Stereoselective oxidation of bis-sulfides catalyzed by peroxygenase from oat

The stereoselective oxidation of two classes of prochiral bis-sulfides, 1,3-dithianes and bis-(phenylthio)alkanes, was achieved by biocatalyzed reactions in the presence of a peroxygenase-containing enzymatic preparation from oat flour. In the oxidation of 2-substituted 1,3-dithianes the corresponding mono-sulfoxides were obtained as exclusive products and the reaction proceeded with preferential formation of trans-(1S,2S)-monosulfoxides in good enantiomeric and diastereoisomeric purity. The (1S,2S)-configuration of the obtained sulfoxides was assigned by correlation with known (1S,2S)-2-phenyl-1,3-dithiane 1-oxide through a combination of circular dichroism data, enantiomer elution orders in chiral HPLC and optical rotations. The enzymatic activity was monitored in different reaction media and in the presence of high concentration of acetonitrile or in organic solvent/ aqueous buffer biphasic system a decrease in reaction rate was observed, however counterbalanced in some cases by increased stereoselectivity. Unlike dithianes, bis-(phenylthio)alkanes were biocatalytically oxidized to the corresponding bis-sulfoxides, obtained as a mixture of chirally active C2-symmetric and meso‑isomers. Some over-oxidation products were detected in the reaction of bis-(phenylthio)methane while 1,2-bis-(phenylthio)ethane was cleanly converted into the corresponding bis-sulfoxide and the best stereoselectivity was obtained by performing the reaction in a biphasic water/tert‑butyl methyl ether reaction medium.