Topology and Nomenclature

1 Vogtle has suggested that synthetic ionophores be classified as follows (21) cor-onands are macromonocyclic compounds with any heteroatoms cryptands are bi- and polymacrocyclic ligands with any heteroatoms and podands are acyclic coronand and cryptand analogs. The term crown ether is reserved for coronands with only oxygens as heteroatoms. Such a classification will be used hereafter. 

The topologies of various cryptands are shown in Fig. 1. Considering the macrobicyclic ligands, each bridgehead may be turned either inward or outward with respect to the molecular cavity (11,12,22). This leads to three topological isomers exo-exo, exo-endo, and endo-endo. Crystal structures of one representative of each have been described. [2.2.2] adopts the endo-endo conformation with both nitrogen lone pairs directed into the cavity, while its bis-borane derivative is exo-exo (23) and the mono-borane derivative of 11.1.11 has the exo-endo form (24) (Fig. 2), with the —BH 1 group outside the cavity. 

The topological requirements of cryptands which favor complexation of alkali and alkaline earth cations may be defined as follows  

The cryptand should define a spherical cavity with heteroatom lone pairs focusing toward the center of the cavity. 

Ligand torsional strain should be minimized by adopting favorable antiperiplanar (180°) —C-VO— and synclinal (60°) —C- C— torsion angles in the oxyethylene chains. 

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