Aldohexose aqueous solution

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. 

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

There is no evidence that aldohexoses in aqueous solution exist to any extent as septanoses. However, in an n.m.r. study, Coxon183 found that 2,3,4,5-tetra-O-methyl-D-glucose in chloroform contains 60% of the /3-septanose and 40% of the acyclic form. 

The tautomeric equilibria of aldopentoses, aldohexoses and hexuloses in DMSO have been examined by C-n.m.r. spectroscopy the mutarotation kinetics of aqueous D-glucose have been investigated by i.r. methods, and Raman spectroscopic data on the tautomeric transformations of glucose in aqueous solution over a wide pH range confirmed that the ring-chain tautomerism depends on pH." The ring shapes of the two fiiranose and two pyranose forms of psicose in aqueous solution, riiown by n.m.r. spectroscopy to be present in almost equal proportions, have been analyzed by MM3."... 

Ketohexoses react with acids much more rapidly than aldohexoses and, hence, give better yields of 5-hydroxymethylfurfural. The reaction proceeds at an appreciable rate in aqueous solutions of fructose, and even glucose, at 100 to 150° without added acids. Since furfurals readily undergo further reactions with the formation of brown-colored products, it is probable that many of the brown colors produced in food processing and in autoclaved solutions result from the intermediary formation of furfurals (60). (See also p. 446). When concentrated hydrochloric or hydrobromic acid is used, the corresponding 5-halogenomethylfurfural is produced. 

Both a- and P-pyranose and fliranose forms exist. Table 22.2 shows the relative amounts for the aldohexoses in aqueous solution. 

When D-tagatose is added to a basic aqueous solution, an equilibrium mixture of monosaccharides is obtained, two of which are aldohexoses and two of which are ketohexoses. Identify the aldohexoses and ketohexoses. 

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). 

Dissolved in an aqueous environment, monosaccharides exist primarily in the cyclic or hemiacetal form, which is more stable in solution than the open chain form. If an aldohexose is the precursor of cyclic hemiacetal, it forms a six-membered pyranoside (Figure 3) if a ketohexose or aldopentose is the precursor of the cyclic hemiacetal, a five-membered furanoside is formed. 

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