Monosaccharide anomeric effect

Anomeric effect of monosaccharides and their derivatives insights from the new QVBMM (quantized valence bond molecular mechanisms) molecular mechanics force field 98H(48)2389. 

Consequences of the conformational anomeric effect are largely expressed in monosaccharides and their derivatives. One recognizes the conformational endo-anomeric effect for pyranosides with a polar X group at C(l) (contrasteric electronic stabilization effect Fig. 7A) and conformational exo-anomeric effect for glycosides (acetals) in which the alkyl group of the exocyclic moiety is synclinal (Fig. 7B, C). 

D-(+)-galactose (15) is an example of the consecutive numbering of the carbon ring atoms in a monosaccharide (disaccharide see p. 253). Carbohydrates can exist in a cyclic and an acyclic structure. For this reason there is a special position in the structure of a monosaccharide, the carbon atom C-l and so called anomeric center. You can see that there is an equilibrium between er-anomer a-15 and / -anomer /3-15 of D-(+)-glucopyranose over the acyclic aldehyde structure 16. Both are cyclic hemi-acetals. The /Tanomer is the preferred conformation, but there are a few effects, like sterical or stereoelectronical effects (anomeric effect, inverse anomeric effect), which have influence on the a /i rate. 

The presence of particular substituents and the nature of the solvent appear to have an effect on the equilibrium composition of particular monosaccharides. As already discussed, the anomeric effect is stronger in... 

The conformational analysis of monosaccharides, disaccharides, and ohgosaccharides is reviewed. Conformational terms are introduced through examination of the conformations of cyclohexane and cyclopentane then applied to the pyranose, furanose, and septanose rings. Concepts such as the anomeric effect are discussed. Topics of current interest, such as hydroxymethyl group and hydroxyl group rotation and disaccharide conformations are summarized. Physical methods for studying conformation are outlined. 

Conformation Chair Anomeric effect Hydrox)miethyl Monosaccharides Disaccharides Oligosaccharides Couphng constants Molecular mechanics Molecular dynamics... 

The dispiroketal protection of monosaccharides is controlled by the stabilizing influence of multiple anomeric effects leading to a single diastereomeric derivative. In certain examples, where there is more than one diequatorial diol pair present in the molecule, as for example in D-glucose derivatives, reaction affords a mixture of diacetals (O Scheme 22). The reaction, often giving crystalline compounds, is carried out by treatment of the polyol with 3,4,3, 4 -tetrahydro-6,6 -bis-2H-pyran in chloroform at reflux in the presence of a catalytic amount of CSA[151]. 

H. Paulsen and F. Leupold, Synthesis of four 5-benzoylcarbonylimino-5- desoxy-pentopyronoses. Conformation and anomeric effect of monosaccharides with nitrogen containing rings, Chem. Ber., 102 (1969) 2804-2821. 

The assignments of H signals then allows the quantitative determination of all forms of monosaccharides in aqueous solution as reproduced in Fig. 4.2.13. The most important hexoses, namely glucose, mannose, and galactose, all occur as pyranoses only. The axial 3-OH group of allose, however, interacts repulsively with the a-OH group on C-1 and thereby overcomes the anomeric effect. The combination of both repulsive effects then also leads to 12% fura-nose. The latter effect is even more pronounced in altrose and ribose (Stod-dart,1971) (Fig. 4.2.13). 

---