Biomimetic oxidation of the antimelanoma agent 4-S-cysteaminylphenol and related catechol thioethers: Isolation and reaction behaviour of novel dihydrobenzothiazinequinones.

Enzymatic and chemical oxidation of 4-S-cysteaminylpheno1 (1) and 4-S-cysteaminylcatechol (2) leads to the formation of the hitherto unknown dihydrobenzothiazine-6,7-quinone 4 via intramolecular cyclisation of the o-quinone 6. Oxidation of 4-S-cysteinylcatechol (3) gives the corresponding dihydrobenzothiazinequinone 9, which undergoes rearrangement with partial decarboxylation to give 6,7-dihydroxybenzothiazine derivatives, isolated as 8 and 5 after reduction and acetylation of the mixture. Oxidation of 2 and 3 with periodate or iodate gives mainly the iodinated quinones 7 and 13, respectively.