Pirkle-concept stationary phases

Initially, chiral stationary phases for chiral liquid chromatography were designed for preparative purposes, mostly based on the concept of three-point recognition .47 Pirkle and other scientists48 developed a series of chiral stationary phases that usually contain an aryl-substituted chiral compound connected to silica gel through a spacer. Figure 1-14 depicts the general concept and an actual example of such a chiral stationary phase. 

Cleveland, T. Pirkle-concept chiral stationary phases for the HPLC separation of pharmaceutical race-mates. J.Liq.Chromatogr, 1995, 18, 649—671... 

In order to broaden the capabilities of the Pirkle concept, both polar and polarizable groups were introduced into the molecule. The most popular of this type of chiral stationary phase are the (R,R) Whelk-01 and the (S,S)Whelk-01 phases, the structures of which are shown below. These phases are more versatile and have a wider field of application than the phases previously described. The phases are covalently bonded to the silica and so they can be used with almost any type of solvent. However, they have been found to operate most effectively in the normal phase mode. It should be noted that the polarizable character of the aromatic ring is essential for the stationary phase to function well. As the Pirkle phases are generally available in both the (R) and (S) configurations, the reversal of the elution order of a pair of enantiomers is possible. This stationary phase was originally designed for the separation of the Naproxen enantiomers but has found a wide application to the separation of epoxides, alcohols, diols, amides, imides and carbamates. 

Figure 4.14 Schematic illustration of the principles underlying design of Pirkle-type chiral stationary phases (CSPs). (a) Illustration of the concept of reciprocity a single enantiomer of a racemate which separates well on the CSP shown on the left, when used to produce a second CSP shown at the right, will usually afford separation of the enantiomers of analytes that are structurally similar to the chiral selector of the first CSP. Reproduced from Pirkle et al, J. Org. Chem. 57 (1992), 3854, Copyright (1992), with permission of the American Chemical Society, (b) Two CSPs that exhibit reciprocal behavior, and (c) enantiomeric recognition model for the more stable diastereomeric complex between (S)-naproxen dimethylamide and the Whelk-0-1 (3R,4R) analog. Note that hydrogen atoms bonded to carbons are omitted for clarity. Reproduced from Wolf and Pirkle (2002), Tetrahedron 58, 3597, copyright (2002), with permission from Elsevier.

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