1,3‐ Alternate Calix[4]arenecrown‐5 Conformers: New Synthetic Ionophores with Better K + /Na + Selectivity than Valinomycin

New 25,27‐dialkoxycalix‐[4]arenecrown‐5 conformers 8, 10 , and 11 have been synthesized and studied. The compounds 8a and 8b , fixed in 1,3‐ alternate structure, have been obtained in 57 and 40% yield, respectively, by reaction of the corresponding 25,27‐dialkoxycalix[4]arenes 7a–b with tetraethylene glycol di‐ p ‐toluenesulfonate in the presence of Cs 2 CO 3 . The cone 10a and 10b and the partial cone 11 conformers were obtained by selective demethylation of the 25,27 ‐dimethoxycalix[4]arenecrown‐5 ( 6a ) and subsequent dialkylation with NaH/DMF and KO t Bu/THF, respectively. In the solid state (X‐ray), compound 6a adopts a flattened cone conformation, which is also found to be most abundant in CD 3 CN and CD 3 OD solution. Upon complexation with potassium picrate compound 6a was converted quantitatively into the 1,3‐ alternate conformation. All new ligands synthesized were used in the extraction of alkali metal cations from H 2 O into CHCl 3 , and as active components in supported liquid membranes and in chemically modified field effect transistors. Results were compared to those obtained with with the natural antibiotic valinomycin 1 . All ligands showed high selectivity for potassium. Ligand 8a , fixed in the 1,3‐ alternate conformation, is more selective than valinomycin and shows the highest K + /Na + selectivity known so far.