Conformations, pyranose rings

The pyranose ring conformations that are important in polysaccharides are the two chair conformations, designated and Q (Fig. 10) to indicate the disposition of atoms above and below the plane of the ring (in older notation the same conformations were denoted by Cl and 1C, respectively). Boat conformations probably have some existence in low proportions in disordered (random coil) polysaccharide chains. 

Fig. 1 Pyranose ring conformations that can be adopted by glycosidase TS.

Fig. 15. Temperature dependence of the equilibrium composition of D-ribose in aqueous (D O) solution p and f signify py-ranose and furanose, and Cl and 1C refer to pyranose ring conformations see Fig. 14.

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... 

In 1991, an important paper was published by Bock et a/.84 that described the steric and electronic effects on the formation of the dispiroketal dihexulose dianhydrides. The authors described the conformation of six dihexulose dianhydrides, as determined by X-ray crystallography or NMR spectroscopy. They concluded that these conformations are dictated by the anomeric and exo-anomeric effects. Thus, the dihexulose dianhydrides are disposed to adopt conformations that permit operation of these effects—even if this results in the dioxane ring having a boat conformation or all three substituents on one pyranose ring being axial. 

The pyranose rings can adopt either of two different chair conformations called C, and C4. Pyranoses usually adopt a chair conformation that puts the majority of bulky groups in the equatorial position, so that steric interactions are minimized. The Ci(d) conformation and the ring numbering system are shown in formula 1. 

Conformation Groups that favor a flattening of the pyranose, a consequence of oxonium ion formation, increase the rate of reactivity. Ley uses 3,4-spiroketals to enforce chairlike structure on the pyranose ring, thus deactivating them toward reaction. 

Our discussion of the conformational preference of H2CF—OH provides the simplest type of explanation for the so called anomeric effect333 which refers to the tendency of an electronegative substituent at C -1 of a pyranose ring to exhibit a greater preference for the axial over the equatorial conformation than it does in cyclohexane. 

The planar Haworth projection formulae bear little resemblance to the shape of the six-membered pyranoses that actually adopt a non-planar ring conformation comparable to that of cyclohexane. The chair form is the most... 

Figure 9.9 Conformation of D-glucose. The pyranoses adopt a non-planar ring conformation and the chair form with the highest number of equatorial rather than axial hydroxyl groups is favoured. It should be noted that a-D-glucopyranose, in contrast to /3-D-glucopyranose, has an axial hydroxyl group.

Monosaccharides have many structural variations that correspond to local minima that must be considered. Acyclic carbohydrates can rotate at each carbon, and each of the three staggered conformers is likely to correspond to a local minimum. The shapes of sugar rings also often vary. Furanose rings usually have two major local minima and a path of interconversion. Experimental evidence shows a clear preference for only one chair form for some pyranose rings, but others could exist in several conformers. For exanqple, the and conformers must all be considered as possible structures for L-iduronate, as discussed by Ragazzi et al. in this book. 

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