NAryl Amino Acid Derived CSPs

The development of second generation Pirkle phases with electron-rich aromatic groups reached it's zenith with the investigations by Pirkle and Pochapsky of compounds derived from the enantiomers of the readily prepared electron-rich N-aryl amino acid derivatives. This is perhaps the best understood chiral recognition system yet studied. In some preliminary studies, it was shown that N-aryl amino acid derivatives were well resolved using the DNB Leu-Ionic CSP structure [V]. Subsequently, the structural requirements for chiral recognition were defined, and a chiral recognition rationale was proposed. The high degree of enantioselectivity observed in this system (a > 10 in some cases) can be attributed to the intimate association of the two components of the more stable diastereomeric adsorbate as illustrated later. This chiral recognition system has been studied at great length using spectroscopic as well as chromato-graphic techniques. An X-ray structure of a 1: 1 complex of soluble analogues of the compounds shows essentially the same adsorbate structure as that initially proposed based upon chromatographic and spectroscopic studies (structure [VI]):

A reciprocal N-(2-naphthyl)valine-derived stationary phase was prepared and shown to provide unprecedented levels of resolution for a variety of racemates containing electron-deficient aromatic groups. The related alanine-derived CSP structure [VII] was subsequently reported, and commercialized, and has proven to be a valuable and widely used research tool:

Subsequent structural refinements led to the preparation and commercialization of the leucine-derived CSP structure [VIII], which generally affords significantly greater enantioselectivity in the separation of DNB-containing enantiomers than does CSP structure [VI]. The predominant effect seems to be a greatly diminished retention of the initially eluted enan-tiomer, presumably owing to the ability of the more

Structure [VI]

Structure [VIII] (N1N-Leu CSP)

bulky leucine sidechain to inhibit approach of the least retained enantiomer from the undesired face of the aromatic ring of the CSP.

These phases have proven to be widely useful for separating the enantiomers of chiral amines, amino alcohols, amino acids, etc., and are sometimes also useful for separating the enantiomers of chiral alcohols, thiols, etc. However, analysis with these phases generally requires that the analyte be derivatized so as to incorporate an electron-deficient aromatic group. Typically, derivatization reagents such as 3,5-di-nitrobenzoyl chloride or 3,5-dinitrophenylisocyanate are used. Despite this somewhat bothersome need to make derivatives, these CSPs remain quite useful and show a tremendous generality.

Recently, a group of CSPs based on proline anilides have been developed in the Pirkle laboratories. These exhibit remarkable enantioselectivities for some DNB derivatives.

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