Chiral Recognition see also Chapters

Several attempts to separate optical isomers by co-precipitation with CyDs have suggested that CyDs are not good reagents for this purpose. CyDs sometimes co-crystallize with racemic compounds, as X-ray structures have demonstrated. In 

The hydroxyl group of both isomers forms hydrogen bonds equally with secondary hydroxyl groups of a-CyD. Another example of a crystalline complex with a racemic compound is the j8-CyD complex with flurbiprofen. In the crystal, two j8-CyD molecules form a head-to-head dimer and a pair of (R)- and (S)-flurbiprofen is packed in the barrel-like cavity (Fig. 7.25A) [163]. In contrast, an excess of (S)-isomer was detected in the j8-CyD complex with racemic fenoprofen [157]. j8-CyD molecules form a same dimer structure in the crystal of the complex with each isomer, but the arrangement of guest molecules in the dimer cavity differs between the two crystals. Two (H)-isomer molecules are included in the head-to-head mode (Fig. 7.25B) while the head-to-tail arrangement is observed in the (S)-isomer complex (Fig. 7.25C) [158]. In fS-CyD complexes with N-acetylphenylalanine, the L-isomer is disordered in the dimer cavity while two molecules of D-isomer are included in a head-to-head mode [177]. 

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