Conformations of Pyranoses

Haworth projections depict pyranose rings as planar. However, as with cyclohexane, 

It is probably no accident that glucose is the most abundant natural monosaccharide because in D-glucose, the larger substituent at each ring carbon is equatorial. The only exception occurs at the anomeric carbon (C-1), where the hydroxyl group maybe axial (in the a anomer) or equatorial (in the /3 anomer). This difference provides one reason why the (3 form is preferred at equilibrium (eq. 16.3). For a ball-and-stick model of /3-D-glucopyranose, see Section 2.9. 

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The predictions for the conformations of pyranoses agree surprisingly well with the conformations actually existing in their crystals, despite the fact that the pyranoid rings are always somewhat distorted in the crystal. Perhaps the structure in solution is closer to the ideal... 

Fig. 7. (a) Chair and boat conformations of pyranose rings (b) stable chair form of [3-D-glucose. 

Fig. 2.—Different conformers of pyranose rings The 4C, chair (I) the B0 3 boat (II) two half-chairs (HI and IV), and a coplanar ring (V).

Figure 1.6 Conformers of pyranoses chair (C), boat (B), skew (S) and half chair (H).

The preferred conformation of pyranoses in solution can be predicted by empirical approaches.14 For example, free energies have been successfully estimated by summing quantitative free-energy terms for unfavourable interactions and accounting for the anomeric effects, which are individually depicted in Figure 1.9. The estimated free energies for both chair conformers can be calculated by summation of the various... 

Draw the structures (using chair conformations of pyranoses) of the following disaccharides. 

In fact, the conformation of pyranoses is dominated by two effects, not present in the cyclohexane, which appear at positions 2 and 6 of the oxane. One of them is characteristic of hexopyranoses and I propose that we call this the coplanar effect in order not to imply a particularly restrictive structure by using the name of an effect already present in methoxyethane. The other effect, present in all pyranoses, is referred to as anomeric. This name, taken from the nomenclature of sugars because it was first recognized in this family, in fact disguises its general nature since it is also present in methyl chloromethyl ether. The consequences of these effects can be modulated by cyclohexane-type interactions, but not to the point where more than a qualitative discussion is necessary. 

As a rule, molecular species that can interconvert by a pathway having an energy barrier (activation energies) less than 75kJ/mol cannot be obtained as a discrete substance at room temperature. This is the case for most conformers. However, energy differences of a few kJ/mol sufQce to allow a stable chair conformer of pyranoses to dominate over less stable ones at equilibrium in solution (e.g. shown for p-o-glucopyranose). 

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