Carbohydrates anomers

Because six membered rings are normally less strained than five membered ones pyranose forms are usually present m greater amounts than furanose forms at equilib rium and the concentration of the open chain form is quite small The distribution of carbohydrates among their various hemiacetal forms has been examined by using H and NMR spectroscopy In aqueous solution for example d ribose is found to contain the various a and p furanose and pyranose forms m the amounts shown m Figure 25 5 The concentration of the open chain form at equilibrium is too small to measure directly Nevertheless it occupies a central position m that mterconversions of a and p anomers and furanose and pyranose forms take place by way of the open chain form as an inter mediate As will be seen later certain chemical reactions also proceed by way of the open chain form... 

Discotic liquid crystals based on carbohydrates are intrinsically chiral due to the chiral nature of their core.49,50 Numerous discotic liquid crystals derived from these mesogens have been reported however, the expression of chirality in the subsequent mesophases has only rarely been seen. An example of the latter is the a-anomer of penta- O -decanoylglucopyranosc (47), which forms a discotic mesophase in which the molecules pack helically in the columns... 

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). 

Uzan et al. have reported an alternative route to 1,2-fused thionocarbamates from 1,2-carbohydrate sulfites.49 The reaction proceeded via formation of a -configured thiocyanate, which further epimerizes to the ot-anomer, ready to undergo cyclisation into the fused OZT (Scheme 36). 

Anomers are enantiomers formed when the symmetric carbonyl carbon is made asymmetrical by the formation of the cyclical form of a carbohydrate. 

Gas-liquid chromatography may be the method of choice when it is necessary to identify or to quantitate one or more carbohydrates especially when they are present in small amounts. Although this technique is often used because it is possible to resolve carbohydrates with very similar structures, the fact that a and /3 anomers and pyranose and furanose forms of the same carbohydrate all give separate peaks is sometimes a disadvantage. 

O. Varela, G. M. de Fina, and R. M. de Lederkremer, The reaction of 2-hydroxyglycal esters with alcohols in the presence of N-iodosuccinimide, stereoselective synthesis of a anomers of alkyl 3-deoxyhex-2-enopyranosides and 3,4-dideoxyhex-3-enopyranosid-2-uloses, Carbohydr. Res., 167 (1987) 187-196. 

In 1987, Chmielewski and coworkers reported for the first time on the preparation of enantiomerically pure hydroperoxides 63a and 63c derived from carbohydrates . The method employed consisted of the oxidation of 2,3-unsaturated glycosides (64a and 64b, see Scheme 29) with hydrogen peroxide in the presence of a M0O3 catalyst. The hydroperoxides 63a and 63c were isolated together with their S-anomers (63b and 63d). 

The most important result, however, is the binding of the trisaccharide tri-N-acetylchitotriose, as the model proposed for substrate binding and enzymic hydrolysis is based on this compound. The three substituted /3-D-glucopyranose residues are labeled A, B, and C in Fig. 2. Two observations are the same as for 2-acetamido-2-deoxy-D-glucose residue C has the same binding as the /3-d anomer, and the resulting shift of amino acid residue 62 (L-tryptophan) is 0.75 A. The hydrogen bonds between lysozyme and carbohydrates A and B are shown in Fig. 2 and listed in Table IV. Residue A, which is located... 

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