Configuration anomeric, determination

Once each sugar residue has been identified and its anomeric configuration determined using a combination of the above-mentioned techniques, all that is required is to identify the sugar sequence and the inter-glycosidic linkage. Proton-proton scalar couplings that are four bonds apart (Vhh) are usually too small to observe. Therefore, it is necessary to make use of either homo-... 

Furanoid Compounds.—An n.m.r. proton-proton coupling constant method has been applied to the anomeric configurational determination of C-glycosidic analogues of 2-deoxy-D-e/yr/iro-pentofuranosides." ... 

The anomeric configuration is set in the reductive lithiation step, which proceeds via a radical intermediate. Hyperconjugative stabilization favors axial disposition of the intermediate radical, which after another single electron reduction leads to a configurationally stable a-alkoxylithium intermediate. Protonation thus provides the j9-anomer. The authors were unable to determine the stereoselectivity of the alkylation step, due to difficulty with isolation. However, deuterium labeling studies pointed to the intervention of an equatorially disposed a-alkoxylithium 7 (thermodynamically favored due to the reverse anomeric effect) which undergoes alkylation with retention of configuration (Eq. 2). 

The anomeric configurations of the sugar residues were determined by chromium trioxide oxidation [14], Oxidation of the fully acetylated polysaccharide and subsequent monosaccharide analysis by GLC indicated that the D-Xyl units are P-linked (oxidized more rapidly) and that die D-GlcA are a-linked (Table II). 

C-Glycosylation was also effected in the T>-arabino series, by treatment of 2,3,5-tri-O-benzyl-a-D-arabinofuranosyl chloride127 (168) with diethyl sodiomalonate. The anomeric diethyl 2-d-arabinofuranosylmalonates were isolated as the acetates (169 and 170), and their anomeric configurations were determined by per-... 

A new NMR method for the determination of the anomeric configuration in mono- and disaccharides has been described.18 The protocol is based on the different cross-correlated relaxation between proton chemical shift anisotropy (CSA) and dipolar relaxation for the a and (3 anomers of sugars. Only the ot-anomers show the presence of CSA (HI or Hl )-proton dipole (H1-H2 or Hl -H2 ) in the longitudinal relaxation of the anomeric protons. The method is of special interest for cases in which vicinal coupling constants between HI and H2 in both anomers a and (3 are similar and small, such as D-mannose, and the non-ambiguous description of the anomeric configuration needs additional measurements. 

In a study of the infrared spectra of acetylated and benzoylated gly-copyranosyl fluorides, it was found that, with few exceptions, such compounds exhibit an absorption band at 802-748 cm-1 if the fluorine atom is axial, whereas no such absorption band is observed if the fluorine atom is equatorial therefore, it is necessary to know the conformation of the molecule before the anomeric configuration can thus be determined.45 It is possible that reinterpretation of some of these observed absorptions may be necessary, because some of the conformations of some of the glycopyranosyl fluorides are now known to exist in the conformation opposite to that originally anticipated without proof, because of the strong anomeric effect of a fluorine substituent (see Section III,2e). 

Determination of the absolute configurations of the 3,6-dideoxy-hexoses involved their degradation, according to Scheme 2, to malic acid, and identification of the latter acid with L-malate dehydrogenase.56 The anomeric configuration in 9—12 was assumed to be the same... 

Furanosyl derivatives split in the manner described only if they have a cts-disposition of die substituents at C-l" and C-2". This reaction may be applied for determining the anomeric configuration in these derivatives.126,127... 

If both anomers of the glycoside are obtained in a reaction, it is necessary to separate them. One separation method that has been used is the preferential extraction101 of the anomers with a volatile solvent. After extraction of the individual glycoside, the solvent is removed by evaporation, and the glycoside may be obtained in crystalline form. A second method of separation utilizes fractional recrystallization, and it may be possible by proper selection of the solvent to obtain both anomers in crystalline form. A third method utilizes chromatography for separating the anomers, and the pure anomers may be obtained from appropriate fractions from the column. The anomeric configuration of the anomers which have been obtained in pure form will need to be determined. Such determinations can be made by measurement of physical constants, from the n.m.r. spectra, and from the susceptibility of the anomer to enzymes of known specificity. 

As the only difference between the two oxidation products is the configuration at the carbon atom that had been the anomeric center, it was possible to determine the configuration of C-l of other N-acetyl-hexopyranosylamines by comparison of the specific rotations of the dialdehydes formed by their oxidation with periodate. In this way, the anomeric configuration of N-acetyl-/3-D-glucopyranosylamine... 

On the other hand, the favored formation (and the yields) of N-acetylglucofuranosylamines were explained on the basis of steric considerations.24,26 The formation of N-acylglycosylamines is a kinetically controlled, irreversible reaction, and the prevalence of the furanose over the pyranose structure has been shown in many irreversible, cyclization reactions of carbohydrates.73 This preponderance, as well as the favoring of a determined anomeric configuration, would be the result of a balance of relative ffee-energies of the transition states in the cyclization step, which would have virtually the same geometry as the final, cyclic structure.74 The five-membered... 

The elucidation of the structure of polysaccharides and of the carbohydrate portion of glycoconjugates involves sugar analysis, linkage analysis, and the determination of anomeric configurations and the sequences of the sugar residues. When non-sugar substituents are present, these should also be identified and located. 

Benzoates Exciton Rule for Determining die Anomeric Configuration in Saccharides... 

L-arabinofuranosyl group in 6, and the anomeric configuration of those in another fraction, defined as 7, were not determined. An alkali-insoluble... 

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