A co-conformational auxiliary approach to axially chiral catenanes and rotaxanes. Having recognized that co-conformational diastereoisomerism is a fundamental property of mechanically axially chiral molecules (Supplementary section 12.3), it became obvious that a co-conformational stereogenic unit could act as a temporary source of chiral information in their synthesis (Fig. ). By forming a mechanical bond selectively on one side of a prochiral unit (route a) and designing the structure such that co-conformational exchange is initially blocked, the mechanically axially chiral catenane product would be formed as a pair of separable diastereomers with identical coconformational configuration (here Rco-c) but opposite mechanical axial configuration (Rma or Sma).