Synthesis, conformational studies and inclusion properties of O-benzylated calixarene analogues of trihydroxy[3.3.3]metacyclophanes

Abstract

O-Benzylation of the flexible macrocycle 1 with benzyl bromide in the presence of NaH in THF under reflux afforded a mixture of two conformers of the tri-O-benzylated products, cone-2a and partial-cone-2a in a ratio of 20∶80 in 80% yield. In contrast, O-alkylation of the triol 1 with 2-(chloromethyl)pyridine in the presence of NaH resulted in exclusive formation of cone-2b. Only when the template metal can hold the 2-pyridyl group(s) and the oxide group(s) on the same side of the [3.3.3]metacyclophane ring, is the conformation immobilized in the cone form. The template effect of the sodium cation plays an important role in this benzylation.

The two-phase solvent extraction data indicated that tris(2-pyridylmethoxy)[3.3.3]metacyclophanes 2b show strong Ag⁺ affinity, high Ag⁺ selectivity being observed for both cone-2b and partial-cone-2b. ¹H NMR Titration of cone-2b with AgSO₃CF₃ clearly demonstrates that a 1∶1 complex is formed with retention of the original symmetry. The pyridine moiety underwent conformational changes upon Ag⁺ complexation with the original outward orientation of the ring nitrogen changing to an inside orientation toward the cyclophane cavity.