Biocatalytic Asymmetric Oxidations with Peroxidases

As enantiomericaUy pure sulfoxides are excellent chiral auxUiaries for asymmetric synthesis, different approaches for biocatalytic asymmetric oxidations at the S-atom have been explored [30, 31]. Asymmetric peroxidase atalyzed sulfoxidation of organic sulfides to sulfoxides in organic solvents opens up attractive opportunities by increased substrate solubility and diminished side reactions [32]. Plant peroxidases located in the cell wall are capable of oxidizing a broad range of structurally different substrates to products with antioxidant, antibacterial, antifungal, antiviral, and antitumor activities [33]. Hydroperoxides and their alcohols have been obtained in excellent e.e. in the biocatalytic kinetic resolution of secondary hydroperoxides with horseradish and Coprinus peroxidase [34]. 

The first example of an asymmetric electroenzymatic synthesis with a peroxidase was carried out with thioanisole and chloroperoxidase from Caldariomyces fumago. 

This catalytic asymmetric oxidation yielded J -methylphenylsulfoxide with a productivity of30g/l/day andane.e. 98% [35]. Chloroperoxidase is the most versatile peroxidase with better stability compared to other peroxidases, because spontaneous oxidation can be suppressed in the presence of ascorbic acid or dihydroxyfu-maric acid, and with better enantioselectivity because substrate access to the heme iron and ferryl oxygen favors stereoselective oxygen transfer [36]. Chloroperoxidase has been used for catalyzing the oxidation of cis-cydopropylmethanols with much higher enantioselectivity than trans-isomers [37]. 

An interesting oxidative coupling mechanism has been proposed for the biotransformation of p-coumaric acid with peroxidase, leading to dimer and trimer structures with stronger antioxidant activities [38]. 

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