Glucose, <7 anomer mutarotation

In the case of glucose, the mutarotation gives 36 percent a, 64 percent p, and negligible strciight chain. The unequal distribution of the two anomers is due to the fact that the -OH on the anomeric carbon of the p form is equatorial, which for a chair conformer is more stable. The -OH on the anomeric carbon in the a anomer is axial, which means this anomer is slightly less stable. 

Figure 1.13 The mutarotation of glucose anomers. The specific rotation of the aqueous equilibrium mixture is +52°...

Figure 9.4 Chair forms of glucose anomers. Note that the -OH group on the anomeric carbon (carbon 1) is axial (less stable) in a-D-glucopyranose, whereas it is equatorial in (3-D-glucopyranose. Mutarotation therefore favors the latter.

Like glucose, galactose mutarotates when it dissolves in water. The specific rotation of a-D-galactopyranose is +150.7°, and that of the 13 anomer is +52.8°. When either of the pure anomers dissolves in water, the specific rotation gradually changes to +80.2°. Determine the percentages of the two anomers present at equilibrium. 

The rate of mutarotation of some sugars (for example, D-glucose and D-xylose) obeys a simple, first-order law corresponding to an equilibrium between two species only. a-n-Glucopyranose and its anomer mutarotate... 

Draw a stepwise mechanism for the acid-catalyzed interconversion of two glucose anomers by mutarotation. 

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. 

The oxidation of a-D-glucose occurs at less than 1% of the rate of oxidation of the /3 anomer. Because these two forms exist in solution in equilibrium in the proportion of 36% (a) and 64% (/3), mutarotation of the a to the /3 form must be allowed to reach equilibrium in the sample and standards for consistent... 

By considering the reversibility of the acetalforming reactions, it is apparent that treatment of either of the two methyl pyranosides with acidic methanol will prodnce the same eqnilibrium mixture. A related equilibration occurs with the anomers of glucose, as seen earlier (see Box 7.1, mutarotation of glucose). 

Certain procedures make it possible to obtain the a and 3 anomers of glucose in pure form. A 1-molar solution of a-D-glucose has a rotation value [a]o of +112°, while a corresponding solution of p-D-glucose has a value of +19°. These values change spontaneously, however, and after a certain time reach the same end point of +52°. The reason for this is that, in solution, mutarotation leads to an equilibrium between the a and p forms in which, independently of the starting conditions, 62% of the molecules are present in the P form and 38% in the a form. 

The term mutarotation means the variation of optical rotation with time, ohserved in a solution of sugar on standing. Let us have a look at this phenomenon in a glucose solution. The pure a anomer of glucose has an m.p. of 146 °C and a specific rotation [a]o +112.2°, and the specific rotation on standing is +52.6°, while pure (3 anomer has an m.p. of 148-155 °C and a specific rotation [a]D + 18.7°, and the specific rotation on standing is + 52.6°. When a sample of either pure anomer is dissolved in water, its optical rotation slowly changes and ultimately reaches a constant value of + 52.6°. Both anomers, in solution, reach an equilibrium with fixed amounts of a (35 per cent), (3 (64 per cent) and open chain ( 1 per cent) forms. 

FIGURE 7-6 Formation of the two cyclic forms of D-glucose. Reaction between the aldehyde group at C-l and the hydroxyl group at C-5 forms a hemiacetal linkage, producing either of two stereoisomers, the a and fi anomers, which differ only in the stereochemistry around the hemiacetal carbon. The interconversion of a and fi anomers is called mutarotation. 

A mutarotational equilibration is demonstrated in Fig. 5.5 by the time dependent 13C NMR spectrum of a solution of D-glucose. Initially, the solution contains almost pure a-D-glucose, but the signals at lower field corresponding to the /i-anomer soon appear. At equilibirium, the relative signal intensities indicate that the /i-anomer is favored (60% P, 40% a Fig. 5.5) [685],... 

Whatever the site of the enzyme may be, Keston et al. have recently produced fairly conclusive evidence that glucose, which is reabsorbed by the kidney, is exposed to mutarotase at some stage of the process (117). Glucose infused into the renal artery spills into urine when the renal threshold is exceeded in the same anomeric form as that administered, whereas reabsorbed glucose in the renal vein is mutarotated. Hill has also shown that the anomer infused in excess is excreted in excess (73). 

Although the crystalline forms of a- and /3-D-glucose are quite stable, in solution each form slowly changes into an equilibrium mixture of both. The process can be observed as a decrease in the optical rotation of the a anomer (+112°) or an increase for the /3 anomer (+18.7°) to the equilibrium value of 52.5°. The phenomenon is known as mutarotation and commonly is observed for reducing sugars. Both acids and bases catalyze mutarotation the mechanism, Equation 20-1, is virtually the same as described for acid- and base-catalyzed hemiacetal and hemiketal equilibria of aldehydes and ketones (see Section 15-4E) ... 

Acetamido-2-deoxy-D-glucose derivatives generally crystallize as the a anomer and mutarotate in polar solvents to a mixture of the two anomers, but the pure /3 anomer may be prepared by acylation of 2-amino-2-deoxy-(3-n-glucose at a low temperature with the acid anhydrides in A, A-dimeth-ylformamide solution.100 Mutarotation in this solvent is very slow, and the reaction product may be removed long before mutarotational equilibrium has been established. 

Define, by reference to glucose, the meaning of each of the following terms (a) a monosaccharide, (b) mutarotation, (c) a pyranose ring system, (d) anomers and (e) a- and (3-glucosides. 

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