- A high-throughput-screening method for the identification of active and enantioselective hydrolases
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A rapid and reliable test for the determination of hydrolase activity and enantioselectivity comprises the conversion of acetic acid released from acetates to NADH by using a commercially available enzymatic test-kit (see scheme). The NAHDH is spectrophotometrically quantified in a microtiter plate format.
- Baumann, Markus,Stuermer, Rainer,Bornscheuer, Uwe T.
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- Rapid screening of hydrolases for the enantioselective conversion of 'difficult-to-resolve' substrates
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Hydrolases showing high enantioselectivity towards three racemic alcohols (1-methoxy-2-propanol, 3-hydroxy-tetrahydrofuran, 3-butyn-2-ol) and pantolactone were identified by a step-wise screening procedure. Initially, those biocatalysts, which exhibited hydrolytic activity towards the corresponding acetates or butyrates, were selected out of >100 enzymes. Here, rapid screening was performed in a pH-indicator-based format in microtiter plates. Subsequently, enantioselectivity of active hydrolases was determined in small scale reactions (~1 mg substrate per reaction) by means of gas chromatography using chiral columns. Enzymes exhibiting highest enantioselectivities were then chosen for preparative scale resolution. Using this strategy, at least one suitable hydrolase was found for 3 out of the 4 model compounds examined, allowing efficient kinetic resolution. Moreover, in all cases enantiocomplementary enzymes were identified thus enabling access to both enantiomers of all substrates.
- Baumann, Markus,Hauer, Bernhard H.,Bornscheuer, Uwe T.
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- Homochiral Metal-Organic Cage for Gas Chromatographic Separations
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Metal-organic cages (MOCs) as a new type of porous material with well-defined cavities were extensively pursued because of their relative ease of synthesis and their potential applications in host-guest chemistry, molecular recognition, separation, catalysis, gas storage, and drug delivery. Here, we first reported that a homochiral MOC [Zn3L2] is explored to fabricate [Zn3L2] coated capillary column for high-resolution gas chromatographic separation of a wide range of analytes, including n-alkanes, polycyclic aromatic hydrocarbons, and positional isomers, especially for racemates. Various kinds of racemates such as alcohols, diols, epoxides, ethers, halohydrocarbons, and esters were separated with good enantioselectivity and reproducibility on the [Zn3L2] coated capillary column. The fabricated [Zn3L2] coated capillary column exhibited significant chiral recognition complementary to that of a commercial β-DEX 120 column and our recently reported homochiral porous organic cage CC3-R coated column. The results show that the homochiral MOCs will be very attractive as a new type of chiral selector in separation science.
- Xie, Sheng-Ming,Fu, Nan,Li, Li,Yuan, Bao-Yan,Zhang, Jun-Hui,Li, Yan-Xia,Yuan, Li-Ming
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p. 9182 - 9188
(2018/07/21)
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- Application of homochiral alkylated organic cages as chiral stationary phases for molecular separations by capillary gas chromatography
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Molecular organic cage compounds have attracted considerable attention due to their potential applications in gas storage, catalysis, chemical sensing, molecular separations, etc. In this study, a homochiral pentyl cage compound was synthesized from a condensation reaction of (S,S)-1,2-pentyl-1,2-diaminoethane and 1,3,5-triformylbenzene. The imine-linked pentyl cage diluted with a polysiloxane (OV-1701) was explored as a novel stationary phase for high-resolution gas chromatographic separation of organic compounds. Some positional isomers were baseline separated on the pentyl cage-coated capillary column. In particular, various types of enantiomers including chiral alcohols, esters, ethers and epoxides can be resolved without derivatization on the pentyl cage-coated capillary column. The reproducibility of the pentyl cage-coated capillary column for separation was investigated using nitrochlorobenzene and styrene oxide as analytes. The results indicate that the column has good stability and separation reproducibility after being repeatedly used. This work demonstrates that molecular organic cage compounds could become a novel class of chiral separation media in the near future.
- Xie, Shengming,Zhang, Junhui,Fu, Nan,Wang, Bangjin,Hu, Cong,Yuan, Liming
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- (β-amino alcohol)(arene)ruthenium(II)-catalyzed asymmetric transfer hydrogenation of functionalized ketones - Scope, isolation of the catalytic intermediates, and deactivation processes
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The asymmetric transfer hydrogenation of functionalized ketones with (β-amino alcohol)(arene)RuII catalysts using 2-propanol as the hydrogen source has been studied. The structure of the catalyst has been systematically screened using a wide variety of [(η6-arene)RuCl2]2 complexes and β-amino alcohols R1CH(OH)CHR2NHR3, some of which were specifically designed for optimized performance, e.g. (1S,2R)-N-(4-biphenylmethyl)norephedrine (9ο). The efficiencies of the catalytic combinations have been evaluated in the reduction of β-oxo esters and ketones bearing heteroatoms at the α-position. The catalyst precursor [{η6-p-cymene}{η2-N,O-(9ο)}RuCl] (35), the 16-electron true catalyst [{η6-p-cymene}{η2-N,O-(9ο1-) }Ru] (36), and the hydride [{η6-p-cymene}{η2-N,O-(9ο)}RuH] (37) involved in the reduction process have been isolated, characterized by NMR and ESI-MS, as well as by X-ray crystallography in the case of 35, and their reactivities have been investigated. The results reveal two general trends regarding this catalytic process: (1) the apparent reaction rate and the enantioselectivity are largely controlled by the nature of the amine functionality of the chiral ligand and the arene ring of the RuII precursor; (2) side reactions occur between the ketone substrate and the active catalytic species that affect the concentration of the latter and consequently the apparent rate; the formation of inactive (β-diketonato)RuII complexes is demonstrated in the case of β-oxo esters.
- Everaere, Kathelyne,Mortreux, André,Bulliard, Michel,Brussee, Johannes,Van Der Gen, Arne,Nowogrocki, Guy,Carpentier, Jean-Fran?ois
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p. 275 - 291
(2007/10/03)
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- N-Benzyl-norephedrine derivatives as new, efficient ligands for ruthenium-catalyzed asymmetric transfer hydrogenation of functionalized ketones
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Significant catalytic activities (up to 600 h-1 at 20°C) and enantiomeric excesses ranging from 56 to 89% for the asymmetric transfer hydrogenation of β-ketoesters, methoxyacetone and 2-acetylpyridine to the corresponding alcohols are achieved in the presence of catalytic combinations of [RuCl2(η6-arene)]2 and N-substituted derivatives of (1S,2R)- norephedrine such as N-benzyl-norephedrine and N-(4-biphenyl)methyl- norephedrine.
- Everaere, Kathelyne,Carpentier, Jean-Francois,Mortreux, Andre,Bulliard, Michel
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p. 4083 - 4086
(2007/10/03)
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- Mechanisms and stereochemistry of acid-induced ring opening of optically active 1,2-propene oxides in the gas phase
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The acid-induced ring opening of (S)-(-)-1,2-propene oxide (1S) and (R)-(+)-1,2-propene oxide (1R) has been investigated in gaseous CH4 and CH3F at 720 torr and in the presence of a nucleophile, NuOH (Nu = H or CH3). The mechanism of the ring-opening reaction has been assessed by modulating the composition of the gaseous mixture. Two reaction pathways are operative in the gas phase, both proceeding through complete inversion of configuration of the reaction center. A first process is detectable only in the CH3F/H2O systems and takes place within a persistent proton-bound complex generated by interaction of the epoxide with the CH3OH2/+ ion, formed by methylation of H2O with (CH3)2F+. Such an intracomplex ring-opening pathway proceeds through proton transfer from the CH3OH2/+ ion to the epoxide followed by motion of the neutral CH3OH moiety around the 1-H-oxonia-2-methyl-cyclopropane structure (H-1R or H-1S) (k8 s-1) before attacking the ring carbons from the rear. In all the other systems with added CH3OH, this intracomplex pathway is preceded by a faster 'extracomplex' pathway involving the attack of an external CH3OH molecule on the proton-bound adduct. The regioselectivity of the intracomplex process is similar to that of the extracomplex pathway. Both are characterized by a slight preference for the Cβ center of H-1 R (or H-1S) (extra-complex path regioselectivity: α/β = 0.72 ± 0.05; intracomplex path regioselectivity: α/β= 0.71 ± 0.05). The regioselectivity of H-1 R (or H-1S) is substantially different from that of the 1-Me-oxonia-2-methyl-cyclopropanes (Me-1 R or Me-1S) toward the same nucleophile NuOH (α/β = 4.13 ± 0.35 (Nu = H); 2.28 ± 0.16 (Nu = CH3)). This difference is attributed to a transition structure wherein the Cα-O bond rupture increases from H-1 R (or H-1S) to Me-1R (or Me-1S) and in passing from CH3OH to H2O. The regio- and stereoselectivity of the gas-phase acid-induced ring opening of 1 S and 1 R are compared with those of related reactions carried out in solution.
- Troiani, Anna,Filippi, Antonello,Speranza, Maurizio
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p. 2063 - 2070
(2007/10/03)
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- Phosphane ligands with two binding sites of differing hardness for enantioselective Grignard cross coupling
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A series of new, chiral phosphanes is presented, individual members of which were designed to serve as ligands in transition-metal mediated asymmetric Grignard cross coupling reactions. These ligands are characterized by a side chain containing one or two oxygen atoms with the capacity to act as binding sites for the incoming Grignard reagent. A number of structural parameters for the compounds was varied to learn about the reaction mechanism. Most of the ligands were tested in two cross coupling reactions, the formation of 3-phenylbut-1-ene and of 2,2′-dimethyl-1,1′-binaphthyl, respectively. Although both systems gave modest enantiomeric excesses it was not possible to make a comparison of their respective abilities.
- Terfort, Andreas,Brunner, Henri
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p. 1467 - 1479
(2007/10/03)
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