Basic Stereochemistry of the 1,4-Dioxa Group in Polyether Complexes

1 Basic Stereochemistry of the 1,4-Dioxa Group in Polyether Complexes 

As we have seen, X-ray studies of the ionophorous antibiotics and their cation complexes were able to explain many of the steric factors that determine the selectivity patterns shown by these ligands. However, more systematic investigations on the relationship between host-cavity size and guest-ion radius could only be carried out using simpler synthetic ligands as models. In 1967, Pedersen reported the synthesis and complexing properties of a new class of compounds named crown ethers which are able to mimic effectively their natural counterpieces. 

The most common crown ethers are shown in Fig. 22. Since application of lUPAC rules to polyethers leads to somewhat cumbersome designations, we will follow the simple crown nomenclature proposed by Pedersen Polyethers are built up from 1,4-dioxa units, O—CH —CH —O. The minimum energy conformation of these units is staggered with torsion angles about C-C bonds being synclinal (60°) and about C-O bonds being antiperiplanar ( 180 °C) (for definitions see Fig. 23). These preferences, however, do not preclude deviations if required by ring formation or cation complexation. 

The latter process may cause major distortions of the torsion an es about C-O bonds whereas in the complexes, C-C bonds are essentially invariant in being synclinal ( all-gauche conformation ) in order to place the ether oxy s close to the cation. It is noteworthy that if C-O bonds are forced into the synclinal conformation they usually adopt torsion angles larger than 70° to avoid short 1,4CH. .. HC contacts 

Interestingly, aliphatic C-C bonds are found to be systematically short in all the crystal structures of cyclic and open-chain oligoethers and in their 

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