THE HYDROGENATION OF HETEROCYCLIC CALIX[4]ARENES, A TRANSFORMATION LEADING TO NOVEL MACROCYCLIC LIGANDS

The simple and efficient synthesis of calix[4]furan and calix[4]pyrrole has been known for more than 125 years. The connectivity of the heterocyclic calix[4]arenes is identical with the skeleton of the "pigments of life". The chemical properties, especially the ability to form complexes with transition metals, are totally different when comparing the artificial systems with the natural products. A direct way to confer metal binding capabilities to compounds derived from heterocyclic calix[4]arenes is the hydrogenation. We review the known hydrogenation reactions of calix[4]furan and calix[4]pyrrole and present our own work in this field. A short description of the metal binding process of our new calix[4]pyrrolidines (= the fully saturated calix[4]pyrrole) with copper will be discussed as a typical example of the metal binding capacities. The new calix[4]pyrrolidines can be structurally classified as new stiffened macrocyclic crown ethers or as saturated analogues of the porphyrin derived "pigments of life".