Stereoselective synthesis of a composite knot with nine crossings

The simultaneous synthesis of a molecular nine-crossing composite knot that contains three trefoil tangles of the same handedness and a 9 7 3 link (a type of cyclic [3]catenane topologically constrained to always have at least three twists within the links) is reported. Both compounds contain high degrees of topological writhe ( w = 9), a structural feature of supercoiled DNA. The entwined products are generated from the cyclization of a hexameric Fe( ii ) circular helicate by ring-closing olefin metathesis, with the mixture of topological isomers formed as a result of different ligand connectivity patterns. The metal-coordinated composite knot was isolated by crystallization, the topology unambiguously proven by tandem mass spectrometry, with X-ray crystallography confirming that the 324-atom loop crosses itself nine times with matching handedness (all Δ or all Λ) at every metal centre within each molecule. Controlling the connectivity of the ligand end groups on circular metal helicate scaffolds provides an effective synthetic strategy for the stereoselective synthesis of composite knots and other complex molecular topologies. A composite knot with nine crossings of the same handedness has been prepared from a hexameric circular helicate in 41% yield in a two-step synthesis. An isomeric cyclic [3]catenane topologically constrained to always have at least three twists within the links is also formed. Both topologies have a high degree of writhe, analogous to that of supercoiled DNA.