Macrocyclic versus acyclic hosts

Podand An acyclic chain-like or branching host with a number of binding sites that are situated at intervals along the length of the molecule, or about a common spacer. 

Macrocycle A cyclic molecule usually with nine or more atoms in the ring and generally in supra molecular chemistry containing a number of binding sites that are arranged around the closed system. 

Allosteric Effect The binding of a guest at one binding site that is influenced by the binding of a second guest, either the same species (homotropic effect) or a different kind of guest (heterotropic effect), bound elsewhere within the host. 

The synthesis of podand hosts is conceptually relatively straightforward and is generally achieved using conventional synthetic methods such as ether, sulfide or amide bond formation however, the synthesis of macrocycles is a little more challenging and is accomplished by one or both of two general procedures, i.e. high-dilution synthesis and template synthesis. 

Sendhoff, N., Mekelburger, H. -B. and Vogtle, F., High-dilution reactions - new synthetic applications, Top. Curr. Chent., 1991, 161, 1-36. 

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