Glucose, <7 anomer pyranose form

The following are the more stable anomers of the pyranose forms of D glucose D mannose and D galactose... 

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 ... 

Many other sugars besides glucose exhibit mutarotation. For example, u-D-galacto-pyranose has [a)i, = +150.7 , and jS-n-galactopyranose has [oJq = +52.8 . If either anomer is dissolved in water and allowed to reach equilibrium, the specific rotation of the solution is +80.2 . What are the percentages of each anomer at equilibrium Draw the pyranose forms of both anomers. 

By contrast, periodate oxidation of D-glucose in water is initiated chiefly at C-l-C-2 of the pyranose form. As anomerization is relatively slow, both anomers are oxidized, but the p anomer is the less reactive and is also cleaved at other glycol groupings. Intermediate products, such as 4-O-formyl-D-arabi-nose (76), are probably oxidized faster than they can cyclize. The overall result is a moderately good yield of 2-(9-formyl-D-glyceraldehyde (81) when 3 molar... 

The simplest way to fix glucose in the pyranose form is to trap it as an acetal. Acid-catalysed condensation with an alcohol, methanol, for example, gives an acetal and, remarkably, the acetal has an axial OR group. Acetal formation is under thermodynamic control (Chapter 11) so the axial compound must be the more stable. This is because of the anomeric effect—so-called because this C atom is called the anomeric position and the acetal diastereoisomers are called anomers. The effect is a bonding interaction between the axial lone pair on the oxygen atom in the ring and the c orbital of the OMe group. 

Section 23.8 Hemiacetal forms of carbohydrates are interconvertible in water. The equilibrium mixture can contain a and p anomers of furanose and pyranose forms. The change from one form to the equilibrium mixture is accompanied by a change in optical rotation called mutarotation. For D-glucose, mutarotation can be described in terms of the interconversion of a-pyranose and P-pyranose forms by way of the open-chain form. 

In solution, the sugars are found predominantly in the cyclic form, more specifically, the hexoses in the pyranose form and the pentoses in the furanose form. The formation of rings creates a new asymmetric center. The two new isomers are distinguished from one another as a and p anomers, because they are epimeric with respect to C-1. In the a anomers, the anomeric hydroxyl group is axially oriented (while the anomeric hydrogen atom is equatorial), whereas in the P anomers it is equatorial. In D-glucose (d-glucopyranose) it follows that... 

The Ribose study was sorni followed by similar investigatiOTis of 2-deoxy-Ribose [71] Fmctose [69], Xylose [68], Eiythrose [97], and Glucose [98]. All observed monosaccharides (five or six carbon units) adopted the pyranose form while C4-sugars adopt furanose conformarimis as Erythrose [97]. These systems do not always follow the general trends observed in solution and in the crystalline phase where pyranose and furanose forms could be detected simultaneously. The absence of OH2 in 2-deoxy-ribose weakens the cooperative networks existing in ribose. Six conformations have been observed, four as p-anomers and two as... 

FIGURE 21.4 Glucose in its cyclic pyranose forms. As explained in t)ie text, two anomers are formed by cyclization of glucose. Tlie molecule wliose newly formed -OH group at Ci is cis to tlie oxygen atom on the lowest chirality center (C5) in a Fischer projection is the a anomer. The molecule whose newly formed -OH group is trans to the oxygen atom on the lowest chirality center in a Fischer projection is the (3 anomer. 

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