Column chromatography problems with

The last several years have seen an enormous growth in the number and use of chiral stationary phases in liquid chromatography [742,780-791]. Some problems with the gas chromatographic approach are that the analyte must be volatile to be analyzed and larger-scale preparative separations are frequently difficult. For entropic reasons relatively high temperatures tend to minimize the stability differences between the diastereomeric complexes and racemization of the stationary phase over time may also occur. The upper temperature limit for phases such as Chirasil-Val is about 230 C and is established by the rate of racemization of the chiral centers and not by column bleed. Liquid chromatography should be s ior in the above... 

Quality assurance measures such as pre-analytical checks on instrumental stability, wavelength calibration, balance calibration, tests on resolution of chromatography columns, and problem diagnostics are not included. For present purposes they are regarded as part of the analytical protocol, and IQC tests their effectiveness together with the other aspects of the methodology. 

One of the major problems with the preparative separation of flavonoids is their sparing solubility in solvents employed in chromatography. Moreover, the flavonoids become less soluble as their purification proceeds. Poor solubility in the mobile phase used for a chromatographic separation can induce precipitation at the head of the column, leading to poor resolution, decrease in solvent flow, or even blockage of the column. 

The present method of preparation is that described by Bruggink and McKillop.4 It has the particular advantages of high yield and manipulative simplicity, and avoids the problem inherent in Hurt-ley s procedure of separation of mixtures of carboxylic acids by fractional crystallization or column chromatography. The method is, moreover, of wide applicability with respect to both the /3-dicarbonyl compound and the 2-bromobenzoic acid. The synthetic scope and limitations of this procedure for the direct arylation of... 

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