Dissymmetric calix[4]arenes with C4- and C2-symmetry. Synthesis, x-ray structures, conformational fixation, and proton NMR spectroscopic studies
作者:Giovanni D. Andreetti、Volker Boehmer、J. Graham Jordon、Moniralsadat Tabatabai、Franco Ugozzoli、Walter Vogt、Artur Wolff
DOI:10.1021/jo00067a040
日期:1993.7
Six dissymmetric calix[4]arenes with C4 symmetry consisting of four 3,4-disubstituted phenolic units have been obtained by condensation of the respective 2- or 6-hydroxymethylated phenols. The regular incorporation 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 example for a calixarene with C2-symmetry consisting of p-cresol and 3,4-dimethylphenol units in alternating sequence was also prepared. Calixarene 4a consisting of 3,4-dimethylphenol units shows a lower energy barrier (DELTAG(double dagger) = 13.4 kcal/mol) for the cone-to-cone inversion in CDCl3 than p-methylcalix[4]arene (DELTAG(double dagger) = 14.6 kcal/mol). The dissymmetric calixarenes 4 could be conformationally fixed by alkylation of the phenolic hydroxyl group. Due to the equivalence 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 distorted cone conformation with C2 symmetry. Dynamic H-1 NMR spectra reveal an energy barrier of DELTAG(double dagger) = 13.3-13.4 kcal/mol for the C2-to-C2 interconversion with the C4-cone as transition state.