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https://www.readbyqxmd.com/read/28319683/stereoselective-total-synthesis-of-oxylipin-from-open-chain-gluco-configured-building-block
#1
Santosh Ramdas Borkar, Indrapal Singh Aidhen
Total synthesis of naturally occurring Oxylipin has been achieved from open chain gluco-configured building block which is readily assembled from inexpensive and commercially available D-(+)-gluconolactone. Grignard reaction and Wittig olefination reactions are key steps for the requisite CC bond formation.
March 3, 2017: Carbohydrate Research
https://www.readbyqxmd.com/read/28319681/stereoselective-acetylation-of-hemicellulosic-c5-sugars
#2
Zachary D Herde, Prathap D John, Dania Alvarez-Fonseca, Jagannadh Satyavolu, Christopher T Burns
The stereoselective peracetylation of α-d-xylose (1) and α-l-arabinose (4) using a combination of triethylamine and acetic anhydride in the presence or absence of a catalytic amount of dimethylaminopyridine (DMAP) is described. The peracetylated d-xylose and l-arabinose alpha pyranose anomers 2α and 5α are obtained in 97% and 56% yields respectively. The peracetylated d-xylose beta pyranose anomer 2β is obtained in 71% yield through simple modification of the reaction conditions. Details regarding synthesis and isolation optimization studies under different conditions are presented below...
March 10, 2017: Carbohydrate Research
https://www.readbyqxmd.com/read/28319377/mechanism-and-origins-of-stereoselectivity-in-the-cinchona-thiourea-and-squaramide-catalyzed-asymmetric-michael-addition-of-nitroalkanes-to-enones
#3
Matthew N Grayson
We report density functional theory calculations which examine the mechanism and origins of stereoselectivity of Soós' landmark discovery from 2005 that cinchona thioureas catalyze the asymmetric Michael addition of nitroalkanes to enones. We show that the electrophile is activated by the catalyst's protonated amine and that the nucleophile binds to the thiourea moiety by hydrogen bonding. These results lead to the correction of published mechanistic work which did not consider this activation mode. We have also investigated the corresponding cinchona squaramide-catalyzed reaction and found that it proceeds by the same mechanism, despite the differences in the geometry of the two catalysts' hydrogen bond donating groups, which demonstrates the generality of this mechanistic model...
March 20, 2017: Journal of Organic Chemistry
https://www.readbyqxmd.com/read/28319220/ni-catalyzed-regio-and-stereoselective-addition-of-arylboronic-acids-to-terminal-alkynes-with-a-directing-group-tether
#4
Madala Hari Babu, Gadi Ranjith Kumar, Ruchir Kant, Maddi Sridhar Reddy
Addition of arylboronic acids to directing group tethered acetylenes in a regio and stereoselective manner using an inexpensive catalytic system is achieved for the first time to access highly sought after allyl/homoallyl alcohol/amine units. The apparent vinylnickel intermediate was successfully trapped by the Michael electrophiles to get defined tri- and tetra-substituted olefins. An interesting selectivity switch was observed with internal alkynes.
March 20, 2017: Chemical Communications: Chem Comm
https://www.readbyqxmd.com/read/28314509/stereo-controlled-synthesis-of-functionalized-tetrahydropyridines-based-on-the-cyanomethylation-of-1-6-dihydropyridines-and-generation-of-anti-hepatitis-c-virus-agents
#5
Ryo Watanabe, Haruki Mizoguchi, Hideaki Oikawa, Hirofumi Ohashi, Koichi Watashi, Hiroki Oguri
Densely functionalized tetrahydropyridines were stereoselectively synthesized from 1,6-dihydropyridines. Exploiting a carbonyl group installed at the C3 position of the 1,6-dihydropyridine system, we devised a strategy for cyanomethylation at C2/C6 and subsequent divergent installation of an allyl group at C3/C5 in a highly regio- and stereo-controlled manner. This versatile protocol for programmable functionalization of the 1,6-dihydropyridine system allows the divergent and streamlined synthesis of multiply-substituted tetrahydropyridines as an important class of biologically and medicinally relevant scaffolds...
March 7, 2017: Bioorganic & Medicinal Chemistry
https://www.readbyqxmd.com/read/28304034/crystalline-magnesium-chloride-electron-donor-complexes-new-support-materials-for-ziegler-natta-catalysts
#6
V H Nissinen, I O Koshevoy, T T Pakkanen
In this study, crystalline magnesium chloride-electron donor complexes were prepared by recrystallization of δ-MgCl2 in the presence of chelating electron donors, including two diethers (1,2-dimethoxyethane; DME and 1,3-dimethoxypropane; DMP) and one diamine (N,N'-diethylethylenediamine; DEEDA). The syntheses and crystal structures of such magnesium chloride complexes with chelating ligands have been rarely reported, even though they can provide important information for the selection of electron donors for stereoselective MgCl2-supported Ziegler-Natta catalysts...
March 17, 2017: Dalton Transactions: An International Journal of Inorganic Chemistry
https://www.readbyqxmd.com/read/28302960/development-of-chemical-and-chemo-enzymatic-glycosylations
#7
Shin-Ichiro Shoda
Glycosidic compounds are indispensable molecules in living systems. Biological phenomena such as cell wall formation, energy storage, and cell recognition strongly depend on the multi-functional characters of these substances. Development of highly regio- and stereoselective glycosylation reactions is necessary to provide sufficient amounts of specific compounds in basic research as well as for applications in industry. This review presents an overview of chemical and chemo-enzymatic glycosylations that have been developed during my forty-year academic career in the field of glyco-science...
2017: Proceedings of the Japan Academy. Series B, Physical and Biological Sciences
https://www.readbyqxmd.com/read/28302508/synthesis-of-clarithromycin-ketolides-chemically-modified-at-the-unreactive-c10-methyl-group
#8
Hany Fakhry Anwar, Mioara Andrei, Kjell Undheim
Chemoselective substitutions in the C10-methyl group of erythromycin A ketolides is reported. The C10-methyl group in the clarithromycin derived substrate 10,11-anhydro-O(6)-methyl-descladinosylerythromycin was activated by conversion into an allyl acetate and thereafter to the corresponding allylic cyanide. Both the allylic acetate and the cyanide reacted with carbonyldiimidazole and ammonia to afford a C11,C12-cyclic carbamate with concurrent elimination of the allylic function to yield a methylene α,β-unsaturated ketone...
February 23, 2017: Bioorganic & Medicinal Chemistry
https://www.readbyqxmd.com/read/28301156/phosphazene-catalyzed-addition-to-electron-deficient-alkynes-the-importance-of-non-linear-allenyl-intermediates-upon-stereoselectivity
#9
Luis Simón, Robert S Paton
An ONIOM QM/MM study on the mechanism of the Michael addition to triple bonds catalyzed by chiral diiminophosphorane catalysts has been performed to understand the stereoselectivity of the product olefin. Our results are consistent with the experimental enantioselectivity, but more importantly, reveal that the Z vs E preference depends on the influence of the catalyst upon the geometry of the allenyl enolate formed in the addition step. These intermediates show an innate preference for a (Z)-configuration, although this can be surpressed by steric interactions due to a catalyst...
March 16, 2017: Journal of Organic Chemistry
https://www.readbyqxmd.com/read/28301144/cinchona-alkaloid-squaramide-catalyzed-sulfa-michael-addition-reaction-mode-of-bifunctional-activation-and-origin-of-stereoinduction
#10
Jinlong Guo, Ming Wah Wong
The mechanism of enantioselective sulfa-Michael addition reaction catalyzed by a cinchona alkaloid-squaramide bifunctional organocatalyst was studied using density functional theory (DFT). Four possible modes of dual activation mechanism via hydrogen bonds were considered. Our study showed that Houk's bifunctional Brønsted acid-hydrogen bonding model, which works for cinchonidine or cinchona alkaloid-urea catalyzed sulfa-Michael addition reaction, also applies to the catalytic system under investigation. In addition, we examined the origin of the stereoselectivity by identifying stereocontrolling transition states...
March 16, 2017: Journal of Organic Chemistry
https://www.readbyqxmd.com/read/28295941/construction-of-fused-pyrrolidines-and-%C3%AE-lactones-by-carbene-catalyzed-c-n-c-c-and-c-o-bond-formations
#11
Xingxing Wu, Lin Hao, Yuexia Zhang, Maiti Rakesh, Rambabu N Reddi, Song Yang, Bao-An Song, Yonggui Robin Chi
A carbene-catalyzed intermolecular C-N bond formation, which initiates a highly selective cascade reaction for the synthesis of pyrrolidine fused β-lactones, is disclosed. The nitrogen-containing bicyclic β-lactone products are obtained with good yields and excellent stereoselectivities. Synthetic transformations of the reaction products into useful functional molecules, such as amino catalysts, can be efficiently realized under mild reaction conditions. Mechanistically, this study provides insights into modulating the reactivities of heteroatoms, such as nitrogen atoms, in challenging carbene-catalyzed asymmetric carbon-heteroatom bond-forming reactions...
March 15, 2017: Angewandte Chemie
https://www.readbyqxmd.com/read/28295752/synthesis-of-an-%C3%AE-2%C3%A2-8-s-linked-gd3-antigen-and-evaluation-of-the-immunogenicity-of-its-glycoconjugate
#12
Chun-Cheng Lin, Ting-Chun Kuan, Hsin-Ru Wu, Avijit K Adak, Ben-Yuan Li, Chien-Fu Liang, Shih-Pin Chiou, Jung-Tung Hung Hung, Alice L Yu Yu, Jih Ru Hwu
Replacing the interglycosidic oxygen atom with a nonhydrolyzable sulfur atom has attracted significant interest because it provides opportunities for developing new glycoconjugate vaccines. Herein, a stereocontrolled and highly convergent method for synthesizing a non-reducing end, inter-S-glycosidic variant of the GD3 (α(2→8)-S-linked GD3) antigen, capable of being resistant to enzymatic hydrolysis was reported. The key steps in the synthesis involved a regio- and stereoselective α(2→3) sialylation of a lactoside acceptor with a C8-iodide-derivatized sialyl donor, followed by an anomeric S-alkylation, which enabled the construction of a terminal S-linked α(2→8) disialyl residue in a stereoselective manner...
March 12, 2017: Chemistry: a European Journal
https://www.readbyqxmd.com/read/28295688/stereocontrolled-synthesis-of-amino-substituted-carbocycles-via-pd-catalyzed-alkene-carboamination-reactions
#13
John P Wolfe, Derick White
Amino-substituted alkylidenecyclopentanes were synthesized through a stereoselective intermolecular Pd-catalyzed alkene carboamination reaction between alkenyl triflates bearing a pendant alkene and exogenous amine nucleophiles. The reactions are effective with a range of different substrate combinations, and proceed with generally high diastereoselectivity. Use of (S)-tBuPhox as the ligand in reactions of achiral substrates provides enantioenriched products with up to 98.5:1.5 er.
March 10, 2017: Chemistry: a European Journal
https://www.readbyqxmd.com/read/28294690/stereoselective-and-non-stereoselective-pharmacokinetics-of-rabeprazole-an-overview
#14
Ranjeet Prasad Dash, Rana Rais, Nuggehally R Srinivas
1. Proton pump inhibitors have been extensively used for the treatment of ailments due to increased gastric acid secretion such as peptic ulcers, gastroesophageal reflux disease etc. 2. There are several approved drugs in the proton pump inhibitor class with the latest entries representing single enantiomer drugs of the previously approved racemic drugs. 3. Despite having a high degree of structural resemblance, rabeprazole, was shown to possess some unique differentiation from other drugs in the class. One of the key distinguishing features of rabeprazole was related to the lesser involvement of polymorphic metabolism in its pharmacokinetic disposition...
March 15, 2017: Xenobiotica; the Fate of Foreign Compounds in Biological Systems
https://www.readbyqxmd.com/read/28294627/total-synthesis-of-%C3%A2-grandilodine-b
#15
Chunyu Wang, Zhonglei Wang, Xiaoni Xie, Xiaotong Yao, Guang Li, Liansuo Zu
The first total synthesis of the opened-type Kopsia alkaloid grandilodine B is reported. Four stereocenters of this alkaloid, three of them quaternary, are stereoselectively generated by a Diels-Alder reaction, a diastereoselective cyanation of tertiary alcohol, and a facial-selective nitrone 1,3-dipolar cycloaddition.
March 15, 2017: Organic Letters
https://www.readbyqxmd.com/read/28290201/regio-and-stereoselective-synthesis-of-functionalized-cyclopentene-derivatives-via-mizoroki-heck-reactions
#16
Alexander Wetzel, Joakim Bergman, Peter Brandt, Mats Larhed, Jonas Brånalt
Pd(0)-catalyzed Mizoroki-Heck alkenylations and arylations of protected aminocyclopentenes, prepared in a few steps from Vince lactam, afforded functionalized cyclopentenes in high yields and stereoselectivities. DFT calculations were performed to rationalize the high diastereoselectivities. Functionalized cyclopentene products were transformed into valuable chiral building blocks, such as cyclic γ-amino acids and carbocyclic nucleoside precursors.
March 14, 2017: Organic Letters
https://www.readbyqxmd.com/read/28288346/synthesis-of-a-disaccharide-repeating-unit-of-the-o-antigen-from-burkholderia-ambifaria-and-its-oligomers
#17
Dongyue Wang, Weiwei Zhuge, Zhongwu Guo, Guofeng Gu
A disaccharide repeating unit of the O-antigen from Burkholderia ambifaria, 6-deoxy-β-d-Alt-(1 → 4)-α-d-Rha-O(CH2)3NH2 (1), and its dimer and trimer, 6-deoxy-β-d-Alt-(1 → 4)-α-d-Rha-(1 → 3)-6-deoxy-β-d-Alt-(1 → 4)-α-d-Rha-O(CH2)3NH2 (2) and 6-deoxy-β-d-Alt-(1 → 4)-α-d-Rha-(1 → 3)-6-deoxy-β-d-Alt-(1 → 4)-α-d-Rha-(1 → 3)-6-deoxy-β-d-Alt-(1 → 4)-α-d-Rha-O(CH2)3NH2 (3), were synthesized via a convergent strategy. The key disaccharyl thioglycoside 4 as a glycosyl donor was stereoselectively assembled by glycosylation of rhammnosyl acceptor 5 with 6-deoxy-altrosyl trichloroacetimidate donor 6b...
March 6, 2017: Carbohydrate Research
https://www.readbyqxmd.com/read/28287263/generation-of-hydrofluoronickelacycles-from-trifluoroethylene-and-ni-0-ligand-effects-on-regio-stereoselectivity-and-reactivity
#18
Kaitie A Giffin, Lorraine A Pua, Sarah Piotrkowski, Bulat M Gabidullin, Ilia Korobkov, Russell P Hughes, R Tom Baker
Treatment of Ni(0) complexes 1a-e with sub-atmospheric pressures of trifluoroethylene (TrFE) affords hydrofluoronickelacyclopentanes L2Ni(C4F6H2) 2a-e (L = PPh3, P(O-o-tol)3, PPh2Me, PPhMe2, PMe3). Fluorine NMR analysis of 2a-e demonstrates predominant formation of three (of the possible six) isomers upon oxidative cycloaddition of TrFE: the cis and trans head-tail isomers and the trans head-head isomer, where the CHF group is defined as the TrFE "head". The respective ratios of L2Ni(C4F6H2) isomers are influenced by the nature of L, with smaller phosphines favoring the thermodynamically preferred (from DFT calculations) trans head-head isomer (cf...
March 13, 2017: Journal of the American Chemical Society
https://www.readbyqxmd.com/read/28286694/highly-stereoselective-biocatalytic-synthesis-of-key-cyclopropane-intermediate-to-ticagrelor
#19
Kari E Hernandez, Hans Renata, Russell D Lewis, S B Jennifer Kan, Chen Zhang, Jared Forte, David Rozzell, John A McIntosh, Frances H Arnold
Extending the scope of biocatalysis to important non-natural reactions such as olefin cyclopropanation will open new opportunities for replacing multi-step chemical syntheses of pharmaceutical intermediates with efficient, clean, and highly selective enzyme-catalyzed processes. In this work, we engineered the truncated globin of Bacillus subtilis for the synthesis of a cyclopropane precursor to the antithrombotic agent ticagrelor. The engineered enzyme catalyzes the cyclopropanation of 3,4-difluorostyrene with ethyl diazoacetate on a preparative scale to give ethyl-(1R, 2R)-2-(3,4-difluorophenyl)-cyclopropanecarboxylate in 79% yield, with very high diastereoselectivity (>99% dr) and enantioselectivity (98% ee), enabling a single-step biocatalytic route to this pharmaceutical intermediate...
November 4, 2016: ACS Catalysis
https://www.readbyqxmd.com/read/28284313/one-step-combined-focused-eppcr-and-saturation-mutagenesis-for-thermostability-evolution-of-a-new-cold-active-xylanase
#20
Juan Pablo Acevedo, Manfred T Reetz, Juan A Asenjo, Loreto P Parra
Enzymes active at low temperature are of great interest for industrial bioprocesses due to their high efficiency at a low energy cost. One of the particularities of naturally evolved cold-active enzymes is their increased enzymatic activity at low temperature, however the low thermostability presented in this type of enzymes is still a major drawback for their application in biocatalysis. Directed evolution of cold-adapted enzymes to a more thermostable version, appears as an attractive strategy to fulfill the stability and activity requirements for the industry...
May 2017: Enzyme and Microbial Technology
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