Alpha anomers

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... 

When a monosaccharide forms a cyclic hemiacetal (or hemiketal), the carbonyl carbon becomes a stereocenter. Thus, cyclization leads to formation of two possible stereoisomers. These isomers are called anomers, and the former carbonyl carbon is called the anomeric carbon. The isomer with the anomeric OH (shown in blue) pointed down is the alpha anomer. The isomer with the anomeric OH (shown in blue) pointed up is the beta anomer. (Figure 12.14)... 

A five-membered cyclic sugar ring is called a furanose. Fructose prefers a fu-ranose ring system, and is formally named fructofuranose. Like glucose, fructose can cyclize and can form either an alpha anomer or a beta anomer. Notice that,... 

Methyl L-sorboside was discovered by Fischer,128 while its antipode was prepared for the first time by Lobry de Bruyn and Van Ekenstein.21 From a comparison of the [ ]d values of the glycosides with that of the ketoses, Hudson and Brauns124 designated both isomers as the alpha anomers. 

Since the methyl L-sorboside ([< ]d — 88°) is more levorotatory than the parent sugar L-sorbose ([< ]d — 43°), it was considered to be the alpha anomer of this L-ketose. In the same manner methyl D-sorboside ]D + 88°) was designated as the alpha form since it was more dextrorotatory than its parent sugar of the D-series, namely D-sorbose ([a]D + 43°). (See also page 113.)... 

Kurosu J, Sato T, Yoshida K, Tsugane T, Shimura S, Kir-imura K, Kino K, Usami S. Enzymatic synthesis of alpha-arbutin by alpha-anomer-selective glucosylation of hydroquinone using lyophilized cells of Xanthomonas campestris WU-9701. J. Biosci. Bioeng. 2002 93 328-330. 

The sugars exist in ring forms in solution, and there are two anomers for each. There is good evidence that glucose isomerase only acts on the alpha anomers. 

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.

Figure 9.17 Alpha and beta anomers of maltose. When only the hydroxyl group on the anomeric carbon of one of the monosaccharide residues is involved in the glyco-sidic linkage, the anomeric hydroxyl group on the other residue is still free. This permits two possible orientations, which are described as either alpha or beta forms of the monosaccharide residue.

Anomerization can conceivably be brought about by dissociation of the Cl-to-ring-oxygen bond, followed by reclosure of the ring to form either the alpha or beta anomer.98... 

This internal nucleophilic addition introduces a new chiral centre into the molecule. The carbon of the new centre is known as the anomeric carbon and the two new stereoisomers formed are referred to as anomers. The isomer where the new hydroxy group and the CH2OH are on opposite sides of the plane of the ring is known as the alpha (a) anomer. Conversely, the isomer with the new hydroxy group and terminal CH2OH on the same side of the plane of the ring is known as the beta (P) anomer (Figure 1.12). 

For polysaccharide, choose connectivity (01-C[l-6]), anomer (alpha or beta), isomer (l or d), and conformation (define phi and psi angle with omega = 180). Add sugars (alsohexoses or aldopentoses) to build polysaccharide chain. 

Anomers (designated by alpha and beta) are sugar diastereomers in which the free carbonyl (—c=o) group opens up to make a chain and two new diastereomers (see fig. 3.1). 

Another advantage of chromatography at elevated temperatures is that the rate of interconversion of the alpha and beta anomers of some saccharides may be increased, resulting in the eUmination of... 

At equilibrium, the (3 anomer of D-glucose predominates, because the —OH group of the anomeric carbon is in the more stable equatorial position of the more stable chair conformation. In a-D-glucose, the —OH group on the anomeric carbon is axial. When remembering the names of D-glucose anomers, some students find it helpful to remember the phrase alpha is axial. 

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