Crystal and molecular structure of 1,3-(dis/tal) derivatives of p-tert-butyl calix[4]arene. Evidence for conformational control and self-inclusion involving weak C-H ... pi interactions

X-Ray crystallographic analyses of four p-tert-butylcalix[4]arenes in which two 1,3-(distal)phenolic groups on the lower rim have been derivatised with cyanomethyl, amino ester and alkyl acetate residues, are reported. Intramolecular hydrogen bonding between the free phenolic groups and their proximal ethereal oxygen atoms is evident in all four compounds. With the exception of the ethyl ester, these calixarenes adopt very distorted cone conformations in the solid state. The ethyl ester adopts a much less distorted cone conformation with the distinguishing feature of self-inclusion of an ethoxy residue of one ester group on the lower rim within the hydrophobic cavity of a neighbouring molecule within the crystal. Factors which control the conformations of these molecules are discussed.