Template effect cryptand synthesis

A template effect due to NH intramolecular hydrogen bonding in the synthesis of [l.l.l]cryptand has been reported by Montanari et al. (Scheme 3). 

In the synthesis of many cryptands, a metal ion has been specifically employed as a template (often chosen to match the cavity size of the target cage) to boost the yield of the product. In other cases, a metal template effect is undoubtedly in operation through the use of an alkali metal reagent even though such a role had not always been anticipated when the synthetic procedure employed was chosen. However, it needs to be noted that apart from the above a number of other reaction types, employing a diverse range of precursors, have been employed for cryptand formation. ... 

The one-pot synthesis of polymers via Ugi-4CR can only be performed if either two bifunctional AA-type components or one AB-type component are used. Moreover, highly concentrated reaction mixtures are needed because otherwise ring formations occur, as thoroughly investigated by Wessjohann and coworkers. Here, the directed synthesis of macrocycles or cryptands under classic or pseudo high dilution conditions, or even without dilution exploiting metal template effects were demonstrated [66, 67]. 

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