Synthesis and conformational behaviour of lower-rim tetraacetylated thiacalix[4]arenes

The acylation of thiacalix4]arenes with AcCl or Ac₂O gave the corresponding lower-rim tetraacetoxy derivatives. In contrast to classical calix4]arenes, tetraacetylated thiacalix4]arenes are conformationally mobile in solution and represent a thermodynamic equilibrium of three different conformers at room temperature. As proven by a dynamic ¹H NMR study, conformational preferences of acetylated thiacalix4]arenes considerably depend on the upper-rim substitution. Hence, t-Bu thiacalixarene prefers 1,3-alternate and 1,2-alternate conformations (43% and 38%, respectively), while the upper-rim unsubstituted compound adopts preferably the partial cone conformation (70%).