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Tetramethoxycalix[4]arenes (4-8) bridged by a polyether chain at the upper r im (upper rim calixcrowns) were synthesized by reaction of 5,17-bis(chloromethyl)-25,26,27,28-tetramethoxyca~ix[4Jarene (3) with poly(oxyethy1ene) glycols of different length (mono to penta). In CDC13 solution at -20 OC, these compounds exist as a mixture of two conformations, viz. the partial cone (P) and the cone (C). The P/C ratio varies between 4.0 and 1.7 depending on the length of the bridge. The upper rim calixcrowns are flexible molecules according to temperature-dependent 'H NMR spectroscopy. For thefirst rime the mechanism of conformational interconversion of calix[4]arenes (4 and 5) was elucidated by quantitative 2-D EXSY NMR spectroscopy. It proves that there are two different pathways (a slow and a fast process) by which the two P conformational topomers can interconvert. One interconversion proceeds via C (slow process), and the other process proceeds most likely via a short-lived 1,3-alternate (A) intermediate (fast process). The rate constants for both pathways were determined; calix(4]crown-2 4, kpA = 0.58, kpc = 0.054, and ICcp = 0.27 s-] at 298 K, and calix[4]crown-3 5, kpA = 5.4, k p C = 0.07, and RCP = 0.18 s-I at 253 K. The overall rate diminishes dramatically going from a nine-atom bridge in 5 to a six-atom bridge in 4, Furthermore, shortening of the bridge has a much larger decelerating effect on the P A than on the P -. C process.

Upper rim calixcrowns: elucidation of the mechanism of conformational interconversion of calix[4]arenes by quantitative 2-D EXSY NMR spectroscopy