ABSTRACT
This chapter reviews the cation-binding behavior of the two major categories of less symmetric crown ethers, i.e., the ring-enlarged and ring-contracted crown ethers, and their functionalized analogs. Historically, crown ethers have adopted a highly symmetric cyclic polyoxyethylene structure ever since the very first synthesis by Pedersen. The deduction of one methylene group between the adjacent two oxygen atoms of a symmetric crown ether gives a ring-contracted 3m—1-crown-m ether of low symmetry. Lariat ethers have been designed by George W. Gokel in order to enhance the cation-binding ability of common crown ethers by introducing a side arm carrying extra donor group and also to mimic in part the dynamic complexation process exhibited by natural ionophores. The effect of molecular symmetry on cation-binding ability of crown-6 series was examined systematically by Ouchi et al. The molecular symmetry of crown ethers has been demonstrated to affect drastically their complexation behavior.
