- Widely applicable background depletion step enables transaminase evolution through solid-phase screening
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Directed evolution of transaminases is a widespread technique in the development of highly sought-after biocatalysts for industrial applications. This process, however, is challenged by the limited availability of effective high-throughput protocols to evaluate mutant libraries. Here we report a rapid, reliable, and widely applicable background depletion method for solid-phase screening of transaminase variants, which was successfully applied to a transaminase from Halomonas elongata (HEWT), evolved through rounds of random mutagenesis towards a series of diverse prochiral ketones. This approach enabled the identification of transaminase variants in viable cells with significantly improved activity towards para-substituted acetophenones (up to 60-fold), as well as tetrahydrothiophen-3-one and related substrates. Rationalisation of the mutants was assisted by determination of the high-resolution wild-type HEWT crystal structure presented herein.
- Planchestainer, Matteo,Hegarty, Eimear,Heckmann, Christian M.,Gourlay, Louise J.,Paradisi, Francesca
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p. 5952 - 5958
(2019/06/19)
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- Evaluation of the Edman degradation product of vancomycin bonded to core-shell particles as a new HPLC chiral stationary phase
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A modified macrocyclic glycopeptide-based chiral stationary phase (CSP), prepared via Edman degradation of vancomycin, was evaluated as a chiral selector for the first time. Its applicability was compared with other macrocyclic glycopeptide-based CSPs: TeicoShell and VancoShell. In addition, another modified macrocyclic glycopeptide-based CSP, NicoShell, was further examined. Initial evaluation was focused on the complementary behavior with these glycopeptides. A screening procedure was used based on previous work for the enantiomeric separation of 50 chiral compounds including amino acids, pesticides, stimulants, and a variety of pharmaceuticals. Fast and efficient chiral separations resulted by using superficially porous (core-shell) particle supports. Overall, the vancomycin Edman degradation product (EDP) resembled TeicoShell with high enantioselectivity for acidic compounds in the polar ionic mode. The simultaneous enantiomeric separation of 5 racemic profens using liquid chromatography-mass spectrometry with EDP was performed in approximately 3?minutes. Other highlights include simultaneous liquid chromatography separations of rac-amphetamine and rac-methamphetamine with VancoShell, rac-pseudoephedrine and rac-ephedrine with NicoShell, and rac-dichlorprop and rac-haloxyfop with TeicoShell.
- Hellinghausen, Garrett,Lopez, Diego A.,Lee, Jauh T.,Wang, Yadi,Weatherly, Choyce A.,Portillo, Abiud E.,Berthod, Alain,Armstrong, Daniel W.
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p. 1067 - 1078
(2018/08/01)
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- Enantioselective synthesis of amines via reductive amination with a dehydrogenase mutant from Exigobacterium sibiricum: Substrate scope, co-solvent tolerance and biocatalyst immobilization
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In recent years, the reductive amination of ketones in the presence of amine dehydrogenases emerged as an attractive synthetic strategy for the enantioselective preparation of amines starting from ketones, an ammonia source, a reducing reagent and a cofactor, which is recycled in situ by means of a second enzyme. Current challenges in this field consists of providing a broad synthetic platform as well as process development including enzyme immobilization. In this contribution these issues are addressed. Utilizing the amine dehydrogenase EsLeuDH-DM as a mutant of the leucine dehydrogenase from Exigobacterium sibiricum, a range of aryl-substituted ketones were tested as substrates revealing a broad substrate tolerance. Kinetics as well as inhibition effects were also studied and the suitability of this method for synthetic purpose was demonstrated with acetophenone as a model substrate. Even at an elevated substrate concentration of 50 mM, excellent conversion was achieved. In addition, the impact of water-miscible co-solvents was examined, and good activities were found when using DMSO of up to 30% (v/v). Furthermore, a successful immobilization of the EsLeuDH-DM was demonstrated utilizing a hydrophobic support and a support for covalent binding, respectively, as a carrier.
- L?we, Jana,Ingram, Aaron A.,Gr?ger, Harald
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p. 1387 - 1392
(2018/03/21)
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- Optimization of 2-alkoxyacetates as acylating agent for enzymatic kinetic resolution of chiral amines
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In this study, the activity of acetic acid esters modified with electron withdrawing 2-alkoxy-groups was investigated as acylating agent in kinetic resolution (KR) of racemic amines. A homologous series of the isopropyl esters of four 2-alkoxyacetic acids (2-methoxy-, 2-ethoxy-, 2-propoxy- and 2-butoxyacetic acids) were prepared and investigated for enantiomer selective N-acylation, catalyzed by lipase B from Candida antarctica, under batch and continuous-flow conditions. In the first set of experiments, isopropyl 2-propoxyacetate showed the highest effectivity with all of the four racemic amines [(±)-1-phenylethylamine, (±)-4-phenylbutan-2-amine, (±)-heptan-2-amine and (±)-1-methoxypropane-2-amine] in the set enabling excellent conversions (≥46%) and enantiomeric excess values (ee ≥ 99%) with each amines in continuous-flow mode KRs under the optimized reaction conditions. In a second set of experiments, KRs of five additional amines – being substituted derivatives of (±)-1-phenylethylamine – further demonstrated the usefulness of isopropyl 2-propoxyacetate – being the best acylating agent in the first set of KRs – in KRs leading to (R)-N-propoxyacetamides with high ee values (≥99.8%).
- Oláh, Márk,Kovács, Dániel,Katona, Gabriel,Hornyánszky, Gábor,Poppe, László
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p. 3663 - 3670
(2018/06/04)
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- Biocatalytic transamination with near-stoichiometric inexpensive amine donors mediated by bifunctional mono- and di-amine transaminases
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The discovery and characterisation of enzymes with both monoamine and diamine transaminase activity is reported, allowing conversion of a wide range of target ketone substrates with just a small excess of amine donor. The diamine co-substrates (putrescine, cadaverine or spermidine) are bio-derived and the enzyme system results in very little waste, making it a greener strategy for the production of valuable amine fine chemicals and pharmaceuticals.
- Galman, James L.,Slabu, Iustina,Weise, Nicholas J.,Iglesias, Cesar,Parmeggiani, Fabio,Lloyd, Richard C.,Turner, Nicholas J.
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supporting information
p. 361 - 366
(2017/08/14)
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- Stereoelectronic effects in the reaction of aromatic substrates catalysed by: Halomonas elongata transaminase and its mutants
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A transaminase from Halomonas elongata and four mutants generated by an in silico-based design were recombinantly produced in E. coli, purified and applied to the amination of mono-substituted aromatic carbonyl-derivatives. While benzaldehyde derivatives were excellent substrates, only NO2-acetophenones were transformed into the (S)-amine with a high enantioselectivity. The different behaviour of wild-type and mutated transaminases was assessed by in silico substrate binding mode studies.
- Contente, Martina Letizia,Planchestainer, Matteo,Molinari, Francesco,Paradisi, Francesca
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p. 9306 - 9311
(2016/10/13)
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- Chiral amine synthesis using ω-transaminases: An amine donor that displaces equilibria and enables high-throughput screening
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The widespread application of ω-transaminases as biocatalysts for chiral amine synthesis has been hampered by fundamental challenges, including unfavorable equilibrium positions and product inhibition. Herein, an efficient process that allows reactions to proceed in high conversion in the absence of by-product removal using only one equivalent of a diamine donor (ortho-xylylenediamine) is reported. This operationally simple method is compatible with the most widely used (R)- and (S)-selective ω-TAs and is particularly suitable for the conversion of substrates with unfavorable equilibrium positions (e.g., 1-indanone). Significantly, spontaneous polymerization of the isoindole by-product generates colored derivatives, providing a high-throughput screening platform to identify desired ω-TA activity. ω-Transaminases (ω-TA) have been employed as biocatalysts in a simple and efficient process for the synthesis of chiral amines. A dual-function diamine donor (ortho-xylylenediamine) serves to displace challenging reaction equilibria towards product formation whilst generating intensely colored by-products, which have allowed the development of liquid-phase and colony-based assays.
- Green, Anthony P.,Turner, Nicholas J.,O'Reilly, Elaine
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supporting information
p. 10714 - 10717
(2015/05/13)
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- Microwave-Enhanced Asymmetric Transfer Hydrogenation of N-(tert-Butylsulfinyl)imines
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Microwave irradiation has considerably enhanced the efficiency of the asymmetric transfer hydrogenation of N-(tert-butylsulfinyl)imines in isopropyl alcohol catalyzed by a ruthenium complex bearing the achiral ligand 2-amino-2-methylpropan-1-ol. In addition to shortening reaction times for the transfer hydrogenation processes to only 30 min, the amounts of ruthenium catalyst and isopropyl alcohol can be considerably reduced in comparison with our previous procedure assisted by conventional heating, which diminishes the environmental impact of this new protocol. This methodology can be applied to aromatic, heteroaromatic and aliphatic N-(tert-butylsulfinyl)ketimines, leading, after desulfinylation, to the expected primary amines in excellent yields and with enantiomeric excesses of up to 96 %. Microwave irradiation promotes the asymmetric transfer hydrogenation of N-(tert-butylsulfinyl)imines in 2-propanol catalysed by a ruthenium complex bearing an achiral β-amino alcohol as ligand. After desulfinylation, α-branched primary amines containing aromatic, heteroaromatic and aliphatic substituents are obtained in excellent yields and with enantiomeric excesses of up to 96 %.
- Pablo, Oscar,Guijarro, David,Yus, Miguel
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p. 7034 - 7038
(2016/02/19)
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- Monoamine oxidase-ω-transaminase cascade for the deracemisation and dealkylation of amines
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Herein we report a one-pot protocol for the deracemisation of chiral benzylic amines employing a novel monoamine oxidase-ω-transaminase cascade, allowing access to enantiopure compounds in >99 % ee. We also demonstrate that the same enzymatic cascade can be employed for the dealkylation of secondary amines with >99 % conversion. Cascade ball: A monoamine oxidase- ω-transaminase cascade has been developed for the deracemisation of chiral benzylic amines, allowing access to the enantiopure compounds in >99 % ee. The same system was also employed for the efficient dealkylation of secondary amines.
- O'Reilly, Elaine,Iglesias, Cesar,Turner, Nicholas J.
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p. 992 - 995
(2014/05/06)
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- Asymmetric synthesis of nonracemic primary amines via spiroborate-catalyzed reduction of pure (E)- and (Z)-O-benzyloximes: Applications toward the synthesis of calcimimetic agents
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Highly enantiopure (1-aryl)- and (1-naphthyl)-1-ethylamines were synthesized by the borane-mediated reduction of single-isomeric (E)- and (Z)-O-benzyloxime ethers using the stable spiroborate ester derived from (S)-diphenyl valinol and ethylene glycol as the chiral catalyst. Primary (R)-arylethylamines were prepared by the reduction of pure (Z)-ethanone oxime ethers in up to 99% ee using 15% of catalyst. Two convenient and facile approaches to the synthesis of new and known calcimimetic analogues employing enantiopure (1-naphthalen-1-yl)ethylamine as chiral precursor are described.
- Ou, Wenhua,Espinosa, Sandraliz,Meléndez, Héctor J.,Farré, Silvia M.,Alvarez, Jaime L.,Torres, Valerie,Martínez, Ileanne,Santiago, Kiara M.,Ortiz-Marciales, Margarita
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p. 5314 - 5327
(2013/07/25)
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- An efficient single-enzymatic cascade for asymmetric synthesis of chiral amines catalyzed by ω-transaminase
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An efficient single-enzymatic cascade approach for the asymmetric synthesis of chiral amines has been developed, which applies the amino donor 3-aminocyclohexa-1,5-dienecarboxylic acid spontaneously tautomerizing to reach reaction completion with excellen
- Wang, Bo,Land, Henrik,Berglund, Per
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supporting information
p. 161 - 163
(2013/02/23)
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- Enzymatic asymmetric synthesis of enantiomerically pure aliphatic, aromatic and arylaliphatic amines with (R)-selective amine transaminases
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Seven (R)-selective amine transaminases (R-ATAs) recently discovered by an in silico-based approach in sequence databases were produced recombinantly in Escherichia coli and subjected to partial purification by ammonium sulfate precipitation. A range of additives and various buffers were investigated to identify best conditions to ensure good storage stability and stable activity during biocatalysis. All enzymes show pH optima between pH 7.5-9. These R-ATAs were then applied in the asymmetric synthesis of twelve aliphatic, aromatic and arylaliphatic (R)-amines starting from the corresponding prochiral ketones using a lactate dehydrogenase/glucose dehydrogenase system to shift the equilibrium. For all ketones, at least one enzyme was found that allows complete conversion to the corresponding chiral amine having excellent optical purities >99% ee. Variations in substrate profiles are also discussed based on the phylogenetic relationships between the seven R-ATAs. Thus, we have identified a versatile toolbox of (R)-amine transaminases showing remarkable properties for application in biocatalysis. Copyright
- Schaetzle, Sebastian,Steffen-Munsberg, Fabian,Thontowi, Ahmad,Hoehne, Matthias,Robins, Karen,Bornscheuer, Uwe T.
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experimental part
p. 2439 - 2445
(2011/11/06)
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- Transaminations with isopropyl amine: Equilibrium displacement with yeast alcohol dehydrogenase coupled to in situ cofactor regeneration
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Enantiopure chiral amines synthesis using ω-transaminases is hindered by an unfavourable equilibrium, but when using isopropylamine as the amine donor the equilibrium can be completely displaced by using a specific dehydrogenase in situ for removal of formed acetone. The Royal Society of Chemistry 2010.
- Cassimjee, Karim Engelmark,Branneby, Cecilia,Abedi, Vahak,Wells, Andrew,Berglund, Per
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supporting information; experimental part
p. 5569 - 5571
(2010/09/10)
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- Asymmetric synthesis of chiral primary amines by transfer hydrogenation of N -(tert -Butanesulfinyl)ketimines
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(Figure presented) The diastereoselective reduction of (R)-N-(tert- butanesulfinyl)ketimines by a ruthenium-catalyzed asymmetric transfer hydrogenation process in isopropyl alcohol, followed by desulfinylation of the nitrogen atom, is an excellent method to prepare highly enantiomerically enriched α-branched primary amines (up to >99% ee) in short reaction times (1-4 h). (1S,2R)-1-Amino-2-indanol has been shown to be a very efficient ligand to perform this transformation. Ketimines bearing either an aryl or a heteroaryl group and an alkyl group as substituents of the iminic carbon atom are very good substrates for this process. The reduction of a dialkyl ketimine could also be achieved, affording the expected amine with moderate optical purity (69% ee). Some amines which are precursors of very interesting biologically and pharmacologically active compounds have been prepared in excellent yields and enantiomeric excesses.
- Guijarro, David,Pablo, Oscar,Yus, Miguel
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supporting information; experimental part
p. 5265 - 5270
(2010/10/21)
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- Ruthenium-catalysed asymmetric transfer hydrogenation of N-(tert-butanesulfinyl)imines
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The ruthenium complex prepared from [RuCl2(p-cymene)]2 and (1S,2R)-1-amino-2-indanol is a very efficient catalyst for the asymmetric transfer hydrogenation of (R)-N-(tert-butanesulfinyl)ketimines in isopropanol. By carefully removing
- Guijarro, David,Pablo, óscar,Yus, Miguel
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experimental part
p. 5386 - 5388
(2009/12/06)
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- Synthesis and Characterization of Nitroaromatic Peptoids: Fine Tuning Peptoid Secondary Structure through Monomer Position and Functionality
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N-Substituted glycine oligomers, or peptoids, have emerged as an important class of foldamers for the study of biomolecular interactions and for potential use as therapeutic agents. However, the design of peptoids with well-defined conformations a priori remains a formidable challenge. New approaches are required to address this problem, and the systematic study of the role of individual monomer units in the global peptoid folding process represents one strategy. Here, we report our efforts toward this approach through the design, synthesis, and characterization of peptoids containing nitroaromatic monomer units. This work required the synthesis of a new chiral amine building block, (S)-1-(2-nitrophenyl)ethanamine (s2ne), which could be readily installed into peptoids using standard solid-phase peptoid synthesis techniques. We designed a series of peptoid nonamers that allowed us to probe the effects of this relatively electron-deficient and sterically encumbered a-chiral side chain on peptoid structure, namely, the peptoid threaded loop and helix. Circular dichroism spectroscopy of the peptoids revealed that the nitroaromatic monomer has a significant effect on peptoid secondary structure. Specifically, the threaded loop structure was disrupted in a nonamer containing alternating V-(S)-1-phenylethylglycine (VVspe) and VVs2ne monomers, and the major conformation was helical instead. Indeed, placement of a single Ns2ne at the N-terminal position of (VVspe)9 resulted in a destabilized form of the threaded loop structure relative to the homononamer (VVspe)9. Conversely, we observed that incorporation of V-(S)-1-(4-nitrophenyl)ethylglycine (VVsnp, a p-nitro monomer) at the VV-terminal position stabilized the threaded loop structure relative to (VVspe)9. Additional experiments revealed that nitroaromatic side chains can influence peptoid nonamer folding by modulating the strength of key intramolecular hydrogen bonds in the peptoid threaded loop structure. Steric interactions were also implicated for the VVs2ne monomer. Overall, this study provides further evidence that aromatic side-chain structure, even if perturbed in a single monomer unit, can strongly influence local peptoid backbone conformation.
- Fowler, Sarah A.,Luechapanichkul, Rinrada,Blackwell, Helen E.
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experimental part
p. 1440 - 1449
(2009/07/30)
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- Asymmetric reduction of oxime ethers promoted by chiral spiroborate esters with an O3BN framework
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Enatioselective reduction of oxime ethers promoted by chiral spiroborate esters with an O3BN framework is reported for the first time. In the presence of (R,S)-1, 11 aralkyloxime ethers are reduced by borane-THF at 0-5 °C to give (S)-1-aralkylamine in high yield and excellent enatiomeric excess (up to 98% ee). Influence of reaction conditions on the enantioselectivity of the reduction is investigated, and a possible mechanism of the catalytic reduction is suggested.
- Chu, Yunbo,Shan, Zixing,Liu, Dejun,Sun, Nannan
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p. 3998 - 4001
(2007/10/03)
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- Highly Enantioselective Hydrogen-Transfer Reductive Amination: Catalytic Asymmetric Synthesis of Primary Amines
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Ammonium formate is the hydrogen source in the catalytic asymmetric reductive amination of ketones presented here (Leuckart-Wallach-type reaction). The reaction proceeds smoothly in methanol in the presence of Ir, Rh, and Ru catalysts. Primary amines were obtained as products in good yields with high enantioselectivities after hydrolytic workup when [((R)-tol-binap) RuCl 2] was used as the catalyst (see scheme). R1, R 2=alkyl, aryl.
- Kadyrov, Renat,Riermeier, Thomas H.
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p. 5472 - 5474
(2007/10/03)
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- Synthesis of enantiomerically pure amino-substituted fused bicyclic rings
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This invention describes various processes for synthesis and resolution of racemic amino-substituted fused bicyclic ring systems. One process utilizes selective hydrogenation of an amino-substituted fused bicyclic aromatic ring system. An alternative process prepares the racemic amino-substituted fused bicyclic ring system via nitrosation. In addition, the present invention describes the enzymatic resolution of a racemic mixture to produce the (R)- and (S)-forms of amino-substituted fused bicyclic rings as well as a racemization process to recycle the unpreferred enantioner. Further provided by this invention is an asymmetric synthesis of the (R)- or (S)-enantiomer of primary amino-substituted fused bicyclic ring systems.
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- Highly efficient resolutions with isopropylidene glycerol 3-carboxy-2-naphthoate
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A number of chiral 1-arylethylamines and 1-alkylethylamines were resolved with the 3-carboxy-2-naphthoate of isopropylidene glycerol 2, previously reported to be an even more efficient resolving agent for 1-phenylethylamine than the corresponding hemiphthalate 1. The results obtained for the 1-arylethylamines confirm such a trend, revealing impressive resolution ability, in particular, for 1-(4-bromophenyl)-, 1-(4-nitrophenyl)- and 1-(2-naphthyl)ethylamine, whose enantiomers were almost quantitatively separated with (S)-2 by a single precipitation of the less soluble (S,S) diastereomeric salt. Additionally, the success of the resolutions of 1-alkylethylamines (1-phenyl-2-propylamine, 1-cyclohexylethylamine and 2-butylamine), which could not be resolved with 1, indicates that the novel carboxy ester 2 has a wider range of application than 1.
- Pallavicini, Marco,Bolchi, Cristiano,Fumagalli, Laura,Valoti, Ermanno,Villa, Luigi
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p. 2277 - 2282
(2007/10/03)
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- Process for preparing optically active amines and optically active carboxylic acids, and intermediates for preparation
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Disclosed is a process for preparing an optically active 1-aryl- or 2-aryl-alkylamines of formulas Ia, Ib and Ic with high optical purity and high yield. The process uses an optically active 1- or 2-naphthylglycolic acid of the general formula II as a resolving agent. Also disclosed is a process for praparing an optically active 1- or 2-naphthylglycolic acid of formula II using an optically active 1-aryl- or 2-aryl-alkylamines of formulas Ia, Ib and Ic as the resolving agents
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- A high-performance, tailor-made resolving agent: Remarkable enhancement of resolution ability by introducing a naphthyl group into the fundamental skeleton1
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A novel resolving agent, 2-naphthylglycolic acid (2-NGA), was designed for p-substituted 1-arylethylamines on the basis of the consideration that a rigid and large naphthyl group would be favorable for the close packing of supramolecular hydrogen-bond sheets formed between the carboxy groups of 2-NGA and the amino groups of p-substituted 1-arylethylamines. Racemic 2-NGA was readily available from commercially available raw materials, and both enantiopure forms could be obtained by simple diastereomeric resolution with enantiopure 1-phenyl-ethylamine. Thus-prepared enantiopure 2-NGA was found to have an excellent resolution ability not only for p-substituted 1-arylethylamines, but also for a wide variety of chiral primary amines. X-Ray crystallographic analyses of the less- and more-soluble diastereomeric salts revealed that this excellent resolution ability of 2-NGA arose from the formation of a supramolecular hydrogen-bond sheet with the primary amine, as we had expected, and also from the possible achievement of an infinite chain of CH... π interaction between its naphthyl group and the aromatic group of the amine, which was formed in the hydrophobic region of the supramolecular hydrogen-bond sheet.
- Kinbara, Kazushi,Harada, Yoshiko,Saigo, Kazuhiko
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p. 1339 - 1347
(2007/10/03)
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- Process for preparation of oxyglutaric acid ester derivatives
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A process for preparing an oxyglutaric acid ester derivative of the formula: STR1 in which each of R1 and R2 is C1-5 alkoxy, C1-7 aralkyloxy, C7-9 halogenated aralkyloxy or phenyl, R4 is a hydroxyl-protecting group, and R5 is C1-10 alkyl which may have a substituent, comprises the steps of reacting a methyl phosphonate derivative or methyl phosphine oxide derivative with an oxyglutaric acid mono-ester to give a reaction product which comprises an oxyglutaric acid derivative having a phosphorus-containing group and a pentenedioic acid mono-ester (by-product), removing the pendenedioic acid mono-ester from the reaction product to isolate the oxyglutaric acid derivative, and converting the isolated oxyglutaric acid derivative into the oxyglutaric acid ester derivative. A process for obtaining an optically active oxyglutaric acid ester derivative is also disclosed.
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- Isopropylidene glycerol hydrogen phthalate: A new resolving agent application to the resolution of 1-arylethylamines
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Hydrogen phthalates of (R)- and (S)-isopropylidene glycerol, obtainable from racemic isopropylidene glycerol by reaction with phthalic anhydride and successive resolution with (S)- and (R)-1-phenylethylamine or, alternatively, from (R)- and (S)-isopropylidene glycerol, were regarded as potential new resolving agents A range of important 1-arylethylamines was selected to test their resolving capability. In particular, trial resolutions were carried out using equivalent amounts of racemic amine and hydrogen phthalate of (R)-isopropylidene glycerol The salts of the S isomers selectively crystallized from methanol or 2-propanol allowing to recover the (S)-1-arylethylamines in high chemical and optical yields. Copyright
- Pallavicini, Marco,Valoti, Ermanno,Villa, Luigi,Piccolo, Oreste
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p. 1117 - 1122
(2007/10/03)
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- Catalytic asymmetric reductive amination of ketones via highly enantioselective hydrogenation of the C=N double bond
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We describe a convenient, chemoselective asymmetric reductive amination procedure for the conversion of ketones to chiral hydrazines and amines. The key step in the three-step process is enantioselective DuPHOS-Rh-catalyzed hydrogenation of the C=N double bond of N-acylhydrazones. Detailed optimization studies revealed the effect of solvent, temperature, and the N- acyl group on the enantioselectivity and catalytic efficiency of the reaction. The reduction products, N-acylhydrazines, were converted to hydrazines or amines through hydrolysis or treatment with samarium(II) iodide, respectively.
- Burk, Mark J.,Martinez, Jose P.,Feaster, John E.,Cosford, Nick
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p. 4399 - 4428
(2007/10/02)
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- Base Catalysed Rearrangements involving Ylide Intermediates. Part 15. The Machanism of the Stevens Rearrangement
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The Stevens rearrangement of acyl-stabilised ammonium ylides has been investigated with regard to stereoselectivity, intramolecularity and the formation of products in addition to the rearrangement product.A detailed study of the effects of reaction conditions upon the rearrangement of the ylide derived from the salt (13) has shown that the stereoselectivity (retention of the configuration of the chiral migrating group) and intramolecularity decrease as solvent viscosity decreases.The rearrangement of the salt (13) in water at 0 deg C is essentially intramolecular with virtually complete retention of the configuration of the migrating group.These results, together with the isolation of products that can be rationalised on the basis of random free-radical coupling, indicate that the rearrangement of acyl-stabilised ammonium ylides normally involves a radical pair mechanism.
- Ollis, W. David,Rey, Max,Sutherland, Ian O.
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p. 1009 - 1027
(2007/10/02)
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