Glucose, <7 anomer configuration

The a anomers of 2-acetamido-2-deoxy-D-glucose and 2-acetamido-2-deoxy-D-galactose have the same nir c.d. bands in 1 1 methanol-water at 0° as the anomeric mixtures have in aqueous solution. This indicates that the anomeric configuration has little influence on the nir c.d. band. 

Figure 9.6 Anomeric forms of D-glucose and D-fructose. The alpha and beta anomers are named with reference to the configuration of the glycosidic hydroxy] group associated with the oxygen bridge.

The probable configuration of isosucrose was systematically discussed by Georg,who thought that the yield was much greater when the beta anomer, rather than the equilibrium mixture or the alpha anomer, of 2,3,4,6-tetraacetyl-D-glucose was used in the condensation. Analogy with tetraacetyl-D-fructopyranosyl chloride, which yielded the acetylated... 

Compounds that have the same chemical formula but have different structures are called isomers. For example, fructose, glucose, mannose, and galactose are all isomers of each other, having the same chemical formula, C6H1206. If two monosaccharides differ in configuration around only one specific carbon atom (with the exception of the carbonyl carbon, see anomers below), they are defined as epimers of each other. (Of course, they are also isomers ) For example, glucose... 

Comparison of the Fischer (a) and Haworth (b) projections for a- and /3-D-glucose. The Haworth projection is a step closer to reality. Chair configurations for the two anomers of D-glucose are the most accurate depiction (c) but they are not always used because of the difficulty in drawing. Note that the largest substituent, —CH2OH, is in an equatorial location in both structures. The differences between the two anomers are shown in color. 

Let s now address the stereochemistry of the cyclization of D-glucose to a pyranose. Note that carbon 1, the hemiacetal carbon, becomes a new stereocenter when the cyclization occurs. Therefore, two diastereomers of the pyranose, with different configurations at the new stereocenter, are formed when D-glucose cyclizes. Such diastereomers are called anomers. The two anomers for the pyranose form of D-glucose are shown in the following equation ... 

Show Haworth projections for a-D-mannopyranose and its /3-anomer. (Remember that D-mannose differs from D-glucose only in its configuration at C-2.)... 

In the structure for maltose shown in Example 2.23, no configuration is given for the anomeric carbon of the glucose unit on the right the structure represents the state of maltose in solution—a mixture of a and (3 anomers. In crystalline maltose, the anomeric hydroxyl is a, and maltose can be described as a-D-glucosyl-(l — 4)-a-D-glucose. 

---