Water-soluble calix[4]resorcinarenes as chiral NMR solvating agents for phenyl-containing compounds

Water-soluble calix[4]resorcinarenes with 3- and 4-hydroxyproline substituent groups are evaluated as chiral NMR solvating agents on a series of monosubstituted phenyl-containing compounds. The substrates interact with the calixresorcinarene through insertion of the aromatic ring into the cavity. Cationic, anionic, and neutral substrates were examined, and all exhibited enantiomeric discrimination in the 1H NMR spectrum with one or more of the calixresorcinarenes. The hydroxyproline derivatives were almost always more effective at causing enantiodifferentiation than the corresponding proline derivative. Presumably the hydroxyl group on the proline moieties is involved in interactions with the substituent groups of the substrate that are important in creating chiral recognition. The enantiomeric discrimination in the 1H NMR spectrum is large enough to permit the analysis of enantiomeric purity.