Abstract
We report on the stereoselective synthesis of both molecular granny and square knots through the use of lanthanide-complexed overhand knots of specific handedness as three-crossing “entanglement synthons”. The composite knots are assembled by combining two entanglement synthons (of the same chirality for a granny knot; of opposite handedness for a square knot) in three synthetic steps: first, a CuAAC reaction joins together one end of each overhand knot. Ring-closing olefin metathesis (RCM) then affords the closed-loop knot, locking the topology. This allows the lanthanide ions necessary for stabilizing the entangled conformation of the synthons to subsequently be removed. The composite knots were characterized by 1H and 13C NMR spectroscopy and mass spectrometry and the chirality of the knot stereoisomers compared by circular dichroism. The synthetic strategy of combining building blocks of defined stereochemistry (here overhand knots of Λ- or Δ-handed entanglement) is reminiscent of the chiron approach of using minimalist chiral synthons in the stereoselective synthesis of molecules with multiple asymmetric centers.
Highlights
Most of the small-molecule knots[1] synthesized to date are trefoil[2] (31)[3] knots, the simplest nontrivial knot topology
The stereochemical relationship between strand crossings in knots is somewhat reminiscent of that between asymmetric centers in orthodox organic molecules (Figure 1): the topological stereoisomerism of knots is determined by the relative orientation of each strand crossing; conventional organic stereoisomerism is determined by the relative handedness of stereogenic elements
Three molecular six-crossing composite knots of different topologies, −31#−31, +31#+31, and +31#− 31, were synthesized stereoselectively through a strategy involving joining together entanglement synthons of particular handedness to form a linear strand with multiple tangles that are each held in place by coordination to lanthanide cations
Summary
Most of the small-molecule knots[1] synthesized to date are trefoil[2] (31)[3] knots, the simplest nontrivial knot topology. It has previously been shown that tris(2,6-pyridinedicarboxamide) ligands can be tied into overhand knots5a,9 (threecrossing entanglements or “trefoil tangles”) upon coordination to lanthanide(III) ions.[10] The handedness of the entanglement can be controlled by introducing asymmetry at the benzylic positions of the ligand backbone: sterics dictate that only the Λ-overhand knot conformation forms from the (R,R,R,R,R,R)strand, while the (S,S,S,S,S,S)-enantiomer generates the Δoverhand knot (Figure 2).[11] Crucially, as long as a lanthanide ion remains coordinated, the strand entanglement remains in place with its handedness retained This makes the coordinated unit a potential “entanglement synthon” with open ends for constructing knots by adding together crossings.
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