Uncovering the Structure of Human Red Hair Pheomelanin: Benzothiazolylthiazinodihydroisoquinolines As Key Building Blocks

Biomimetic oxidation of the pheomelanin precursor-5-S-cysteinyldopa in the presence of Zn2+; ions led to the, isolation of two isomeric products, one of which could be identified as the benzothiazolythiazinodihydroisoquinoline 5, while the other proved too unstable for a complete characterization. Both these products were converted into more stable oxidized forms, which after ethylchloroformate derivatization, were characterized as the ethyl ester/ethoxycarbonyl isoquino. lines 8 and 9. Compound. 5 exhibited,absorption characteristics,similar to those of red hair pheomelanin, including a main band around 400 tun in acids Similarly to red hair pheomelanin and synthetic pigments, 5 afforded on chemical degradation a thiazolylpridinecarboxylic add fragment Model chemical studies allowed the proposal of a formation mechanisin for the benzothiazole and dihydroisoquinoline systems in compound 5: