Search results for "Selectivity"
showing 10 items of 1148 documents
Investigations on isomerization and rearrangement of polycyclic arenes under oxidative conditions – Anodic versus reagent-mediated reactions
2019
Abstract Electro-organic conversions at an active molybdenum anode enable the formation of fused arenes. High chemoselectivity was achieved under anodic conditions, and a reagent-induced selectivity was observed by comparison with results of MoCl5-mediated reactions. Polycyclic arenes like phenanthrenes, triphenylenes, chrysenes, or helicenes were selectively obtained in yields up to 87% and in some cases unusual rearrangements were crucial for the product formation.
Catalytic Dehydration of Fructose to 5-Hydroxymethylfurfural in Aqueous Medium over Nb2O5-Based Catalysts
2021
The catalytic dehydration of fructose to 5-hydroxymethylfurfural (HMF) in water was performed in the presence of pristine Nb2O5 and composites containing Nb and Ti, Ce or Zr oxides. In all experiments, fructose was converted to HMF using water as the solvent. The catalysts were characterized by powder X-ray diffraction, scanning electron microscopy, N2 physical adsorption, infrared and Raman spectroscopy and temperature-programmed desorption of NH3. Experimental parameters such as fructose initial concentration, volume of the reacting suspension, operation temperature, reaction time and amount of catalyst were tuned in order to optimize the catalytic reaction process. The highest selectivit…
Direct Synthesis of Aluminosilicate IWR Zeolite from a Strong Interaction between Zeolite Framework and Organic Template.
2019
A large amount of zeolite structures are still not synthetically available or not available in the form of aluminosilicate currently. Despite significant progress in the development of predictive concepts for zeolite synthesis, accessing some of these new materials is still challenging. One example is the IWR structure as well. Despite successful synthesis of Ge-based IWR zeolites, direct synthesis of aluminosilicate IWR zeolite is still not successful. In this report we show how a suitable organic structure directing agent (OSDA), through modeling of an OSDA/zeolite cage interaction, could access directly the aluminum-containing IWR structure (denoted as COE-6), which might allow access to…
Unveiling the Chemistry of Higher-Order Cycloaddition Reactions within the Molecular Electron Density Theory
2022
The higher-order cycloaddition (HOCA) reaction of tropone with cyclopentadiene (Cp) has been studied within the Molecular Electron Density Theory. The Electron Localization Function (ELF) analysis of the electronic structure of tropone and Cp characterizes the structural behaviors of the two conjugated unsaturated systems, while the conceptual DFT reactivity indices classify tropone as a strong electrophile and Cp as a strong nucleophile participating in polar cycloaddition reactions of reverse electron density flux. Eight competitive reaction paths have been characterized for this cycloaddition reaction. The most favorable one allowing the formation of the formal out [6 + 4] cycloadduct ha…
Lipase-catalysed preparation of acyl derivatives of the germacranolide cnicin
2009
Several acyl derivatives of cnicin were obtained through lipase-catalysed acylation and alcoholysis reactions. In most reactions lipases showed a regioselective behaviour affording only one product. Longer chain acyl derivatives were prepared at lower temperature than the used in lipase-catalysed reactions, to preclude side products formation. The enzymatic approach let to prepare a family of novel acetyl and fatty acid derivatives of cnicin which are not obtainable following traditional organic synthetic procedures. Fil: Monsalve, Leandro Nicolas. Consejo Nacional de Investigaciones Científicas y Técnicas. Oficina de Coordinación Administrativa Ciudad Universitaria. Unidad de Microanálisis…
Food pickiness in the elderly: Relationship with dependency and malnutrition
2014
Special Issue: 5th European Conference on Sensory and Consumer Research (Eurosense) 2012 "A sense of inspiration"; International audience; Among factors contributing to malnutrition in the elderly, the present study aimed at assessing the impact of food selectivity (also referred as food "pickiness") on the nutritional status of the elderly. A survey with 559 French people over 65 years old was conducted to collect data on food selectivity, dependency and nutritional status. Food selectivity was assessed by asking respondents to tick each food they dislike among a list of familiar foods. Since some foods could be ticked as disliked because elderly people experience physical difficulties in …
On the Mechanistic Origins of the pH-Dependency in Au-Catalyzed Glycerol Electro-Oxidation: Insight from First Principles Calculations
2021
Electrocatalytic oxidation of glycerol (EOG) is an attractive approach to convert surplus glycerol to value-added products. Experiments have shown that EOG activity and selectivity depend on the electrocatalyst, but also on the electrode potential, the pH, and the electrolyte. For broadly employed gold (Au) electrocatalysts, experiments have demonstrated high EOG activity under alkaline conditions with glyceric acid as a primary product, whereas under acidic and neutral conditions Au is rather inactive producing only small amounts of dihydroxyacetone. In the present computational work, we have performed an extensive mechanistic study to understand the pH- and potential-dependency of Au-cata…
Stereoselective synthesis of the naturally occurring styryllactones (+)-goniofufurone and (+)-cardiobutanolide.
2004
The naturally occurring gamma-lactones (+)-goniofufurone 1 and (+)-cardiobutanolide 2, two pharmacologically active products from Goniothalamus species (Annonaceae), have been synthesized in enantiopure form using l-erythrulose as the chiral starting material. Key steps of these syntheses were a stereoselective anti boron aldol reaction and an asymmetric allylboration.
3.13 Carbohydrate Derived Auxiliaries: Amino Sugar and Glycosylamine Auxiliaries
2012
The stereodifferentiating potential of carbohydrate auxiliaries is illustrated by examples of oxazolidinones derived from amino sugars and aldimines of glycosylamines. Taking advantage of the bicyclic structure of amino sugar-derived oxazolidinones, reactions of their unsaturated or deprotonated N -acyl derivatives show an efficient exo - versus endo -differentiation. High diastereoselectivity is achieved in the syntheses of β -branched or α -substituted carboxylic acid derivatives. Imines of glycosylamines, in particular of 2,3,4,6-tetra- O -pivaloyl- β - d -galactopyranosylamine, are characterized by a conformation stabilized through the exo -anomeric effect and by their ability to coordi…
Highly regioselective synthesis of a 3-O-sulfonated arabino Lewisa asparagine building block suitable for glycopeptide synthesis
2006
Using the stannylene method, the trisaccharide 2-acetamido-3-O-[6-O-benzyl-beta-D-galactopyranosyl]-4-O-[2,3,4-tri-O-benzyl-beta-D-arabinopyranosyl]-6-O-benzyl-2-deoxy-beta-D-glucopyranosyl azide was regioselectively sulfonated and, after reduction of the anomeric azide, coupled to Fmoc alpha-allyl aspartate. After Pd(0)-catalyzed deallylation, the sulfatyl Lewis(a) asparagine building block was obtained, suitable for solid-phase glycopeptide synthesis applying the fluoride labile PTMSEL linker system.