Pyranoses rings

Most free pentoses, hexoses, and heptoses occur primarily in less strained pyranose rings, but the furanose ring is also quite important. The furanose ring is formed in the same way as the pyranose ring and also occurs in a and P forms. This is demonstrated with L-arabinose, which is commonly found in polysaccharides in the form of a-L-arabinofuranosyl units (see Fig. 2). 

Fig. 3. Equilibiium mistuie of D-glucose forms in solution. Pyranose ring forms predominate.

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 19.6 The phosphoglncoisomerase mechanism involves opening of the pyranose ring (Step A), proton abstraction leading to enediol formation (Step B), and proton addition to the double bond, followed by ring closure (Step C). 

In 1947, L-rhamnose was first recognized by Stacey as a constituent of Pneumococcus Type II specific polysaccharide. This finding was confirmed, in 1952, by Kabat et al. and in 1955 again by Stacey when 2,4- and 2,5-di-O-methyl-L-rhamnose were synthesized and the former was shown to be identical with a di-O-methylrhamnose, obtained by hydrolysis of the methylated polysaccharide. This result indicated a pyranose ring structure for the rhamnose units in the polysaccharide. Announcement of the identification of D-arabinofuranose as a constituent of a polysaccharide from M. tuberculosis aroused considerable interest. The L-enantiomer had been found extensively in polysaccharides, but reports of the natural occurrence of D-arabinose had been comparatively rare. To have available reference compounds for comparison with degradation products of polysaccharides, syntheses of derivatives (particularly methyl ethers) of both d- and L-arabinose were reported in 1947. 

The following schematic representation of pyranose ring closure in D-glucose shows the reorientation at C-5 necessary to allow ring formation this process corresponds to the change from Fischer to modified Fischer projection. 

Thus, it seems that the concept of anomeric electronic activation-deactivation at the anomeric center taking precedence over armed-disarmed in the remainder of the pyranose ring might have reasonable validity, but, in many instances, the difference in reactivity of the p-nitrophenyl thioglycoside versus the p-acetamidophenyl thioglycoside is not enough to make this work (Scheme 1l).94... 

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. 

The benzoyl groups are all axial, and are oriented with their planes approximately normal to the mean plane of the pyranose ring. The bond lengths are normal. The C-5-0-5, O-5-C-l, C-l-O-1, O-l-C(Bz) bond-lengths are 142.0,138.4,143.5,136.0 pm, which are characteristic of 1-O-benzoyl or 1-O-acetyl substitution. 

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