Column chromatography competitiveness

The oxyamination of C-2 C-3 and C-3-C-4 unsaturated sugars gave mixtures of regioisomeric hydroxy amides, i.e., 12, 13 and 14, 158s-91. The cis dihydroxylation was often a competitive reaction, especially for C-2-C-3 unsaturated sugars, e.g., formation of 1888-91. The products were generally separated by column chromatography. 

Direct alkylation of 5-substituted tetrazoles is often the best method of obtaining simple 2,5-disubstituted tetrazoles. The 1,5-isomers, which are formed as competitive side products, are usually separated with ease by fractional crystallization or better by column chromatography on alumina or silica gel which gives excellent separations of most tetrazole isomers (8UCR(S)174). Another convenient synthesis of 2,5-disubstituted tetrazoles (241) is the formazan method (Scheme 31). By coupling aldehyde arylsulfonylhydrazones (240) with diazonium salts in basic solutions, compounds (241) may be obtained directly in yields of up to 78% (76BCJ1920). 

There is a dearth of competitive adsorption data, in a large part because they are difficult to measme, but also because little interest has been devoted to them, as, until recently, there were few problems of importance whose solution depended on their understanding. Besides the static methods, which are extremely long and tedious and require a large amoimt of material, the main methods of measurement of competitive isotherms use column chromatography. Frontal analysis can be extended to competitive binary isotherms [14,73,93-99], as well as pulse techniques [100-104]. The hodograph transform is a powerful method that permits an approach similar to FACP for competitive binary isotherms [105,106]. 

Included in this chapter is also a discussion of the modernised versions of liquid-solid column chromatography which have enabled this technique to hold its own against the competition offered by TLC, the latter having the advantage of being a convenient and rapid chromatographic technique. 

Competitive Cyclizing and Noncyclizing Dimerizations. When reactions such as noncyclizing dimerization were prolonged for 40-50 h, then 1,2,3-tri-phenylnaphthalene (mp 148-150 °C), (Z,Z)-1,2,3,4-tetraphenyl-l,3-butadiene (mp 146-148 °C), and (E)-l,2-diphenylethene (mp 124-125 °C) could be isolated from the hydrolyzed reaction mixture by column chromatography on aluminum oxide and elution with a benzene-hexane gradient. The principal product was still the (E,E)-1,2,3,4-tetraphenyl-l,3-butadiene. 

Amine (1 4.5 mmol) was added slowly to the mixture of carbon disulfide (CS2, 5 mmol) and the Michael acceptor (2 4 mmol) into a test tube placed in an ice bath. The reaction mixture was stirred at 0 °C for 30 min, and then warmed to room temperature with crmtinued stirring for stirred for another 1-12 h to complete the reaction. On competition of the reaction, excess of CS2 and amine was removed under reduced pressure to afford dithiocarbamate derivative 4 in the almost pure form in some cases, further purification was carried out by reciystallization or short-column chromatography on silica gel (EtOAc-petrol ether). All compounds were characterized on the basis of NMR spectral data. 

Competitive reaction of benzenethiol, benzhydrazide and aniiine with phenyl acrylate. Pheny acrylate (0.148 g, 1.0 mmol) was added to a solution of benzenethiol (0.11 mL, 1.0 mmol), benzhydrazide (0.136 g, 1.0 mmol) and aniline (0.093 g, 1.0 nunol) in THF at room temperature. The solution was stirred for S min and was evaporated. The crude product was purified by silica gel column chromatography using benzene as eluent to give pure phenyl 3-phenylthiopropionate (2). The yield was 0.237 g (92%). 

Displacement Development A complete prediction of displacement chromatography accounting for rate factors requires a numerical solution since the adsorption equilibrium is nonlinear and intrinsically competitive. When the column efficiency is high, however, useful predictious can be obtained with the local equilibrium theoiy (see Fixed Bed Transitions ). 

The competitive adsorption isotherms were determined experimentally for the separation of chiral epoxide enantiomers at 25 °C by the adsorption-desorption method [37]. A mass balance allows the knowledge of the concentration of each component retained in the particle, q, in equilibrium with the feed concentration, < In fact includes both the adsorbed phase concentration and the concentration in the fluid inside pores. This overall retained concentration is used to be consistent with the models presented for the SMB simulations based on homogeneous particles. The bed porosity was taken as = 0.4 since the total porosity was measured as Ej = 0.67 and the particle porosity of microcrystalline cellulose triacetate is p = 0.45 [38]. This procedure provides one point of the adsorption isotherm for each component (Cp q. The determination of the complete isotherm will require a set of experiments using different feed concentrations. To support the measured isotherms, a dynamic method of frontal chromatography is implemented based on the analysis of the response curves to a step change in feed concentration (adsorption) followed by the desorption of the column with pure eluent. It is well known that often the selectivity factor decreases with the increase of the concentration of chiral species and therefore the linear -i- Langmuir competitive isotherm was used ... 

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