Second Generation Pirkle CSPs with Electron Rich Aromatic Groups

Studies in the Pirkle laboratories and elsewhere showed that the DNB amino acid-derived CSPs were capable of separating the enantiomers of a wide variety of analytes possessing electron-rich aromatic groups. At this point, it was only natural to again turn to the principle of reciprocity and prepare and evaluate 'reciprocal' materials based on some of these structures. A number of reciprocal phases were prepared and evaluated, although they lacked the requisite generality required of a commercial CSP. Nevertheless, these studies revealed many important design features that would prove useful later.

The enantiomers of aryl hydantoins are well resolved on DNB amino acid-derived CSPs, and several hydantoin-based CSPs were prepared to study the mechanisms for chiral recognition of these types of analytes and to probe the ability of this type of phase to afford separation of various racemates.

Separation of the enantiomers of a number of aryl-substituted phthalides was studied using DNB amino acid-derived CSPs such as CSP structures [III] and [V], and a detailed study of the effect of analyte structure on chromatographic performance led to some optimized structures for chiral recognition. One such compound was used to prepare a reciprocal aryl phthalide CSP which, as expected, showed a general ability to resolve a number of racemates containing electron-deficient aromatic systems.

Another class of compounds which are well resolved on the DNB amino acid-derived CSPs are general amide structures bearing at least one aromatic ring. In an extensive series of studies, Pirkle, Hyun and co-workers prepared and evaluated more than 10 CSPs derived from various analogues of (l-naph-thyl)ethylamine (a-NEA) in which a variety of different tether geometries and substituent patterns were explored. Study of this group revealed a number of interesting and useful principles of CSP design. However, the CSPs themselves were somewhat limited in that they were useful primarily for separation of the enantiomers of analytes bearing electron-deficient aromatic groups. Similar CSPs were prepared and studied by Oi and co-workers.

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