Synthesis and Ion Selectivity of Conformers Derived from Hexahomotrioxacalix[3]arene

The reaction of 7,15,23-tri-tert-butyl-25,26,27-trihydroxy-2,3,10,11,18,19-hexahomc-3,11,19-trioxacalix-[3]arene (1H₃) with ethyl bromoacetate yielded the cone conformer (cone-1Es₃) as a minor product and the partial-cone conformer (partial-cone-1 Es₃) as a major product. Interconversion between these two conformers did not take place, indicating that the OCH₂CO₂Et group is bulky enough to inhibit the oxygen-through-the-annulus rotation. The conformer distribution was affected by the metal cation present in the base, suggesting the importance of the metal template effect. When NaH was used in THF, the partial-cone/cone ratio was 1.6–3.8; when CS₂CO₃ was used in acetone, partial-cone-1Es₃ was produced quantitatively. Two-phase solvent extraction established that cone-1Es₃ shows Na⁺ selectivity, whereas partial-cone-1Es₃ shows K⁺ selectivity. Cone-1Es₃ showed high affinity toward RNH₃⁺ cations, indicating that the structure of cone-1Es₃ with C₃ symmetry is very suitable for the binding of RNH₃⁺ cations with C₃ symmetry. ¹H NMR studies established that the phenyl units in cone-1Es₃ are more or less flattened but stand up when a cationic guest is included. This paper thus demonstrates that IH₃ is useful as a basic skeleton for the design of new ionophores, particularly those suitable for the recognition of guests with C₃ symmetry.