Mannose 0 anomer

The following are the more stable anomers of the pyranose forms of D glucose D mannose and D galactose... 

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. 

Exceptions to this rule are the mannose, rhamnose and talose anomers. Closely spaced dipoles in the /3-anomers are believed to be responsible for this change45 46. 

Compounds that have the same chemical formula but have different structures are called isomers. For example, fructose, glucose, mannose, and galactose are all isomers of each other, having the same chemical formula, C6H1206. If two monosaccharides differ in configuration around only one specific carbon atom (with the exception of the carbonyl carbon, see anomers below), they are defined as epimers of each other. (Of course, they are also isomers ) For example, glucose... 

At the completion of the reaction, the excess n-pentenyl alcohol can be recovered by distillation and saved for future glycosidation. As expected, the reaction gives a mixture of anomers. For example, the ot/p ratio for glucose is 2 1, whereas mannose gives almost exclusively the a-product [6],... 

An analogous phenomenon has also been observed with D-mannose pentaacetate and D-altrose pentaacetate. The increase in intensity of the Ai peak in the mass spectra of a-(d or l) anomers may be explained as due to the unfavorable steric position of the OAc group at C-l, analogous to that in the methyl ethers. 

A kinetic study of the methyl glycosidation of D-mannose was also made by Mowery.48 The more stable anomer, the a-D-mannofuranoside, was formed at a higher initial rate (see Table III) the proportions of both furanosides in the final equilibrium mixture was too small to permit accurate comparison of isomer distribution. The conformational stability of the D-mannofuranosides may be compared with that of the D-lyxo-furanosides the furanoid structures are similar, except for the bulky two-carbon group at 04 of the hexoside. This similarity is shown in the very small proportion of n-lyxofuranosides (see Table V) and of D-manno-furanosideo (see Table III) in the final equilibrium mixtures, and also in the initial formation of D-mannopyranosides48 and of D-lyxofuranosides.u... 

The data in Table VIII are presented in the order of the conformational stability observed for methyl glycosidation (see Table I on p. 99 and Table II on p. 105). In each of the three cases where data for both anomers are available, the trans-1,2 anomer is the more stable, and a lower rate-constant is observed. This is the same pattern as that observed in Table II. However the order of conformational stability given earlier is not observed here. The arabinofuranosides show the maximum stability, and the D-galactofuranosides and L-fucofuranosides, having similar conformations, show a similar stability, However, the furanosides of D-lyxose, n-mannose, and i>rhamnose show an unexpectedly high stability which is almost as great as those of the furanosides of the first-mentioned sugars. These data lead to the conclusion that the conformational stability for transition complexes... 

An anomeric carbon with an axial hydroxy group usually resonates at higher field than the corresponding C-1 with an equatorial hydroxy group (rule (5)). This rule allows one to prove, experimentally, that arabinose exists in the C4 and not in the 4C, conformation, in which most sugars occur in solution [88, 726]. However, rule (5) is not valid for the mannose, rhamnose and talose anomers. This was attributed to the presence of three closely spaced dipoles (Reeves effect) present in the /J-anomeric configuration of these three sugars [683, 684] (Fig. 5.7). 

In the presence of dilute base, D-glucose rearranges to a mixture containing the anomers of D-glucose (—64%), D-fructose (—31%), and D-mannose (3%). This interconversion undoubtedly involves enolization of the hexoses by way... 

The compound of Micheel and Micheel was obtained in small amount after treatment of 2,3,4,6-tetra-O-acetyl-a-D-mannosyl bromide with tri-methylamine the bulk of the material remained unchanged. The product apparently resulted from hydrolysis by moisture from the air. The composition was established by analyses, but the identification was reported with a question mark. A 2,3,4,6-tetra-O-acetyl-D-mannose with different physical constants (m.p. 93°, [c ]d +26.3°) was obtained by Levene and Tipson63b by the deliberate addition of water and silver carbonate to tetra-O-acetyl-D-mannopyranosyl bromide its structure was confirmed by conversion to the known pentaacetates. If Micheel and Micheel were correct in their identification, the two acetates could be anomers. The acetylation and deacetylation reactions performed by Maurer and Bohme are additional evidence in favor of this relationship. 

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