Allose aqueous solution

The 2,3,4,5-tetra-0-acetyI-a(deht/do-pentoses are hydrated to the extent of — 90% in 7 3 oxolane -deuterium oxide.80 On the other hand, the 2,3,4,5-tetra-O-methylaldohexoses are hydrated74 to a lesser extent in aqueous solution at 37 °. When the carbon chain is in the planar, zigzag form, hydration is favored, and the ratio of aldehydrol to free aldehyde is — 2 1 (mannose and galactose) but when the chain is in a sickle form, hydration is impeded by 1,3-parallel interactions, and the ratio is —1 2 (glucose and allose). 

Fusion of an oxirane ring to a pyranose ring also deforms it, and thereby lowers its stability. The composition of 2,3-anhydro-D-mannose in aqueous solution,165 as determined by g.l.c. of the trimethylsilyl derivatives, is 23 7 65 5. This is remarkably similar to the composition of a solution of 2,3-O-isopropylidene-L-rhamnose. For 2,3-anhydro-D-allose, the ratios are166 41 12 5 42 (or 41 5 12 42). In this case, although the proportion of furanose forms is substantial, there is no clear preponderance of the //-furanose form, presumably because OH-1 and OH-2 are trans but OH-1 is quasi-equatorial by contrast, in the (preponderant)... 

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. 

Eschenmoser et al. have also postulated that the Btlrgi-Dunitz trajectory must be taken into account in discussions of the kinetic preference for formation of allose 2,4,6-triphosphate in the hexose series and of ribose 2,4-diphosphate in the pentose series, by aldomerization of glycolaldehyde phosphate in aqueous NaOH solution [37]. In this analysis, however, an additional interaction between the donor and acceptor substituents (b <-> f in Figure 6.40) is supposed to increase when the approach is non-perpendicular. 

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