Cycloenantiomeric rotaxanes

Yamamoto, C., Okamoto, Y., Schmidt. T., Jager, R., Vogtle, F. Enantiomeric resolution of cycloenantiomeric rotaxane, topologically chiral catenane, and pretzel-shaped molecules observation of pronounced circular dichroism, J. Am. Chem. Soc. 119 (1997), 10547-10548. 

Undoubtedly, the most important stereochemical feature is chirality. The [2]-rotaxane depicted schematically in Figure 2.12a is made with a macrocycle and a dumbbell that are not themselves chiral but oriented by appropriate substitution patterns on these components. Vogtle and co-workers described cycloenantiomeric rotaxanes that are based on the amide templating functionality (Figure 2.13).36 Racemic [2]-rotaxane 34 was synthesized in 20% yield by reaction of unsymmetrical acid dichloride 32 with trityl aniline 33 in the presence of macrocycle 31. The object and its mirror image are created by different sequences of the amide and sulfonamide linkers in both the dumbbell and the macrocyclic component. If the order of the atoms within the dumbbell is examined, in one enantiomer the sulfonamide nitrogen atom is... 

Figure 49. The first cycloenantiomeric [2]rotaxane - the different atom sequence of wheel with respect to the unsymmetric axle creates this rare case of chirality.

Figure SO. Circular dichrogram of the cycloenantiomeric [l]rotaxane 100 in trifluoroetha-nol.

Again it was surprising that enantioseparation by HPLC was successful because the conformational flexibility of the wheel and the thread entail reduced structural dissymmetry of the enantiomers compared with more rigid molecules. For the rotaxanes 80m and 100 separation factors a were found to be 1.48 and 1.69, respectively. The circular dichrogram of the cycloenantiomeric [ljrotaxane 100 is given in Figure 50. 

Amide-linked catenanes and rotaxanes can play a major role in the study of rare forms of chirality, e.g. topological chirality and cycloenantiomerism [60]. Resolution of enantiomeric catenanes, rotaxanes, and pretzelanes has been achieved by HPLC on chiral column materials, but further work must be performed to determine absolute configurations and to realize new chiral skeletons composed of achiral building blocks. Topological asymmetric synthesis still belongs to dreams of the future yet should be kept in mind. 

In addition, chiral dendrimers (see Section 4.2) can be resolved with the aid of HPLC into their enantiomers, if the silica gel material used as stationary phase has optically active substances bound to its surface [9]. Since the chiral stationary phase (CSP) [10] undergoes different intensities of interaction with the enantiomeric dendrimers, they are retained to different degrees, and in the ideal case two completely separated (baseline separated) peaks are obtained. This separation technique was successfully applied inter alia to racemic mixtures of planar-chiral dendro[2.2]paracyclophanes, cycloenantiomeric dendro[2] rotaxanes, topologically chiral dendro[2]catenanes [11] as well as topologically chiral, dendritically substituted molecular knots (knotanes) [12] (Section 4.2.3). 

Figure 2.13. Preparation, by templated threading, of the chiral, cycloenantiomeric [2]-rotaxane...

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