CyDs macrocyclic structures

The macrocyclic structure of j8-CyD is not affected by methylation of the 02 H hydroxyl groups because the intramolecular 03H-02 hydrogen bonds retain the... 

On the basis of X-ray analysis, methylated or acetylated CyDs are sometimes considered to be more flexible than the native ones (see Chapter 7). Such a conclusion seems unfounded since X-ray diffraction can yield straightforwardly only the structure of the macrocyclic ring averaged over time and space, not its mobility. As a matter of fact, native CyDs are more flexible, and thus more difficult to freeze, than permethylated ones. This fact is frequently overlooked since the above mentioned averaging is not taken into account. As shown above, NMR spectra in solution and in the solid state are much more sensitive to CyD flexibUity and clearly prove their nonrigidity. 

The classic chemistry of native CyDs is now a kind of completed area, described in a book [1] and several reviews that summarize state-of-the-art syntheses of these macrocycles [2-4], while syntheses of several novel CyD structures or their analogues are given in the literature [5-8). Studies of CyDs as enzyme models, involving syntheses of numerous selectively substituted derivatives, have been reviewed [9-11]. Numerous journals and proceedings can also be mentioned here [12, 13). 

Figure 7.1 shows the structure of CyDs consisting of 6-9 glucose units. Some average parameters describing the macrocyclic conformation of native CyDs are summarized in Table 7.1. The pyranose ring of each glucose unit in native CyDs is relatively rigid and assumes the Ci chair conformation. Some structural characteristics of CyDs are illustrated in Fig. 7.2. Primary hydroxyl groups have rotational...

A crystal of the J-CyD hydrate shows the cage-type packing structure [21). In the solid state, d-CyD forms complexes with several macrocyclic compounds, such as cydononanone to cydopentadecanone [183]. Currently only the structure of the cydoundecanone complex has been reported. The crystal shows a typical channel-type packing structure consisting of dimeric d-CyD with the head-to-head mode [78]. Four cydoundecanone molecules are induded in the dimer cavity (Fig. 7.19B). Two guest molecules are perpendicularly located at the center of the dimer cavity and each of the other two molecules is coaxially included at the primary hydroxyl end of the cavity. 

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