The stereoselective synthesis of chiral 1,3‐diols with the aid of biocatalysts is an attractive tool in organic chemistry. Besides the reduction of diketones, an alternative approach consists of the stereoselective reduction of β‐hydroxy ketones (aldols). Thus, we screened for an alcohol dehydrogenase (ADH) that would selectively reduce a β‐hydroxy‐β‐trifluoromethyl ketone. One potential starting material for this process is readily available by aldol addition of acetone to 2,2,2‐trifluoroacetophenone. Over 200 strains were screened, and only a few yeast strains showed stereoselective reduction activities. The enzyme responsible for the reduction of the β‐hydroxy‐β‐trifluoromethyl ketone was identified after purification and subsequent MALDI‐TOF mass spectrometric analysis. As a result, a new NADP+‐dependent ADH from Pichia pastoris (PPADH) was identified and confirmed to be capable of stereospecific and diastereoselective reduction of the β‐hydroxy‐β‐trifluoromethyl ketone to its corresponding 1,3‐diol. The gene encoding PPADH was cloned and heterologously expressed in Escherichia coli BL21(DE3). To determine the influence of an N‐ or C‐terminal His‐tag fusion, three different recombinant plasmids were constructed. Interestingly, the variant with the N‐terminal His‐tag showed the highest activity; consequently, this variant was purified and characterized. Kinetic parameters and the dependency of activity on pH and temperature were determined. PPADH shows a substrate preference for the reduction of linear and branched aliphatic aldehydes. Surprisingly, the enzyme shows no comparable activity towards ketones other than the β‐hydroxy‐β‐trifluoromethyl ketone. 相似文献
A dual catalytic system has been developed following the screening of a series of chiral primary amine catalysts and chiral phosphoric acid catalysts for the Michael addition of cyclic ketones to nitroolefins bearing only one α‐substituent. The resulting γ‐nitro ketones, which contain a substituent on the carbon connected to the nitro group, were formed in excellent yields (>80%) with high levels of stereoselectivity (up to 94:6 dr and 98% ee) when the reaction was performed in benzene at 0 °C with 10 mol% of the optimal amine/phosphoric acid combination (1:1) as a catalyst. Subsequent reduction of the nitro group followed by intramolecular reductive amination could afford optically active cis‐octahydroindole analogues bearing a non‐functional substituent at their 2‐position.
This review surveys the recent developments in enantioselective titanium‐catalyzed cyanation reactions of carbonyl compounds, covering the literature since the beginning of 2008. It well demonstrates that this reaction constitutes an important tool in organic synthesis, still attracting a considerable interest due to the potential use of enantiopure cyanohydrins as natural products and useful synthetic intermediates in the synthesis of biologically active molecules. Abbreviations: Ac: acetyl; Ar: aryl; BINOL: 1,1′‐bi‐2‐naphthol; BINOLAM: bis(diethylaminomethyl)‐1,1‘‐binaphthol; Bn: benzyl; Cy: cyclohexyl; DFT: density functional theory; DMAP: 4‐(dimethylamino)pyridine; ee: enantiomeric excess; Naph: naphthyl; r.t.: room temperature; salen: 1,2‐bis(salicylidenamino)ethane; TADDOL: α,α,α′,α′‐tetraphenyl‐2,2‐dimethyl‐1,3‐dioxolane‐4,5‐dimethanol; TEA: trimethylamine; THF: tetrahydrofuran; TMS: trimethylsilyl; Tol: tolyl.
针对无线传感网安全管理领域中存在的密钥生成困难、节点抗俘性能较差等不足,提出了一种基于反克隆机制的WSN区域密钥分发算法。首先,基于区域成型存在的簇头-簇节点的分层特性,采用反克隆机制进行节点层次认证,引入正交机制进行节点密钥分发,降低因簇头-簇成员节点信息交互不畅而导致的安全抖动风险。随后,采取密文非对称传输方式进行密钥冲激响应,通过逆向分发进一步降低密钥被破解的概率,减少安全确认过程中存在的信令频繁收发现象。仿真结果表明:与当前常用的WSN安全领域常用的基于多项式和矩阵的无线传感器网络密钥管理安全方案算法(Polynomial and Matrix Based Key Management Security Scheme in Wireless Sensor Networks, P-MB算法)和基于块LU分解的无线传感器网络密钥预分配算法(Key Pre-distribution Approach Using Block LU Decomposition in Wireless Sensor Network, PD-BLU算法)相比,本算法可显著降低节点被俘获的概率,减少网络信令传... 相似文献