Synthesis and Ion Selectivity of All Conformational Isomers of Tetrakis((ethoxycarbonyl)methoxy)calix[4]arene

We have found that the conformer distribution in tetra-O-alkylation of 5,11,17,23-tetra-tert-butylcalix[4]-arene-25,26,27,28-tetrol by ethyl bromoacetate is remarkably affected by the metal cation present in the base. In general, the cone conformer predominantly results when the base contains template metal cations whereas the partial-cone and 1,3-altemate conformers result when the base contains nontemplate metal cations. In acetone solvent one can realize the change from the 100% cone selectivity to the 100% partial-cone selectivity. By combining the metal template effect with a protection-deprotection method with a benzyl group, we developed synthetic routes to all of the four conformers. Two-phase solvent extraction established that the cone conformer shows Na⁺ selectivity whereas the remaining three conformers show K⁺ selectivity. ¹H NMR studies established that the 1,3-altemate conformer can form both a 1:1 and a 2:1 metal/calixarene complex and the two metal-binding sites display negative allostericity. This paper thus demonstrates that the metal selectivity of ionophoric calix[n]aryl esters can be changed not only by the change in the ring size but also by the conformational change.