A biomimetic heteroditopic receptor for zwitterions in protic media

The efficient synthesis of calix[6]cryptothiourea 6 was achieved through a two-step sequence that involves a key [1+1] macrocyclization step. It was shown by NMR spectroscopy that this heteroditopic receptor can bind zwitterions in protic media with an outstanding selectivity for β-alanine betaine G5, which is likely due to a high complementarity between the two partners. This result constitutes a rare example of cavity complexation of a zwitterion by a calix[6]arene. In comparison with the parent urea-based receptors, 6 behaves as a much more efficient host for betaines. This strengthening of the binding properties is due to the better preorganization of the tripodal hydrogen-bonding cap as well as to the higher acidity of the thiourea groups and their poor ability to self-associate. Remarkably, host 6 is able to perform solid-liquid as well as liquid-liquid extraction of G5. Finally, 6 provides an excellent structural model for the binding site of glycine betaine G4 encountered in natural systems.