Aldohexoses conformations

Other aldohexoses behave similarly m adopting chair conformations that permit the CH2OH substituent to occupy an equatorial orientation Normally the CH2OH group is the bulkiest most conformationally demanding substituent m the pyranose form of a hexose... 

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. 

Problem 22.57 (a) Write shorthand open structures for a ketohexose (I) and an aldohexose (II) which form the same osazone as D-(-)-altrose (III), the C epimer of o-mannose, and for the aric acid (IV) formed by HNOj oxidation of the o-aldohexoses (III) and o-talose (V). (b) Write shorthand and conformational structures for -D-altropyranose (VI). ... 

All stereocenters in 1,6-anhydrohexopyranoses are of inverted orientation compared to those in the parent 4Ci(d) or 1C4(l) conformations of the corresponding hexopyranoses for example, see 21, 23, and l,6-anhydro-/J-D-glucopyranose (22). In chemical properties, these compounds resemble to a certain degree the methyl /f-D-hexopyranosides. They are relatively stable in alkaline media, but are readily hydrolyzed by acids. In aqueous acid solution, an equilibrium is established between the 1,6-anhydrohexo-pyranose and the corresponding aldohexose, whose composition correlates with expectations from conformational analysis and calculations from thermodynamic data.121 Extreme values, 0.2 and 86%, are observed respectively with 1,6-anhydro-/f-D-glucopyranose (22) and l,6-anhydro-/f-D-idopyranose (the latter has all hydroxyl groups in equatorial disposition). 

Most molecules tend to favor one conformer over the others based on the stereochemistry of the particular monosaccharide and the steric bulk of the groups that are appended to it. For example, most aldohexoses prefer the chair conformation that places the bulky C5 hydroxymethyl group in the equatorial position. Having said that, the energy barrier between the two possible chair conformations is... 

Changes in ring conformation vastly alter the relative position of groups within the same molecule. In cases involving interaction between or reaction with two functional groups, ring shapes may determine the course of reactions. The formation of certain hexosans, (1,5)/3(1,6), has been explained upon the basis of the conformational behavior of the aldohexose.10... 

Chair conformations can be drawn by recognizing the differences between the sugar in question and glucose. The following procedure is useful for drawing D-aldohexoses. 

The nature of the sugars in a nonenzymic browning reaction determines their reactivity. Reactivity is related to their conformational stability or to the amount of open-chain structure present in solution. Pentoses are more reactive than hexoses, and hexoses more than reducing disaccharides. Nonreducing disaccharides only react after hydrolysis has taken place. The order of reactivity of some of the aldohexoses is mannose is more reactive than galactose, which is more reactive than glucose. 

The most stable conformation of the pyranose ring of most D-aldohexoses places the largest group, CH2OH, in the equatorial position. An exception to this is the aldohexose D-idose. Draw the two possible chair conformations of either the a or p anomer of D-idose. Explain why the more stable conformation has the CH2OH group in the axial position. 

Paper electrophoresis in molybdate (pH 5.0) and wolframate (pH 5.0) solutions has revealed that only those aldohexoses and aldopentoses will migrate which possess in their cyclic forms, most probably their pyranose form, a cfs-cfs-l,2,3-triol system and, thus, in at least one of their chair conformations, one equatorial hydroxyl group neighbored by two axial hydroxyl groups (13). The suggestion that the pyranose form... 

---