Altrose aqueous solution

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. 

The assignments of H signals then allows the quantitative determination of all forms of monosaccharides in aqueous solution as reproduced in Fig. 4.2.13. The most important hexoses, namely glucose, mannose, and galactose, all occur as pyranoses only. The axial 3-OH group of allose, however, interacts repulsively with the a-OH group on C-1 and thereby overcomes the anomeric effect. The combination of both repulsive effects then also leads to 12% fura-nose. The latter effect is even more pronounced in altrose and ribose (Stod-dart,1971) (Fig. 4.2.13). 

For the same conformational reasons, the P anomers of d aldoses dominate in aqueous solution when the Cf atom has the D configuration (ribose, xylose, allose, glucose, gulose, galactose). The a anomers, however, predominate when the atom has the l configuration (arabinose, lyxose, altrose, mannose, idose, talose). 

Studies of the D-aldohexapyranoses showed that the classical C chair is the most likely conformer for all of those -pyranoses. The or-anomers have one more axial group in the Cf orm, and so allose and altrose are likely to have C4 con-formers present as well. or-D-Idose in aqueous solution should contain equal amounts of both chairs and the °S2 form as well. 

We have studied the relative reactivity of cellulose and mixed polysaccharides (III) and (IV) in reactions with aqueous and alcoholic solutions of NaOH. The data on the composition of alkali compounds of polysaccharides are given in Table 6. As seen from the data presented in Table 6, the amount of bound alkali in preparations of alkaline compounds of mixed polysaccharides is le than in alkali cdlulose obtained under the same conditions, the DS with respect to NaOH decreasing with increasing content of altrose units in polysaccharide (III) and of 3,6-anhydroglucose units in polysaccharide (IV). 

Conversion of either pentaacetate to crystalline acetochloro-o-n-altrose was effected in good yield by the action of titanium tetrachloride in chloroform solution. Replacement of the chlorine atom by a hydroxyl group was accomplished by shaking the acetochloro compound with silver carbonate and aqueous acetone. The resulting tetraacetate appeared as the /3-modification, mutarotating from [ajo —6.0 to - -12.9° in chloroform solution. 

Further structural studies of this material have been reported.78 In strain 49 and X6C61, this hexose is replaced by the C-5 epimer, a-o-Galp. L-Altrose was purified by preparative paper chromatography after hydrolysis of the polymer, and the L-configuration was confirmed by its optical rotation. L-Altrose is transformed into 1,6-anhydroaltrose on treatment with aqueous acid the acid catalyzed equilibrium is reached when the latter compound constitutes 60-65% of the total carbohydrate in solution. Apparently, the enantiomeric D-altrose has not been found in Nature. 

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