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101.
Hammed H. A. M. Hassan 《Central European Journal of Chemistry》2005,3(4):803-829
A concise method for a stereocontrolled synthesis of a set of selectively protected disaccharides is reported. Coupling of
the donor 11 onto acceptors 23 and 24, promoted by trimethylsilyl triflate-N-iodosuccinimide (TMSOTf-NIS), generated the disaccharides 25 and 26. Under typical conditions, condensation of the fully protected donor 12 onto acceptors 23 and 24 produced the disaccharides 27 and 28. The building blocks 25–28 were prepared in moderate yields having exclusive β-stereoselectivity. A unique pattern of protecting groups distinguished
clearly between positions to be sulfated and functional groups remaining as free hydroxyl groups. Acetyl and/or levulinoyl
esters temporarily protected the positions to be sulfated, while benzyl ethers were used for permanent protection. The anomeric
positions were protected as allyl ethers, whereas the 4′-positions were masked as p-methoxybenzyl (PMB) ethers. The orthogonality of the PMB and allyl groups can then be used for further elongation of the
chain by recurrent deprotection and activation steps. The hydroxyl group, OH-6, of glucosamine moieties was protected as a
TBDPS ether to avoid oxidation. A five-step deprotection/sulfonation sequence was applied to the disaccharide 27 to generate the corresponding sulfated [β-D-GlcUA-2-OSO3Na-(1→4)-β-D-Glc pNAc]-(1→O-Pro) 34. 相似文献
102.
Nagwa Rashed Abdel Moneim El Massry El Sayed H. El Ashry Adel Amer Hans Zimmer 《Journal of heterocyclic chemistry》1990,27(3):691-694
A series of 1-aryl-s-triazolo[4,3-a]quinoxalin-4-ones, 3 , were synthesized via the pyrolysis of the corresponding hydrazones, 6 . Thus, the cyclodehydrogenation occurred by refluxing them in an inert solvent (e.g. ethylene glycol) to give the triazoloquinoxalin-4-ones in a satisfactory yield. Using DMSO as a solvent for the above transformation afforded as a minor by-product an S-ylid. In contrast to earlier findings, annelation of a six-membered ring was successful and achieved through the pyrolysis of the pyruvate hydrazones derived of the quinoxalin-4-ones at ?230° to give the as-triazino[4,3-a]quinoxalin-5-ones, 4 . The reaction of 5 with acetylacetone afforded 3-(3′,5′-dimethylpyrazol-1-yl)-2(1H)-quinoxalinone, 10 . The structural assignments for the new compounds were based on their elemental analysis and spectroscopic data as well as an independent synthesis. 相似文献
103.
Mohamed El Guendouzi Abderrahim Dinane Abdelfetah Mounir 《Journal of solution chemistry》2002,31(2):119-129
The mixed aqueous electrolyte system of ammonium and lithium chlorides has been studied by the hygrometric method at 25°C. The relative humidities of this system are measured at total molalities from 0.3 to 6 mol-kg– 1 for different ionic-strength fractions y of NH4Cl with y = 0.33, 0.50, and 0.67. The data obtained allow the deduction of new water activities and osmotic coefficients. The experimental results are compared with the predictions of the ECA (extended composed additivity) law proposed in our previous work. The Zdanovskii–Stokes–Robinson (ZSR), the Robinson–Stokes (RS), Reilly–Wood–Robinson (RWR), the Pitzer, and the Lietzke–Stoughton (LS II) models are also compared with our results. Predictions made using these models are, in general, consistent with our results. From these measurements, new Pitzer mixing ionic parameters are determined and used to predict the solute activity coefficients in the mixture for different ionic-strength fractions. 相似文献
104.
105.
The ability of titanium(IV) chloride as a catalyst to promote the Pechmann condensation reaction with a range of phenols and β-keto esters is described.The reaction was carried out by addition of TiCl4 to a mixture of the phenol and the β-keto ester with thorough stirring in the absence of a solvent and represents an improvement on the classical Pechmann conditions. The yields of coumarins obtained via this novel protocol were significantly higher than those using the conventional method and the reaction duration was reduced to a few minutes or even a few seconds. 相似文献
106.
Atomic hydrogen electrosorption is reported at crystallite sites of polyacrylate-capped Pt nanoparticles (d = 2.5 +/- 0.6 nm), by assembling nanostructured electrodes of polyacrylate-Pt nanocrystallites layer-by-layer in a cationic polyelectrolyte, poly(diallyldimethylammonium chloride). Cyclic voltammetry in 1 M H2SO4 revealed a strongly adsorbed hydrogen state and a weakly adsorbed hydrogen state assigned to adsorption at (100) and (110) sites of the modified nanocrystallites, respectively. Resolving hydrogen adsorption states signifies that surface capping by the carboxylate groups is not irreversibly blocking hydrogen adsorption sites at the modified Pt nanoparticle surface. Adsorption peak currents increased with increasing the number of layers up to 16 bilayers, indicating the feasibility of nanoparticle charging via interparticle charge hopping and the accessibility of adsorption states within the thickness of the nanoparticle/polyelectrolyte multilayers. Despite similarity in hydrogen adsorption in the cyclic voltammorgrams in 1 M H2SO4, negative shifts in adsorption potentials were measured at the nanocrystallite Pt-polyelectrolyte multilayers relative to a polycrystalline bulk Pt surface. This potential shift is attributed to a kinetic limitation in the reductive hydrogen adsorption as a result of the Pt nanoparticle surface modification and the polyelectrolyte environment. 相似文献
107.
A series of potential indicator dyes is evaluated for use in the development of optical sensors for measuring sulfur dioxide in gaseous samples. Rhodamine B isothiocyanate is selected on the basis of relative sensitivity to dynamic quenching by sulfur dioxide and oxygen. A solid-state fluorometer is described for monitoring the sulfur dioxide induced fluorescence quenching of sensing membranes composed of silicone and rhodamine B isothiocyanate. A modulated blue LED is coupled with the lock-in detection of a photodiode detector to provide high signal-to-noise ratios. The limit of detection is 0.114+/-0.009% for sulfur dioxide in a carrier stream of nitrogen gas. Selectivity measurements indicate no interference from several common gases (HCl, NH(3), NO, and CO(2)). Oxygen alters the sensor response when comparing signals for sulfur dioxide in 0, 20 and 100% oxygen environments. 相似文献
108.
New simple, rapid, accurate, and selective methods are described for microdetermination of the nitro and nitroso groups in organic compounds. These are based on reduction with cadmium metal and 0.05 M HCl whereby 6 and 4 equivalents of the cadmium ions are released per nitro and nitroso groups, respectively. The cadmium ions are measured by atomic absorption spectrometry at 228.8 nm, potentiometrically by titration with EDTA using the cadmium ion selective electrode, and visually by EDTA titration using Eriochrome Black T indicator. Results with an average recovery of 98% and a mean standard deviation of 1.3% are obtainable and no interferences are caused by many nitrogen, oxygen, and sulfur functional groups. 相似文献
109.
Up to 100 microg of sample can be collected from gold or silver alloys by rubbing the specimen with the ground hemispherical tip of a 4-mm Pyrex glass rod. Gold alloys are then dissolved in potassium cyanide solution containing hydrogen peroxide; silver alloys are exposed to vapours of nitric acid. Procedures for transfer, ring oven separation and identification of alloy constituents in the sample solutions are described. 相似文献
110.
Zusammenfassung Zu den schwierigsten Aufgaben der toxikologischen Analyse gehört die rasche und sichere Isolierung und Identifizierung organischer Basen aus dem Untersuchungsmaterial. Mit einem entsprechenden Extraktionsverfahren oder durch Fällung mit Tetraphenylborat in einem systematischen Analysengang können sehr günstige Ausbeuten erzielt werden. Die Reinigung der Rohextrakte oder -fällungen erfolgt prinzipiell mittels Dünnschichtchromatographie. Aus den Tetraphenylboratkomplexen werden dabei auf den Kieselgelschichten die reinen Basen in Freiheit gesetzt. An Hand derR
f- Wert-Tabellen ist eine Identifizierung möglich, wobei Substanzen mit gleichemR
f- Wert durch fraktionierte Extraktion bei verschiedenem pH getrennt werden können. Eine weitere Identifizierung kann mit Hilfe der UV-Spektren erfolgen.
Summary Among the most difficult problems of toxicological analysis are the rapid and reliable isolation and identification of organic bases from the material under examination. Very satisfactory yields can be obtained by means of a suitable extraction procedure or through precipitation with tetraphenyl borate in a systematic scheme of analysis. The purification of the crude extracts or precipitates is accomplished fundamentally by thin layer chromotography. The pure bases are liberated on the silica layers from the tetraphenyl borate complexes. An identification is possible through reference toR f- tables; substances with likeR f value can be separated by fractional extraction at different pH values. A further identification can be made with the aid of UV spectra.
Résumé L'isolement rapide et sûr des bases organiques dans une substance à l'étude et leur identification représentent l'un des problèmes les plus difficiles de l'analyse toxicologique. On peut atteindre des rendements très satisfaisants avec un procédé d'extraction analogue ou par précipitation par le tétraphénylborure dans un processus d'analyse systématique. La purification de l'extrait brut ou des produits de précipitation s'effectue principalement au moyen de la chromatographie en couche mince. Les bases pures sont libérées des complexes au tétraphénylborure en opérant sur couches en gel de silice. On peut faire une identification en se servant des valeurs desR f figurant dans les tables, ce qui permet de séparer les substances de mêmeR f par extraction fractionnée à des pH différents. Une identification plus poussée peut avoir lieu à l'aide des spectres UV.相似文献