DISSYMMETRIC CALIX[4]ARENES WITH C4-SYMMETRY AND C2-SYMMETRY - SYNTHESIS, X-RAY STRUCTURES, CONFORMATIONAL FIXATION, AND H-1-NMR SPECTROSCOPIC STUDIES

Six dissymmetric calix[4]arenes with C4 symmetry consisting of four 3, 4-disubstituted phenolic units have been obtained by condensation of t he respective 2- or 6-hydroxymethylated phenols. The regular incorpora tion of the phenolic units into the macrocycle was established on the basis of H-1 NMR spectra and in one case also by X-ray analysis. One e xample for a calixarene with C2-symmetry consisting of p-cresol and 3, 4-dimethylphenol units in alternating sequence was also prepared. Cali xarene 4a consisting of 3,4-dimethylphenol units shows a lower energy barrier (DELTAG(double dagger) = 13.4 kcal/mol) for the cone-to-cone i nversion in CDCl3 than p-methylcalix[4]arene (DELTAG(double dagger) = 14.6 kcal/mol). The dissymmetric calixarenes 4 could be conformational ly fixed by alkylation of the phenolic hydroxyl group. Due to the equi valence of all four phenolic units, mono- and 1,3-di-derivatives could be obtained in addition to tetra derivatives. A tetraester derivative in the cone conformation, like the parent calix[4]arene, crystallizes as a racemate. In solution these tetraester derivatives exist in a di storted cone conformation with C2 symmetry. Dynamic H-1 NMR spectra re veal an energy barrier of DELTAG(double dagger) = 13.3-13.4 kcal/mol f or the C2-to-C2 interconversion with the C4-cone as transition state.