Trefoil knots synthesis

As seen above, the randomness of the Mobius strip approach and numerous difficult steps in Schill s directed approach were highly limiting factors in a trefoil knot synthesis. Both these major obstacles might be circumvented by the use of an unambiguous templated synthesis procedure. 

The successful synthesis of various catenanes by the strategy depicted in Figure 12 soon brought us to imagine a molecular trefoil knot synthesis by extending the former synthetic concept from one to two copper ions. As shown in Figure 14, two bis-chelating molecular threads (A) can be interlaced on two transition... 

We have seen how elegantly transition metals can template the formation of knots, but what about Nature s favourite templating interaction, the hydrogen bond A remarkably efficient molecular trefoil knot synthesis based on this interaction was reported by Vogtle and co-workers, who made a knotane in 20% yield [39]. This amazing route (Fig. 11) was uncovered serendipitously during the synthesis of catenanes. The crystal structure of the compound was the definitive proof for the structure, because neither NMR nor mass spectrometry could tell it apart conclusively from the macrocycles that are also formed. 

A remarkably efficient molecular trefoil knot synthesis based on organic templates was reported by VOgtle and coworkers, who serendipitously made and astutely identified the knotane. In the course of the synthesis of [ ]catenanes, reaction of the simple precursors 9 and 10 in the presence of triethylamine under high dilution conditions (10 mM in dichloromethane) produced three colorless products macrocycle 11 (15% yield, 1 1 ratio of reagents... 

Figure 14. Template synthesis of a trefoil knot (two metal centers involved).

Figure 16. Precursor and reaction scheme leading to the dicopper(I) trefoil knot (Cu2(K-86)2+). Formation of the double helix is essential this step is the weak point of the present synthesis.

High-Yield Synthesis of a Dicopper(I) Trefoil Knot using Ring-Closing Metathesis Methodology... 

Figure 24. The template-directed synthesis of the trefoil knot 69-6PF6 and of its isomeric trivial knot 70-6PF6-...

Figure 10.88 X-ray crystal structure of a hydrogen-bonded trefoil knot produced in a one-pot synthesis.104...

Ashton, P. R., Matthews, 0. A., Menzer, S., et ai, Molecular meccano, 27 - A template-directed synthesis of a molecular trefoil knot. Liebigs Annalen-Recueil 1997, 2485-2494. 

Trefoil knots are the classic examples of topologically chiral structures. The rational synthesis of molecular trefoil knots, suggested as long ago as 1953,81 was finally achieved in 1989, by Christiane Dietrich-Buchecker and Jean-Pierre Sauvage82 (Figure 18) the enantiomers of this knot were subsequently resolved.83... 

Template synthesis of a free trefoil knot and an unknotted product. 

Fig. 29. Template synthesis of the trefoil knot. The molecular thread is a metal-ligand that contains two chelating sites and the black dot represents a transition-metal ion in a pseudo-tetrahedral environment...

Fig. 11 Left. Synthesis of a trefoil knot thanks to hydrogen bonding. Right. Crystal structure of the compound, with the non-covalent interactions indicated by dashed lines...

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