Conformational studies pyranoid ring

To date, only a few solution calculations for carbohydrates have been attempted (one such study of mannitol and sorbitol is described in the chapter by Grigera in this volume), but the results of these early studies bear out the expectation that solvation effects in carbohydrate systems can be both significant and difficult to predict. In the case of pyranoid rings, molecular solvation is further complicated by the close juxtaposition of these groups in essentially fixed relative orientations (assuming no conformational changes in the ring). Under such circumstances, molecular stereochemistry could play important physical roles, as is... 

Fusion of a second ring to a pyranoid ring often limits the number of possible conformations that the pyranoid part may adopt. For example, trans-fusion of a benzylidene acetal ring at C-4 and C-6 of a hexopyranose system effectively prevents attainment of the IC(d) conformation, although skew or boat conformations may be possible. Thus, a comparative study of the n.m.r. spectra of 36 methyl 4,6-0-... 

Although there is some uncertainty as to the conformational effect of one cis-fiised, 5-membered acetal ring, evidence from both i.r.-and n.m.r.-spectral studies clearly indicates that the presence of two such cis-fiised rings forces the cyclohexane or pyranoid ring to adopt a skew conformation. Thus, the pyranoid ring in derivatives of 1,2 3,4-di-0-isopropylidene-j3-L-arabinose and -a-D-galactopyranose (34), and of 2,3 4,5-di-0-isopropylidene-/3-D-fructopyranose (35), and the... 

N.m.r. studies on pyranoid sugar derivatives containing an epoxide ring indicate that the compounds adopt a half-chair conformation similar to that observed for unsaturated, pyranoid sugars. From an analysis of the spin-coupling data for some methyl 2,3- and 3,4-anhydroaldopyranosides, it was found that the particular half-chair form adopted is determined by the anomeric effect and the tendency of the 5-(hydroxymethyl) or 5-methyl group to assume the equatorial... 

It has been shown that stereoelectronic effects play an important role in many enzyme-catalyzed biochemical reactions, as, for example, in ribozymes [2-4], serine proteases [5], lysozymes [6], and ribonucleases [3, 7], While most of the studies on stereoelectronic effects were conducted on pyranoid forms of sugars, the relationship between stereoelectronic effects and furanoid ring conformations, to the best of our knowledge, is almost totally neglected. This is a big void since the conformations of furanoid forms of sugars are of great importance for overall conformations of ribo- and deoxyribonucleic acids (RNA and DNA). 

Early structural studies were conducted by Kuhn and co-workers who determined the ring size of l-aminodeoxy-o-fructose derivatives by methylation and degradation experiments [63]. For the l-(p-toluidinyl)fructose derivative they proved with this method the consequently most stable pyranoid conformation. 

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