Allose composition

There is also an axial methoxyl group flanked by two equatorial hydroxyl groups in both of the pyranose forms of 3-O-methyl-D-allose. The pyranose forms are thereby destabilized, and, in solution, the proportions of the furanose forms are more than doubled,88 to give the composition 14 65 7.5 13.5 at 31 °. 

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 presence of substituents outside the ring usually does not affect the composition. Thus, D-allose 6-phosphate, D-altrose 6-phosphate, and d-manno-heptulose 7-phosphate have practically the same composition as the parent sugars.51... 

Various sugars epimerize at different rates. For example, maimose is converted much slower than glucose or fructose [9] and it is also formed more slowly. Idose reacts very rapidly in the presence of bases, altrose rapidly but allose slowly [23]. The most investigated case is, of course, the glucose-maimose-fruc-tose system. The products are formed in a ratio of approximately 4 1 4. More accurate figures have been quoted but the composition will depend on the nature of the base and of the starting material True equilibrium, as already stated, is probably never obtained [9]. 

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