Anomer, aldopyranose

Calculated Conformational Free Energies" and the Proportion of the a Anomer in Aqueous Equilibrium Solution of Aldopyranoses... 

Aldopyranose anomers with axial C-1—H bond are more readily oxidized by bromine than those with equatorial C-l—H bond. A recent examination of this reaction led to the conclusion that "these rate differences could be reflected in relatively small ground state epimeric differences in the polarization of the CH bond, especially since the polarization could well be magnified in the transition state" (22). 

Cracetoxyl bond. The reaction is applicable to fully acetylated aldopyranoses of 1 2-trans configuration and gives stereospecifically acetyl glycosyl chlorides of 1 2-trans configuration (the less stable anomers). 

The conformations of the aldopyranoses in aqueous solution have been investigated by n.m.r. spectroscopy " (see Table IV). The chair form favored by the various aldohexopyranoses appears to be controlled by the tendency of the 5-(hydroxymethyl) group to assume the equatorial orientation. Hence, all the /3-D anomers exist preponderantly in the CJ(d) conformation, as the alternative, 1C(d) conformation appears to be highly destabilized by the si/n-diaxial interaction between the 1-hydroxyl and 5-(hydroxymethyl) groups. Although this interaction is not found in the 1C(d) conformation of the a-D anomers, most of the a-D anomers nevertheless adopt the C1(d) conformation, presumably because of a favorable anomeric effect (see p. 103). Solutions of a-D-idopyranose and a-D-altropy-ranose appear to contain appreciable proportions of each chair con-former at equilibrium. 

Correlations for Certain Aldopyranose Derivatives. In the range 960 to 730 cm Barker et al. (I954a,b,c) have found bands eharacteristie of several aldopyranoses and their derivatives. They identified (1954a), for D-glucopyranose derivatives, three principal sets of bands, which are presented in Table 6.2. These were as follows for a anomers, type la at 917 13 cm type 2a at 844 8 cm" and type 3a at 766 10 cm" and for P anomers, type lb at 920 5 cm" S type 2b at 891 7 cm" and type 3b at 774 9 cm" ... 

Experiments with aldopyranoses have shown that the actual course of the reaction is somewhat more complex than we have indicated. Bromine water specifically oxidizes the jS anomer, and the initial product that forms is a 8-aldonolactone. This compound may then hydrolyze to an aldonic acid, and the aldonic acid may undergo a subsequent ring closure to form a y-aldonolactone ... 

An anomer is called a if the absolute configurations R or S) of the anomeric carbon and the last chiral carbon are opposite. Otherwise the anomer is called p. In the representations brought up to date according to Haworth, carbons 1 and 5 of the p-aldopyranoses are R. ... 

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