The template effect and synthesis of ionophorous macrocycles

Abstract

The template role of alkali and alkaline-earth metals has been investigated in detail for the synthesis of some unsubstituted cyclic polyethers – ’crown ethers’. It was found that the concept of a template effect was more applicable to the role of alkali metals than alkaline-earth metals in the synthesis of crown ethers. The template effect however should not be used in isolation, but rather in conjunction with other factors, such as the charge density of the cation, solvent dependence and the base strength of the reaction medium In order to maximise macrocyclic yields.
Optimum synthetic conditions were used to make a series of alkyl and acid substituted crown ethers; chosen because of their similarity to natural ionophorous antibiotics. The ion-transporting abilities of these compounds was examined by use of a bulk chloroform membrane experiment and liposome studies. In the bulk chloroform membrane studies there was found to be a broad correlation between the fit of a cation:crown ether complex and the transport rate. The introduction of lipophilic substituents decreased sodium and potassium transport, but a certain degree of lipophilization assisted lithium transport. Preliminary work on the synthesis of acid substituted crown ethers suggested that transport may occur via different mechanism from that of crown ethers with non-ionizable functional groups. A limited study of liposome experiments showed that bulk chloroform studies, whilst very useful, may not be directly applicable to biological membranes.

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