Macrocyclic Selectors, Cyclodextrins, and Antibiotics

A broad range of macrocyclic compounds can be used as chiral selectors for enantioselective HPLC. Besides synthetic crown ethers, derivatized cyclodextrins and cyclic antibiotics are also used as chiral stationary phases. Enantiomer separation employing these compounds is often based on host-guest interactions [15], whereby the cyclic molecules form an inclusion compound or an association complex with the analyte. 

Chirally substituted crown ethers (e.g., Crownpack CR produced by Daicel) are useful for enantiomer separations of primary amines and amino adds under 

Cyclodextrin phases can be operated in RP and normal-phase modes, as well as in the polar ionic mode (see Table 3). Aqueous eluent systems are better suited for the formation of inclusion complexes of apolar or aromatic moieties in the inner cavity of the macrocycle. The selectivity of the cyclodextrins depends on ring size, pH, ionic strength, the nature and concentration of modifier, and temperature. 

When cyclodextrins are used in normal-phase mode, chiral recognition is based on the interaction of the outer functional groups through hydrogen bonds, dipole-dipole interactions, and Jt-Jt interactions. The normal-phase mode is used for analytes that show a low solubility in aqueous eluents here, mostly alkane/alcohol mixtures are used. 

In the polar mode, the mobile phase is very often acetonitrile containing some protic additives, such as methanol, acetic acid or triethylamine. The formation of 

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