Covalently bound chiral substitution

The knots based on neutral, purely organic molecules are obviously not prone to classical diastereomer resolution, and, while chromatographic methods were not suitable for the separation of the two enantiomers of the metal-templated trefoil knot, they have been proved successful in the amide-containing knots. As far as these knotted molecules are concerned, it must be noted that they incorporate classical stereogenic centers (carbon atoms), which makes them very different from the copper-based systems in terms of chirality. In the first instance, the separation of the two enantiomers of six different knots was achieved with a colunm that was not conunercially available (chiral-AD type). Trichloromethane was needed to obtain an optimal separation. The silica gel and the chiral stationary phase were covalently bound so that the material did not bleed out when the lipophilic eluent was used. Moreover, comparison of the experimental CD of the pure enantiomers of a knot with a theoretically calculated CD (based on X-ray structure and a fiiUy optimized AMI geometry) permitted assignment of the absolute configuration of this knot. The latter preparation of soluble knots based on substitution of the 5-position of the pyridine moieties in 13 afforded molecules that were soluble in solvents which could be used in commercially available chiral columns." On the other hand, the substitution of a racemic mixture of knots with chiral auxiliaries allows the separation of the diastereomeric product." ... 

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