 5,5',6,6'-tetrahydroxy-2,4'-biindolyl (2) in the presence of Zn2+ ions. The tetramer, 5,5',5' ',5' '',6,6',6' ',6' ''-octaacetoxy-2,4':2',3' ':2' ',4' ''-tetraindolyl (acetylated 7), incorporates an unprecedented 2,3'-biindolyl substructure suggestive of a different positional reactivity of the 5,6-dihydroxyindole system when framed into a dimeric scaffold.