- Synthesis of aryl 3-O-β-cellobiosyl-β-D-glucopyranosides for reactivity studies of 1,3-1,4-β-glucanases
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A series of substituted aryl β-glycosides derived from 3-O-β-cellobiosyl-D-glucopyranose with different phenol-leaving group abilities as measured by the pK(a) of the free phenol group upon enzymatic hydrolysis has been synthesised. Aryl β-glycosides with a pK(a) of the free phenol leaving group>5 were prepared by phase-transfer glycosidation of the per-O-acetylated α-glycosyl bromide with the corresponding phenol, whereas the 2,4-dinitrophenyl β-glycoside was obtained by condensation of 1-fluoro-2,4-dinitrobenzene with the partially acetylated trisaccharide followed by acid de-O-acetylation. The aryl β-glycosides have been used for reactivity studies of the wild-type Bacillus licheniformis 1,3-1,4-β-d-glucan 4-glucanohydrolase. The Hammett plot log k(cat) versus pK(a) is biphasic with an upward curvature at low pK(a) values suggesting a change in transition-state structure depending on the aglycon. Copyright (C) 1998 Elsevier Science Ltd.
- Planas, Antoni,Abel, Mireia,Millet, Oscar,Palasi, Josep,Pallares, Cristina,Viladot, Josep-Lluis
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- RADICAL-ANIONS OF AROMATIC COMPOUNDS. XV. ELECTRONIC STRUCTURE OF THE RADICAL-ANIONS AND THE DIRECTION OF PARTIAL REDUCTION OF POLYNITROPHENOLS AND N,N-DIALKYLANILINES. THE EFFECT OF pH ON THE REGIOSELECTIVITY
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The radical products formed during the reduction of 2,3-, 2,5-, 3,4-, and 2,4-dinitrophenols in DMFA and aqueous solutions at various pH values were studied by the ESR method.The radical-anions of the phenols and corresponding phenolates exist in two forms, which differ in the position of the negatively charged nitro group in relation to the substituent (OH or O-).The direction of partial reduction of the dinitrophenols by the complex of titanium trichloride with Trilon B was investigated, and at was shown that it can be varied by variation of the pH of the medium.The effect of the pH on the relative content of the reaction products (isomeric aminonitrophenols) correlates with the effect of this factor on the ratio of the two forms of the radical-anions of the initial phenols.It was shown for the case 2,4,6-trinitro-N,N-dimethylaniline that it is possible to change the direction of partial reduction of N,N-dialkylpolynitroanilines by varying the pH.
- Bilkis, I. I.,Goidin, V. V.,Uskov, S. I.,Shteingarts, V. D.
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- Synthese von Di- und Tri(o-phenylendiamin)-Liganden
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The o-phenylenediamine ligand trisamine, TRIPACEN (4) and bisamine, DIPACEN (8) are obtained by Williamson coupling of the potassium salt of 2,3-dinitrophenole (1) to tris(3-chlorpropyl)amine (2) or benzylbis(3-chlorpropyl)amine (5), respectively, followed by reduction of the nitro functions with Sn/HCl or Pd(OH)2/H2.
- Hahn, Ekkehardt F.,Dittler-Klingemann, Andreas
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p. 779 - 782
(2007/10/02)
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- The Rearrangement of Aromatic Nitro Compounds. Part 2. The Rearrangement os Substituted Nitrophenols in Trifluoromethanesulphonic Acid
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o-Nitrophenols with an additional substituent (Y = NO2, Cl, or Me) in the 3-position rearrange in trifluoromethanesulphonic acid at 100 deg C to give mainly the product with the nitro group in the opposite ortho position; no more than 1-4percent of other products are formed.The reactions give first-order kinetics, are acid-catalysed and (at least when Y = NO2) are intramolecular.The rate of rearrangement varies with the 3-substituent in the order Me > Cl > NO2.The results are discussed in terms of a rate-determining migration of the nitro group in the Wheland intermediate formed by protonation at the 2-position.A much slower rearrangement occurs with 3,4-dinitrophenol under the same conditions to give a small yield of 2,5-dinitrophenol accompanied by decomposition of the substrate.
- Bullen, John V.,Ridd, John H.
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p. 1675 - 1679
(2007/10/02)
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- Effective Charge Distribution for Attack of Phenoxide Ion on Aryl Methyl Phosphate Monoanion: Studies Related to the Action of Ribonuclease
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The reaction of phenoxide ion with aryl methyl phosphate monoanions and aryl diethyl phosphates obeys second-order kinetics in aqueous solution at 39 deg C and 1 M ionic strength.The second-order rate constants (M-1s-1) for these reaction obey the following Broensted equations: log k2 = -0.51pKlg + 0.72 (r = 0.970) (aryl diethyl phosphate) log k2 = -0.64pKlg - 2.53 (r = 0.898) (aryl methyl phosphate monoanion) The monoanion is some 104-fold less reactive toward attack by phenolate ion than is the diethyl ester with 4-nitrophenol leaving group, consistent in part with the operation of an electrostatic effect.The similarity between the Broensted exponents in both reactions indicates that the effective charge change on the leaving oxygen from ground state to transition state is similar in both cases; this indicates that the oxyanion in the monoanion case does not assist leaving group expulsion.The date are consistent with little coupling between proton abstraction from the 2'-hydroxyl group and the leaving group expulsion in the ribonuclease reaction.
- Ba-Saif, Salem A.,Davis, Andrew M.,Williams, Andrew
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p. 5483 - 5486
(2007/10/02)
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