Spherands, hemispherands and calixspherands

This preorganization principle not only affects the thermodynamic stability of the complex but also has a pronounced effect on the kinetic stability of the alkali complexes of the spherands. The anisyl groups in the spherands provide the preorganized ligating donor sites, and they shield the cavity from solvent molecules. As a result, the rates of complexation by the spherands are decreased and the rates of decomplexation are decreased even more when compared with related flexible crown ethers. 

In addition to the spherands, Cram synthesized compounds in which at least half of the binding sites may be considered to be preorganized, the so-called hemispherands.6 In the parent molecule 2, the molecular cavity is composed of a rigid m-teranisyl moiety in which the oxygen binding sites are conformationally organized prior to complexation, and a flexible polyether 

---