Pyranose sugars, conformation

The incorporation of heteroatoms can result in stereoelectronic effects that have a pronounced effect on conformation and, ultimately, on reactivity. It is known from numerous examples in carbohydrate chemistry that pyranose sugars substituted with an electron-withdrawing group such as halogen or alkoxy at C-1 are often more stable when the substituent has an axial, rather than an equatorial, orientation. This tendency is not limited to carbohydrates but carries over to simpler ring systems such as 2-substituted tetrahydropyrans. The phenomenon is known as the anomeric ect, because it involves a substituent at the anomeric position in carbohydrate pyranose rings. Scheme 3.1 lists... 

FIGURE 7.9 (a) Chair and boat conformations of a pyranose sugar, (b) Two possible chair conformations of /3-D-glncose. 

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... 

In the pyranose sugar crystal structures, intramolecular hydrogen bonds are formed between C(n)OH and C(n+2)OH groups when the molecular conformation is such that the C-OH bonds are in the syndiaxial orientation, i.e. ... 

Boat and chair conformations are found A. in pyranose sugars. 

The exact sugar conformation in the ES complex of GH 33 enzymes seems to vary from substrate to substrate. Thus, complexes of 4-methylumbelliferyl N-acetyl-a-neuraminide in complex with the (inactive) acid-base mutant of the T. cruzi tf(2 .s-sialidase reveal a conformation, whereas the complex with sialyllactose has a 82 5 ring these are next to each other on the pseudoro-tational itinerary (Figure 2.6b). A possible conformational trajectory for the pyranose ring in these enzymes is 82 5 (ES complex) (probably most stable conformation of the sialosyl cation, with NElAc and OH pseudoequa-torial) Cs (glycosyl-enzyme intermediate). This would result in most of the motion of the substrate relative to enzyme in the course of catalysis being in the carbon which is being substituted, in the normal way. 

The calculated free-energies given in Table XV are relative to an imaginary hexopyranose that has no non-bonded interactions. The work provides a basis for predicting the stabilities of aldohexo-pyranoses in conformations that may apply to certain reaction intermediates, as well as to sugars in solution. 

Verstraeten has made the interesting suggestion that the ring-breathing vibration of the pyranoid ring causes infrared absorption (at 781 5 cm.- ) only when there is instability in the conformation adopted by the pyranose sugar, but the basis of this correlation was not explained. 

The incorporation of heteroatoms can result in stereoelectronic effects that have a pronounced effect on conformation and, ultimately, on reactivity. It is known from numerous examples in carbohydrate chemistry that pyranose sugars substituted... 

Of the infinite conformations that pyranose sugars can have, there are two extremes that resemble chairs. The two chair conformations are usually nonequivalent, such that one is the most favored conformation and the other is the least favored conformation. There are two types of bonds around each of the carbon atoms in the ring. These bonds are either within the plane of the ring and are called equatorial bonds, or they are perpendicular to the plane of the ring and are called axial bonds [19]. 

Whereas the furanose ring forms of sugars exist as planar rings it should be emphasized that this is not so in the pyranose sugars. If the normal valency angles are to be maintained in the latter then a non-planar conformation must be adopted. ... 

The relationship between the Haworth formulae and the two chair forms for the more common pyranose sugars is illustrated in the following formulae. In drawing conformational formulae the direction of the bonds is determined by the tetrahedral valencies of the carbon atoms and the glycosidic carbon links in the Ci conformation must be drawn as in (CXXX) and not as in (CXXXI). 

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