Aldopyranoses conformations

Scheme 1.—Occurrence of Ring Protons Expected to Have a Major Influence on the Relaxation Rates (R,) of the Anomeric Protons (H-1) of Aldopyranoses and Derivatives (4-25, Table I) in the C, or C4, Conformation.

The calculated, relative free-energies of the aldo-hexo- and -pento-py-ranoses are shown in Table I these values are relative to an imaginary aldopyranose in which there are no conformational interactions. From these values, the a / ratio can be calculated despite the simplicity and... 

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

In 1937 Isbell published an important paper on the conformational analysis of aldopyranoses, in which several of the forms were depicted (see Fig. 2). They comprise a 4C, chair (I), afl03 boat (II), two half-chairs (III and IV), and a coplanar pyranose (V). He correctly favored the chair form (I) and predicted that, in the case of saccharides, the chair would tend to assume a somewhat flatter conformation than that of carbocycles such as cyclohexane because of the smaller bond angles of oxygen as compared to those of carbon (105° instead of 109.5°). We now know that such coplanarity is strongly avoided because of the strain that would be produced and because of the repulsive forces between the substituents. 

Angyal SJ (1968) Conformational analysis in carbohydrate chemistry. I. Conformational free energies. The conformation and a / ratios of aldopyranoses in aqueous solution. Aust J Chem 21 2737-2746. Angew Chem Intern Edit 8 157-166 (1969)... 

The above category should include the jS-D-aldopyranoses (and their )3-l enantiomorphs), which preferably assume the normal chair conformation (CT VII), as well as the a-n sugars (and their enantiomorphs), such as a-D-ar.abinose, which are more stable in the reverse chair conformation IC (VIII). 

A notable exception has been the application of i.r. spectroscopy by Isbell and Tipson to the analysis of the conformations of methyl aldopyranosides," acetylated methyl aldopyranosides," " and fully acetylated aldopyranoses in the solid state. Their approach was based on (J) the assumption that the pyranoid sugar derivatives predicted by Reeves to exist in only one of the chair conformations in solution would adopt that same conformation in the crystalline state, and (2) the observation that axially and equatorially attached groups at the anomeric carbon atom give rise to different vibrations. 

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. 

The estimated free-energies for both chair conformations of the free aldopyranoses were then calculated by summation of the various steric interactions and the anomeric effect of the anomeric hydroxyl group. The energies calculated are given in Table IV (see p. 85), together with the conformations predicted and those indicated by experiment. When the free-energy difference between the two ehair forms was less than 0.7 kcal.mole", both conformers were considered to be present in comparable amounts at equilibrium. 

In a further quantitative check of the validity of the calculated free-energy values, the predicted a,p anomeric equilibria of the aldopyranoses in aqueous solution were found to be in reasonable agreement with those determined experimentally by studies of their mutarotation, oxidation by bromine, and n.m.r. spectra. In addition, the position of the equilibria between aldohexopyranoses (and 3-deoxy-aIdohexopyranoses) and their 1,6-anhydrides, and between heptuloses and their 2,7-anhydrides, as determined by gas-liquid chromatography, were in good agreement with data estimated from conformational interaction-energies. 

R. H. Marchessault and S. Perez, Conformations of the hydroxymethyl group in crystalline aldopyranoses. Biopolymers, 18 (1979) 2379-2374. 

For D-aldopyranose in general, the conformation Ci is usually preferred. A summary of the different representations of D-glucose and the most probable conformation of a-D-glucose are given in Figure 8. 

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