Aldopentopyranoses, tetraacetates, conformations

Study of the eight stereoisomeric aldopentopyranose tetraacetates showed that significant population of both chair conformers is the rule... 

Table VII. Conformational Equilibria of D-Aldopentopyranose Tetraacetates in Acetone- at 31°C...

The introduction of substituents and the effects of solvation profoundly influence the conformational equilibria of monocyclic pyranoid sugars and their derivatives, because of the changes in steric and electronic interactions that result. The conformations of numerous polysubstituted pyranoid sugars in various solvents have been investigated, mostly by application of the n.m.r. spectroscopic method. Six aldopentopyranose tetraacetates were examined in chloroform solution near room temperature they were considered to exist almost entirely in the CJ(d) conformation, except for the jS-D-ribo derivative, which appeared to contain substantial proportions of each chair form, and the -L.-arahino derivative which was estimated to contain mostly the CJ(l) conformation. 2-Deoxy-j8-D-e/T/thro-pentopyranose triacetate was considered to exist mainly in the 1C conformation. 

The only aldopentopyranose tetraacetates for which conformational freeze-outs were observed at temperatures down to —100° were the p-D-ribo and the (3-D-lyxo derivatives. ... 

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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