New artificial receptors from selectively functionalized calix[4]arenes

The development of new synthetic methods for the monoalkylation of calix[4]arenes at the lower rim allows the synthesis of a new class of trihydroxamate siderophores. Three chelating hydroxamic acid units are introduced through a sequence of reactions which blocks the macrocycle in the cone conformation. The new ligands obtained form neutral 1 : 1 complexes (FeL) with iron (III), which are stable in EtOH/H2O 9: 1 at pH 2-7. Calix[4] arene his-crown ethers are prepared by exploiting the selective 1,2-(proximal) functionalization of calix[4] arenes at the lower rim. These ligands are, however, less effective in complexing alkali metal cations compared with the 1,3-calix [4] arene crown-ethers which, in their partial cone structure, offer a better shielding for the complexed cations. Rigid upper rim-bridged calix [4] arenes potentially useful for the inclusion of neutral molecules are prepared by exploiting the selective 1,3-diformylation of calix[4]arene at the upper rim. Finally a new chloromethylation method for calix [4] arenes blocked in the cone conformation is described together with the synthesis of new cavitands.