Pyranose sugars, chair conformations

FIGURE 7.9 (a) Chair and boat conformations of a pyranose sugar, (b) Two possible chair conformations of /3-D-glncose. 

The pyranoid monosaccharides provide a wide range of asymmetric molecules for study by the c.d. spectroscopist. However, these compounds are not without their difficulties. In aqueous solution, these compounds exist in a complex equilibrium involving the two possible chair conformers of the pyranoses, the furanoses, a and p anomers, and the acyclic form, as well as septanoses for aldohexoses and higher sugars. 

All of the crystalline pyranoses thus far examined adopt a chair conformation. A boat conformation has not yet been found for crystalline monocyclic compounds of sugars. Fused-ring systems seem to be required for part of the molecule to adopt a boat form, as in sedo-heptulosan (5) (where a chair form is also a part of a boat form (fused to the boat form)27 and l,6-anhydro-/3-D-glucopyranose. 

It is most straightforward to begin with a-L-sorbopyranose (1) (see Figure 1) because this sugar shows virtually no mutarotational change in water (I, 7) thus, it remains basically in the a-pyranose form. By analogy with the established (8) structure of the crystalline material, this sugar exists in solution in the 1C(l) pyranose chair conformation (1) because all substituents are thereby optimally oriented—i.e., the... 

The influence of the 4,6-O-benzylidene acetal in p-mannosylations was believed to be due to its torsional disarming effect. In an earlier work, Bert Fraser-Reid and coworkers found that a 4,6-O-benzylidene acetal locks the pyranose ring in a stable chair conformation [41], Therefore, it disfavors the formation of an intermediate oxocarbenium ion, which requires rehybridization and flattening of the sugar ring, usually leading to ahalf-chair conformation. This torsional effect, combined with the strong endo-mom nc effect in mannose, favors the a-triflate intermediate. 

Both tandem spiro-frameworks 50/51 bear a certain resemblance to trisaccharides. Generally, in the aldol adducts from 3-hydroxyaldehydes the respective pyranose conformations are induced by the preference of the hydroxy functions generated during C-C coupling for an equatorial position. Therefore, the opposite absolute configurations induced by the aldolases consequently force a change of relative orientation of the sugar chairs with respect to the bicyclic... 

Boat and chair conformations are found A. in pyranose sugars. 

Moving clockwise around the pyranose ring, the next example is provided by the deamination of 4-amino-1,6-anhydro-4-deoxy-/3-n-mannose (LXXII), which gives 1,6 3,4-dianhydro- 3-D-talose (LXXIII). There is some evidence indicating that the parent compound 1,6-anhydro-/3-D-mannopyranose, which cannot adopt the usual chair conformation (Cl), favors the reverse chair conformation (1C) to the alternative boat form (SB) Thus, we can assume that the above derivative also favors the reverse chair conformation LXXII and that the anhydro sugar is formed through the mechanism involving axial and antiparallel substituents. 

When a pyranose is in the chair conformation in which the CH2OH group and the C-1 OH group are both in the axial position, the two groups can react to form an acetal. This is called the anhydro form of the sugar. (It has lost water.) The anhydro form of D-idose is shown here. In an aqueous solution at 100 °C, about 80% of D-idose exists in the anhydro form. Under the same conditions, only about 0.1% of D-glucose exists in the anhydro form. Explain. 

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