Synthesis of Monothiacalix[4]arene Using the Fragment Condensation Approach

The article describes a simple and scalable preparation of 2-monothiacalix[4]arene 7, the simplest representative of the mixed-bridged (CH₂ and S) calix[4]arenes. The synthesis is based on the condensation of linear building blocks (bisphenols), which are relatively readily available, and allows, depending on the conditions, the use of two alternative reaction routes that provide macrocycle 7 in high yield. The dynamic behavior of the basic macrocyclic skeleton was investigated using NMR spectroscopy at variable temperatures. High-temperature measurements showed that compound 7 undergoes a cone-cone equilibrium with activation free energy ΔG^# of the inversion process of 63 kJ·mol^-1. Interestingly, the same barrier for the oxidized sulfone derivative 14 shows a value of 60 kJ·mol^-1, indicating weakened hydrogen bonds at the lower rim of the calixarene. The same was also confirmed at low temperatures, when barriers to changing the direction of the cyclic hydrogen bond arrays (flip-flop mechanism) were determined (compare ΔG^# = 44 kJ·mol^-1 for 7 vs. ΔG^# = 40 kJ·mol^-1 for 14).