Glucose, <7 anomer chair conformation

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. 

The other anomer of glucose, in contrast, has its anomeric proton (/31) in the axial position. In the preferred chair conformation of glucose, protons occupy all of the other axial positions leading to presumed diaxial interactions between H-l (/31) and H-3 (/33) and between H-l (/31) and H-5 (/35).The ROESY spectrum reveals three interactions with the anomeric proton, /SI, at 4.67 ppm. The H-2 COSY interaction is, of course, present, and the two-diaxial NOE interactions to H-3 (/33) and H-5 (/S5) are quite evident. 

Q Draw and identify the structures of glucose, its anomers, and its epimers, as Fischer projections and as chair conformations. 

Draw the Fischer projections and the chair conformations of the anomers and epimers of glucose from memory. Identify and name these sugars based on how they differ from the structure of glucose. Problems 23-52 and 53... 

Glucose has all substituents larger than a hydrogen atom in the more roomy equatorial positions, making it the most stable and thus most prevalent monosaccharide. The p anomer is the major isomer at equilibrium, moreover, because the hemiacetal OH group is in the equatorial position, too. Figure 27.7 shows both anomers of D-glucose drawn as chair conformations. 

Chair conformations of a-D-glucopyranose and )3-D-glucopyranose. Because a-D-glucose and /3-D-glucose are different compounds (they are anomers), they have different specific rotations. 

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. 

Fig. 37 (a) Fisher projection of D-glucose. (b) Haworth projections of a and p anomers. (c) Ci chair conformations, (d) Newman projections of the plausible conformations of the hydroxymethyl group around C5-C6 and Cg-O bonds. (From [225])... 

The polysaccharides are, after the proteins and the nucleic acids, the third important group of biological polymers. From the point of view of the quantum-mechanical calculations of their conformational problems, mention may be made in the first place of the work of Neely 13> who has studied by the Extended Hiickel Theory the relative conformational stabilities of a few of the different possible chair and boat conformations of D-glucose, II. Within the conformations studied, the chair one, known under the designation Cl, appears so far the most stable. Experimentally, it is apparently the most stable of all. Moreover, the (3-anomer comes out as 9 kcal/mole more stable than the a-one. Although this difference is far too large with respect to experiment, it is in the proper direction. 

Studies of the structures of the cyclic hemiacetal forms of D-(-+-)-glucose using X-ray analysis have demonstrated that the actual conformations of the rin are the chair forms represented by conformational formulas 6 and 7 in Fig. 22.3. This shape is exactly what we would expect from our studies of the conformations of cyclohexane (Chapter 4), and it is especially interesting to notice that in the j8 anomer of D-glucose all of the large substituents, —OH and —CH2OH, are equatorial. In the a anomer, the only bulky axial substituent is the —OH at Cl. 

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