Aldopentopyranoses conformation

Conformational Equilibria of Acylated Aldopentopyranose Derivatives and Favored Conformations of Acyclic Sugar Derivatives... 

T1he conformational studies (I) on acyclic sugar derivatives and on aldopentopyranose derivatives that have been conducted in our laboratories during the last few years are surveyed, and some of our more recent results in each of these areas are introduced. For each aspect the sugar derivatives were examined in solution by proton magnetic resonance (PMR) spectroscopy, and the data obtained were used to provide conformational information. Acyclic systems will be treated first. 

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...

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

The aldopentopyranoses are not constrained to the Ci conformation by the presence of a hydroxymethyl group. As a result, the equitibria between the Ci and conformers are more closely balanced, with the D-xylopyranoses present in aqueous solution mainly as " Ci conformers (25), the D-arabinopyranoses mainly as 4 conformers (26), and the others as mixtures [136,137,138]. 

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. ... 

Conformational Equilibria of Aldopentopyranose Derivatives in Acetone-d Solution at 31°... 

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. 

The internal energies of each of the sixteen possible Ci conformations of the aldopentopyranoses (i.e. the ol- and j8-anomers of both the d- and L-forms) have been derived from Monte-Carlo calculations that take into account contributions made by van der Waals attractive and repulsive forces, polar interactions, and... 

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