A Trefoil Knot

AND ITS LARGER OCTALACTAM ANALOGUE 26, WHEN THE REACTION SEQUENCE SHOWN IN SCHEME 8 IS REVERSED AND EXTENDED DIAMINE 25 IS REACTED WITH 2,6-PYRIDINE DICARBOXYLIC ACID DICHLORIDE. 

The third method which provides evidence for a knotted structure is mass spectrometry.42 With electrospray ionization (ESI) it is possible to ionize the knot and other similar molecules by protonation and to transfer them into the highly diluted gas phase of a mass spectrometer. In a so-called tandem-MS experiment, the parent ion, i.e. the protonated knot, is isolated and subjected to collisions with a stationary gas inside the 

The Next Surprise Rotaxane Synthesis Mediated by a Template Based on Hydrogen-Bonded Anions 

SCHEME 14 AXLE CENTERPIECES 30 AND 31 THAT WERE USED AS CONTROL COMPOUNDS FOR A MORE DETAILED INVESTIGATION OF THE MECHANISM OF AMIDE ROTAXANE FORMATION. 

Since axle centerpiece 30 does not contain any functional group which is capable of forming strong interactions with the wheel, it seems 

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Figure 6. The Mobius strip approach (three half-twists) to a trefoil knot.

Figure 9. The 6-center attempt to form a trefoil knot [43, 44],...

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. 

Figure 10. The hook and ladder approach to a trefoil knot, as proposed by Walba [51].

Figure 11. Sokolov s strategy for constructing a trefoil knot on an octahedral tris-chelate complex [39].

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

Figure 3. Template-directed approach to a trefoil knot and its isomeric trivial knot. Note that the starting acyclic components must incorporate terminal reactive functions which can only lead to intercomponent rather than intracomponent coupling.

Scheme 1. Pictorial representation of a catenane 1, a rotaxane 2, and a trefoil knot 3.

Figure 9 illustrates die simplest knot, a trefoil knot, drawn in a DNA context, so that the strand has been assigned a polarity. Each of the nodes has been sur-... 

Figure 9. The relationship between a half-turn and a node. A trefoil knot has been drawn with thick lines its polarity is shown by the arrowheads along the knot. A dashed box has been drawn about each node, so that the strands of the knot divide the boxes into four regions, two between antiparallel strands, two between parallel strands. A half-turn of base pairs is drawn between antiparallel strands the helix axes are shown as double-headed arrows and dyad axes normal to them are represented by dotted lines ending in two ellipses.

Du, S.M., Seeman, N.C. (1994) The construction of a trefoil knot from a DNA branched junction motif. Biopolymers 34, 31-37. 

Scheme 12 A trefoil knot 27 forms as a byproduct together with the expected macrocycle 18...

Scheme 13 Tentative template mechanism leading to the formation of a trefoil knot. After...

Figure 10.54 Topological isomerism, diastereoisomerism and chirality as related to a [2] catenane, a trefoil knot and a doubly interlocked [2] catenane.

Finally, a trefoil knot has also been reported based on Stoddart s blue box donor-acceptor stacking templating methodology (Section 10.7.3).105... 

Fig. 30. The topological equivalence between two presentations of a trefoil knot. Note that the minimum number of crossings is 3...

In the field of molecular knots templated by transition metal ions, another remarkable achievement deserves to be mentioned. Hunter and co-workers recently prepared a trefoil knot in a very satisfactory fashion by first wrapping a string-like molecule around a single transition metal ion with an octahedral coordination sphere (Zn2+) so as to obtain an open knotted structure and, subsequently, they could link the two ends of the string to form a real trefoil knot [38]. This nice piece of work again confirms the power of transition metals when it comes to preparing topologically novel species. 

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...

Fig. 12 Representation of the synthetic pathway to a trefoil knot by hydrogen bond-mediated weaving...

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