Aqueous Solution Aldoses

Composition in Aqueous Solution Aldoses 1. Aldohexoses and Aldopentoses 

Glucose, having all of its substituents equatorial in its pyranose forms, shows the highest proportion of pyranoses ( 99%) in its equilibrium 

The equilibrium compositions of aqueous solutions of some aldoheptoses are listed in Table III. Because the additional carbon atom in the side chain does not introduce additional steric interactions, the composition of solutions of heptoses is similar to that of the homomorphous hexoses, with only one exception, namely, n-glycero-n-ido-heptose, 92 a-D-Idopyranose in solution is a mixture of the 4Ci and 1C4 conformant) S. J. Angyal and R. J. Beveridge, Carbohydr. Res., 65 (1978) 229-234. 

It has been noted92 that, in aqueous solutions of the D-glycero-L-hep-toses, the a- to y -pyranose ratio is somewhat higher than that for the homomorphous hexoses, whereas, for the D-gh/cero-D-heptoses the ratio is the same, or even slightly lower. No explanation is apparent for this observation. 

The two aldotetroses, erythrose and threose, differ from the other aldoses in their behavior.23 Ring formation, to give furanoses, can occur only through the primary hydroxyl group, and is therefore less favored than with the higher sugars. Consequently, considerable proportions of the aldehydo and aldehydrol forms are found in solution. Like all a- and /J-hydroxyaldehydes, the aldehydo form of the aldotetroses readily forms dimers in concentrated solutions of the tetroses, the signals of the dimers are readily visible in their n.m.r. spectra. In the syrupy state, the tetroses consist mainly of dimers, rather than of furanoses they have never been crystallized. 

The composition of D-glucose has been determined over a wide range of temperature by Franks and coworkers27 and by Maple and Allerhand4 (see Table II). Both sets of data are self-consistent, but the a fi pyranose ratio recorded by Maple and Allerhand is considerably higher, for example, 39.4 0.8% of a-pyranose versus 35.5 1% at 37°. These authors added 11% of 1,4-dioxane to the solutions they used for recording the, 3C-n.m.r. 

During an investigation of the properties of furanoses, Serianni and coworkers6 studied the composition of sugars which cannot form pyranoses. Starting with the aldotetroses6 [p. 36], they studied the 5-deoxypentoses 9 some 5-0-substituted pentoses,9 29 and the 2-pentuloses.12 All these compounds will be discussed in this Section. 

As found typically for glucose and idose, nearly equal proportions of the two furanose forms were observed39 for 5-deoxy-5-fluoro-D-glucose (45 55) and -L-idose (47 53). 

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The reaction is used for the chain extension of aldoses in the synthesis of new or unusual sugars In this case the starting material l arabinose is an abundant natural product and possesses the correct configurations at its three chirality centers for elaboration to the relatively rare l enantiomers of glucose and mannose After cyanohydrin formation the cyano groups are converted to aldehyde functions by hydrogenation m aqueous solution Under these conditions —C=N is reduced to —CH=NH and hydrolyzes rapidly to —CH=0 Use of a poisoned palladium on barium sulfate catalyst prevents further reduction to the alditols... 

In aqueous solution the rate law for oxidation of simple a-ketols, such as acetoin and benzoin and various aldoses and ketoses o. sog-sioj markedly different from that of Wiberg and Nigh (vide supra), viz. 

Diols and polyols can participate in equilibria with boric acid in aqueous solution. The stability of polyolborates is determined by the number of OH groups in cis positions. Complexes with polyols are more stable than with diols, and 1,2-diol complexes are more stable than their 1,3-diol counterparts (Table 10) since the resulting five-membered chelate ring is unstrained.75120 In the case of 1,3,5-triols stable cage-like structures (5) and (6) are favored. Open-chain or five-membered cyclic polyols form more stable chelate complexes than their six-membered counterparts.120 Thus, chelates from alditols and ketohexoses are more stable than the corresponding aldose chelates (Table 10). Many polyols allow quantitative titrimetric determination of boric acid. Of these, mannitol remains the most widely used reagent on the basis of availability, cost and ease of handling.75... 

For simplicity, we have thus far represented the structures of aldoses and ketoses as straight-chain molecules (Figs 7-3, 7-4). In fact, in aqueous solution, aldotet-roses and all monosaccharides with five or more carbon atoms in the backbone occur predominantly as cyclic (ring) structures in which the carbonyl group has formed a covalent bond with the oxygen of a hydroxyl... 

Aldaric acids may be prepared from aldoses or aldonic acids by oxidation in aqueous solution with oxygen over platinum-charcoal255 or platinum-on-alumina.256 The effect of such promoters as bismuth or gold has also been studied.257 Hydrogen peroxide in the presence of iron salts has been used for the oxidation of uronic acids to aldaric acids.258... 

In general, a six-membered pyranose form is preferred over a five-membered furanose form because of the lower ring strain, and these cyclic forms are very much favoured over the acyclic aldehyde or ketofte forms. As can be seen in Table 1.3, at equilibrium, the anomeric ratios of pyranoses differ considerably between aldoses. These observations are a direct consequence of differences in anomeric and steric effects between monosaccharides. The amount of the pyranose and furanose present in aqueous solution varies considerably for the different monosaccharides. Some sugars, such as D-glucose, have undetectable amounts of furanose according H-NMR spectroscopic measurements whereas others, such as D-altrose, have 30% furanose content under identical conditions. 

Table 1.3 Composition of some aldoses at equilibrium in aqueous solution...

Aldose-bisulfite addition compounds are the least stable and decompose even in aqueous solution on heating. 

The conversion of a carbohydrate C-nitroalcohol to the corresponding sugar is achieved simply by adding an aqueous solution of the sodium oci-nitroalcohol to a moderately concentrated aqueous sulfuric acid solution at room temperature. A copious evolution of nitrous oxide occurs during the addition and the resulting sugar then can be obtained from the reaction solution in yields of from 60 to 80 percent, depending upon the ease of isolation of the particular aldose produced. 

Trifluoroacetates have been applied to the analysis of sugars in the same way as acetates. Imanari et al. [445] analysed aldoses after a prior reduction, as follows. A 0.5-ml volume of 1% NaBH4 in water was added to 0.5 ml of an aqueous solution containing 100-500 pg of a mixture of aldoses. The solution was allowed to stand at room temperature for 30 min and the excess of borohydride was decomposed by adding 0.5 ml of Amberlite CG-120 (H+). The resin was removed by filtration and the filtrate was evaporated to dryness. Borate was removed by the three-fold addition of 1 ml of methanol and subsequent evaporation. The residue was vacuum dried and dissolved in 0.1 ml of ethyl acetate and 0.1 ml of TFA anhydride. After 30 min at room temperature, 1—2 pi were taken for analysis. The resulting derivatives were more volatile than acetates and provided symmetric peaks on a column packed with 2% of XE-1105 (see Fig. 5.30). 

Hydrogenation of aldoses to alditols (polyhydric alcohols) is usually performed in an aqueous solution with nickel or ruthenium as catalyst, as seen in the examples shown in eqs. 5.9-5.11.17-19... 

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