Chiral stationary phases structural features

The term Molecular Clip has been coined for molecules of type 2. That these molecules do indeed possess the geometric features of a clip is apparent from the X-ray structure of the tetramethoxy derivative 3a (Fig. 2) [lla,b]. The o-xylylene moieties of this molecule define a tapering cavity, the walls of which are at an angle of 39.5 with the centers of the benzene rings 6.67 A apart. The carbonyl groups of the glycoluril moiety, which are hydrogen-bond acceptor sites, are separated by 5.52 A. It was also possible to obtain a crystal structure of the chiral dibromo-derivative 4 of clip 3 (Fig. 3). This compound was separated into its enantiomers by HPLC on a chiral stationary phase [12]. 

The use of chiral stationary phases (CSP) in liquid chromatography continues to grow at an impressive rate. These CSPs contain natural materials such as cellulose and starch as well as totally synthetic materials, utilizing enantioselective and retentive mechanisms ranging from inclusion complexation to Ti-electron interactions. The major structural features found in chiral stationary phases include cellulose, starch, cyclodextrins, synthetic polymers, proteins, crown ethers, metal complexes, and aromatic w-electron systems. 

The presence of polar or polarizable groups close to the chiral centre can promote hydrogen bonding or 7i-overlap with the stationary phase and enhance the resolution. This was again shown to be a significant structural feature in the separation of diastereomeric amides of acyclic isoprenoid acids [11]. The introduction of an olefinic group close to the chiral centre increased the separation factors. Similarly, separation factors were decreased... 

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