[1]rotaxanes and pretzelanes: Synthesis, chirality, and absolute configuration

The synthesis of aliphatically bridged [I][n]rotaxanes and [n]pretzelanes i n preparative yields and the dependency of their chiroptical properties on the length [n] of their bridge are reported. A cycloenantiomeric bis(sulpho namide)[2]rotaxane with a sulphonamide group in its axle and its wheel was intramolecularly dialkylated by homologous bifunctional oligomethylene reag ents to form chiral [I][n]rotaxanes bearing bridges of different lengths [n ] between the axle and the wheel. Intramolecular dialkylation by 1,omega-di bromoalkanes 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 macrocy cles. Their yields decrease with decreasing length of the bridge. The short est 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 onl y three methylene groups ([1][3]rotaxane). The structural changes of the [1 ][n]rotaxanes with decreasing bridge length is expressed by an increasing h igh-field shift in the H-1 NMR spectra. Enantiomeric resolution of the race mates of both series was achieved in seven cases for the [I][n]rotaxanes an d two for the [n]pretzelanes by use of chiral HPLC columns. The circular di chrograms of both compound families show a strong dependency on the length of the bridge. However, the shortest bridges displayed some additional unex pected deviations. A new specification of the absolute configuration of sup ramolecules, such as [n]catenanes, [n]rotaxanes and [n]pretzelanes is intro duced together with some nomenclature additions.