Conformation figure-eight

Solid-state X-ray structural studies of the tetrakis HCl salt of 8.5 (i.e., 8.5b) revealed that the molecule does not exist in a planar, circular conformation as represented in Scheme 8.1.1. Rather, as depicted in Figures 8.1.2 and 8.1.3, the molecule adopts a helical twist and overall figure-eight conformation. This twisting makes 8.5b chiral by virtue of conformation and serves to define a pair of enantiomeric atropisomers, at least in the solid state. In solution, detailed H... 

Figure 8.1.2 Schematic Representation of Structure 8.5a in a Figure-eight Conformation...

Proton NMR spectroscopic studies of the non-conjugated macrocycle 8.12 revealed the existence of a C2-symmetry element within the macrocycle. However, on the basis of the diastereotopicity observed for the ethyl side-chains on the periphery of the macrocycle, it was concluded that this molecule, like turcasarin, preferentially adopts a chiral figure-eight conformation. The same conclusion was... 

A single crystal X-ray structure (Figure 8.2.4) of 8.20a confirmed that the proposed figure-eight conformation is manifest in the solid state. In this case, however, H NMR spectroscopic studies revealed that this molecule undergoes rapid conformational inversion in solution (i.e., the enantiomeric forms interconvert on... 

It is likely that anion-protonated macrocycle interactions also play a critical role in establishing the three-dimensional geometry of turcasarin tetrahydrochlor-ide. ° The quadruply protonated form of this macrocycle (10.44-4HC1) is found to adopt a helix-like figure-eight twist both in solution and in the solid state. While the determinants of conformation in solution are potentially difficult to deconvolute . 

Zn(II). Molecular shape is clearly related to metal-binding activity. Patellamide C adopts a figure eight conformation when uncomplexed and in the presence of Zn(II). However, when Cu(II) is added, the conformation switches to a square form that can accommodate Cu(II) preferentially. When one or two of the oxazoline rings in patellamide A were synthetically opened, Ca bound with low affinity. " " Similarly, synthetic ester analogues of ascidiascylcamide led to the formation of a relatively stable complex. More recently, westiellamide has also been shown to bind Cu(II) in mononuclear and dinuclear clusters." ... 

Figure 2 Shape of cyanobactins. These molecules adopt a number of conformations that are important to metal binding and activity, including (clockwise from upper left) triangle, square, lissoclinamide turn, and figure eight. This figure was adapted from a previous figure, reprinted with permission from P. Wipf P. C. Fritch S. J. Geib A. M. Sefler, J. Am. Chem. Soc. 1998, 120, 4105-4112, copyright 1998 American Chemical Society.

Similarly, the rate constant for dehydrochlorination of 9, which can react by syn-coplanar elimination, was found to be about eight times greater than the rate constant for 10, which can only react from an anti-clinal conformation (Figure 10.20). ... 

Figure 35 The figure-eight conformation observed in the crystal structure the chloride complex of 61. Each pocket of the figure eight contains a chloride template held by six H-bonds.

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