Cholapod anion receptors can achieve high affinities while maintaining compatibility with nonpolar media. Previously they have been shown to transport anions across cell and vesicle membranes. In the present work, the scope of the architecture is expanded and structure-selectivity relationships are investigated. Eight new receptors have been synthesized, with up to six H-bond donor centers. Using Cram's extraction method, these compounds plus five known examples have been tested for binding to seven monovalent anions (tetraethylammonium salts, wet chloroform as solvent). Association constants in excess of 10(10) M(-1) have been measured for several pairings. Selectivities vary with receptor geometry, as expected. More remarkably, they also depend on receptor strength: more powerful receptors show a wider range of binding free energies, and therefore a greater spread of Ka(X-)/Ka(Y-). This "affinity-selectivity" effect can be derived from empirical relationships for H-bond strengths, and could prove widely operative in supramolecular chemistry.

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Receptors 5-11, bearing guanidinium, carbamate and (in some cases) other functional groups, were synthesized from cholic acid 1. These cations were shown to extract chiral carboxylate anions from aqueous buffer into chloroform with significant enantioselectivities. The most successful receptors bore two carbamate substituents and achieved, in the best cases, ratios (:) of 7-10:1 for a series of five N-acetyl α-amino acids.