Biocatalyst and bioprocess engineering for alkene epoxidation.

摘要

The epoxidation of electron rich olefins is generally carried out using preformed peroxyacids or metal catalysts in the presence of stoichiometric amounts of hydrogen peroxide or tert-butyl hydroperoxide. The drawbacks of these methods include the high cost and toxicity of catalysts, environmental concerns of solvents and the potential side reactions at the harsh oxidizing conditions employed. A novel method for the perhydrolysis of carboxylic acids employs a lipase in the presence of hydrogen peroxide in an organic solvent at ambient conditions.;This chemo-enzymatic reaction was optimized using Novozyme 435, the immobilized form of the lipase from Candida antarctica, and the complex, Urea-Hydrogen Peroxide (UHP). The absence of water minimizes further undesired reactions of the oxidized product. Among many solvents evaluated, ethyl acetate was chosen due to the high conversions obtained in this solvent. On recycling, the enzyme maintained its activity after five rounds of epoxidations as compared to the rapid loss of activity when aqueous H2O2 was used as the oxidant. Several other alkenes, such as indene, were epoxidized by this method with yields ranging from 75 to 100%.;The enzymatic kinetic resolution of these readily available racemic epoxides by epoxide hydrolases furnishes chiral epoxides and 1,2-diols, important building blocks for the synthesis of biologically active compounds. The poor solubility of epoxides and the spontaneous hydrolyses in aqueous systems are major limitations of the usual methods of hydrolysis in aqueous reaction media using whole microbial cells. To overcome these problems, the use of organic solvents for these hydrolyses was studied.;Crude enzyme preparations from Beauveria bassiana and Diplodia gossypina were immobilized by entrapment in calcium alginate beads for the resolution of styrene oxide and indene oxide in the presence of organic solvents. The immobilized enzyme could be successfully employed in the resolution of styrene oxide at more than 20 g/L with 100% enantio-selectivity in the presence of hexane. The immobilized enzyme could be re-used up to five times without any significant loss in activity.