Liquid chromatography on triacetylcellulose, 4. Determination of enantiomeric purity in spite of incomplete chromatographic separation

Abstract

A novel procedure for the measurement of enantiomeric purity P is derived, whereby the mixture to be analyzed is subjected to liquid chromatography (LC) on an optically active sorbent. Extensive peak overlap does not prohibit the application of the method, whereas the complete absence of separation does. In addition, the mixture is subjected to LC on an achirdsorbent. The same photometer and polarimeter detections are used for both experiments. At the same time, an x/y recorder plots the rotation angle a uersus the absorbance A , thus generating the slopes C, and C+ in Fig. 1 which result in P = C,/C+. -Optically active samples of 1,2, 3, and 4 were analyzed for P. The findings agree satisfactorily with the results of other methods. Fliissigkeits-Chromatographie an Triacetylcellulose, 4 l) Neuartige Bestimmung der enantiomeren Reinheit durch Fliissigkeits-Chromatographie trotz intensiver Peak-UherlappungEine neuartige Methode fur die Messung der enantiomeren Reinheit P wird abgeleitet, wobei die zu analysierende Mischung der Flussigkeits-Chromatographie (LC) an einem optisch aktiuen Sorbens unterworfen wird. Die Anwendung des Verfahrens wird durch intensive Peak-Uberlappung nicht verhindert, wohl aber durch vollstandiges Fehlen einer Trennung. Auljerdem wird die Mischung der LC an einem achiralen Sorbens unterworfen. Fur beide Experimente wird dieselbe photometrische und polarimetrische Detektion verwendet. Ein x/y-Schreiber tragt dabei den Drehwinkel a gegen die Extinktion A auf, wobei die Steigungen C, und C , in Fig. 1 und damit P = C,/C+ erhalten werden. -Optisch aktive Proben von 1 , 2 , 3 und 4 wurden im Hinblick auf P analysiert . Die Befunde stimmen zufriedenstellend mit Ergebnissen anderer Methoden uberein.The enantiomeric purity P of chiral substrates is of interest for several purposes: Calculation of chiroptical data, i. e. [a] and A&, of the pure enantiomers, which are needed for understanding such data; evaluation of educts and products of reactions, the mechanisms of which are under study; evaluation of the products of enantioselective syntheses; characterization of substrates to be checked for biological effects.It is worthwhile to strive for new techniques of P determination because none of the known methodsz) is universally applicable. E. g., conversion of enantiomers to diastereoisomers and subsequent analysis of the latter require a functional group in the substrate and rely on complete reaction, the absence of racemization, and the knowledge of P of the reagent2). Absolute methods2) do not need a standard for P, i. e., they analyze the mixture of enantiomers by association with an optically active auxiliary, the enantiomeric purity of which does not enter the

Other Modes of Detection

“…Mannshreck et al [186,187] designed a method for enantiomeric purity determinations in cases of incomplete chiral separations. They prepared a plot of optical rotation versus UV absorbance over the time a peak elutes.…”

Enantiomers of Polychlorinated Biphenyls Semipreparative Enrichment by Liquid Chromatography

“…A bigger column would be desirable in order to exploit this splitting on a larger scale. No splitting (3,4) and moderate splittings (1, 2) were detected on triacetylcellulose. Since this sorbent is more easily accessible, we tried semipreparative enrichment of the enantiomers of 1 and 2 on a column (30 x 2.5 cm) containing 80 g of this material.…”