Carbohydrates favored conformations

Horton et al. were interested in the synthesis of tetra-C-substituted carbocycles, and extensively studied asymmetric Diels-Alder reactions employing acyclic unsaturated carbohydrate derivatives. Thus, the thermal cycloaddition of the D-arabinose-derived c/i-dienophile (Z)-100 with cyclopentadiene gave endo-adduct 101 in excellent optical purity. The high diastereo-facial differentiation in this reaction arises from a highly favored conformation at the allylic center [76,77] (Scheme 10.33). Therefore, conformer 102 seems to be more favored than 103, where the latter suffers from severe allylic strain between the methoxycarbonyl and the C4-acetoxy group... 

The first interpretation of the conformational anomeric effect, given by Edward,2 invoked more favorable electrostatic interactions in the axial anomers than in the equatorial anomers of carbohydrates (Fig. 8). 

The anomeric effect, a stereoelectronic effect, is explained in terms of lone pair-lone pair repulsion, dipole-dipole interactions and by M.O. theory. The equatorial positions of a carbohydrate are favored by sterically demanding substituents. However, electronegative groups at the anomeric center prefer the axial position because of the stereoelectronic effects. This fact is known as the anomeric effect.13 If there is a positive charge at the anomer substituent of a carbohydrate, the equatorial conformation is preferred. To explain this result a reverse anomeric effect was proposed and first detected at A-(tt-glycopyranosyl)pyridinium ions 31 and 32.14... 

Owing to the great impact that p.m.r. spectroscopy has made on structural carbohydrate chemistry, it is not feasible to discuss here all of the reports in which this technique has been used merely to confirm assigned structures. In the present Chapter, some selectivity has been exercised in favor of novel structural and conformational applications, innovative techniques, information that could not be obtained by other methods, and a fundamental understanding of the magnetic parameters of carbohydrate molecules. 

Carbohydrates (disaccharides) in aqueous solution are favorable examples for ROA studies furnishing an informative band structure over a wide range of the vibrational spectrum. Their complex and highly coupled normal modes generate strong ROA bands that produce patterns characteristic of the various types of structural units. ROA data can be used to confidently specify the central component of carbohydrate stereochemistry. For disaccharides [92] ROA can provide the nature and conformation of the glycosidic link, and can also probe extended secondary structures of oligosaccharides [93]. 

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