Regioselective Synthesis of 3‐Hydroxyorthanilic Acid and Its Biotransformation into a Novel Phenoxazinone Dye by Use of Laccase

A natural phenoxazinone derivative, cinnabarinic acid ( 2 , CA, R = CO 2 H), could be obtained in vitro with the aid of purified laccase from the fungi Pycnoporus cinnabarinus by oxidative dimerization of 3‐hydroxyanthranilic acid ( 1 , 3‐HAA, R = CO 2 H). With the aim of gaining access to a new class of water‐soluble chromophores and potential bioactive molecules, the regioselective sulfonation of 2‐hydroxyaniline was investigated. Straightforward insertion of a sulfonate group into a carbon–metal bond provided an efficient and selective process for the synthesis of 3‐hydroxyorthanilic acid ( 3 , 3‐HOA, R = SO 3 H). This sulfonated compound was then subjected to laccase oxidation in order to mimic the cinnabarinic acid synthesis. A practical HPLC method to study the oxidized products was developed, and the major product of biotransformation – namely 2‐amino‐3‐oxo‐3 H ‐phenoxazin‐1,9‐disulfonic acid ( 4 , LAO, R = SO 3 H) – was isolated and identified as the sulfonate analogue of cinnabarinic acid.(© Wiley‐VCH Verlag GmbH & Co. KGaA, 69451 Weinheim, Germany, 2008)