title: Preparation of eumelanin-related metabolites 5,6-dihydroxyindole, 5,6-dihydroxyindole-2-carboxylic acid, and their O-methyl derivatives.
abstract: 5,6-Dihydroxyindole (5,6DHI) and 5,6-dihydroxyindole-2-carboxylic acid (5,6DHI2C) are ultimate precursors of the black melanin, eumelanin. These indolic metabolites and their O-methyl derivatives are excreted in urine of melanoma patients at high levels and of healthy persons at low levels. We describe here a simplified procedure for preparing milligram to subgram quantities of 5,6DHI and 5,6DHI2C and their O-methyl derivatives. Dopachrome generated in situ by ferricyanide oxidation of dopa at pH 6.5 underwent spontaneous decarboxylation to give 5,6DHI in 40% isolation yield, while treatment of dopachrome with alkali at pH 13 afforded 5,6DHI2C in 38% isolation yield. Two isomeric O-methyl derivatives of 5,6DHI were prepared by treatment with diazomethane, while those of 5,6DHI2C were prepared by treatment with diazomethane followed by alkaline hydrolysis of the methyl esters. 5,6DHI and 6-hydroxy-5-methoxyindole were also obtained by heating the corresponding carboxylic acids in decalin. 5-Hydroxy-6-methoxyindole and 6-hydroxy-5-methoxyindole-2-carboxylic acid could also be prepared by debenzylation of the commercially available O-benzyl derivatives.
authors.affiliation: School of Hygiene, Fujita-Gakuen Health University, Aichi, Japan.
authors.forename: K
authors.initials: K
authors.lastname: Wakamatsu
authors.forename: S
authors.initials: S
authors.lastname: Ito
mesh_terms: D000468:Alkalies; D003653:Decarboxylation; D006358:Hot Temperature; D006863:Hydrogen-Ion Concentration; D006868:Hydrolysis; D007211:Indoles / Q000138:chemical synthesis*; D008543:Melanins / Q000138:chemical synthesis; D008745:Methylation
chemical_list: D000468:Alkalies; D007211:Indoles; D008543:Melanins; C041877:eumelanin; C030692:5,6-dihydroxy-2-indolylcarboxylic acid; C033871:5,6-dihydroxyindole