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21.
Inside Cover: Exploiting Aromatic Interactions for β‐Peptide Foldamer Helix Stabilization: A Significant Design Element (Chem. Eur. J. 16/2014)
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A Phillips type high density polyethylene was extruded six times without additives and the compression molded plates prepared from the granules were stored in distilled water for 12 months. Specimens withdrawn from the containers at regular intervals were thoroughly characterized with various methods including the determination of weight changes, color, MFI, functional group content (FTIR), molecular weight (GPC), thermal (DSC) and mechanical (tensile) properties. The results proved that all reactions taking place during the storage of HDPE in distilled water are related to each other; the correlation of all functional groups formed or consumed in them is surprisingly close. The amount of oxygen present determines the direction of reactions, larger oxygen content leads to chain scission, to an increase of methyl content and to the formation of carbonyl groups. Most of these reactions go through double bonds, their number decreases during storage. In spite of the large number of reactions proposed in the literature, only one or two dominating reactions determine the changes in the chain structure of the polymer and thus the properties of the final product under the conditions of this study. Any variation in the conditions of storage is reflected in the properties of the polymer. Stabilizers used under extractive conditions must be stable against hydrolysis and should trap oxygen centered radicals. 相似文献
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The first direct enzymatic method is reported for the synthesis of cis and trans beta-amino acid enantiomers through the lipase-catalyzed enantioselective hydrolysis of alicyclic beta-amino esters in organic media. High enantioselectivities (E usually >100) were observed when the Candida antarctica lipase B catalyzed reactions were performed with H2O (0.5 equivalents) in iPr2O at 65 degrees C. The resolved products, obtained in good yields (>or=42%), could be easily separated. 相似文献
24.
Conformationally restricted, orthogonally protected 2,4-diaminocarboxylates with a cyclopentane skeleton were efficiently synthesized from beta-lactam 6, the syntheses involving strategies of diastereoselective epoxidation of the beta-lactam and the corresponding monoprotected amino esters with opposite selectivities followed by regioselective opening of the oxirane ring with sodium azide. The enantiomers were also prepared. This new class of compounds can be regarded not only as conformationally constrained beta,gamma-diamino acid derivatives but also as potential functionalized carbocyclic nucleoside precursors. 相似文献
25.
Isoxazoline-fused 2-aminocyclopentanecarboxylate derivatives were regio- and stereo-selectively synthesized by nitrile oxide 1,3-dipolar cycloaddition to cis- or trans-ethyl-2-aminocyclopent-3-enecarboxylates. The compounds were prepared in enantiomerically pure form by enzymatic resolution of the racemic bicyclic β-lactam. 相似文献
26.
Racemic 1-methyl-6,7-dimethoxy-1,2,3,4-tetrahydroisoquinoline 1 and 1-methyl-1,2,3,4-tetrahydro-ß-carboline 3 were resolved through lipase-catalysed asymmetric acylation on the secondary amino group. High enantioselectivities (E?>200) were observed when the acylation of racemic 1 was performed with phenyl allyl carbonate in the presence of Candida rugosa lipase in toluene at 40?°C or with Candida antarctica lipase B in tert-butyl methyl ether at 50?°C. Excellent enantioselectivity (E?>200) characterised the CAL-B-catalysed acylation of racemic 3 with phenyl allyl carbonate in the presence of triethylamine in tert-butyl methyl ether at 50?°C. The product (R)-carbamates (ee?>97%) were hydrolysed into the corresponding (R)-enantiomers of the free amines 1 and 3 (ee?=?99%) with the use of Pd2(dba)3·CHCl3 catalyst. 相似文献
27.
Zoltán Demjén Béla Pukánszky Enik? Földes József Nagy 《Journal of colloid and interface science》1997,190(2):427
A study of eight silane coupling agents showed very different effect of these compounds on the mechanical properties of PP/CaCO3composites. The application of aminofunctional silane coupling agents resulted in the reactive coupling of the two inactive components leading to increased strength and decreased deformability. A detailed study of the interaction between CaCO3and the various coupling agents was carried out in order to find an explanation for the strong coupling effect. The amount of coupling agent creating a monolayer coverage was determined by a dissolution method for each coupling agent. The obtained values changed between 0.3 and 1.0 wt% calculated for the CaCO3. An attempt was made to determine the orientation of the adsorbed molecules to the filler surface. Most of the coupling agents are oriented perpendicularly to the surface with the exception of a methacryl functional silane compound. Possible interactions between hydrolyzed or condensed silane coupling agents and the filler were studied by Fourier transform infrared spectroscopy using transmitting (FTIR-TS) and diffuse reflectance (DRIFT) modes, as well as gel permeation chromatography (GPC). The results showed that bulky organofunctional groups form a caged, polycyclic, low-molecular-weight structure on the surface, while silanes with smaller groups tend to condense into open, ladder type, high-molecular-weight polysiloxane chains. Polymer/filler adhesion, however, depends primarily on the chemical character of the organofunctional group. Aminofunctional silane coupling agents adhere well to the filler surface and react also with the polymer. In the case of similar functionality the size of the organofunctional group determines the strength of the adhesion. 相似文献
28.
In the frame of substrate specificity, CAL-B-catalysed asymmetric N-alkoxycarbonylations of 1-substituted tetrahydro-ß-carbolines (Me, Et, Pr, iPr) have been studied. High enantioselectivities (>200) were observed, when alkoxycarbonylation of racemic compounds (±)-1,3,5,7 were performed in DIPE in the presence of phenyl allyl carbonate and Et3N at 60?°C using ultrasound shaking method. The reaction time increased considerably with increasing substituent size on C1; however, the isopropyl-substituted compound proved to be too bulky for the optimum activity of CAL-B. The (R)-carbamate enantiomers were hydrolysed using Pd2(dba)3.CHCl3 and the enantiomers of the free amines were obtained with excellent ee (>99%). 相似文献
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Háznagy-Radnai Erzsébet Léber Petra Tóth Enikő Janicsák Gábor Máthé Imre 《平面色谱法杂志一现代薄层色谱法》2005,18(4):314-318
JPC – Journal of Planar Chromatography – Modern TLC - Stachys palustris L. (Lamiaceae) contains the iridoids harpagide and acetylharpagide. The presence of other components, for example... 相似文献