Epimerization monosaccharides

Reaction of nitromethane and monosaccharide-derived dialdehydes is a useful tool that has been broadly used for the preparation of nitro and amino sugars, and carbocycles.30 Dialdehydes can easily be obtained by oxidative cleavage of conveniently protected monosaccharides with sodium periodate. Their subsequent Henry reaction with a nitroalkene, commonly nitromethane, usually gives isomeric mixtures that require the isolation of the major isomer.31 Thus, treatment of the D-ribose derivative 27 with sodium periodate gave dialdehyde 28, which was subjected to a Henry reaction with nitromethane, to afford nitrosugar 29 as an epimeric mixture (Scheme 11).32... 

When sugars are treated with aqueous ammonia for a short time at low temperature in the absence of a catalyst, the reaction is arrested before heterocyclic compounds can be formed in appreciable proportion, and the products are mainly epimerization products of the sugars, probably formed by way of their 2,3-enediols. These epimerization products are summarized in Table I which shows the reactions of D-glucose, D-fructose, lactose, maltose, and melibiose with aqueous ammonia for a short time at low temperature. A dark-colored, high polymer is also formed in some instances (the browning reaction). In the ammoniacal solution, the monosaccharides are epimerized the disaccharides are epimerized and, in addition, may be hydrolyzed to monosaccharides that can also be epimerized hence, the variety of products obtained may be considerable. 

Epimerization at C-2 of UDP-GlcNAc, leading to the UDP derivative of 2-acetamido-2-deoxy-D-mannose, was also observed115 118 the monosaccharide was identified as a constituent of several capsular polysaccharides from Streptococcus pneumonia119-122 and Neisseria meningitidis group A (Ref. 123), the teichuronic acid of Bacillus cereus,124 and the O-specific... 

The hexosylulose derivative 7c may also be converted into the GDP derivative of colitose (3,6-dideoxy-L-xy/o-hexose, 3,6-dideoxy- L-galact-ose ),161,195 a component of lipopolysaccharides from several enterobacterial strains. Another monosaccharide of this group, ascarylose (3,6-dideoxy-L-arabino-hexose, 3,6-dideoxy- L-mannose ), which is present in Yersinia pseudotuberculosis lipopolysaccharides, was found to be synthesized as the CDP derivative through epimerization and reduction of CDP-6-deoxy-D-xy/o-hexos-4-ulose.168 171... 

Other monosaccharide components (of bacterial polysaccharides) that are structurally related to D-ribose include D-riburonic acid,232 identified in the exocellular polysaccharide produced by a strain of Rhizobium meliloti, and D-arabinose, frequently present as the furanose, in polysaccharides of mycobacterial cell-wall.233,234 L-Xylose235,236 should probably be included in the group, as it may be derived from D-arabinose through epimerization at C-4. Biosynthesis of these monosaccharides was not investigated. 

Kinetics of oxidation of four pentoses by bromamide-T were conducted in alkaline medium at different temperatures and the overall activation parameters have been calculated.52 Aldonic acids were the oxidation products, and a mechanism was suggested in which formation of the enediol anion of the sugar is the rate-limiting step. As aldoses may undergo epimerization in alkaline solutions, the oxidation of monosaccharides with bromamide-T was also performed in hydrochloric acid solution.53 Kinetic parameters revealed a low reactivity of ketoses relative to aldoses, and indicated that the cyclic forms of the latter are involved in the oxidations. 

Mechanism 23-2 Base-Catalyzed Epimerization of Glucose 1115 Mechanism 23-3 Base-Catalyzed Enediol Rearrangement 1115 23-9 Reduction of Monosaccharides 1116... 

Figure 2 Mammalian monosaccharides and nucleotide sugars. (A) Chemical structures, common names, and abbreviations of the 11 monosaccharides found in mammalian glycans as well as ManNAc, the precursor for sialic acids. (B) Examples of the three classes of nucleotide sugars are provided by UDP-GIcNAc (Glc, GIcA, Gal, GalNAc, and Xyl also use UDP), GDP-Man (Fuc also is linked to GDP), and CMP-Neu5Ac/Gc (IdoA is produced by postsynthetic epimerization of GIcA and therefore does not require a nucleotide sugar).

In the structures, introduced so far, an O4 rhomb or O5 patterns turned out to be ideal building blocks for carbohydrate-metal complexes. The most important monosaccharide, D-glucose, in any of its hemiacetal forms does not exceed the simple 02 diol pattern ( a consequence is the epimerization reaction of glucose to 8-mannofuranose in the presence of trivalent metal ions). 

In contrast, the separation of the various monosaccharides is much easier, which is surprising because the affinities of these compounds are not very different. However, when the pH value of the mobile phase is lowered to a value corresponding to the pK value of the respective compounds, even small differences in the dissociation behavior contribute to the separation. Thus, the chromatogram in Fig. 3-108 is obtained using a dilute sodium hydroxide solution of c = 0,001 mol/L. Under these conditions, even epimeric compounds such as glucose, mannose, and galactose are baseline resolved. Two important aldopentoses - arabinose and xylose - can also be determined in the same analysis. Remarkable is the elution order, which cannot be correlated with the carbon chain length of the monosaccharides. 

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