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High Enantiomeric Excess (high + enantiomeric_excess)
Selected AbstractsComparative Study of Cyanobacteria as Biocatalysts for the Asymmetric Synthesis of Chiral Building BlocksENGINEERING IN LIFE SCIENCES (ELECTRONIC), Issue 2 2006J. Havel Abstract The three representative cyanobacteria, Synechococcus PCC7942, Anabaena variabilis, and Nostoc muscorum, were studied for their ability to asymmetrically reduce the prochiral ketones 2,-3,-4,-5,-6,-pentafluoroacetophenone, ethyl 4-chloroacetate, 4-chloroacetophenone, and ethylbenzoylacetate to the corresponding chiral alcohols. Photosynthesis as well as respiration was applied for intracellular regeneration of the NAD(P)H cofactor. It was shown for the first time that all cyanobacteria were able to reduce the prochiral ketones asymmetrically without light for cofactor regeneration. By comparison of the cell specific product formation capacities of cyanobacteria with typical heterotrophic whole cell biocatalysts in batch processes, it is shown that comparable or, in some cases, better performances at high enantiomeric excess (ee > 99.8,%) are obtained. As a consequence of a generally strong product inhibition, in situ product removal must be applied in order to restore process efficiency when using cyanobacteria as biocatalysts. [source] Combining Designer Cells and Click Chemistry for a One-Pot Four-Step Preparation of Enantiopure ,-HydroxytriazolesADVANCED SYNTHESIS & CATALYSIS (PREVIOUSLY: JOURNAL FUER PRAKTISCHE CHEMIE), Issue 13 2010Wiktor Szymanski Abstract The multistep catalytic process using designer cells, either added as freshly prepared suspensions or as stable lyophilized powder, and click reaction can be performed in one pot. The sequence of four reactions allows the production of both enantiomers of ,-hydroxytriazoles with high enantiomeric excess. [source] Tandem Enzyme/Gold-Catalysis: From Racemic ,-Allenic Acetates to Enantiomerically Enriched 2,5-Dihydrofurans in One PotADVANCED SYNTHESIS & CATALYSIS (PREVIOUSLY: JOURNAL FUER PRAKTISCHE CHEMIE), Issue 14-15 2009Martta Asikainen Abstract We report the first example of a tandem kinetic resolution/cycloisomerization of racemic allenic acetates in the presence of Burkholderia cepacia lipase (PS Amano SD) and catalytic amounts of chloroauric acid (HAuCl4) which affords 2,5-dihydrofurans, as well as unreacted starting material, in one pot with high enantiomeric excess and moderate to good yield. [source] Productive Asymmetric Styrene Epoxidation Based on a Next Generation Electroenzymatic MethodologyADVANCED SYNTHESIS & CATALYSIS (PREVIOUSLY: JOURNAL FUER PRAKTISCHE CHEMIE), Issue 14-15 2009Reto Ruinatscha Abstract We have established a novel and scalable methodology for the productive coupling of redox enzymes to reductive electrochemical cofactor regeneration relying on efficient mass transfer of the cofactor to the electron-delivering cathode. Proof of concept is provided by styrene monooxygenase (StyA) catalyzing the asymmetric (S)-epoxidation of styrene with high enantiomeric excess, space-time yields, and current efficiencies. Highly porous reticulated vitreous carbon electrodes, maximized in volumetric surface area, were employed in a flow-through mode to rapidly regenerate the consumed FADH2 cofactor required for StyA activity. A systematic investigation of the parameters determining cofactor mass transfer revealed that low FAD concentrations and high flow rates enabled the continuous synthesis of the product (S)-styrene oxide at high rates, while at the same time the accumulation of the side-products acetophenone and phenylacetaldehyde was minimized. At 10,,M FAD and a flow rate of 150,mL,min,1, an average space-time yield of 0.35,g,L,1,h,1 could be achieved during 2,h with a final (S)-styrene oxide yield of 75.2%. At two-fold lower aeration rates, the electroenzymatic reaction could be sustained for 12,h, albeit at the expense of lower (59%) overall space-time yields. Under these conditions, as much as 20.5% of the utilized current could be channeled into (S)-styrene oxide formation. In comparison with state-of-the-art electroenzymatic methodologies for the same conversion, (S)-styrene oxide synthesis could be improved up to 150-fold with respect to both reaction time and space-time yield. These productivities constitute the most efficient reaction reported for asymmetric in vitro epoxidations of styrene. [source] Enantioselective Silylcyanation of Aldehydes and Ketones by a Titanium Catalyst Prepared from a Partially Hydrolyzed Titanium Alkoxide and a Schiff Base LigandADVANCED SYNTHESIS & CATALYSIS (PREVIOUSLY: JOURNAL FUER PRAKTISCHE CHEMIE), Issue 10 2009Kazuhiko Yoshinaga Abstract In the presence of small amount (0.2,1.0 mol%) of a titanium complex catalyst prepared from a partially hydrolyzed titanium alkoxide and an optically active tridentate Schiff base ligand, the enantioselective silylcyanation of aldehydes and ketones proceeded in a short reaction time at room temperature to afford the corresponding optically active cyanohydrin derivatives in excellent chemical yield with high enantiomeric excess (86,97% ee). The results indicate that partially hydrolyzed titanium alkoxides are a promising titanium source for the preparation of efficient catalysts for asymmetric synthesis. [source] Nitrilase-Catalyzed Enantioselective Synthesis of Pyrrolidine- and Piperidinecarboxylic AcidsADVANCED SYNTHESIS & CATALYSIS (PREVIOUSLY: JOURNAL FUER PRAKTISCHE CHEMIE), Issue 8-9 2007Margit Winkler Abstract The enantioselective synthesis of the non-proteinogenic amino acids ,-proline and nipecotic acids from their readily available nitriles is achieved in high enantiomeric excess by commercially available nitrilases. The presented procedure comprises not more than 4 steps, thus considerably reducing the multiple steps generally required. Amide formation is also observed for specific heterocyclic nitriles. [source] Bifunctional-Thiourea-Catalyzed Diastereo- and Enantioselective Aza-Henry ReactionCHEMISTRY - A EUROPEAN JOURNAL, Issue 2 2006Xuenong Xu Dr. Abstract Bifunctional thiourea 1,a catalyzes aza-Henry reaction of nitroalkanes with N -Boc-imines to give syn -,-nitroamines with good to high diastereo- and enantioselectivity. Apart from the catalyst, the reaction requires no additional reagents such as a Lewis acid or a Lewis base. The N-protecting groups of the imines have a determining effect on the chirality of the products, that is, the reaction of N -Boc-imines gives R adducts as major products, whereas the same reaction of N -phosphonoylimines furnishes the corresponding S adducts. Various types of nitroalkanes bearing aryl, alcohol, ether, and ester groups can be used as nucleophiles, providing access to a wide range of useful chiral building blocks in good yield and high enantiomeric excess. Synthetic versatility of the addition products is demonstrated by the transformation to chiral piperidine derivatives such as CP-99,994. [source] Asymmetric Hydrogenations (Nobel Lecture 2001)ADVANCED SYNTHESIS & CATALYSIS (PREVIOUSLY: JOURNAL FUER PRAKTISCHE CHEMIE), Issue 1-2 2003William Abstract The start of the development of catalysts for asymmetric hydrogenation was the concept of replacing the triphenylphosphane ligand of the Wilkinson catalyst with a chiral ligand. With the new catalysts, it should be possible to hydrogenate prochiral olefins. Knowles and his co-workers were convinced that the phosphorus atom played a central role in this selectivity, as only chiral phosphorus ligands such as (R,R)-DIPAMP, whose stereogenic center lies directly on the phosphorus atom, lead to high enantiomeric excesses when used as catalysts in asymmetric hydrogenation reactions. This hypothesis was disproven by the development of ligands with chiral carbon backbones. Although the exact mechanism of action of the phosphane ligands is not incontrovertibly determined to this day, they provide a simple entry to a large number of chiral compounds. [source] | ||||||||||