Recognition of amides by new rigid calix[4]arene-based cavitands

The synthesis of new hosts specifically designed for the recognition of ami des, characterized by two binding regions: a rigid calix[4]arene cavity and a sidearm, inserted at its rim, able to form strong hydrogen bonds, is des cribed. The binding abilities of the new receptors toward amides of general structure (RCONRR3)-C-1-R-2 have been investigated in CDCl3 solution by H- 1 NMR spectroscopy. When the additional binding site is the N-phenylureido group spaced by a methylene unit from the apolar cavity, binding constants up to 756 M-1 were measured. Neither the two separate potential binding sit es, nor the model host, where the calix[4]arene skeleton is flexible show d etectable binding ability toward the series of guests examined. The rigidit y of the calix[4]arene apolar cavity is the key control element in determin ing the efficiency of these molecular recognition processes. The presence o f NH groups in the guest controls the efficiency and selectivity of binding .