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https://www.readbyqxmd.com/read/28440624/acid-base-catalysis-in-glycosidations-a-nature-derived-alternative-to-the-generally-employed-methodology
#1
Peng Peng, Richard R Schmidt
Inverting glycosyltransferases enforce in the active site an intramolecular, acid-base catalyzed glycosidation that, due to proximity of the donor anomeric carbon and the acceptor hydroxyl group, follows an SN2-type reaction. Spacers, tethering donor and acceptor via nonreacting functional groups, led in intramolecular glycosidations to excellent yields and, independent of the donor anomeric configuration, to either the α- or the β-anomer. The requirement of a demanding protecting group pattern confines the application of this efficient method...
April 25, 2017: Accounts of Chemical Research
https://www.readbyqxmd.com/read/28438657/the-thiol-of-human-serum-albumin-acidity-microenvironment-and-mechanistic-insights-on-its-oxidation-to-sulfenic-acid
#2
Jenner Bonanata, Lucía Turell, Laura Antmann, Gerardo Ferrer-Sueta, Santiago Botasini, Eduardo Méndez, Beatriz Alvarez, E Laura Coitiño
Human serum albumin (HSA) has a single reduced cysteine residue, Cys34, whose acidity has been controversial. Three experimental approaches (pH-dependence of reactivity towards hydrogen peroxide, ultraviolet titration and infrared spectroscopy) are used to determine that the pKa value in delipidated HSA is 8.1 ± 0.2 at 37°C and 0.1M ionic strength. Molecular dynamics simulations of HSA in the sub-microsecond timescale show that while sulfur exposure to solvent is limited and fluctuating in the thiol form, it increases in the thiolate, stabilized by a persistent hydrogen-bond (HB) network involving Tyr84 and bridging waters to Asp38 and Gln33 backbone...
April 21, 2017: Free Radical Biology & Medicine
https://www.readbyqxmd.com/read/28423201/highly-enantioselective-allylation-of-ketones-an-efficient-approach-to-all-stereoisomers-of-tertiary-homoallylic-alcohols
#3
Marcus Brauns, Marvin Mantel, Julie Schmauck, Marian Guder, Martin Breugst, Joerg Pietruszka
Optimized protecting group for allylboronates allows the use of ketones to synthesize all isomers of quaternary homoallylic alcohols with high enantioselectivities. All symmetric isomers of the allylboronate can be prepared in high yields and diastereoselectivities using SN2' reactions. The improved reactivity of the novel protecting group is verified via 1H-NMR kinetics. Mechanistic studies using DFT calculations are conducted to investigate the new findings. Thus, the stereochemical outcome and enhanced reactivity can be rationalized...
April 19, 2017: Chemistry: a European Journal
https://www.readbyqxmd.com/read/28414466/enantioselective-synthesis-of-chiral-%C3%AE-azido-and-%C3%AE-aryloxy-quaternary-stereogenic-centers-via-the-phase-transfer-catalyzed-%C3%AE-alkylation-of-%C3%AE-bromomalonates-followed-by-sn2-substitution
#4
Doyoung Kim, Min Woo Ha, Suckchang Hong, Cheonhyoung Park, Byungsoo Kim, Jewon Yang, Hyeung-Geun Park
A new efficient synthetic method for chiral α-azido-α-alkylmalonates and α-aryloxy-α-alkylmalonates was developed. The enantioselective α-alkylation of diphenylmethyl tert-butyl α-bromomalonate under phase-transfer catalytic conditions [(S,S)-3,4,5-trifluorophenyl-NAS bromide, 50% KOH, toluene, and -40 °C) provided the corresponding α-bromo-α-alkylmalonates in high chemical yields (≤98%) and high optical yields (≤99% ee). The resulting α-alkylated products were converted to α-azido-α-alkylmalonates (≤96%, ≤97% ee) and α-aryloxy-α-alkylmalonates (≤79%, ≤93% ee) by SN2 substitution with sodium azide and aryloxides, respectively...
April 20, 2017: Journal of Organic Chemistry
https://www.readbyqxmd.com/read/28398749/dynamic-behavior-and-isomerization-equilibria-of-distannenes-synthesized-by-tin-hydride-olefin-insertions-characterization-of-the-elusive-mono-hydrido-bridged-isomer
#5
Shuai Wang, Madison L McCrea-Hendrick, Cory M Weinstein, Christine A Caputo, Elke Hoppe, James C Fettinger, Marilyn M Olmstead, Philip P Power
The tin(II) hydride [AriPr6Sn(µ-H)]2 (AriPr6 = C6H3-2,6(C6H2-2,4,6-iPr3)2) (1a) reacts with two equivalents of ethylene or t-butylethylene at ca. 25 °C to yield Sn2(AriPr6)2R2 (R = ethyl or t-butylethyl), which exist either as a symmetric distannene AriPr6(R)SnSn(R)AriPr6 (2a or 5a) or an unsymmetric stannylstannylene AriPr6SnSnR2AriPr6 (3a). In contrast, the less crowded Sn(II) hydride [AriPr4Sn(µ-H)]2 (AriPr4 = C6H3-2,6(C6H3-2,6-iPr2)2) (1b) reacts with excess ethylene to give AriPr4(CH2CH3)2Sn(CH2CH2)Sn(CH2CH3)(CHCH2)AriPr4 (4) featuring five ethylene equivalents, one of which is dehydrogenated to an alkenyl, -CH=CH2 group...
April 11, 2017: Journal of the American Chemical Society
https://www.readbyqxmd.com/read/28395126/mapping-the-trajectory-of-nucleophilic-substitution-at-silicon-using-a-peri-substituted-acenaphthyl-scaffold
#6
Emanuel Hupf, Marian Olaru, Ciprian I Rat, Christian B Hübschle, Enno Lork, Simon Grabowsky, Stefan Mebs, Jens Beckmann
The nucleophilic substitution reaction of second order (SN2) at a silicon atom is scrutinized using snapshots along a pseudo-reaction coordinate. Phosphine and fluoride represent both attacking and leaving groups in the modeled SN2 reaction. In the experimentally obtained 5-Ph2P-Ace-6-SiMe2F (1) the phosphine moiety and the fluorosilane moiety are forced into immediate proximity through an acenaphthyl scaffold, i.e. they exhibit peri-interactions that serve as the model of the reactant ion-molecule complex and starting point for a theoretical PES scan...
April 10, 2017: Chemistry: a European Journal
https://www.readbyqxmd.com/read/28394615/steric-effects-of-solvent-molecules-on-sn2-substitution-dynamics
#7
Xu Liu, Jing Xie, Jiaxu Zhang, Li Yang, William L Hase
Influences of solvent molecules on SN2 reaction dynamics of microsolvated F(-)(H2O)n with CH3I, for n = 0-3, are uncovered by direct chemical dynamics simulations. The direct substitution mechanism, which is important without microsolvation, is quenched dramatically upon increasing hydration. The water molecules tend to force reactive encounters to proceed through the prereaction collision complex leading to indirect reaction. In contrast to F(-)(H2O), reaction with higher hydrated ions shows a strong propensity for ion desolvation in the entrance channel, diminishing steric hindrance for nucleophilic attack...
April 20, 2017: Journal of Physical Chemistry Letters
https://www.readbyqxmd.com/read/28375546/catalytic-cracking-of-lactide-and-poly-lactic-acid-to-acrylic-acid-at-low-temperatures
#8
Frédéric G Terrade, Jan van Krieken, Bastiaan J V Verkuijl, Elisabeth Bouwman
Despite being a simple dehydration reaction, the industrially relevant conversion of lactic acid to acrylic acid is particularly challenging. For the first time, the catalytic cracking of lactide and poly(lactic acid) to acrylic acid under mild conditions is reported with up to 58 % yield. This transformation is catalyzed by strong acids in the presence of bromide or chloride salts and proceeds through simple SN 2 and elimination reactions.
April 4, 2017: ChemSusChem
https://www.readbyqxmd.com/read/28362011/effects-of-microsolvation-on-a-sn2-reaction-indirect-atomistic-dynamics-and-weakened-suppression-of-reactivity
#9
Li Yang, Xu Liu, Jiaxu Zhang, Jing Xie
Systematic studies of microsolvation in the gas phase have enriched our knowledge of solvent effects. Here, the dynamics of a prototype SN2 reaction of a hydrated fluoride ion with methyl iodide is uncovered employing direct dynamics simulations that show strikingly distinct features from those determined for an unsolvated system. An indirect scattering is found to prevail, which occurs dominantly by forming hydrated F(-)(H2O)-HCH2I and F(-)(H2O)-CH3I pre-reaction complexes at low energies, but proceeds through their water-free counterparts at higher energies...
April 12, 2017: Physical Chemistry Chemical Physics: PCCP
https://www.readbyqxmd.com/read/28358153/domino-reaction-of-cyclic-sulfamidate-imines-with-morita-baylis-hillman-acetates-promoted-by-dabco-a-metal-free-approach-to-functionalized-nicotinic-acid-derivatives
#10
Debashis Majee, Soumen Biswas, Shaikh M Mobin, Sampak Samanta
A facile, green, metal-free new one-pot synthetic strategy has been developed for easy access to a wide array of medicinally promising functionalized pyridines having an ester, a nitrile or an acetyl group at the C-3 position in good to excellent yields via a domino SN2/elimination/6π-aza-electrocyclization/aromatization reaction of several 4-aryl/hetero-aryl-substituted 5-membered cyclic sulfamidate imines with a broad range of MBH acetates of acrylate/acrylonitrile/MVK in 2-MeTHF promoted by DABCO as an organobase under an O2 atmosphere...
April 11, 2017: Organic & Biomolecular Chemistry
https://www.readbyqxmd.com/read/28338332/benchmark-ab-initio-characterization-of-the-complex-potential-energy-surface-of-the-f-ch3ch2cl-reaction
#11
Viktor Tajti, Gábor Czakó
We compute benchmark structures, frequencies, and relative energies for the stationary points of the potential energy surface of the F(-) + CH3CH2Cl reaction using explicitly correlated ab initio levels of theory. CCSD(T)-F12b geometries and harmonic vibrational frequencies are obtained with the aug-cc-pVTZ and aug-cc-pVDZ basis sets, respectively. The benchmark relative energies are determined using a high-level composite method based on CCSD(T)-F12b/aug-cc-pVQZ frozen-core energies, CCSD(T)-F12b/cc-pCVTZ-F12 core electron correlation effects, and CCSD(T)-F12b/aug-cc-pVDZ zero-point energy corrections...
April 3, 2017: Journal of Physical Chemistry. A
https://www.readbyqxmd.com/read/28332678/tandem-sn2-nucleophilic-substitution-oxidative-radical-cyclization-of-aryl-substituted-allylic-alcohols-with-1-3-dicarbonyl-compounds
#12
Zhen Zhang, Cheng Li, Shao-Hua Wang, Fu-Min Zhang, Xue Han, Yong-Qiang Tu, Xiao-Ming Zhang
A novel and efficient tandem SN2' nucleophilic substitution/oxidative radical cyclization reaction of aryl substituted allylic alcohols with 1,3-dicarbonyl compounds has been developed by using Mn(OAc)3 as an oxidant, which enables the expeditious synthesis of polysubstituted dihydrofuran (DHF) derivatives in moderate to high yields. The use of weakly acidic hexafluoroisopropanol (HFIP) as the solvent rather than AcOH has successfully improved the yields and expanded the substrate scope of this type of radical cyclization reactions...
April 11, 2017: Organic & Biomolecular Chemistry
https://www.readbyqxmd.com/read/28328225/palladium-catalyzed-%C3%AE-mesylation-of-simple-amide-via-primary-sp-3-c-h-activation
#13
Ren Zhao, Wenjun Lu
A β-mesylation of primary sp(3) C-H bonds from simple amides with methanesulfonic anhydride (Ms2O) has been established successfully at 80 °C in a Pd(OAc)2 (catalyst)/K2S2O8 (oxidant)/CF3CH2OH (solvent) system. These amide substrates involve N-monosubstituted linear, branch, or cyclic alkanes, and electron-deficient benzyl compounds. The β-mesylated amide products can be converted easily to β-fluoroamides or β-lactams through inter- or intramolecular SN2 processes.
April 7, 2017: Organic Letters
https://www.readbyqxmd.com/read/28327767/regioselective-sn2-reactions-for-rapid-syntheses-of-azido-inositols-by-one-pot-sequence-specific-nucleophilysis
#14
Arthi Ravi, Syed Zahid Hassan, Ajithkumar N Vanikrishna, Kana M Sureshan
Triflates of myo-inositol undergo facile solvolysis in DMSO and DMF yielding SN2 products substituted with O-nucleophiles; DMF showed slower kinetics. Axial O-triflate undergoes faster substitution than equatorial O-triflate. By exploiting this difference in kinetics, solvent-tuning and sequence-controlled nucleophilysis, rapid synthesis of three azido-inositols of myo-configuration from myo-inositol itself has been achieved.
March 22, 2017: Chemical Communications: Chem Comm
https://www.readbyqxmd.com/read/28296406/synthesis-of-the-abc-tricyclic-system-of-daphnicyclidin-a
#15
Jian Long Li, Hong Wei Shi, Qi Wang, Yong Hai Chai, Jun Yang
A substrate-stereocontrolled synthesis of the ABC tricyclic system of daphnicyclidin A is developed. The key reactions include an efficient tandem N-allylation-SN2' reaction to assemble 2,3,4-cis trisubstituent pyrrolidine ring C and two intramolecular Horner-Wadsworth-Emmons reactions to construct cycloheptanone ring A and piperidine ring B.
April 7, 2017: Organic Letters
https://www.readbyqxmd.com/read/28282046/unexpected-mechanochemical-complexity-in-the-mechanistic-scenarios-of-disulfide-bond-reduction-in-alkaline-solution
#16
Przemyslaw Dopieralski, Jordi Ribas-Arino, Padmesh Anjukandi, Martin Krupicka, Dominik Marx
The reduction of disulfides has a broad importance in chemistry, biochemistry and materials science, particularly those methods that use mechanochemical activation. Here we show, using isotensional simulations, that strikingly different mechanisms govern disulfide cleavage depending on the external force. Desolvation and resolvation processes are found to be crucial, as they have a direct impact on activation free energies. The preferred pathway at moderate forces, a bimolecular SN2 attack of OH(-) at sulfur, competes with unimolecular C-S bond rupture at about 2 nN, and the latter even becomes barrierless at greater applied forces...
February 2017: Nature Chemistry
https://www.readbyqxmd.com/read/28273206/amf-naf-sncl2-solution-reduces-in-situ-enamel-erosion-profilometry-and-cross-sectional-nanoindentation-analysis
#17
Thayanne Monteiro Ramos-Oliveira, Camila Vieira Silva, Paula Mendes Acatauassu Nunes, Cecília Pedroso Turssi, Peter Rechmann, Patricia Moreira de Freitas
This in situ study aimed to investigate the effect of a tin-containing fluoride solution in preventing enamel erosion. Also, its effects on the partly demineralized zone were assessed for the first time. Thirteen volunteers participated in this 2-phase study, wearing removable intra-oral appliances containing four sterilized bovine enamel slabs, for 8 days, where 2 treatment protocols were tested using samples in replicas (n = 13): CO - no treatment (negative control) and FL - AmF/NaF/SnCl2 solution (500 ppm F-, 800 ppm Sn2+, pH = 4...
March 6, 2017: Brazilian Oral Research
https://www.readbyqxmd.com/read/28264144/template-mediated-stabilization-of-a-dna-g-quadruplex-formed-in-the-hiv-1-promoter-and-comparative-binding-studies
#18
Laureen Bonnat, Laure Bar, Béatrice Génnaro, Hugues Bonnet, Olivier Jarjayes, Fabrice Thomas, Jérôme Dejeu, Eric Defrancq, Thomas Lavergne
G-rich DNA oligonucleotides derived from the promoter region of the HIV-1 long terminal repeat (LTR) were assembled onto an addressable cyclopeptide platform through sequential oxime ligation, a thiol-iodoacetamide SN2 reaction, and copper-catalyzed azide-alkyne cycloaddition reactions. The resulting conjugate was shown to fold into a highly stable antiparallel G4 architecture as demonstrated by UV, circular dichroism (CD), and NMR spectroscopic analysis. The binding affinities of six state-of-the-art G4-binding ligands toward the HIV-G4 structure were compared to those obtained with a telomeric G4 structure and a hairpin structure...
April 24, 2017: Chemistry: a European Journal
https://www.readbyqxmd.com/read/28262888/computational-study-of-c-sp-3-o-bond-formation-at-a-pd-iv-centre
#19
Allan J Canty, Alireza Ariafard, Nicole M Camasso, Andrew T Higgs, Brian F Yates, Melanie S Sanford
This report describes a computational study of C(sp(3))-OR bond formation from Pd(IV) complexes of general structure Pd(IV)(CH2CMe2-o-C6H4-C,C')(F)(OR)(bpy-N,N') (bpy = 2,2'-bipyridine). Dissociation of (-)OR from the different octahedral Pd(IV) starting materials results in a common square-pyramidal Pd(IV) cation. An SN2-type attack by (-)OR ((-)OR = phenoxide, acetate, difluoroacetate, and nitrate) then leads to C(sp(3))-OR bond formation. In contrast, when (-)OR = triflate, concerted C(sp(3))-C(sp(2)) bond-forming reductive elimination takes place, and the calculations indicate this outcome is the result of thermodynamic rather than kinetic control...
March 14, 2017: Dalton Transactions: An International Journal of Inorganic Chemistry
https://www.readbyqxmd.com/read/28260380/highly-emissive-divalent-ion-doped-colloidal-cspb1-xmxbr3-perovskite-nanocrystals-through-cation-exchange
#20
Ward van der Stam, Jaco J Geuchies, Thomas Altantzis, Karel H W van den Bos, Johannes D Meeldijk, Sandra Van Aert, Sara Bals, Daniel Vanmaekelbergh, Celso de Mello Donega
Colloidal CsPbX3 (X = Br, Cl, and I) perovskite nanocrystals (NCs) have emerged as promising phosphors and solar cell materials due to their remarkable optoelectronic properties. These properties can be tailored by not only controlling the size and shape of the NCs but also postsynthetic composition tuning through topotactic anion exchange. In contrast, property control by cation exchange is still underdeveloped for colloidal CsPbX3 NCs. Here, we present a method that allows partial cation exchange in colloidal CsPbBr3 NCs, whereby Pb(2+) is exchanged for several isovalent cations, resulting in doped CsPb1-xMxBr3 NCs (M= Sn(2+), Cd(2+), and Zn(2+); 0 < x ≤ 0...
March 22, 2017: Journal of the American Chemical Society
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