[1]rotaxanes and pretzelanes: synthesis, chirality, and absolute configuration [In Process Citation]

The synthesis of aliphatically bridged [1](n)rotaxanes and (n)pretzelanes in preparative yields and the dependency of their chiroptical properties on the length (n) of their bridge are reported. A cycloenantiomeric bis(sulphonamide)[2]rotaxane with a sulphonamide group in its axle and its wheel was intramolecularly dialkylated by homologous bifunctional oligomethylene reagents to form chiral [1](n)rotaxanes bearing bridges of different lengths (n) between the axle and the wheel. Intramolecular dialkylation by 1,omega-dibromoalkanes of a topologically chiral bis(sulphonamide)[2]catenane with a sulphonamide group in both of the macrolactam rings leads to pretzel shaped molecules ((n)pretzelanes) with homologous bridges between the two macrocycles. Their yields decrease with decreasing length of the bridge. The shortest bridge isolated so far in reasonable amounts consists of six methylene groups ((6)pretzelane). Remarkably, a covalent connection of axle and wheel in a [2]rotaxane was successful even with much shorter bridges-down to only three methylene groups ([1](3)rotaxane). The structural changes of the [1](n)rotaxanes with decreasing bridge length is expressed by an increasing high-field shift in the 1H NMR spectra. Enantiomeric resolution of the racemates of both series was achieved in seven cases for the [1](n)rotaxanes and two for the (n)pretzelanes by use of chiral HPLC columns. The circular dichrograms of both compound families show a strong dependency on the length of the bridge. However, the shortest bridges displayed some additional unexpected deviations. A new specification of the absolute configuration of supramolecules, such as [n]catenanes, [n]rotaxanes and (n)pretzelanes is introduced together with some nomenclature additions.