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.