Aldopentopyranoses equilibria

First, the anomeric equilibria of the aldopentopyranose tetraacetates were examined by NMR spectroscopy and by optical rotation (25). Each of the acetylated anomeric forms of the aldopentopyranose tetraacetates was allowed to attain equilibrium in a 1 1 mixture of acetic anhydride and acetic acid containing perchloric acid as a catalyst (Table II). The... 

Table XII. Comparison of Equilibrium Positions for the Peracetylated and Perbenzoylated Aldopentopyranoses...

Solvent polarity has a negligible effect on conformational equilibrium in the aldopentopyranose tetrabenzoates, but a regular effect is observed for methyl glycoside derivatives. 

Durette and Horton295(c) studied, by nuclear magnetic resonance spectroscopy, the anomeric equilibria at 27° of the aldopentopyranose tetraacetates in 1 1 acetic anhydride-acetic acid that was 0.1 M in perchloric acid. They found the following values for the ratios of the anomeric tetraacetates (/3la) at equilibrium, with the free energy A G° values for the J3 equilibrium (in parentheses) D-ribopyra-nose, 3.4 (—0.73 0.03) D-arabinopyranose, 5.4 (—1.01 0.03) D-xylopyranose, 0.23 (+0.89 0.03) and D-lyxopyranose, 0.20 (+ 0.98 0.05). 

D-Aldopentopyranose derivative % Cl Equilibrium data %1C K = C1I1C ACgio, kcal.mole forlC(D) JSCI(D) Refer- ences... 

From a consideration of detailed results on the conformational equilibria of aldopentopyranose derivatives, it has been pointed out92- 9 that a more sophisticated model is required before conformational populations can be reliably predicted, at least with acylated derivatives. Even with adjustment of the original parameters in order to take revised values for the anomeric equilibrium of D-lyxopyranose tetraacetate and the conformational equilibrium of /3-D-arabinopyranose tetraacetate into account, the observed data cannot be accommodated within the framework of this model, except on a very broad, qualitative basis. Other possible factors that should be considered " include polar contributions from substituents other than that on C-1, attractive interactions between syn-diaxial acyloxy groups, non-bonded interactions between atoms that have unshared pairs of electrons,repulsive interactions between gauche-vicinal groups, the effect of solvent pressure, and differences between the molar volume of conformers. 

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