Organic solvents, biocatalytic asymmetric

Biphasic systems proved to be advantageous as well in the biocatalytic synthesis of (-)-l-trimethylsilylethanol which was performed by asymmetric reduction of acetyltrimethylsilane with an isolate from Rhodotorula sp. AS2.2241 [144]. Immobilized cells were employed due to the easy separation of the product as well as the improved tolerance against unfavorable factors. In an aqueous/organic solvent biphasic system higher product yield and enantiomeric excess were achieved as compared to an aqueous monophasic system. Several organic solvents were examined, and isooctane was found to be the most suitable organic phase for the reaction. 

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 peroxidaseorganic 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]. 

Hydrolases were also successfully coupled with co-TAs for the production of 3-cyclohexylamine derivatives from prochiral bicyclic diketones (Scheme 2.17) [41]. The reaction sequence was based on three biocatalytic steps— namely, stereoselective hydrolysis of a C—C bond using a p-diketone hydrolase (6-oxocamphor hydrolase (OCH) from Rhodococcus sp.), followed by Candida antarctica lipase B (C AL-B)-catalyzed esterification, and finally asymmetric amination by stereocomplementary (R)-selective ArRmutll or (S)-selective VF-co-TA. The first two reactions were performed simultaneously in an organic solvent (diisopropyl etirer H O MeOH 97.5/2.5/1). The ami-nation step could also be conducted in an organic solvent after removing the hydrolases by filtration, avoiding the change of the reaction media. 

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