Complexation of quaternary ammonium ions by tetraester derivatives of [3.1.3.1]homooxacalixarene in mobile and in fixed conformation

In the tetraalkylation of p-tert-butyl[3.1.3.1]homooxacalixarene with BrCH2CO2R and K2CO3 in acetone, the initially formed cone conformer is converted into the more stable 1,4-alternate conformer when R = Me or Et, but not when R = i-Pr or t-Bu. In the case of R = i-Pr, derivatives in fixed 1,4-alternate conformation and in partial cone conformation were also isolated. Compounds in fixed cone conformation are good ligands for tetramethylammonium, acetylcholine, and N-methylpyridinium salts in CDCl3, but the partial cone isomer proved to be somewhat better and even the 1,4-alternate conformer turned out to be active. The possible involvement of the ester functions as additional binding sites is discussed; moreover, an insight into the energetics of the complexation and conformational isomerization processes is given.