Finally, we considered the stereochemical nature of catenanes in which one or both prochiral units are replaced with covalent stereocentres (Supplementary section 14). Such structures represent logical alternative precursors to axially chiral catenanes if they could be prepared diastereoselectively and the in-plane substituents subsequently symmetrized. Furthermore, there has been a suggestion that the latter class might contain both mechanical axial and mechanical topological stereogenic units .