Copolymerisation of 5,6-dihydroxyindole and 5,6-dihydroxyindole-2-carboxylic acid in melanogenesis: isolation of a cross-coupling product.

Under biologically relevant conditions, co-oxidation of 5,6-dihydroxyindole (1) and 5,6-dihydroxyindole-2-carboxylic acid (2) affords, in addition to a complex mixture of homopolymers of the two indoles, a small but significant amount of a cross-coupling product which was isolated as the tetra-O-acetyl derivative and assigned the biindolyl structure 3. Co-oxidation of 5,6-dihydroxyindole and its 2-carboxy derivative under biomimetic conditions afforded, besides homopolymers of the two indoles, a mixed-type dimer which was isolated in the O-acetyl form and formulated as 3