Synthesis of Oxacalix4arene and the Study of Its Dynamic Behavior

The preparation of a new member of the mixed-bridged calixarenes, monooxacalix4arene, is described. The key step is the synthesis of an oxygen-bridged bisphenol, which was obtained by an Ullmann-type cross-coupling reaction using 2,2,6,6-tetramethylheptane-3,5-dione/CuI as a catalytic system in a microwave reactor. Final fragment condensation under high-dilution conditions allowed the isolation of oxacalix4arene in 36% yield. The dynamic behavior of the new macrocycle was studied using VT NMR experiments, and the energy barriers of the basic dynamic processes (cone–cone interconversion and flip–flop motion of hydrogen bonds at the lower rim) were determined. Preliminary attempts to immobilize the macrocycle in a specific conformation revealed a surprising effect of the oxygen bridge on the conformational outcome of the alkylation reactions (cone vs partial cone).