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

The article describes a simple and scalable preparation of 2-monothiacalix[4]arene , the simplest representative of the mixed-bridged (CH 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 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 undergoes a - equilibrium with activation free energy Δ of the inversion process of 63 kJ·mol. Interestingly, the same barrier for the oxidized sulfone derivative shows a value of 60 kJ·mol, 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 ( mechanism) were determined (compare Δ = 44 kJ·mol for vs. Δ = 40 kJ·mol for ).