Pyranose ring, monosaccharides with

The chemistrv of sugars rcarbohydiatttsi is a major subject area, covered by many specialist texts. Note monosaccharides can exist as open-chain or as cyclic forms, where the oxygen atom forms part of the ring. Those with six-atom rings are called pyranoses. and those with five-atom rings are described as furenoses. 

To fully establish the structure of a disaccharide, we must determine (1) the identity of the component monosaccharides (2) the type of ring junction, furanose or pyranose, in each monosaccharide, as it exists in the disaccharide (3) the positions that link one monosaccharide with the other and (4) the anomeric configuration (a or fi) of this linkage. 

As we saw in Chapter 6, six-membered rings are generally more stable than five-membered rings, primarily because of increased torsional strain in the latter. Therefore, it is not surprising that the pyranose form of a monosaccharide is usually more stable than the furanose form. At equilibrium, glucose exists primarily as die pyranose (>99.8%), with little, if any, furanose (<0.2%) present. There is also a trace amount (0.02%) of the uncyclized aldehyde present. Of course, this equilibrium depends on the structure of the monosaccharide, and some other sugars have larger amounts of the furanose form. 

Cyclization forms the more stable ring size in a given molecule. The most common monosaccharides, the aldohexoses like glucose, typically form a pyranose ring, so our discussion begins with forming a cyclic hemiacetal from D-glucose. 

In this section the selected and most representative examples (including also several older ones) of the preparation of unsaturated monosaccharides with the double hond present in the sugar ring will be described. The material will be divided into three main classes 2,3-, 3,4-, and 4,5-unsaturated sugars in both pyranose and furanose forra... 

MUTAROTATION The a- and /T forms of monosaccharides are readily interconverted when dissolved in water. This spontaneous process, called mutarotation, produces an equilibrium mixture of a- and /1-forms in both furanose and pyranose ring structures. The proportion of each form differs with each sugar type. Glucose, for example, exists primarily as a mixture of a- (38%) and j8- (62%) pyranose forms (Figure 7.11). Fructose is predominantly found in the a-and /J-furanose forms. The open chain formed during mutarotation can participate in oxidation-reduction reactions. 

The ring forms of monosaccharides are derived by reaction of the aldehyde or ketone group with one of the carbons at the other end of the chain. It is possible to have a six-membered (pyranose) ring or a five-membered (furanose) ring. See also polysaccharide. 

Cyclic monosaccharides with six-membered and five-membered rings are called pyranoses and furanoses, respectively. 

Fig. 2 Pyranose Cj chair representation of some common monosaccharides. The standard orientation of the pyranose ring, with the endocyclic oxygen on the top right corner, has been adonted...

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