Asymmetric Reduction of Activated Alkenes by Pentaerythritol Tetranitrate Reductase: Specificity and Control of Stereochemical Outcome by Reaction Optimisation
Asymmetric Reduction of Activated Alkenes by Pentaerythritol Tetranitrate Reductase: Specificity and Control of Stereochemical Outcome by Reaction Optimisation
复制标题
DOI：
10.1002/adsc.200900574
复制
发表时间：
2009-11-01
影响因子：
5.4
通讯作者：
Scrutton, Nigel S.
中科院分区：
文献类型：
--
作者：
Fryszkowska, Anna;Toogood, Helen;Scrutton, Nigel S.
我们发现，季戊四醇四硝酸还原酶（PETNR），一个成员的'烯'还原酶老黄酶家族，催化不对称还原的各种工业相关的活化α，β-不饱和烯烃，包括烯酮，烯醛，马来酰亚胺和硝基烯烃。我们已经合理化了广泛的底物特异性和立体化学的结果，这些减少通过参考分子模型的酶-底物复合物的基础上的结晶复合物的PETNR与2-环己烯酮4A。产物的光学纯度是可变的（49- 99%ee），这取决于底物类型和取代基的性质。通常，对于在C β具有立体中心的反应产物观察到高的对映选择性（> 99%ee）。然而，对于以两种异构形式存在的底物（例如，柠檬醛11 a或硝基烯烃18- 19 a），E/Z-形式还原的对映体发散过程可能导致产物的对映体纯度较低。我们还证明，在C α立体中心的产品获得的光学纯度差是由于非酶的外消旋。在与酮基异佛尔酮3a的反应中，我们表明，通过反应优化，特别是通过缩短反应时间和通过控制溶液pH值，防止产品外消旋化。我们建议这是一个通用的策略，用于改善回收光学纯产品与其他生物催化转化，其中有潜在的产品外消旋化。
We show that pentaerythritol tetranitrate reductase (PETNR), a member of the 'ene' reductase old yellow enzyme family, catalyses the asymmetric reduction of a variety of industrially relevant activated alpha,beta-unsaturated alkenes including enones, enals, maleimides and nitroalkenes. We have rationalised the broad substrate specificity and stereochemical outcome of these reductions by reference to molecular models of enzyme-substrate complexes based on the crystal complex of the PETNR with 2-cyclohexenone 4a. The optical purity of products is variable (49-99% ee), depending on the substrate type and nature of substituents. Generally, high enantioselectivity was observed for reaction products with stereogenic centres at C beta (> 99% ee). However, for the substrates existing in two isomeric forms (e.g., citral 11a or nitroalkenes 18-19a), an enantio-divergent course of the reduction of E/Z-forms may lead to lower enantiopurities of the products. We also demonstrate that the poor optical purity obtained for products with stereogenic centres at C alpha is due to non-enzymatic racemisation. In reactions with ketoisophorone 3a we show that product racemisation is prevented through reaction optimisation, specifically by shortening reaction time and through control of solution pH. We suggest this as a general strategy for improved recovery of optically pure products with other biocatalytic conversions where there is potential for product racemisation.