Aldohexose solution

Note The dipping solution, which can also be used as a spray solution, can be employed with silica gel, kieselguhr, cellulose, RP, NH2 and CN phases. Sugars (exceptions include, for example, fructose, melezitose and raffinose) yield brilliantly colored zones on an almost colorless background when the spray solution is employed. Aldohexoses appear brown, aldopentoses bright red and hexuronic acids orange in color [3]. The detection limit differs for different substances it ranges from 10 ng (pyridoxal) over 100 ng (cinnamaldehyde) up to 2 pg (citral). 

The pyranoid monosaccharides provide a wide range of asymmetric molecules for study by the c.d. spectroscopist. However, these compounds are not without their difficulties. In aqueous solution, these compounds exist in a complex equilibrium involving the two possible chair conformers of the pyranoses, the furanoses, a and p anomers, and the acyclic form, as well as septanoses for aldohexoses and higher sugars. 

Epimerization. In weakly alkaline solutions, glucose is in equilibrium with the ketohexose D-fructose and the aldohexose D-mannose, via an enediol intermediate (not shown). The only difference between glucose and mannose is the configuration at C-2. Pairs of sugars of this type are referred to as epi-mers, and their interconversion is called epimerization. 

IV, Composition in Aqueous Solution Aldoses 1. Aldohexoses and Aldopentoses... 

By means of the cyanohydrin reaction, higher sugars of the heptose. octosc, and nonosc types have been prepared. A monosaccharide such as an aldohexosc may be converted into the next lower monosaccharide, such as an aldopeniosc. by oxidation to the acid, which corresponds to the aldohexose. then treating the calcium salt solution of this acid with a solution or ferrous acetate plus hydrogen peroxide. Carbon dioxide is evolved and aldopentose formed. 

A more detailed study on the structure of D-glucose based upon physical data and chemical reactivity has revealed that the open-chain formulation of the aldopentoses, aldohexoses, ketopentoses and ketohexoses is however an oversimplification. Thus, for example, in solution D-glucose exists as an equilibrium... 

All stereocenters in 1,6-anhydrohexopyranoses are of inverted orientation compared to those in the parent 4Ci(d) or 1C4(l) conformations of the corresponding hexopyranoses for example, see 21, 23, and l,6-anhydro-/J-D-glucopyranose (22). In chemical properties, these compounds resemble to a certain degree the methyl /f-D-hexopyranosides. They are relatively stable in alkaline media, but are readily hydrolyzed by acids. In aqueous acid solution, an equilibrium is established between the 1,6-anhydrohexo-pyranose and the corresponding aldohexose, whose composition correlates with expectations from conformational analysis and calculations from thermodynamic data.121 Extreme values, 0.2 and 86%, are observed respectively with 1,6-anhydro-/f-D-glucopyranose (22) and l,6-anhydro-/f-D-idopyranose (the latter has all hydroxyl groups in equatorial disposition). 

The nature of the sugars in a nonenzymic browning reaction determines their reactivity. Reactivity is related to their conformational stability or to the amount of open-chain structure present in solution. Pentoses are more reactive than hexoses, and hexoses more than reducing disaccharides. Nonreducing disaccharides only react after hydrolysis has taken place. The order of reactivity of some of the aldohexoses is mannose is more reactive than galactose, which is more reactive than glucose. 

The amount of reducible form present in solutions of several aldohexoses and aldopentoses was measured by Cantor and Penis-ton by polarographic methods. Their measurements showed a... 

Like the 5-amino aldopentoses, the 5-amino aldohexoses have a pronounced tendency to form the pyranose ring in alkaline solution. In acid solution, three molecules of water are eliminated per molecule, to give the corresponding derivative of 3-pyridinol. 5-Amino-5-deoxyaldohexopyranoses are, however, distinctly more stable, as the Amadori rearrangement and pyridine formation occur at pH 5.7—6.2. With the pentose analogs, these reactions begin at pH 7—8. Because of the reactive a-amino alcohol arrangement at C-1, the 5-amino-5-... 

In the aldohexose series, 5-(benzylamino)-l,2-0-cyclohexylidene-5-deoxy-L-idurononitrile gives, on acid hydrolysis, an almost quantitative yield of N-benzyl-2-cyano-5-hydroxypyridinium chloride. On partial, catalytic hydrogenation of this aminonitrile, 5-(benzylamino)-I,2-0-cyclohexylidene-5-deoxy-L- do-hexodialdo-l,4-furanose is obtained. This compound is reducible with sodium borohydride to crystalline 5-(benzylamino)-l,2-0-cyclohexylidene-5-deoxy-L-idofur-anose which, on removal of the cyclohexylidene group with acid, forms the intermediate 5-(benzylamino)-5-deoxy-L-idopyranose this then loses three molecules of water per molecule, to give N-benzyl-5-hydroxy-2-(hydroxymethyl)pyridinium chloride. It is therefore clear that the transformation of 5-aminoaldoses into pyridine derivatives in acid solution is not prevented by the monoalkylation of the amino group. 

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