The asymmetric reduction of ketones is a frequently used synthesis route towards chiral alcohols. Amongst available chemo- and biocatalysts the latter stand out in terms of product enantiopurity. Their application is, however, restricted by low reaction output, often rooted in limited enzyme stability under operational conditions. Here, addition of 2-hydroxypropyl-β-cyclodextrin to bioreductions of o-chloroacetophenone enabled product concentrations of up to 29% w/v at full conversion and 99.97% e.e. The catalyst was an E. coli strain co-expressing NADH-dependent Candida tenuis xylose reductase and a yeast formate dehydrogenase for coenzyme recycling. Analysis of the lyophilized biocatalyst showed that E. coli cells were leaky with catalytic activity found as free-floating enzymes and associated with the biomass. The biocatalyst was stabilized and activated in the reaction mixture by 2-hydroxypropyl-β-cyclodextrin. Substitution of the wild-type xylose reductase by a D51A mutant further improved bioreductions. In previous optimization strategies, hexane was added as second phase to protect the labile catalyst from adverse effects of hydrophobic substrate and product. The addition of 2-hydroxypropyl-β-cyclodextrin (11% w/v) instead of hexane (20% v/v) increased the yield on biocatalyst 6.3-fold. A literature survey suggests that bioreduction enhancement by addition of cyclodextrins is not restricted to specific enzyme classes, catalyst forms or substrates.

酮的不对称还原是制备手性醇的常用合成途径。在可用的化学和生物催化剂中，后者在产品对映纯度方面脱颖而出。然而，它们的应用受到低反应输出的限制，通常根源于操作条件下酶稳定性有限。在这里，将 2-羟丙基-β-环糊精添加到邻氯苯乙酮的生物还原中，使完全转化时的产品浓度高达 29% w/v 和 99.97% ee 催化剂是一种大肠杆菌菌株，共表达 NADH 依赖性念珠菌细木糖还原酶和酵母甲酸脱氢酶用于辅酶循环。冻干生物催化剂的分析表明，大肠杆菌细胞泄漏，催化活性被发现为自由漂浮的酶，并与生物质相关。生物催化剂在反应混合物中被 2-羟丙基-β-环糊精稳定和活化。用 D51A 突变体替代野生型木糖还原酶进一步改善了生物还原。在之前的优化策略中，己烷作为第二相加入，以保护不稳定的催化剂免受疏水底物和产物的不利影响。添加 2-羟丙基-β-环糊精 (11% w/v) 代替己烷 (20% v/v) 将生物催化剂的产率提高了 6.3 倍。一项文献调查表明，通过添加环糊精来增强生物还原作用并不限于特定的酶类、催化剂形式或底物。