Sugars crystalline, conformation

All of the crystalline pyranoses thus far examined adopt a chair conformation. A boat conformation has not yet been found for crystalline monocyclic compounds of sugars. Fused-ring systems seem to be required for part of the molecule to adopt a boat form, as in sedo-heptulosan (5) (where a chair form is also a part of a boat form (fused to the boat form)27 and l,6-anhydro-/3-D-glucopyranose. 

Rotation around a linkage between two sugars may take place. This influences the shape of the disaccharide, and affects the conformation that the polysaccharide will adopt. When a carbohydrate polymer is obtained in crystalline form, characterization of its shape is possible by using X-ray diffraction. However, it is not at all certain that this X-ray conformation will be that of the active form in the biological environment. Nevertheless, it constitutes a basis for formulating hypotheses concerning the shape in a biological environment. 

It is most straightforward to begin with a-L-sorbopyranose (1) (see Figure 1) because this sugar shows virtually no mutarotational change in water (I, 7) thus, it remains basically in the a-pyranose form. By analogy with the established (8) structure of the crystalline material, this sugar exists in solution in the 1C(l) pyranose chair conformation (1) because all substituents are thereby optimally oriented—i.e., the... 

However, the preparation of an appropriate crystal can prove to be more difficult than the spectroscopy itself. Naturally, sugars are typically a highly crystalline family. But in order to purify them, contemporary chemists would sooner rely on the more systematic and powerful chromatographic methods than on the uncertain search for the ideal solvent for crystallization. There is also a more fundamental problem in that a conformation in the crystal may not be the... 

In crystalline furanose 8.16, the chlorine atom adopts a pseudo-axial orientation and the nitrogen atom, a pseudo-equatorial one. The orientation of the N-O bond is given by the Newman projection along the C-l-N bond, 8.18. A compact model indicates that one face of the N=O bond is very hindered. It is probable that the same conformation exists in solution and that this is what imposes the endo approach as shown in 8.19 in which the cyclohexadiene tries to avoid the sugar support of the nitroso chloro derivative. 

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