No abstract

Template-assisted reactions make use of the self-organization of two (or more) molecules, one being the host, the other acting as a guest [l]. By that means the reactants adopt a conformation that facilitates the formation of a generally macrocyclic product. Although the term 'template effect' was introduced in the early sixties [2], only the enormous impetus of supramolecular chemistry [3] within the last two decades has led to the chemical breakthrough of this elegant strategy. The application of the template effect not only raises yields, but also often enables the preparation of otherwise unobtainable products.The progress in synthesis of catenanes, rotaxanes, and knots is exemplary for the effect of templates. Early syntheses of these molecules with mechanical bonds used statistical or multistep procedures [4]. Yields were very low and thus catenanes and rotaxanes had the standing of laboratory curiosities. The pioneering work of Sauvage et al. [5] and Stoddart et al. [6] introduced the template strategy to the synthesis of mechanically interlocked molecules by using ionic coordination (I) and ionic n-n donor-acceptor complexes (II) combined with hydrogen bonding, respectively (Figure 1). In 1992 Hunter et al. [7a] and Vogtle et al. [8]established, independently of each other, a third, yet nonionic template effect founded on hydrogen-bonding and 7t-n interactions (111), the same interactions that are involved in many biological processes.The synthesis of molecules with mechanical bonds via the nonionic template strategy is all the more fascinating if one considers naturally occurring DNA catenanes [9] or proteins forming knotted structures [lo]. Starting from simple twoand one-step syntheses of catenanes by the use of neutral templates, it took only a few years to develop not only amide-based rotaxanes, but also nanoscale assemblies of several mechanically bound building blocks, topologically chiral 'pretzelanes' and cycloenantiomeric rotaxanes as will be described in the following section of this progress report.

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