A Pyranose sugars

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

Carbohydrate polymers derive from nature s capacity for converting carbohydrate molecules into polyacetals. Figure 1 shows how chemical bonding between a pyranose sugar s C-1 and with one of several different hydroxyls can occur in four different ways. In addition, the configuration of the bond at C-1 may be axial or equatorial (a or 3) leading to steric isomers. Thus 2 glucopyranose... 

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 a similar manner, ketones can react with alcohols to form hemiketals. The analogous intramolecular reaction of a ketose sugar such as fructose yields a cyclic hemiketal (Figure 7.6). The five-membered ring thus formed is reminiscent of furan and is referred to as a furanose. The cyclic pyranose and fura-nose forms are the preferred structures for monosaccharides in aqueous solution. At equilibrium, the linear aldehyde or ketone structure is only a minor component of the mixture (generally much less than 1%). 

Pyranose (Section 25.5) The six-membered-ring form of a simple sugar. 

It is noteworthy that D-fructose, which has a pyranose structure in the free crystalline state, assumes a furanose configuration whenever it combines with another sugar to form an oligosaccharide or polysaccharide. Apparently the ketohexose L-sorbose shows the same behavior. 

Fig. 1. Higher-order sugars L-g/yceroD-manno-Heptose (1, open-chain and 2, a-pyranose) and A -acetylneurami ilic acid (3, a-pyranose form). Structures 4-8 depict D-glucose (4) as its open-chain Fischer projection, and as the a-oxetose (5), a-furanose (6), a-pyranose (7), and a-septanose (8) ring forms.

Scheme 47 Synthesis of a 2-C-branched-chain sugar via a pyranose-fused butenolide...

The catalytic hydrogenation of difluorovinylic compounds affords C-difluoro-methyl glycosides and 2-, 3-, or 5-C-difluoromethyl sugars vide supra) (Figure 6.29). In the a-pyranose series, hydrogenation of the exocyclic double bond occurs on the jS... 

A diamino sugar, 2,4-diamino-2,3,4,6-tetradeoxy-D-arafomo-hexo-pyranose (kasugamine), is a component of the antibiotic kasugamycin. Two key steps in a synthesis182 of this sugar were the preparation of a bromodeoxy sugar and a subsequent displacement reaction by azide... 

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