Monosaccharides isomerizations

Lactose on hydrolysis gives glucose and an isomeric monosaccharide galactose, which may be given the symbol Ga-r. The lactose molecule may be represented as Ga-r-G-r, and it has therefore also a free potential aldehyde group and is a reducing sugar like maltose. 

Fructose—Dextrose Separation. Emctose—dextrose separation is an example of the appHcation of adsorption to nonhydrocarbon systems. An aqueous solution of the isomeric monosaccharide sugars, C H 2Dg, fmctose and dextrose (glucose), accompanied by minor quantities of polysaccharides, is produced commercially under the designation of "high" fmctose com symp by the enzymatic conversion of cornstarch. Because fmctose has about double the sweetness index of dextrose, the separation of fmctose from this mixture and the recycling of dextrose for further enzymatic conversion to fmctose is of commercial interest (see Sugar Sweeteners). 

Isomerization. Both the carbonyl group and the adjacent hydroxyl group are involved in isomerization of monosaccharides. This reaction can be catalyzed by either a base or an enzyme. By this reaction, an aldose is converted into another aldose and a ketose, and a ketose is converted into two... 

The dissertation26 by de Bruijn, Monosaccharides in alkaline medium isomerization, degradation and oligomerization and other publications by... 

The reversible reactions are initiated by an equilibrium between neutral and ionized forms of the monosaccharides (see Fig. 6). The oxyanion at the anomeric carbon weakens the ring C-O bond and allows mutarotation and isomerization via an acyclic enediol intermediate. This reaction is responsible for the sometimes reported occurrence of D-mannose in alkaline mixtures of sucrose and invert sugar, the three reducing sugars are in equilibrium via the enediol intermediate. The mechanism of isomerization, known as the Lobry de Bruyn-... 

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... 

The most important natural monosaccharide, D-glucose, is an aliphatic aldehyde with six C atoms, five of which carry a hydroxyl group (1). Since C atoms 2 to 5 represent chiral centers (see p. 8), there are 15 further isomeric aldohexoses in addition to D-glucose, although only a few of these are important in nature (see p.38). Most natural monosaccharides have the same configuration at C-5 as D-glyceraldehyde-they belong to the D series. 

Rare or unnatural monosaccharides have many useful applications as nonnutritive sweeteners, glycosidase inhibitors and so on. For example, L-glucose and L-fructose are known to be low-calorie sweeteners. In addition, rare or unnatural monosaccharides are potentially useful as chiral building blocks for the synthesis of biologically active compounds. Therefore, these compounds have been important targets for the development of enzymatic synthesis based in the use of DHAP-dependent aldolases alone or in combination with isomerases. Fessner et al. showed that rare ketose-1-phosphates could be reached not only by aldol addition catalyzed by DHAP-dependent aldolases, but by enzymatic isomerization/ phosphorylation of aldoses [35]. Thus, for example, L-fructose can be prepared... 

Treatment of monosaccharides with alkali usually results in a preponderance of products that are not isomeric with the original sugar. Their molecular-weight distribution indicates that they must... 

All the monosaccharides except dihydroxyacetone contain one or more asymmetric (chiral) carbon atoms and thus occur in optically active isomeric forms (pp. 17-19). The simplest aldose, glyceraldehyde, contains one chiral center (the middle carbon atom) and therefore has two different optical isomers, or enantiomers (Fig. 7-2). 

The above example demonstrates the potentialities of mass spectrometry as a method for elucidating the position of isopropylidene groupings. This problem has often to be solved in synthetic monosaccharide chemistry, especially when the compound may form several isomeric isopropylidene derivatives and the chemical methods are tedious and, sometimes, unreliable. 

Total Number of Isomeric Disaccharide Fragments of the Polysaccharides Composed of the Most Common Monosaccharides"... 

Mutation, where repositioning of a phosphate group in the monosaccharide phosphate molecule, e.g. the isomerization of glucose 6-phosphate and glucose 1-phosphate (Figure 8.2), is achieved via an intermediate diphosphate. 

Glucose is an aldohexose, where aldo- indicates that it is an aldehyde, -hex- designates the number of carbons, and -ose is the suffix used for carbohydrates. Some other common monosaccharides are fructose, a 2-ketohexose that is isomeric with glucose, and ribose, an aldopentose that contains one fewer carbons than glucose. 

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