J8-anomer

The relaxation of certain 13C nuclei in some uronic acids and derivatives is affected by Gd3+. For example, on addition of gadolinium nitrate to solutions of heparin or dermatan sulfate in D20, the C-l and C-6 signals of the uronic acid residues are selectively diminished. This property would be expected from a-L-idopyranosyluronic acid residues, as the C-l and C-6 signals of sodium a-D-idopyranuronate are similarly lessened in intensity, whereas those of the j8 anomer are... 

The roles of nucleophilic assistance and stereoelectronic control in determining endo-versus exo-cyclic cleavage of pyranoside acetals have been investigated for a series of a- and j8-anomers.15 Exocyclic cleavage of a-anomers, via a cyclic oxocarbenium ion, is predicted by the theory of stereoelectronic control, and was found exclusively for the cases studied. The endocyclic route, with an acyclic ion, is predicted for the /1-structures, and a measurable amount was found in all cases, but its extent was dependent on temperature, solvent, and the nature of the aglycone group. 

The presence of anisotropic motion along the molecular length is suggested for trisaccharide 15 as the anomeric carbons of the a and j8 anomers show different Tl values.85 A smaller T, for the C-l carbon of the a anomer (0.11s) relative to that of the j3 anomer (0.24 s) indicates a close correspondence between the equatorial C-H vector in the a-anomer and the favored axis of rotation. By contrast, relaxation studies on sucrose14,106 (7) did not reveal anisotropic motion (however, see below). 

Man-3 (see later). The doublets for this H-l for both the a and /3 form of 5 are well resolved, reflecting the relatively large Jli2 value (0.95 Hz) for this compound. For the H-2 atom of Man-3, this large J 1>2 value leads to two doublets of doublets, at 8 4.244 (a anomer of 5) and at 8 4.233 (j8 anomer of 5), respectively, instead of to two doublets having relatively broad lines. Nevertheless, the pattern of the H-2 resonance of an a-linked Man residue is clearly distinguishable from that of a /3-linked one. 

The presence of GlcNAc-5 is revealed by small shift-decrements of H-l of Man-3 in both anomers of 25, whereas GlcNAc-5 does not affect the chemical shift of H-l of Man-3 in either the a or the j8 anomer of 26. 

Fig. 32. — Structural-reporter-group Regions of the Resolution-enhanced, 500-MHz, H-N.m.r. Spectrum of a Mixture Containing Compounds 41 and 30 in the Ratio of 23 2. [The bold numbers in the spectrum refer to the corresponding residues in the structures, and the italic numbers to the compounds in the mixture, each of which occurs as an anomeric mixture in the ratio of a /3 = 2 1. Signals of corresponding protons in the various components of this mixture coincide, unless otherwise indicated. The relative-intensity scale of the N-acety 1-proton region differs from that of the other parts of the spectrum, as indicated. The HOD resonance, as well as the H-l signals of Man-3 for both anomers and that of GlcNAc-2 for the j8 anomer of 41, and also of 30, have been omitted from the spectrum their positions are indicated by arrows.]...

The a-anomer gives better selectivity for the 2-OH than does the j8-anomer (3 2). Note that the conditions used to remove the 4,6-0-benzylidene group are sufficiently mild to retain the sulfate.2... 

The influence of various factors on the proportion of the and j8 anomers obtained by the Hilbert-Johnson reaction has been studied systematically by Sorm et al. the first paper28,34 of the series dealt with the synthesis of 2 -deoxyuridine and thymidine. Thus, the reaction of an equimolecular mixture of 2,4-dimethoxypyrimidine... 

The reactivity (in acetonitrile as solvent) of various 2,4-dialkoxy-pyrimidines19 towards 2,3,5-tri-O-benzoyl-D-ribofuranosyl chloride (59) was found to decrease in the order 2,4-dibenzyloxy-, 2,4-bis(p-methoxybenzyloxy)-, 2,4-dimethoxy-, 2,4-diethoxy-, 2,4-bis(2,4-di-methoxybenzyloxy), 2,4-diisopropoxypyrimidine. In all these cases, the j8 anomer represented the sole reaction product. In view of this stereospecificity, the reaction of the halogenose (59) with the above 2,4-dialkoxypyrimidines represents a procedure of choice for preparation of uridine or cytidine and their derivatives. When the Hilbert-... 

Johnson reaction of the halogenose (59) with the bases (54 or 55) was performed in benzene, a small amount of the corresponding anomer accompanied the main reaction product, the j8 anomer. Formation of... 

Likewise, if the solution is kept saturated with respect to the /3 anomer, the increase in solubility of the j8 anomer is represented by ... 

Replacement of secondary sulphonate esters which are adjacent to the anomeric carbon atom is not normally possible with azide ion owing to a combination of the electron-withdrawing effect of the acetal and unfavourable polar effects in the transition state . The latter effect is best visualized by considering the Newman projections (128) and (129) along the C(ij-C(2) bond for the 5 12 transition states of both a- and j8-anomers. In the case of the a-anomer the developing dipoles of the transition state are almost parallel and opposed to the C(i)— methoxyl and C j-ring oxygen dipoles. The situation is similar,... 

Cellulose is formed by elimination of water from adjacent j8-anomers of the two cyclic forms of glucose (Eq. 15.2), hence it is a dehydro polymer. 

There are fewer ketoses than there are aldoses because ketoses have one less chiral carbon. The most prevalent of the ketoses are dihydroxyacetone, ribu-lose, xylulose, and fructose (see Figure 4). All four of these sugars are important intermediates in metabolism. Fructose is, along with glucose, part of sucrose or table sugar. Fructose is a ketohexose, and the only one of the four ketoses that can assume a ring structure. Like ribose, fructose forms a five-membered (or furanose) ring and has a and j8 anomers. 

The half-life for nonenzymic interconversion of the anomers is about 0.4 sec. Stopped-flow kinetic measurements, using the isomerase from yeast, indicate that both the a- and j8-anomers of glucose 6-phosphate are used directly by the enzyme to give the a- and )8-anomers of fructose 6-phosphate, although the a-anomer is consumed at least 20-fold faster than the )3-anomer. In addition, the enzyme is capable of catalyzing the interconversion (anomerization) of the a-and jS-anomers. With the a-anomer of glucose 6-phosphate, the anomerization reaction is approximately twice as fast as the isomerization reaction. 

In the procedure of Laine and Sweeley [405], methoximation and trimethylsilyla-tion are combined to avoid variation in the ratios of a- and j8-anomers of the pyranose and furanose forms. This approach was used to handle successfully 33 sugar derivatives, including aldoses, ketoses, polyols, acidic forms and A -acetylated amino sugars, on a fused silica capillary column [406]. Obviously, the problems of multiple peak formation can be overcome through the use of highly efficient capillary columns. The application of capillary columns in carbohydrate analysis, used probably for the first time by Tesarik [407] in 1972, has now become widespread. 

Stereospecificity. Sucrose, a major product of photosynthesis in green leaves, is synthesized by a battery of enzymes. The substrates for sucrose synthesis, D-glucose and D-fructose, are a mixture of a and j8 anomers as well as acyclic compounds in solution. Nonetheless, sucrose consists of a-D-glucose linked by its carbon-1 atom to the carbon-2 atom of /3-D-fructose. How can the specificity of sucrose be explained in light of the potential substrates ... 

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