Aldopyranoses effects

The effect of alkoxyl or phosphate groups on the acidic degradation of sugars has not yet been systematically studied. The aldopyranoses substituted at 0-1, that is, the aldopyranosides, are only degraded... 

Calculations of the 13C-13C coupling constants for all aldopyranoses of the D-series have allowed some general relationships with respect to stereochemistry and coupling constants to be postulated <2003RJ0663, 2003RJ01194>. Data have been reported for the 13C NMR substituent effects at the a, f3, it, and S positions for 2-halomethyl-2-hydroxy-tetrahydrofurans and 2-halomethyl-5,6-tetrahydro-4/7-pyrans. These studies showed that additivity rules allow the prediction of the chemical shifts for each carbon <1996SPL631>. 

Effects of Ring Substituents on the Anomeric Equilibrium of Aldopyranoses and Derivatives... 

The electron-withdrawing effect of the anomeric center, which inhibits displacements of primary sulfonyloxy groups at C-l of fructose and sorbose, strongly hinders displacements at C-2 of aldopyranose derivatives. An alternative or additional effect, proposed by Hough and Richardson,347 may well consist in unfavorable dipolar interactions which would arise in the formation of the SN2 transition state. 

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. 

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