Silica chromatography, structural characterization

Compound 165 was reduced with SDMA and the product was hydrolyzed with acid to effect ring contraction, affording 4-deoxy-4-C-[(jR,S)-ethylphosphinyl]-o ,/ -D-ribo- and -L-lyxo-furanoses (166), which were characterized by conversion into the peracetates. After separation by chromatography on silica gel, structures 167-170 were established for these peracetates by 400-MHz, -n.m.r. spectroscopy and high-resolution mass spectrometry the structures of these products, their probable conformations, and the yields from 165 are summarized in Scheme 7. 

This is liquid-solid chromatography in which the surface of microparticulate silica or other adsorbent constitutes the polar stationary phase. The silica particles are characterized by their shape (irregular or spherical), size and size distribution, and pore structure (mean pore diameter,... 

C NMR-spectroscopy indicated a 3/4-ratio of 87 13. Diastereomers 3/4 can be separated by careful medium-pressure silica gel chromatography (petroleum ether/ether 95 5)3b or by fractional crystallization of the diastereomerlc chloroacetates (vide supra). For structural characterization see Note 19. 

Monomeric sulfur imide SNH is not known, but a coordination compound Fe2(CO)6(HNS) (72) has been isolated and converted to the corresponding anion [Fe2(CO)6(/r-SN)] with n-BuLi. The complex (72) is obtained from the reaction of Fe3(CO)i2 with MeySiNSNSiMes, followed by chromatography on silica. The HNS unit behaves as a six-electron ligand see Counting Electrons). Mononuclear complexes of the type (RNS)Cr(CO)5 (R = MeyN, PhyN), in which the ligand is bonded to the metal via sulfur, have also been structurally characterized. ... 

Silica, alumina, and silica-alumina surfaces are of great importance for catalysis and chromatography. Reactivity of these materials is determined by the structure of the surface and its relative acidity, and considerable effort is being expended to characterize it. Of particular interest are the surface hydroxyl groups. Among the methods used for their study the most powerful are IR spectroscopy and titration with acid-base indicators. Conventional NMR can cope with the observation of adsorbed species, where a considerable amount of motional averaging is present MAS NMR must be used to study the surface directly. 

Methyl Esters. These esters can be isolated and purified by thin-layer chromatography on silica gel G with a solvent system of petroleum ether (b.p. 30-60°)-diethyl ether (80 20, v/v). Usually these esters exhibit an /Rvalue in the range of 0.80-0.85. Their characterization can be achieved in a manner similar to that described in Chapter 4. Thus, a combination of gas-liquid chromatography and mass spectrometry will afford a detailed insight into the structure and composition of these long-chain fatty acid esters. 

Ketones or aldehydes (1 mmol) were added to a stirred mixture of diethyl phophoramidate 1 (1 mmol) and 2-chloro5,5-dimethyl-l,3,2-dioxaphosphane 3 (1 mmol) at 50-60 °C. After stirring for the corresponding amount of time, the mixture went slimy and the reactions were stopped. The crude products were purified by flash chromatography on silica gel (started with ethyl acetate-petroleum ether 4 1, and then pure ethyl acetate as eluent). Their structures were characterized by XH NMR, 31P NMR, 13C NMR and elemental analysis. 

Silica exists in a broad variety of forms, in spite of its simple chemical formula. This diversity is particularly true for divided silicas, each form of which is characterized by a particular structure (crystalline or amorphous) and specific physicochemical surface properties. The variety results in a broad set of applications, such as chromatography, dehydration, polymer reinforcement, gelification of liquids, thermal isolation, liquid-crystal posting, fluidification of powders, and catalysts. The properties of these materials can of course be expected to be related to their surface chemistry and hence to their surface free energy and energetic homogeneity as well. This chapter examines the evolution of these different characteristics as a function not only of the nature of the silica (i.e., amorphous or crystalline), but also as a function of its mode of synthesis their evolution upon modification of the surface chemistry of the solids by chemical or heat treatment is also followed. 

A unified theory recently proposed to explain the manner of sorption and the form of sorption isotherm in gas, liquid, and ion-exchange chromatography is presented in some detail. Selectivity in reversed-phase high-pressure liquid chromatography is explored at length. Several chapters deal with characterization of bonded phases, relationship of column-packing structure and performance, variability of reversed-phase packing materials, and the differences between silica-based reversed-phase and poly(styrene-divinylbenzene) columns. A short review is included to cover various approaches used in HPLC to achieve the desired selectivity for resolution of enantiomeric compounds. 

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