- Amino Acid-Functionalized Metal-Organic Frameworks for Asymmetric Base–Metal Catalysis
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We report a strategy to develop heterogeneous single-site enantioselective catalysts based on naturally occurring amino acids and earth-abundant metals for eco-friendly asymmetric catalysis. The grafting of amino acids within the pores of a metal-organic framework (MOF), followed by post-synthetic metalation with iron precursor, affords highly active and enantioselective (>99 % ee for 10 examples) catalysts for hydrosilylation and hydroboration of carbonyl compounds. Impressively, the MOF-Fe catalyst displayed high turnover numbers of up to 10 000 and was recycled and reused more than 15 times without diminishing the enantioselectivity. MOF-Fe displayed much higher activity and enantioselectivity than its homogeneous control catalyst, likely due to the formation of robust single-site catalyst in the MOF through site-isolation.
- Newar, Rajashree,Akhtar, Naved,Antil, Neha,Kumar, Ajay,Shukla, Sakshi,Begum, Wahida,Manna, Kuntal
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p. 10964 - 10970
(2021/03/29)
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- Chiral Iron(II)-Catalysts within Valinol-Grafted Metal-Organic Frameworks for Enantioselective Reduction of Ketones
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The development of highly efficient and enantioselective heterogeneous catalysts based on earth-abundant elements and inexpensive chiral ligands is essential for environment-friendly and economical production of optically active compounds. We report a strategy of synthesizing chiral amino alcohol-functionalized metal-organic frameworks (MOFs) to afford highly enantioselective single-site base-metal catalysts for asymmetric organic transformations. The chiral MOFs (vol-UiO) were prepared by grafting of chiral amino alcohol such as l-valinol within the pores of aldehyde-functionalized UiO-MOFs via formation of imine linkages. The metalation of vol-UiO with FeCl2 in THF gives amino alcohol coordinated octahedral FeII species of vol-FeCl(THF)3 within the MOFs as determined by X-ray absorption spectroscopy. Upon activation with LiCH2SiMe3, vol-UiO-Fe catalyzed hydrosilylation and hydroboration of a range of aliphatic and aromatic carbonyls to afford the corresponding chiral alcohols with enantiomeric excesses up to 99%. Vol-UiO-Fe catalysts have high turnover numbers of up to 15 ?000 and could be reused at least 10 times without any loss of activity and enantioselectivity. The spectroscopic, kinetic, and computational studies suggest iron-hydride as the catalytic species, which undergoes enantioselective 1,2-insertion of carbonyl to give an iron-alkoxide intermediate. The subsequent σ-bond metathesis between Fe-O bond and Si-H bond of silane produces chiral silyl ether. This work highlights the importance of MOFs as the tunable molecular material for designing chiral solid catalysts based on inexpensive natural feedstocks such as chiral amino acids and base-metals for asymmetric organic transformations.
- Akhtar, Naved,Antil, Neha,Begum, Wahida,Chauhan, Manav,Kumar, Ajay,Manna, Kuntal,Newar, Rajashree
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p. 10450 - 10459
(2021/08/31)
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- An Enantioconvergent Benzylic Hydroxylation Using a Chiral Aryl Iodide in a Dual Activation Mode
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The application of a triazole-substituted chiral iodoarene in a direct enantioselective hydroxylation of alkyl arenes is reported. This method allows the rapid synthesis of chiral benzyl alcohols in high yields and stereocontrol, despite its nontemplated nature. In a cascade activation consisting of an initial irradiation-induced radical C-H-bromination and a consecutive enantioconvergent hydroxylation, the iodoarene catalyst has a dual role. It initiates the radical bromination in its oxidized state through an in-situ-formed bromoiodane and in the second, Cu-catalyzed step, it acts as a chiral ligand. This work demonstrates the ability of a chiral aryl iodide catalyst acting both as an oxidant and as a chiral ligand in a highly enantioselective C-H-activating transformation. Furthermore, this concept presents an enantioconvergent hydroxylation with high selectivity using a synthetic catalyst.
- Abazid, Ayham H.,Clamor, Nils,Nachtsheim, Boris J.
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p. 8042 - 8048
(2020/09/21)
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- Multigram Scale Enzymatic Synthesis of (R)-1-(4′-Hydroxyphenyl)ethanol Using Vanillyl Alcohol Oxidase
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The enantioselective oxyfunctionalisation of C?H bonds is a highly interesting reaction, as it provides access to chiral alcohols that are important pharmaceutical building blocks. However, it is hard to achieve using traditional methods. One way in which it can be achieved is through the action of oxidative enzymes. Although many reports of the oxyfunctionalisation capabilities of enzymes at an analytical scale have been published, reports on the use of enzymes to achieve oxyfunctionalisation on a synthetically relevant scale are fewer. Here, we describe the scale-up of the conversion of 4-ethylphenol to (R)-1-(4′-hydroxyphenyl)ethanol using the flavin-dependent enzyme vanillyl alcohol oxidase. The process was optimised by testing different reaction media and substrate and enzyme concentrations and by performing it under an oxygen atmosphere. Under optimised reaction conditions, 4.10 g (R)-1-(4′-hydroxyphenyl)ethanol at 97% ee was obtained from 10 g 4-ethylphenol (isolated yield 36%). These results highlight some of the challenges that can be encountered during scale-up of an enzymatic oxyfunctionalisation process to a synthetically relevant scale and will be of use for the development of enzymatic processes for the synthesis of industrially relevant compounds. (Figure presented.).
- Ewing, Tom A.,Kühn, Jasmin,Segarra, Silvia,Tortajada, Marta,Zuhse, Ralf,van Berkel, Willem J. H.
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supporting information
p. 2370 - 2376
(2018/06/20)
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- Multienzyme One-Pot Cascade for the Stereoselective Hydroxyethyl Functionalization of Substituted Phenols
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The operability and substrate scope of a redesigned vinylphenol hydratase as a single biocatalyst or as part of multienzyme cascades using either substituted coumaric acids or phenols as stable, cheap, and readily available substrates are reported.
- Payer, Stefan E.,Pollak, Hannah,Schmidbauer, Benjamin,Hamm, Florian,Juri?i?, Filip,Faber, Kurt,Glueck, Silvia M.
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supporting information
p. 5139 - 5143
(2018/09/13)
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- N,O- vs N,C-Chelation in Half-Sandwich Iridium Complexes: A Dramatic Effect on Enantioselectivity in Asymmetric Transfer Hydrogenation of Ketones
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Cyclometalation of [Cp?IrCl2]2 with methyl (S)-2-phenyl-4,5-dihydrooxazole-4-carboxylate in the presence of NaOAc selectively led to a N,C- or N,O-chelated Cp?Ir(III) complex, depending on whether or not water was present in the reaction. While derived from the same precursor, these two complexes behaved in a dramatically different manner in asymmetric transfer hydrogenation (ATH) of ketones by formic acid, with the N,O-chelated complex being much more selective and active. The sense of asymmetric induction is also different, with the N,O-complex affording S while the N,C-analogue R alcohols. Further study revealed that the nature of the base additive considerably impacts the enantioselectivity and the effective HCOOH/amine ratios. These observations show the importance of ligand coordination mode and using the right base for ATH reactions.
- Zhou, Gang,Aboo, Ahmed H.,Robertson, Craig M.,Liu, Ruixia,Li, Zhenhua,Luzyanin, Konstantin,Berry, Neil G.,Chen, Weiping,Xiao, Jianliang
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p. 8020 - 8026
(2018/09/06)
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- Synthesis and characterization of novel silica coated magnetic nanoparticles with tags of β-cyclodextrin: application as an eco-friendly and chiral micro-vessel catalyst in the enantioselective reduction of ketones
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In this work, we report the synthesis of a novel, green and recoverable organic–inorganic magnetic nanocomposite by grafting β-cyclodextrin on the surface of a silica-coated magnetic nanoparticle, Fe3O4@SiO2/Pr-β-CD. FT-IR spectroscopy, transmission electron microscopy, CHN analysis, thermogravimetric analysis, vibrating sample magnetometer and X-ray diffraction analyses confirmed its structure. The magnetic core–shell structured modified silica microsphere has been successfully used as a chiral micro-vessel catalyst for the enantioselective reduction of ketones by NaBH4. The described catalyst was regenerated and reused without any significant changes in the yield and enantiomeric excess.
- Jafari Nasab, Mina,Kiasat, Ali Reza
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p. 2719 - 2728
(2018/02/06)
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- A Rational Active-Site Redesign Converts a Decarboxylase into a C=C Hydratase: "tethered Acetate" Supports Enantioselective Hydration of 4-Hydroxystyrenes
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The promiscuous regio- and stereoselective hydration of 4-hydroxystyrenes catalyzed by ferulic acid decarboxylase from Enterobacter sp. (FDC-Es) depends on bicarbonate bound in the active site, which serves as a proton relay activating a water molecule fo
- Payer, Stefan E.,Pollak, Hannah,Glueck, Silvia M.,Faber, Kurt
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p. 2438 - 2442
(2018/03/13)
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- Selective oxidation of aliphatic C-H bonds in alkylphenols by a chemomimetic biocatalytic system
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Selective oxidation of aliphatic C-H bonds in alkylphenols serves significant roles not only in generation of functionalized intermediates that can be used to synthesize diverse downstream chemical products, but also in biological degradation of these environmentally hazardous compounds. Chemo-, regio-, and stereoselectivity; controllability; and environmental impact represent the major challenges for chemical oxidation of alkylphenols. Here, we report the development of a unique chemomimetic biocatalytic system originated from the Gram-positive bacterium Corynebacterium glutamicum. The system consisting of CreHI (for installation of a phosphate directing/ anchoring group), CreJEF/CreG/CreC (for oxidation of alkylphenols), and CreD (for directing/anchoring group offloading) is able to selectively oxidize the aliphatic C-H bonds of p-And m-Alkylated phenols in a controllable manner. Moreover, the crystal structures of the central P450 biocatalyst CreJ in complex with two representative substrates provide significant structural insights into its substrate flexibility and reaction selectivity.
- Du, Lei,Dong, Sheng,Zhang, Xingwang,Jiang, Chengying,Chen, Jingfei,Yao, Lishan,Wang, Xiao,Wan, Xiaobo,Liu, Xi,Wangi, Xinquan,Huang, Shaohua,Cui, Qiu,Feng, Yingang,Liu, Shuang-Jiang,Li, Shengying
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p. E5129 - E5137
(2017/07/04)
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- Ansa-Ruthenium(II) Complexes of R2NSO2DPEN-(CH2)n(η6-Aryl) Conjugate Ligands for Asymmetric Transfer Hydrogenation of Aryl Ketones
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New 3rd generation designer ansa-ruthenium(II) complexes featuring N,C-alkylene-tethered N,N-dialkylsulfamoyl-DPEN/η6-arene ligands, exhibited good catalytic performance in the asymmetric transfer hydrogenation (ATH) of various classes of (het)aryl ketones in formic acid/triethylamine mixture. In particular, benzo-fused cyclic ketones furnished 98 to >99.9% ee using a low catalyst loading.
- Ki?ic, Andrea,Stephan, Michel,Mohar, Barbara
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p. 2540 - 2546
(2015/08/18)
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- Hydroxy functionalization of non-activated Ci-H and Ci=C Bonds: New perspectives for the synthesis of alcohols through biocatalytic processes
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New perspectives through enzymes: Recent breakthroughs have been achieved in the selective hydroxy functionalization of non-activated Ci-H and Ci=C bonds. Enzymes turned out to be suitable catalysts for the ω-hydroxylation of (substituted) alkanes and regioselective hydroxylation of aromatic hydrocarbons with atmospheric oxygen as the oxidant, and the asymmetric addition of water to non-activated alkenes.
- Groeger, Harald
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p. 3067 - 3069
(2014/04/03)
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- The reaction mechanism of chiral hydroxylation of p -OH and p -NH 2 substituted compounds by ethylbenzene dehydrogenase
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Ethylbenzene dehydrogenase (EbDH; enzyme commission (EC) number: 1.17.99.2) is a unique biocatalyst that hydroxylates alkylaromatic and alkylheterocyclic compounds to (S)-secondary alcohols under anaerobic conditions. The enzyme exhibits a high promiscuity catalyzing oxidation of over 30 substrates, inter alia, para-substituted alkylphenols and alkylanilines. Secondary alcohols with OH and NH2 substituents in the aromatic ring are highly valuable synthons for many biologically active compounds in the fine chemical industry. EbDH hydroxylates most of the studied compounds highly enantioselectively, except for five substrates that harbour OH and NH2 groups in the para position, which exhibit a significant decrease in the percent enantiomeric excess (% ee). This phenomenon is inconsistent with the previously suggested enzyme mechanism, but it may be linked to a stabilization of the carbocation intermediate by deprotonation of the OH or NH2 substituent in the active site that yields a transient quinone (imine) ethide species. This would initiate an alternative reaction pathway involving the addition of a water molecule to a C=C double bond. This hypothesis was cross-validated by density functional theory (DFT) cluster modelling of the alternative reaction pathway with 4-ethylphenol, as well as by experimental assessment of the pH dependency of enantiomeric excesses. The results reported herein suggest that the alternative reaction pathway may significantly contribute to the overall reaction if the carbocation intermediates are stabilized by deprotonation.
- Dudzik, Agnieszka,Kozik, Bartlomiej,Tataruch, Mateusz,Wojcik, Anna,Knack, Daniel,Borowski, Tomasz,Heider, Johann,Witko, Malgorzata,Szaleniec, MacIej
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p. 775 - 786
(2013/10/08)
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- Highly enantioselective double reduction of phenylglyoxal to (R)-1-phenyl-1,2-ethanediol by one NADPH-dependent yeast carbonyl reductase with a broad substrate profile
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The activity and enantioselectivity of a carbonyl reductase from Pichia pastoris GS115 were evaluated with a series of carbonyl compounds including aryl aldehydes, ketones, α- and β-ketoesters. This recombinant enzyme possessed a broad substrate profile with the ability of reducing both aldehydes and ketones. Especially, the enzyme catalyzed the double reduction of phenylglyoxal to (R)-1-phenyl-1,2-ethanediol with 99% yield and 99% ee by coupling with d-glucose dehydrogenase for the regeneration of cofactor NADPH, representing the first example of effective reduction of both aldehyde and ketone functional groups in one molecule by using only one enzyme. Furthermore, this study provides valuable information for guiding the future application of this versatile biocatalyst.
- Li, Zhe,Liu, Weidong,Chen, Xi,Jia, Shiru,Wu, Qiaqiang,Zhu, Dunming,Ma, Yanhe
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p. 3561 - 3564
(2013/04/24)
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- Asymmetric enzymatic hydration of hydroxystyrene derivatives
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More than one activity: Owing to their hydratase activity, phenolic acid decarboxylases catalyze the regio- and stereoselective addition of H 2O across the C=C double bond of hydroxystyrene derivatives yielding (S)-4-(1-hydroxyethyl)phenols with up to 82 % conversion and 71 % ee. Based on structure analysis and molecular docking simulations, a catalytic mechanism for this novel enzymatic reaction is proposed. Copyright
- Wuensch, Christiane,Gross, Johannes,Steinkellner, Georg,Gruber, Karl,Glueck, Silvia M.,Faber, Kurt
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supporting information
p. 2293 - 2297
(2013/04/10)
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- Enantioselective biocatalytic reduction of non-protected hydroxyacetophenones
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Direct enantioselective reduction of -, - and -hydroxyacetophenone without protection of the hydroxy moiety was carried out in the presence of (R)- and (S)-alcohol dehydrogenases as bio-catalysts. Whereas reduction of -hydroxyacetophenone gave only low to
- Neupert, Adrian,Ress, Tina,Wittmann, Jrgen,Hummel, Werner,Grger, Harald
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experimental part
p. 337 - 340
(2010/09/06)
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- Immobilization of a modified tethered rhodium(III)-p-toluenesulfonyl-1,2- diphenylethylenediamine catalyst on soluble and solid polymeric supports and successful application to asymmetric transfer hydrogenation of ketones
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Catalyst immobilization through covalent attachment onto a support is one strategy to provide recyclable systems. Here, soluble and surface-functionalized solid polymers were used as supports for a modified tethered rhodium(III)-p-toluenesulfonyl-1,2-diphenylethylenediamine [Rh(III)-TsDPEN] complex. The supported catalysts were applied to the asymmetric transfer hydrogenation of phenyl ketones in aqueous solution of sodium formate. High ee values (up to 99%) and good activities were achieved. It was discovered that the solid polymer-supported catalyst could be recycled at least four times without a significant decrease of the activity when a mixture of sodium formate and formic acid was used as the hydrogen source. This catalytic system provides a promising approach towards an ecologically and economically rational production of enantioenriched building blocks.
- Dimroth, Jonas,Keilitz, Juliane,Schedler, Uwe,Schomaecker, Reinhard,Haag, Rainer
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supporting information; scheme or table
p. 2497 - 2506
(2011/02/23)
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- Enantioselective reduction of prochiral ketones employing sprouted Pisum sativa as biocatalyst
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Sprouted green peas have been used for the first time as biocatalysts for enantioselective reduction of prochiral ketones. The reactions are highly enantioselective to furnish chiral alcohols in good yields. The sprouted peas as biocatalysts are a cheap and easy way for generating some interesting chiral alcohols. This process is efficient and convenient to produce chiral secondary alcohols in water. Georg Thieme Verlag Stuttgart.
- Yadav, Jhillu S.,Subba Reddy, Basi V.,Sreelakshmi, Chittamuru,Rao, Adari Bhaskar
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experimental part
p. 1881 - 1885
(2010/01/16)
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- Immobilized Manihot esculenta preparation as a novel biocatalyst in the enantioselective acetylation of racemic alcohols
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The enzymatic preparation obtained from a discard of Manihot esculenta roots has been successfully immobilized on calcium alginate hydrogels. This preparation has been tested as a chiral biocatalyst in the enzymatic acylation of a set of racemic aromatic alcohols. Depending on the reaction conditions, excellent enantioselectivities can be achieved. Some parameters that can alter the biocatalytic properties of the enzyme, such as solvent, temperature, acyl donor and substrate structure have been studied exhaustively in order to establish a deeper knowledge of this novel biocatalyst.
- Machado, Luciana L.,Lemos, Telma L.G.,de Mattos, Marcos Carlos,de Oliveira, Maria da Conceicao F.,de Gonzalo, Gonzalo,Gotor-Fernandez, Vicente,Gotor, Vicente
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p. 1418 - 1423
(2008/12/20)
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- Enzymatic process for the preparation of optically active alcohols from ketones using tuberous root Daucus carota
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The present invention relates to an enzymatic process for the preparation of optically active chiral alcohols using tuberous root Daucus carota; particularly invention relates to an enzymatic process for the preparation of optically active alcohols by enantioselective reduction of corresponding ketones using tuberous root Daucus carota.
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Page/Page column 4; 8
(2008/06/13)
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- Preparative asymmetric reduction of ketones in a biphasic medium with an (S)-alcohol dehydrogenase under in situ-cofactor-recycling with a formate dehydrogenase
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The substrate range of a novel recombinant (S)-alcohol dehydrogenase from Rhodococcus erythropolis is described. In addition, an enzyme-compatible biphasic reaction medium for the asymmetric biocatalytic reduction of ketones with in situ-cofactor regeneration has been developed. Thus, reductions of poorly water soluble ketones in the presence of the alcohol dehydrogenase from R. erythropolis and a formate dehydrogenase from Candida boidinii can be carried out at higher substrate concentrations of 10-200 mM. The resulting (S)-alcohols were formed with moderate to good conversion rates, and with up to >99% ee.
- Gr?ger, Harald,Hummel, Werner,Rollmann, Claudia,Chamouleau, Francoise,Hüsken, Hendrik,Werner, Helge,Wunderlich, Christine,Abokitse, Kofi,Drauz, Karlheinz,Buchholz, Stefan
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p. 633 - 640
(2007/10/03)
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- Biocatalytic asymmetric hydrogen transfer employing Rhodococcus ruber DSM 44541
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Nonracemic sec-alcohols of opposite absolute configuration were obtained either by asymmetric reduction of the corresponding ketone using 2-propanol as hydrogen donor or by enantioselective oxidation through kinetic resolution of the rac-alcohol using acetone as hydrogen acceptor employing whole lyophilized cells of Rhodococcus ruber DSM 44541. The microbial oxidation/reduction system exhibits not only excellent stereo- and enantioselectivity but also a broad substrate spectrum. Due to the exceptional tolerance of the biocatalyst toward elevated concentrations of organic materials (solvents, substrates and cosubstrates), the process is highly efficient. The simple preparation of the biocatalyst and its ease of handling turns this system into a versatile tool for organic synthesis.
- Stampfer, Wolfgang,Kosjek, Birgit,Faber, Kurt,Kroutil, Wolfgang
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p. 402 - 406
(2007/10/03)
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- Biocatalytic oxidation of 4-vinylphenol by Nocardia
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Nocardia species NRRL 5646 stereospecifically hydrates 4-vinylphenol (15) to S-1-(4′-hydroxyphenyl)ethanol (17), and further oxidizes 17 to 4′-hydroxyacetophenone (18). Labeled metabolites 17 and 18 obtained from incubations in D2O and H218O support initial enzymatic tautomerization of 15 to a reactive quinone methide (16), which adds water in the first reaction. Commitment to catalysis is high in the hydration reaction, while the alcohol dehydrogenation reaction appears to be reversible. The stereochemical features of water addition, alcohol oxidations, and ketone reductions with growing culture biocatalysis were established by chiral HPLC. Alcohol oxidations or ketone reductions in 12 000 × g supernatants preferentially require NADP+-NADPH,H+ as co-factors. The alcohol dehydrogenase has broad substrate specificity, favoring the oxidation of primary alkanols and 4-hydroxybenzyl alcohols.
- Lee, Kyung-Seon,Rosazza, John P.N.
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p. 582 - 588
(2007/10/03)
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- Efficient enantioselective reduction of ketones with Daucus carota root
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A novel and efficient reduction of various prochiral ketones such as acetopehones, α-azido aryl ketones, β-ketoesters, and aliphatic acyclic and cyclic ketones to the corresponding optically acive secondary alcohols with moderate to excellent chemical yield was achieved by using Daucus carota, root plant cells under extremely mild and environmentally benign conditions in aqueous medium, has been described. Many of these optically active alcohols are the potential chiral building blocks for the synthesis of pharmaceutically important molecules and asymmetric chiral ligands. Hence, this biocatalytic approach is found to be the most suitable for the preparation of a wide range of chiral alcohols and gave inspiration for the development of a new biotechnological process.
- Yadav,Nanda,Thirupathi Reddy,Bhaskar Rao
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p. 3900 - 3903
(2007/10/03)
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- A new procedure for deconvolution of inter-/intramolecular intrinsic primary and α-secondary deuterium isotope effects from enzyme steady-state kinetic data
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The A2B2 flavocytochrome p-cresol methylhydroxylase (PCMH) from Pseudomonas putida oxidizes 4-methylphenol (p-cresol) to 4-hydroxybenzyl alcohol in a process requiring scission of an α-C-H bond with concomitant reduction of covalently bound FAD in each A subunit. Values of k(cat)/K were determined from steady-state kinetic data for the reactions of PCMH with the following substrates: 4-methylphenol, 4-(2H1)methylphenol, 4- (2H2)methylphenol, and 4-(2H3)methylphenol. A procedure was devised to extract the intrinsic primary deuterium and intrinsic α-secondary deuterium kinetic isotope effects from these values of k(cat)/K. The primary effect, P, is 6.71 ± 0.08, and the secondary effect, S, is 1,013 ± 0.014. The magnitudes of these effects are discussed in terms of an early or late transition state, hydrogen tunneling, coupled motion between the leaving and remaining hydrogens of the methyl group, and a H- expulsion mechanism versus a substrate radical mechanism versus a covalent substrate-FAD intermediate mechanism. The reaction of 4-ethylphenol with PCMH produces 4-vinylphenol and (-)-S-1-(4-hydroxyphenyl)ethanol (~100% enantomeric excess). The evidence indicates that these are formed from a common intermediate, presumably a p- quinone methide. From the partition ratios for the formation of the alcohol and 4-vinylphenol from 4-ethylphenol, 4-(1',1'-2H2)ethylphenol, and 4- (2',2',2'-2H3)ethylphenol, the primary isotope effect for conversion of the p-quinone (2',2',2'2H3)methide to 4-(2',2'-2H2)vinylphenol was estimated to be about 2, and the α-secondary isotope effect for conversion of p- quinone (1'-2H1)methide to 1-(4-hydroxyphenyl)-(1'-2H1)ethanol was found to be inverse (=0.83), as expected for sp2 to sp3 hybridization change at the α-carbon. Values of k(cat)/K were determined for 4-ethylphenol, R,S- (±)-4-(1'-2H1)ethylphenol (abbreviated R,S-D), S-(-)-4-(1'- 2H1)ethylphenol (S-D), R-(+)- 4-(1'-2H1)ethylphenol (R-D), and 4- (1',1',2H2)ethylphenol (D2). The (D2)(k(cat)/K) value was found to be 5.1- 6.1, the same as determined in an earlier study. Unexpectedly, the values for (R,S-D)(k(cat)/K), (S-D)(k(cat)/K), and (R-D)(k(cat)/K) were all about the same (~1.7), indicating that there is nearly an equal probability for pro-R or pro-S C-H bond scission. An apparent flux ratio for the pro-S path/pro-R path was estimated to be 0.78 ± 0.02. The same procedure devised to determine values for P and S for 4-methylphenol was used to determine these values for the 4-ethylphenol reaction (commitment to catalysis = 0); P = 5.98 ± 0.12 and S = 0.967 ±0.021. These values are essentially the same as those determined for 4-methylphenol. Thus, the chemical mechanisms for both substrates are assumed to be similar.
- McIntire, William S.,Everhart, E. Thomas,Craig, John C.,Kuusk, Vladislav
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p. 5865 - 5880
(2007/10/03)
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- A Facile General Route to Enantiomeric 1-(4-Hydroxyphenyl)alkanols, and an Improved Synthesis of 4-Vinylphenol
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Optically pure 1-(4-hydroxyphenyl)alkanols, the phenolic hydroxy groups of which are protected, have been obtained by an improved resolution procedure.Since subsequent deprotection of these is accompanied by complete elimination to the phenolic styrenes, an efficient synthesis of 4-vinylphenol from the racemic protected alcohols by simultaneous deprotection and elimination at 0 deg C has been developed. The target chiral 1-(4-hydroxyphenyl)alkanols have been prepared by treatment of the O-protected 4-hydroxyphenyl alkyl ketone with the enantiomers of chlorodiisopinocampheylborane at 0 deg C, when asymmetric reduction and simultaneous deprotection gives the enantiomeric diols in >99.7percent e.e. and high chemical yield.
- Everhart, E. Thomas,Craig, J. Cymerman
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p. 1701 - 1707
(2007/10/02)
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