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排序方式: 共有107条查询结果,搜索用时 250 毫秒
1.
Communicated by J. B. McLeod 相似文献
2.
Garner P Anderson JT Cox PB Klippenstein SJ Leslie R Scardovi N 《The Journal of organic chemistry》2002,67(17):6195-6209
The development of an effective chiral auxiliary for hydroxyalkyl radicals is delineated. Both the 2-tetrahydropyranyl (THP) and tri-O-benzyl-2-deoxy-alpha-D-glucopyranosyl (GLU) auxiliaries resulted in diastereoselective radical additions to methyl acrylate at -78 degrees C (ds = 6/1 and 11/1, respectively). The developing stereochemistry at the radical center was completely under auxiliary control. Correlation experiments showed that the D-GLU auxiliary led to attack on the radical Si-face. The selectivity of these radical additions dropped-off considerably when the more reactive 2-nitropropene trap was employed. Computational studies suggested that the observed facial selectivity was due primarily to entropic factors in the transition state but that a smaller temperature-dependent enthalpic contribution was also involved. It was hypothesized that incorporation of a quaternary center at C-6 (THP numbering) would restore the facial selectivity with more reactive radical traps by restricting the orientations available to the incoming alkene. In the event, the trans-6-tert-butyltetrahydropyranyl (tBu-THP) auxiliary resulted in very good diastereoselection with 2-nitropropene (ds = 35/1 at -78 degrees C, 15/1 at 0 degrees C, and 8/1 at RT) as did the tri-O-benzyl-6,6-dimethyl-2-alpha-D-deoxyglucopyranosyl (diMe-GLU) auxiliary during additions to ethyl alpha-trifluoroacetoxyacrylate (ds = 10/1 at 0 degrees C). A protocol for recovery of the sugar-derived chiral auxiliaries was also established. This work sets the stage for the development of a novel approach to 1, 3, 5.(2n + 1) polyols based on iterative radical homologation as well as the application of these pyranosidic auxiliaries to other synthetically important reactions. 相似文献
3.
Alessandro D'Aprano Camillo La Mesa Noemi Proietti Bianca Sesta Sonia Tatone 《Journal of solution chemistry》1994,23(12):1331-1346
The interactions between octyl--D-glucoside and glycine in water have been investigated by surface tension, viscosity, and density measurements. The results show that the -amino acid causes an unexpected lowering of the critical micellar concentration of octyl--D-glucoside. Such a finding has been interpreted in temss of dipole-dipole interactions between the hydrophilic site of the surfactant and the peptidic cosluttes. From three to seven amino acid molecules have been estimated to be coordinated with each glucoside unity in the micellar state. The research has been extended to glycine oligopeptides and L-lysine. The latter compound has effects similar to those observed with glycine whereas diglycine and triglycine show weaker effects on the micellization process. 相似文献
4.
Enantioselective addition of aryllithium reagents to aromatic imines mediated by 1,2-diamine ligands
A variety of optically enriched amines have been obtained by addition of aryllithium reagents to aromatic imines using N,N′-tetramethylcyclohexane-1,2-diamine as chiral ligands. Enantiomeric excesses up to 90% could be obtained. 相似文献
5.
Noemi Merayo Ana Balea Elena de la Fuente Ángeles Blanco Carlos Negro 《Cellulose (London, England)》2017,24(2):677-692
Although the positive effect that cellulose nanofibers (CNF) can have on paper strength is known, their effect on flocculation during papermaking is not well understood, and most relevant studies have been carried out in presence of only cationic starch. Flocculation is the key to ensuring retention of fibers, fines, and fillers, and furthermore floc properties have a great influence on paper quality. The aim of this research is to study the interactions between CNF and flocculants by assessing the effect of two types of CNF, from eucalyptus and corn, on the flocculation process induced by three different retention systems [a dual system, polyvinylamine (PVA), and cationic starch as reference]. The results showed that CNF interacted with the flocculants in different ways, affecting flocculation efficiency and floc properties. In general, addition of CNF increased floc stability and minimized overdosing effects. Moreover, presence of CNF increased floc size for given PVA dose; therefore, CNF addition could contribute to improve the wet end in the paper machine if combined with the optimal flocculant and dose. 相似文献
6.
Wilson C. Brooks Noemi D. Paguigan Huzefa A. Raja Franklin J. Moy Nadja B. Cech Cedric J. Pearce Nicholas H. Oberlies 《Magnetic resonance in chemistry : MRC》2017,55(7):670-676
Analysis of complex mixtures is a common challenge in natural products research. Quantitative nuclear magnetic resonance spectroscopy offers analysis of complex mixtures at early stages and with benefits that are orthogonal to more common methods of quantitation, including ultraviolet absorption spectroscopy and mass spectrometry. Several experiments were conducted to construct a methodology for use in analysis of extracts of fungal cultures. A broadly applicable method was sought for analysis of both pure and complex samples through use of an externally calibrated method. This method has the benefit of not contaminating valuable samples with the calibrant, and it passed scrutiny for line fitting and reproducibility. The method was implemented to measure the yield of griseofulvin and dechlorogriseofulvin from three fungal isolates. An isolate of Xylaria cubensis (coded MSX48662) was found to biosynthesize griseofulvin in the greatest yield, 149 ± 8 mg per fermentation, and was selected for further supply experiments. Copyright © 2016 John Wiley & Sons, Ltd. 相似文献
7.
Dr. Noemi Linares Dr. Erika De Oliveira Jardim Dr. Geetu Sharma Dr. Elena Serrano Prof. Alexandra Navrotsky Prof. Dr. Javier García-Martínez 《Chemistry (Weinheim an der Bergstrasse, Germany)》2019,25(43):10045-10048
With the aim of understanding the thermochemistry of the introduction of mesoporosity in zeolites by using surfactants, high temperature oxide melt solution calorimetry was used to determine the change in the enthalpy of formation of USY zeolite before and after the introduction of mesoporosity. Our results confirm that this process only slightly destabilizes the zeolite by the additional surface area. However, this can be overcome by the stabilizing effect of the interactions between the surfactant and the zeolite framework. 相似文献
8.
Noemi Bognanni Francesco Bellia Maurizio Viale Nadia Bertola Graziella Vecchio 《Molecules (Basel, Switzerland)》2021,26(6)
Over the years, cyclodextrin uses have been widely reviewed and their proprieties provide a very attractive approach in different biomedical applications. Cyclodextrins, due to their characteristics, are used to transport drugs and have also been studied as molecular chaperones with potential application in protein misfolding diseases. In this study, we designed cyclodextrin polymers containing different contents of β- or γ-cyclodextrin, and a different number of guanidinium positive charges. This allowed exploration of the influence of the charge in delivering a drug and the effect in the protein anti-aggregant ability. The polymers inhibit Amiloid β peptide aggregation; such an ability is modulated by both the type of CyD cavity and the number of charges. We also explored the effect of the new polymers as drug carriers. We tested the Doxorubicin toxicity in different cell lines, A2780, A549, MDA-MB-231 in the presence of the polymers. Data show that the polymers based on γ-cyclodextrin modified the cytotoxicity of doxorubicin in the A2780 cell line. 相似文献
9.
10.
[reaction: see text] Elusive nitroacetic acid esters and amides were obtained through a halogen exchange reaction of the corresponding bromoacetic acid derivatives with polymer-supported nitrite anion. The process is flawed by a side product catalyzed degradation of the products. Domino processes turned out to be a powerful tool for overcoming such drawbacks, converting a substandard reaction into an efficient multicomponent preparation of 4-hydroxy-4,5-dihydroisoxazoles. 相似文献