酶立体选择性的定向进化及其高通量筛选方法

定向进化技术已成为开发新型生物催化剂的有力工具,特别是在对酶结构或催化机理信息缺乏的情况下。酶的立体选择性是个比较难处理的参数,其在定向进化过程中的技术瓶颈是建立快速有效的高通量筛选方法。本文概述了在酶立体选择性的定向进化方面所取得的进展,着重论述了酶立体选择性的高通量筛选方法。     

Laboratory Evolution of Stereoselective Enzymes: A Prolific Source of Catalysts for Asymmetric Reactions

Asymmetric catalysis plays a key role in modern synthetic organic chemistry, with synthetic catalysts and enzymes being the two available options. During the latter part of the last century the use of enzymes in organic chemistry and biotechnology experienced a period of rapid growth. However, these biocatalysts have traditionally suffered from several limitations, including in many cases limited substrate scope, poor enantioselectivity, insufficient stability, and sometimes product inhibition. During the last 15 years, the genetic technique of directed evolution has been developed to such an extent that all of these long‐standing problems can be addressed and solved. It is based on repeated cycles of gene mutagenesis, expression, and screening (or selection). This Review focuses on the directed evolution of enantioselective enzymes, which constitutes a fundamentally new approach to asymmetric catalysis. Emphasis is placed on the development of methods to make laboratory evolution faster and more efficient, thus providing chemists and biotechnologists with a rich and non‐ending source of robust and selective catalysts for a variety of useful applications.     

Mechanism and scope of salen bifunctional catalysts in asymmetric aldehyde and α-ketoester alkylation

Metal complexes of C₂-symmetric Lewis acid/Lewis base salen ligands provide bifunctional activation resulting in rapid rates in the enantioselective addition of diethylzinc to aldehydes (up to 92% ee). Further experiments probed the reactivity of the individual Lewis acid and Lewis base components of the catalyst and established that both moieties are essential for asymmetric catalysis. These catalysts are also effective in the asymmetric addition of diethylzinc to α-ketoesters. This finding is significant because α-ketoesters alone serve as their own ligands to accelerate racemic 1,2-carbonyl addition of Et₂Zn and racemic carbonyl reduction. The latter proceeds via a metalloene pathway, and often accounts for the predominant product. Singular Lewis acid catalysts do not accelerate enantioselective 1,2-addition over these two competing paths. The bifunctional amino salen catalysts, however, rapidly provide enantioenriched 1,2-addition products in excellent yield, complete chemoselectivity, and good enantioselectivity (up to 88% ee). A library of the bifunctional amino salens was synthesized and evaluated in this reaction. The utility of the α-ketoester method has been demonstrated in the synthesis of an opiate antagonist.     

Identification of new catalysts for the asymmetric reduction of imines into chiral amines with polymethylhydrosiloxane using high-throughput screening

The use of high-throughput techniques allowed the rapid identification of new catalysts for the enantioselective reduction of imines using polymethylhydrosiloxane (PMHS) as a reducing agent. By a simple modification of the chiral ligand structure that came out of the screening, the enantioselectivity of the reduction was increased from 40% ee to 60% ee.     

Self-supported heterogeneous titanium catalysts for enantioselective carbonyl-ene and sulfoxidation reactions

A new strategy for the heterogenization of chiral titanium complexes was developed by the in situ assembly of bridged multitopic BINOL ligands with [Ti(OiPr)4] without using a support. The assembled heterogeneous catalysts (self-supported) showed excellent enantioselectivity in both the carbonyl-ene reaction of alpha-methylstyrene with ethyl glyoxylate (up to 98 % ee) and the oxidation of sulfides (up to >99 % ee). The catalytic performance of these heterogeneous catalytic systems was comparable or even superior to that attained with their homogeneous counterparts. The spacers between the two BINOL units of the ligands in the assembled catalysts had significant impact on the enantioselectivity of the carbonyl-ene reaction. This demonstrates the importance of the supramolecular structures of the assemblies on their catalytic behavior. In the catalysis of sulfoxidation, the self-supported heterogeneous titanium catalysts were highly stable and could be readily recycled and reused for over one month (at least eight cycles) without significant loss of activity and enantioselectivity (up to >99.9 % ee). The features of these self-supported catalysts, such as facile preparation, robust chiral structure of solid-state catalysts, high density of the catalytically active units in the solids, as well as easy recovery and simple recycling, are particularly important in developing methods for the synthesis of optically active compounds in industrial processes.     

Enantioselective alkynylation of aldehydes catalyzed by [2.2]paracyclophane-based ligands

[reaction: see text] [2.2]Paracyclophane-based ketimine ligands were evaluated as catalysts for the enantioselective addition of in situ-prepared alkynylzinc reagents to aldehydes. The initial high activity and enantioselectivity of these ligands could be improved by an additive screening. The final protocol gives chiral propargyl alcohols in up to >98% ee.     

Self-supported chiral catalysts for heterogeneous enantioselective reactions

The development of heterogeneous chiral catalysts for enantioselective reactions is highly desirable in order to overcome some drawbacks of homogeneous catalysts. Different from the conventional approaches by using various types of supports or biphasic systems for the recovery and reuse of homogeneous catalysts, a conceptually new strategy for heterogenization of homogeneous chiral catalysts, that is, a "self-supporting" approach, has been developed to use homochiral metal-organic coordination polymers generated by the self-assembly of chiral multitopic ligands with metal ions, and thus obviates the use of any support. In this concept article, the success of this "self-supporting" strategy will be exemplified in heterogeneous catalysis of asymmetric carbonyl-ene, sulfoxidation, epoxidation, and asymmetric hydrogenation reactions.     

Combinatorial chemistry approach to chiral catalyst engineering and screening: rational design and serendipity

An efficient asymmetric catalyst relies on the successful combination of a large number of interrelated variables, including rational design, intuition, persistence, and good fortune-not all of which are necessarily well-understood; this renders such practice largely empirical. As a result, the possibility of using combinatorial chemistry methods in asymmetric catalysis research has been widely recognized to be highly desirable. In this account, we attempt to show the principle and application of combinatorial approach in the discovery of chiral catalysts for enantioselective reactions. The concept focuses on the strategy for the creation of a modular chiral catalyst library by two-component ligand modification of metal ions on the basis of molecular recognition and assembly. The self-assembled chiral catalyst with two different ligands indeed exhibited synergistic effects in terms of both enantioselectivity and activity in comparison with its corresponding homocombinations in many reactions. The examples described in this paper demonstrated the powerfulness of combinatorial approach for the discovery of novel chiral catalyst systems, particularly for the development of highly efficient, enantioselective, and practical catalysts for enantioselective reactions. We hope this concept will stimulate further work on the discovery of more highly efficient and enantioselective catalysts, as well as unexpected classes of catalysts or catalytic enantioselective reactions in the future with the help of a combinatorial chemistry approach.     

Enantioselective enzymes for organic synthesis created by directed evolution

A new concept for the creation of enzymes displaying improved enantioselectivity in a given reaction is described; it is based on "evolution in the test tube". Accordingly, proper molecular biological methods for random mutagenesis, gene expression, and high-throughput screening systems for the rapid assay of enantioselectivity are combined. Several rounds of mutagenesis and screening are generally necessary in order to create mutant enzymes that show high degrees of enantioselectivity, as in the case of the lipase-catalyzed hydrolytic kinetic resolution of a chiral ester in which the original enantioselectivity of 2 % ee (E = 1) increases to > 90% ee (E = 25).     

Complementing Pyridine‐2,6‐bis(oxazoline) with Cyclometalated N‐Heterocyclic Carbene for Asymmetric Ruthenium Catalysis

A strategy for expanding the utility of chiral pyridine‐2,6‐bis(oxazoline) (pybox) ligands for asymmetric transition metal catalysis is introduced by adding a bidentate ligand to modulate the electronic properties and asymmetric induction. Specifically, a ruthenium(II) pybox fragment is combined with a cyclometalated N‐heterocyclic carbene (NHC) ligand to generate catalysts for enantioselective transition metal nitrenoid chemistry, including ring contraction to chiral 2H‐azirines (up to 97 % ee with 2000 TON) and enantioselective C(sp³)?H aminations (up to 97 % ee with 50 TON).     

Palladium-catalyzed asymmetric arylation,vinylation, and allenylation of tert-cyclobutanols via enantioselective C-C bond cleavage

A novel enantioselective C-C bond cleavage has been achieved using palladium catalysts and chiral N,P-bidentate ligands in the asymmetric arylation, vinylation, and allenylation of tert-cyclobutanols. In these reactions, the enantioselective beta-carbon elimination of Pd(II) alcoholate formed in situ is the key step. Treatment of tert-cyclobutanols with arylating reagents in toluene in the presence of Pd(OAc)(2), a chiral ferrocene-containing N,P-bidentate ligand, and Cs(2)CO(3) affords optically active gamma-arylated ketones in excellent yields with high enantioselectivity (up to 95% ee). When vinylating reagents are used in place of arylating ones, the asymmetric vinylation also proceeds to afford optically active gamma-vinylated ketones in high yields with good to high enantioselectivity. When propargylic acetates are used, which are known to generate (sigma-allenyl)palladium complexes with Pd(0) species, asymmetric allenylation occurs to afford optically active gamma-allenylated ketones in moderate to good yields with moderate to high enantioselectivity.     

过渡金属催化的有机硼试剂对醛酮的不对称加成研究进展

手性芳基醇是一类重要的合成砌块,广泛存在于许多生物活性分子以及天然产物中,因此,高效高选择性地构建该类化合物是有机化学家们一直关注的研究热点.金属试剂对羰基化合物的不对称加成是构建手性芳基醇的一个简单高效的方法,其中,有机硼试剂由于其方便易得、稳定、低毒、官能团耐受性好等优点而被广泛用于醛、酮的不对称加成反应中.本文综述了过去二十年来过渡金属催化的有机硼试剂对醛、酮的不对称加成反应研究进展,并介绍了一些方法在生物活性手性分子合成中的应用.     

Recent applications of chiral ferrocene ligands in asymmetric catalysis

Despite the impressive progress achieved in asymmetric catalysis during the last decade, an increasing number of new catalysts, ligands, and applications are reported every year to satisfy the need to embrace a wider range of reactions and to improve the efficiency of existing processes. Because of their availability, unique stereochemical aspects, and wide variety of coordination modes and possibilities for the fine-tuning of the steric and electronic properties, ferrocene-based ligands constitute one of the most versatile ligand architectures in the current scenario of asymmetric catalysis. Over the last few years ferrocene catalysts have been successfully applied in an amazing variety of enantioselective processes. This Review documents these recent advances, with special emphasis on the most innovative asymmetric processes and the development of novel, efficient types of ferrocene ligands.     

Determination of Enantiomeric Excess of Glutamic Acids by Lab-made Capillary Array Electrophoresis

The techniques of combinatorial chemistry have recently been applied to the discovery of new asymmetric catalyst for a variety of organic transformations1-3. Using combina- torial methods, it is straightforward to generate thousands of potential asymmetri…     

Discovery of exceptionally efficient catalysts for solvent-free enantioselective hetero-Diels-Alder reaction.

Combinatorial coordination chemistry strategy combined with high-throughput screening techniques has been successfully applied to engineering practical enantioselective catalysts for asymmetric hetero-Diels-Alder reaction. The reaction of Danishefsky's diene with a variety of aldehydes can be carried out with 0.1-0.005 mol % of H4-BINOL/Ti/H4-BINOL or H4-BINOL/Ti/H8-BINOL catalysts at room temperature under solvent- and MS-free conditions to afford dihydropyrone derivatives with up to quantitative yield and 99.8% ee.     

New P,N-ferrocenyl ligands for rhodium-catalyzed hydroboration and palladium-catalyzed allylic alkylation

A set of nine new chiral P,N-ferrocenyl ligands for metal-catalyzed enantioselective reactions has been prepared. The rhodium-catalyzed hydroboration of styrene with catechol borane proceeded with high regioselectivity (up to 97:3) or with high enantioselectivity (up to 92% ee) depending on the catalyst. Good results were also obtained in the palladium-catalyzed asymmetric alkylation of 1,3-diphenylallylic systems (up to 94% ee).     

Enantiocatalytic activity of substituted 5-benzyl-2-(pyridine-2-yl)imidazolidine-4-one ligands

Currently, asymmetric synthesis represents one of the main streams of organic synthesis. Although an extensive research has been carried out in this area, the synthesis of chiral compounds with the required enantiomeric purity is still a challenging issue. Herein, we focus on the preparation of new enantioselective catalysts based on pyridine-imidazolidinones. The substituted 5-benzyl-2-(pyridine-2-yl)imidazolidine-4-ones 5–8 were prepared by condensation of chiral amino acid amides (α-methylDOPA and α-methylphenylalanine) with 2-acetylpyridine and pyridine-2-carbaldehyde. The individual isomers of the described ligands 5–8 were separated chromatographically. The copper(II) complexes of these chiral ligands were studied as enantioselective catalysts for the asymmetric Henry reaction of substituted aldehydes with nitromethane or nitroethane. The ligands containing a methyl group at the 2-position of the imidazolidinone ring 6a and 8a exhibit a high degree of enantioselectivity (up to 91% ee). The nitroaldols derived from nitroethane (2-nitropropan-1-ols) were obtained with a comparable enantiomeric purity to derivatives of 2-nitroethanol. This group of ligands represents a new and promising class of enantioselective catalysts, which deserve further attention.     

Ordered Mesoporous Silica as Supports in the Heterogeneous Asymmetric Catalysis

Enantioselective synthesis of organic compounds has been studied by homogeneous catalysts for several years. However, these catalysts have yet to make a significant impact on industrial scales for fine chemical synthesis. A primary reason is the designing of a homogeneous asymmetric catalyst, which requires relatively bulky ligands and catalyst recovery and recycling often causes problems. One of the convincing ways to overcome this problem is to immobilise the asymmetric catalyst onto a solid support and the resulting heterogeneous asymmetric catalyst system can, in principle, be readily re-used. A large number of supports such as inorganic oxides including zeolites, alumina, zirconia, silica and organic polymers have been employed as supports in heterogeneous asymmetric catalysis. Therefore, in this review article we have summarized the work done by us in our laboratory on the immobilization of chiral transition metal complexes such as Ru, Ir, Mn and Ti onto ordered mesoporous silica and its asymmetric catalysis. All these immobilized catalysts were well characterized by different physicochemical techniques to confirm the structural retention of the support as well as the active metal complex after immobilization. This report includes our asymmetric catalytic investigations in enantioselective reactions such as hydrogenation of ketones, olefins, oxidation of sulfides and oxidative kinetic resolution of alcohols and sulfoxides through immobilized heterogeneous catalyst systems.     

面手性二芳基膦-噁唑啉配体的开发及其在不对称催化反应中的应用

膦-噁唑啉配体形成的催化剂具有出色的催化活性和对映选择性, 被广泛应用于各种不对称催化反应中. 具有面手性的膦-噁唑啉配体是其中重要的一类. 综述了面手性二芳基膦-噁唑啉配体的开发, 并按反应类型介绍了它们在不对称催化反应中的应用.     

Multiple dendritic catalysts for asymmetric transfer hydrogenation

The first and second generation multiple dendritic ligands based on chiral diamine were synthesized in a convergent approach and were well-characterized by NMR and MS techniques. Their ruthenium complexes prepared in situ had good solubility in the reaction medium (azeotrope of formic acid and triethylamine) and demonstrated high catalytic activity and enantioselectivity comparable to monomeric catalysts in the asymmetric transfer hydrogenation of ketones and imines. Quantitative yields and for some cases a slightly higher enantioselectivity (up to 98.7% ee) were obtained in the dendritic catalysis. Considering the high local catalyst concentrations at the periphery, diones were tested for the possible synergic reactivity between catalytic units at the surface, while no apparent differences were noted.