Aldoses branched-chain sugars

A considerable contribution to oin knowledge of the chemistry of dicarbonyl sugars, a group of sensitive and reactive compounds, has been made by Theander, who used, for the most part, acetals of D-glucose, D-mannose, and n-galactose. Carbonyl derivatives of aldoses and aldosides are of importance in the synthesis of branched-chain sugars. ... 

If this reaction is appUed to a ketose it should result not in epimerization but in the formation of a branched-chain sugar, a 2-C-(hydroxymethyl)aldose, because the carbon atom which migrates carries a hydroxymethyl group. PreUmi-nary results show that this is indeed the outcome of the reaction [53]. 

The Bilik reaction applied to 2-ketoses yield 2-hydroxymethyl aldoses in which the tertiary carbon originates from C2 of the ketose and the C2 hydroxyl is on the opposite side to the C3 hydroxyl of the ketose (in the Fischer projection). Thus, o-fructose yield o-hamamelose. The position of equilibrium, however, lies towards the straight-chain sugar, although it can be pulled over somewhat towards the branched-chain aldose by the addition of borate. The mechanism in Figure 6.9 again explains the main reaction, but not the formation of sorbose as a by-product, which probably arises from a metal ion-promoted hydride shift, as there is no isotope exchange with solvent. The Bilik reaction can be applied to the production of l-deoxy-o-xylulose from 2-C-methyl-D-erythrose the reaction is particularly clean and only the two... 

The individual 2-C-(hydroxymethyl)aldoses were obtained after a combined separation of the epimers via their phenylhydrazones followed by chemisorption chromatography on a cation-exchange resin in the Ba form [50]. Then, on treatment with molybdic acid at increased temperature, 2-C-(hydroxymethyl)-D-mannose (16c) rearranged to D- /Mco-hept-2-ulose (15 c) as expected. Under similar conditions, 2-C-(hydroxymethyl)-D-glucose (15 d) was transformed to d-ma o-hept-2-ulose (16d). The analysis of the reaction equilibria by NMR spectroscopy revealed the presence of small quantities of the respective starting branched-chain aldoses 15d (3.5%) and 16c (8%). The same equilibria were also obtained from the side of the model ketoses 15 c and 16 d. Thus, for example, the equilibrium of the sugars 15c and 16c at their total 2% concentration in a 0.2%... 

The data obtained suggest that conversions of the branched-chain aldoses 15 d and 16c to the respective ketoses 15c and 16d are not irreversible but their reaction equilibria are strongly shifted to the side of the ketoses. Thus, between the interconverting pairs of sugars, 15 c 16c and 16d 15d, respective thermo-... 

Molybdic acid treatment of the branched-chain aldose 40 results in a 1 12 equilibrium mixture with sedoheptulose (41), a proportion of which is present in the form of the 2,7-anhydro derivative 42. For isolation purposes, 41 is completely converted in 0.5 M H2SO4 into the commercially available form of the sugar 42 (Scheme 16), which is thus obtained in 63% yield (in relation to the starting 40). 

Monosaccharides, aldoses and ketoses, are divided - according to the number of carbons in the chain - into trioses, tetroses, pentoses, hexoses and higher sugars, and into aldotrioses, ketotrioses and so on. The carbon chain of monosaccharides present in foods is usually linear, but branched-chain monosaccharides also exist. 

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