Search results for "SELECTIVITY"
showing 10 items of 1148 documents
Preparation of Polymer-Supported Polyazamacrocycles. The Role of the Polymeric Matrix in the Preparation of Polymer-Supported Polyazamacrocycles
2000
Different approaches have been studied for the preparation of resins containing different polyazamacrocycles. Preparation of monolithic resins by polymerization of vinylic derivatives of the corresponding macrocycles is shown to be a more versatile strategy for this purpose. The use of energy-dispersive analysis by X-ray has revealed to be a very useful tool for the rapid evaluation of the interaction of those materials with both anions and cations and has allowed the corresponding selectivity trends to be obtained in a fast and simple way. Important effects of the polymeric matrix are observed in some cases.
First Diastereoselective Synthesis of (−)-Methyl Thyrsiflorin A, (−)-Methyl Thyrsiflorin B Acetate, and (−)-Thyrsiflorin C
2000
An efficient procedure for the synthesis of scopadulan diterpenes, using (+)-podocarp-8(14)-en-13-one 13 as starting material, is reported. This procedure has been used for the diastereoselective synthesis of (-)-methyl thyrsiflorin A (8), (-)-methyl thyrsiflorin B acetate (9), and (-)-thyrsiflorin C (7). Key steps in our strategy are the intramolecular cyclopropanation of diazoketone 19 and the regioselective cleavage of the cyclopropane ring.
Improved Regioselectivity in Pyrazole Formation through the Use of Fluorinated Alcohols as Solvents: Synthesis and Biological Activity of Fluorinated…
2008
The preparation of N-methylpyrazoles is usually accomplished through reaction of a suitable 1,3-diketone with methylhydrazine in ethanol as the solvent. This strategy, however, leads to the formation of regioisomeric mixtures of N-methylpyrazoles, which sometimes are difficult to separate. We have determined that the use of fluorinated alcohols such as 2,2,2-trifluoroethanol (TFE) and 1,1,1,3,3,3-hexafluoro-2-propanol (HFIP) as solvents dramatically increases the regioselectivity in the pyrazole formation, and we have used this modification in a straightforward synthesis of fluorinated analogs of Tebufenpyrad with acaricide activity. Fil: Fustero, Santos. Universidad de Valencia; España Fil…
Oligo(1,4-phenylenepyrazole-3,5-diyl)s
2003
The bifunctional nucleophile methylhydrazine reacts in an alkaline medium in a regioselective mode with chalcones to yield 2-pyrazolines, which can be oxidized by DDQ to the corresponding 1H-pyrazoles. From oligo(chalcone)s this reaction yields cross-conjugated compounds with an alternating sequence of 1,4-disubstituted benzene rings and 3,5-disubstituted 1H-pyrazole rings. (© Wiley-VCH Verlag GmbH & Co. KGaA, 69451 Weinheim, Germany, 2003)
Electrical Pumping of Potassium Ions Against an External Concentration Gradient in a Biological Ion Channel
2013
We show experimentally and theoretically that significant currents can be obtained with a biological ion channel, the OmpF porin of Escherichia coli, using zero-average potentials as driving forces. The channel rectifying properties can be used to pump potassium ions against an external concentration gradient under asymmetric pH conditions. The results are discussed in terms of the ionic selectivity and rectification ratio of the channel. The physical concepts involved may be applied to separation processes with synthetic nanopores and to bioelectrical phenomena. (C) 2013 AIP Publishing LLC.
Selective Gas and Vapor Sorption and Magnetic Sensing by an Isoreticular Mixed-Metal–Organic Framework
2012
A novel isoreticular oxamato-based manganese(II)-copper(II) open metal-organic framework H(2)O@iso1 featuring a pillared square/octagonal layer structure with alternating open and closed octagonal pores has been rationally prepared. The open-framework topology is responsible for a large selectivity in the separation of small gas (CO(2) over CH(4)) and vapor molecules (CH(3)OH over CH(3)CN and CH(3)CH(2)OH). H(2)O@iso1 displays a long-range three-dimensional ferromagnetic ordering with a drastic variation of the critical temperature as a function of the guest molecule [T(C)2.0 K (CO(2)@iso1 and CH(4)@iso1) and T(C) = 6.5 (CH(3)OH@iso1) and 21.0 K (H(2)O@iso1)].
A Model of Smart G-Quadruplex Ligand
2012
An unprecedented strategy to control the quadruplex- vs duplex-DNA selectivity of a ligand is reported. We designed a compound whose structure can rearrange when it interacts with a G-quadruplex, thereby controlling its affinity. Thus, the first "smart G-quadruplex ligand" is reported, since this ligand experiences a structural change in the presence of quadruplexes but not in the presence of duplexes, ensuring a high level of quadruplex selectivity.
Reaction between Indazole and Pd-Bound Isocyanides-A Theoretical Mechanistic Study
2018
The mechanism of the addition of indazole (Ind)&mdash
Structure of Rhodococcus erythropolis limonene-1,2-epoxide hydrolase reveals a novel active site
2003
Epoxide hydrolases are essential for the processing of epoxide-containing compounds in detoxification or metabolism. The classic epoxide hydrolases have an alpha/beta hydrolase fold and act via a two-step reaction mechanism including an enzyme-substrate intermediate. We report here the structure of the limonene-1,2-epoxide hydrolase from Rhodococcus erythropolis, solved using single-wavelength anomalous dispersion from a selenomethionine-substituted protein and refined at 1.2 A resolution. This enzyme represents a completely different structure and a novel one-step mechanism. The fold features a highly curved six-stranded mixed beta-sheet, with four alpha-helices packed onto it to create a …
Tandem Nucleophilic Addition−Intramolecular Aza-Michael Reaction: Facile Synthesis of Chiral Fluorinated Isoindolines
2010
A highly stereoselective synthesis of fluorinated 1,3-disubstituted isoindolines is described. To this end, a tandem reaction consisting of a diastereoselective addition of fluorinated nucleophiles to Ellman's N-(tert-butanesulfinyl)imines followed by an intramolecular aza-Michael reaction has been developed. This strategy allows for the construction of isoindolines bearing several degrees of fluorination (mono-, di-, or trifluoromethyl as well as heavier fluorinated groups). In the majority of all cases, the products are formed as single isomers.