Statistical mixture design selective extraction of compounds with antioxidant activity and total polyphenol content from Trichilia catigua

Statistical design mixtures of water, methanol, acetone and ethanol were used to extract material from Trichilia catigua (Meliaceae) barks to study the effects of different solvents and their mixtures on its yield, total polyphenol content and antioxidant activity. The experimental results and their response surface models showed that quaternary mixtures with approximately equal proportions of all four solvents provided the highest yields, total polyphenol contents and antioxidant activities of the crude extracts followed by ternary design mixtures. Principal component and hierarchical clustering analysis of the HPLC–DAD spectra of the chromatographic peaks of 1:1:1:1 water–methanol–acetone–ethanol mixture extracts indicate the presence of cinchonains, gallic acid derivatives, natural polyphenols, flavanoids, catechins, and epicatechins.     

Chemistry of novolac resins. X. Polymerization studies of HMTA and strategically synthesized model compounds

The reactions between hexamethylenetetramine (HMTA) and compounds which model novolac resins have been studied by ¹³C‐ and ¹⁵N‐NMR techniques. The dimer and tetramer compounds vary in molecular size and structure and react with HMTA to yield benzylamines and benzoxazine as the major initial‐formed intermediates and convert to methylene linked compounds at increased temperatures. The reaction of the compounds with only ortho reactive sites paralleled the 2,4‐xylenol–HMTA case reported by us previously; however, increasing molecular weight favored the formation of benzylamines and not benzoxazines. Those compounds with only para reactive sites paralleled the 2,6‐xylenol–HMTA case. The reactivity of the systems containing both ortho and para reactive sites depends on the ratio of ortho/para sites and various aspects such as the chemical structure and molecular weight of the compound, the HMTA level, and the melting point and pH of the system. These results parallel those obtained from novolac/HMTA systems. The xylenol/HMTA reactions formed similar products but showed quite different relative reaction rates by varying the HMTA ratio and structures of the materials. The importance of careful selection of model systems is also discussed. © 1999 John Wiley & Sons, Inc. J Polym Sci A: Polym Chem 37: 1347–1355, 1999     

Efficient Filtering and Tabu Search on a Consistent Neighbourhood for the Frequency Assignment Problem with Polarisation

This article presents a specific filtering algorithm for the Frequency Assignment Problem with Polarisation, which combines arc-consistency and path-inverse-consistency adapted to the specificities of the constraints. The effectiveness of this filtering algorithm enabled us to improve the Tabu Search on a Consistent Neighbourhood (CN-Tabu) using two different approaches. So, after a short recall of this general methodology and a presentation of its obtained results on the FAPP, we propose a behavioural study of the two approaches by comparing the results.     

Access to diversely α-substituted cyclopentenones from α-chlorocyclopentenones

α-Chlorocyclopentenones can readily be transformed into a variety of α-substituted cyclopentenones via their dimethyltrimethylene acetals.     

A molecular orbital study of bond energies in compounds of sulfur and fluorine

CNDO/2 molecular orbital theory is employed in a study of the binding energies of the molecules SF, SF₂, SF₄, SF₆, their cations and anions, and of the molecules SSF₂, FSSF and S₂F₁₀. Computed energies, when rescaled according to energy partitioning concepts, compare favorably with available experimental data. Ionization energies and electron affinities are calculated for SF, SF₂, SF₄ and SF₆.

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Zusammenfassung Mit Hilfe der CNDO/2 Theorie werden die Bindungsenergien der SF, SF₂, SF₄ und SF₆ Moleküle, von deren positiven und negativen Ionen und von SSF₂, FSSF und S₂F₁₀ berechnet. Die berechneten Energien stimmen gut mit experimentellen Daten überein, wenn sie nach Energieaufteilungsprinzipien wiederberechnet werden. Ferner werden die Ionisierungsenergien und Elektronenaffinitäten für SF, SF₂, SF₄ und SF₆ angegeben.

     

Synthesis of Reversed C‐Acyl Glycosides through Ni/Photoredox Dual Catalysis

The incorporation of C‐glycosides in drug design has become a routine practice for medicinal chemists. These naturally occurring building blocks exhibit attractive pharmaceutical profiles, and have become an important target of synthetic efforts in recent decades. 1 Described herein is a practical, scalable, and versatile route for the synthesis of non‐anomeric and unexploited C‐acyl glycosides through a Ni/photoredox dual catalytic system. By utilizing an organic photocatalyst, a range of glycosyl‐based radicals are generated and efficiently coupled with highly functionalized carboxylic acids at room temperature. Distinctive features of this transformation include its mild conditions, impressive compatibility with a wide array of functional groups, and most significantly, preservation of the anomeric carbon: a handle for further, late‐stage derivatization.     

Spray-drying as a tool to disperse conductive carbon inside Na<Subscript>2</Subscript>FePO<Subscript>4</Subscript>F particles by addition of carbon black or carbon nanotubes to the precursor solution

In this work, Na₂FePO₄F-carbon composite powders were prepared by spray-drying a solution of inorganic precursors with 10 and 20 wt% added carbon black (CB) or carbon nanotubes (CNTs). In order to compare the effect of CB and CNT when added to the precursor solutions, the structural, electrochemical, and morphological properties of the synthesized Na₂FePO₄F-xCB and Na₂FePO₄F-xCNT samples were systematically investigated. In both cases, X-ray diffraction shows that calcination at 600 °C in argon leads to the formation of Na₂FePO₄F as the major inorganic phase. ⁵⁷Fe Mössbauer spectroscopy was used as complementary technique to probe the oxidation states, local environment, and identify the composition of the iron-containing phases. The electrochemical performance is markedly better in the case of Na₂FePO₄F-CNT (20 wt%), with specific capacities of about 100 mAh/g (Na₂FePO₄F-CNT) at C/4 rate vs. 50 mAh/g for Na₂FePO₄F-CB (20 wt%). SEM characterization of Na₂FePO₄F-CB particles revealed different particle morphologies for the Na₂FePO₄F-CNT and Na₂FePO₄F-CB powders. The carbon-poor surface observed for Na₂FePO₄F-CB could be due to a slow diffusion of carbon in the droplets during drying. On the contrary, Na₂FePO₄F-CNT shows a better CNT dispersion inside and at the surface of the NFPF particles that improves the electrochemical performance.     

Synthesis and inhibition properties of conformational probes for the mutase-catalyzed UDP-galactopyranose/furanose interconversion

UDP-galactose mutase is a flavoenzyme that catalyzes the isomerization of UDP-galactopyranose into UDP-galactofuranose, a key step in the biosynthesis of important bacterial oligosaccharides. Several mechanisms for this unique ring-contraction have been proposed, one of them involving a putative 1,4-anhydrogalactopyranose as an intermediate in the reaction. The purpose of this study was to probe the mutase binding site with conformationally restricted analogues of its substrate. Thus, we describe the straightforward synthesis of two C-glycosidic UDP-galactose derivatives: analogue 1, presenting a galactose moiety locked in a bicyclic (1,4)B boat conformation, and UDP-C-Galf 2, where the galactose residue is locked in the conformation of the mutase substrate. The two molecules were found to be inhibitors of UDP-galactose mutase at levels depending on the redox state of the enzyme. Strong inhibition of the native enzyme, but a low one of the reduced mutase, were observed with UDP-C-Galf 2, whereas 1 displayed intermediate inhibition levels under both native and reducing conditions. These data provide evidence of a significant conformational difference of the mutase binding pocket in the reduced enzyme and in the native one, the enzyme switching from a low Galf-affinity state (reduced enzyme) to a very strong one (native enzyme). It is remarkable that the mutase binds the boat-locked analogue 1 with similar affinities in both its conformational states. These results support a mechanism involving the formation of 1,4-anhydrogalactopyranose as a low-energy intermediate. An alternative explanation would be that the distortion of the galactose moiety during the cycle contraction transiently brings the carbohydrate into a conformation close to a (1,4)B boat.     

Postcondensation and oxidation processes in molten polyamide 6

Polyamide samples were heated under vacuum or mixed in a Brabender plastograph. UV absorbance, chain end concentration, and molecular weight were studied. Postcondensation was observed for polyamide heated under vacuum. For polyamide samples mixed in the plastograph, atmosphere, shear rate, and temperature changed. Melt viscosity and intrinsic viscosity are in good correlation in a log-log plot. Oxidation effect on molecular weight and amine chain end concentration could be well related to UV absorbance. The oxygen diffusion into the molten polyamide is a critical parameter. The oxygen concentration in the polyamide mixed under air is ca. 20 times higher than when mixed under nitrogen. The introduction of preoxidized material in the molten polyamide or mixing in the presence of oxygen have similar effects. Postcondensation and oxidation strongly influence the melt behavior of polyamides. © 1993 John Wiley & Sons, Inc.     

Etude physico-chimique de poly(methyl-indenes) prepares par polymerisation cationique

Poly(4,7-dimethyl indene) and poly(6-methyl indene) have been fractionated by selective precipitation and preparative G.P.C. Fractions have been studied by various physical methods, chiefly osmometry, light-scattering, viscometry and analytical G.P.C. This study has been made in a “good solvent” and led to Mark-Houwink relations for the polymers. Moreover, the determination of radii of gyration and branching indices indicates that there are long branches, particularly in the case of poly(4,7-dimethyl indene). They are very probably produced through Friedel-Crafts attacks on phenyl rings by carbocations of growing chains. The fact that these attacks are much easier with 4,7-dimethyl indene is in perfect agreement with previous quantum chemistry calculations.