SYNTHESIS, STRUCTURAL CHARACTERIZATION, AND ALKALI-METAL COMPLEXATIONOF THE 6 POSSIBLE (1,3)-BRIDGED AND (1,2)-BRIDGED P-TERT-BUTYLCALIX[4]CROWN-5 CONFORMERS BEARING ALPHA-PICOLYL PENDANT GROUPS

Complementary synthetic strategies leading to the six possible regiois omers and conformational isomers of is[(alpha-picolyl)oxy]-p-tert-buty lcalix[4]crown-5 (1-6) are reported. The overall conformation of 1-6 w as deduced by NMR spectroscopy and further proven by single-crystal X- ray analysis for (1,3)-bridged cone and (1,2)-bridged 1,2-alternate co nformers 1 and 6. Compounds 1-6 form 1:1 complexes with alkali-metal c ations. The complexation sites and solution structures for Na+ and Kcomplexes have been determined by H-1 NMR titration experiments. The b inding affinities of ligands 1-6 for alkali-metal ions have been asses sed by phase transfer and stability constant measurements using spectr ophotometric, potentiometric, and calorimetric techniques. All 1,3-bri dged calixcrowns 1-3 show a strong affinity for K+ (partial cone 2 and 1,3-alternate 3 also for Rb+), conformer 3 being slightly more select ive than the naturally occurring ionophore valinomycin. Conversely, 1, 2-bridged calixcrowns 4-6 are less efficient and their selectivities v ary from K+ for cone 4 to both K+ and Rb+ for partial cone 5 and to Cs + for 1,2-alternate conformer 6.