CALIXARENES .36. CONFORMATIONAL CHARACTERISTICS OF P-TERT-BUTYLCALIX[6]ARENE ETHERS

The conformational characteristics of a variety of O-substituted p-tert-butylcalix[6]arenes have been studied by means of the H-1 NMR patterns arising from their ArCH2Ar methylene protons and from the position of the C-13 NMR lines arising from the carbons bearing these protons. On the basis of this information, the tribenzyl trimethyl ether 13, the bis-calixarenes 15 and 16, the monobenzyl ether 3, the monobenzyl pentamethyl ether 10, and the hexakis(p-cyanobenzyl) ether 19b are established as having cone conformations at ambient temperature. The tetrabenzyl dimethyl ether 14, the tetrabenzoyl esters 17a-d, the tetraarylmethyl ethers 18a-d, and the bridged ether 21, on the other hand, are found to possess 1,2,3-alternate conformations. The conformational mobilities of these compounds were assessed by variable temperature H-1 NMR spectral measurements, leading to the classification of the structures as immobile, semimobile, or mobile with respect to conformational interconversion via the ''lower rim through the annulus'' pathway.