Separation selectivity chromatography

Another class of configurationally stable a-mctallo amines is derived from the N-tert-butoxy-carbonyl-protected piperidines 32 and 3516, l7. Addition of the lithiated piperidines to aldehydes leads to mixtures of the anti- and. yin-diastereoiners. Although the diastereoselectivity is low, the diastereomers can be readily separated by chromatography since the. vyn-isomer is often in a cyclized form 34. The stereochemistry of the products obtained from piperidines 32 are consistent with an equatorial a-lithiation followed by addition to the aldehyde with retention of configuration. However, with piperidine 35 selective axial lithiation is observed. 

In contrast, transmetalation of the lithium enolate at —40 C by treatment with one equivalent of copper cyanide generated a species 10b (M = Cu ) that reacted with acetaldehyde to selectively provide a 25 75 mixture of diastereomers 11 and 12 (R = CH3) which are separable by chromatography on alumina. Other diastereomers were not observed. Similar transmetalation of 10a (M = Li0) with excess diethylaluminum chloride, followed by reaction with acetaldehyde, produced a mixture of the same two diastereomers, but with a reversed ratio (80 20). Similar results were obtained upon aldol additions to other aldehydes (see the following table)49. 

The diastereomeric a-alkoxy complexes (1 )-15 and (S)-15, separable by chromatography, were each converted to the corresponding aluminum enolates and reacted with 2-methylpropanal (17)49. Enolate (/ )-16 selectively provided a mixture of two diastereomers with the (Fe/ ,2, 3 i )-complex (/ )-18 identified as the major constituent of a 94 6 mixture. The two chiral auxiliaries of complex (S j-lS exerted antagonistic effects and an undefined mixture of all four possible diastereomers was obtained. 

The popularity of reversed-phase liquid chromatography (RPC) is easily explained by its unmatched simplicity, versatility and scope [15,22,50,52,71,149,288-290]. Neutral and ionic solutes can be separated simultaneously and the rapid equilibration of the stationary phase with changes in mobile phase composition allows gradient elution techniques to be used routinely. Secondary chemical equilibria, such as ion suppression, ion-pair formation, metal complexatlon, and micelle formation are easily exploited in RPC to optimize separation selectivity and to augment changes availaple from varying the mobile phase solvent composition. Retention in RPC, at least in the accepted ideal sense, occurs by non-specific hydrophobic interactions of the solute with the... 

Hu, W. Z., Haddad, P. R., Tanakar, K., and Hasebe, K., Modulation of the separation selectivity of inorganic anions in electrostatic ion chromatography using acidic eluents, Analyst, 125, 241, 2000. 

Krokhin, O. V., Adamov, A. V., Hoshino, H., Shpigun, O. A., and Yotsuyanagi, T. Separation selectivity of some ethylenediaminetetraacetic acid and cyclohexane-1,2,-diaminetetraacetic acid complexes in column and ion electroki-netic chromatography, /. Chromatogr. A, 850, 269, 1999. 

Chiral glycolates. The chiral dioxolanes 1 and 2 are prepared by reaction of 8-phenylmenthone with a protected derivative, (CH3)3SiOCH2COOSi(CH3)3, of glycolic acid catalyzed by trimethylsilyl triflate. They are obtained in about a 1 1 ratio and are separable by chromatography. Alkylation of the enolates of 1 and 2 proceeds with marked diastereofacial selectivity. After separation of the major... 

The use of dansyl derivatives is not recommended for routine analysis of free amino acids but is very suitable in the identification of an unknown amino acid that has been selectively extracted from the original sample and is present in small quantities. Both kinds of derivative can be easily separated by chromatography or electrophesis and no locating reagent is required for either because the DNP derivatives are themselves yellow in colour and the dansyl derivatives are fluorescent. 

Chromatographic methods of separation are distinguished by their high selectivity, that is their ability to separate components of closely similar physical and chemical properties. Many mixtures which are difficult to separate by other methods may be separated by chromatography. The range of materials which can be processed covers the entire spectrum of molecular weights, from hydrogen to proteins. 

Electrokinetic chromatography (EKC) using microemulsion is one of the most powerful tools for the rapid measurement of log P w with high reproducibility. Because it is relatively easy to manipulate the pseudostationary phases of EKC, a lot of phases have been reported for the measurement not only of physicochemical properties but also of the separation selectivity, such as polymer micelles (64) and double-chain surfactant vesicles (56-58,60,61). These phases are also interesting in terms of the correlation to bioactivity. 

Baltazar, Q.Q., Leininger, S.K., and Anderson, J.L., Binary ionic liquid mixtures as gas chromatography stationary phases for improving the separation selectivity of alcohols and aromatics compounds, /. Chromatogr. A., 1182, 119-127, 2008. 

In most cases only a few odorants selected for identification appear as clear peaks in the gas chromatogram. The majority of the odorants are concealed by peaks of the volatiles predominating in the extract. To enrich the odorants the extract is separated into the acid and the neutral/basic fractions and the latter is separated by chromatography on silica gel [21, 27]. If necessary, the fractions... 

Critical separations in chromatography should be investigated at the appropriate level. Specificity can best be demonstrated by the resolution of two chromographic peaks that elute close to each other. In the potency assay, one of the peaks would be the analyte peak. Figure 2.4 illustrates the selectivity of a method to resolve known degradation peaks from the parent peak. Based on the... 

Critical separations in chromatography should be investigated at an appropriate level. For critical separations, selectivity can be demonstrated by the resolution of the two components that elute closest to each other. Peak purity tests using diode array or mass spectrometric detectors may be useful to show that the analyte chromatographic peak is not attributable to more than one component. 

Selective enzymatic hydrolysis. The traditional strategy in sequence determination is to cut protein chains into smaller pieces which can be separated by chromatography or electrophoresis and sequenced individually. Enzymatic cleavage is especially useful because of its specificity. Trypsin, a so-called endo-... 

Khaledi, M. G., J. K. Strasters, A. H. Rodgers, and E. D. Breyer. 1990. Simultaneous enhancement of separation selectivity and solvent strength in reversed-phase liquid chromatography using micelles in hydro-organic solvents nal. Chem62 130-136. 

In order to allow selection of these peaks, the separation is monitored by an LC detector, in most cases a conventional UV detector, which displays a chromatogram of the separation. The chromatography software must allow selection of certain positions in the chromatogram for further measurement. 

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