Fructose furanose form stability

The enzymatic synthesis of sucrose also throws light on the formation of the furanose form of fructose in the sucrose molecule. The fact that sucrose is directly formed from D-glucose-l-phosphate and D-fructose supports Isbell and Pigman s34 and Gottschalk s85 evidence that the latter monosaccharide occurs in solution in an equilibrium mixture of furanose and pyranose forms. This makes it unnecessary to postulate a special mechanism of stabilization of a five membered (furanose) ring before the formation of compound sugars containing the D-fructose molecule.86... 

Another interesting example of hindered internal rotation is related to the relative stability of the /3-furanose form of D-fructose (12a) in dimethyl sulfoxide solutions,83139-141 in marked contrast to the fact that the /3-pyranose form (13) is by far the predominant tautomer in water. From the, 3C 7, relaxation data83 (see Table HI) of the three major tautomers of D-fructose—a-furanose (12b), jS-furanose (12a), and 0-pyranose (13)—it can be concluded that these tautomers tumble isotropically in both D20 and (CD3 )2SO solutions. The T, values83 for the second-... 

The essentially nonreversible formation of D-fructose 1-phosphate in the muscle-aldolase system is probably attributable to thermodynamic stabilization. D-Fructose 1-phosphate can form a stable pyranose structure, whereas D-fructose 1,6-diphosphate can exist only in the less stable furanose or acyclic forms.72(,) Only when the cleavage products are removed is the monophosphate effectively split under the influence of aldolase. 

The observed stability constants (Afobs) for compounds 140( =3)-145( =g) and 146(pyrene) with saccharidcs, which have the ability to form a furanose ring with a jjn-periplanar anomeric hydroxyl pair (o-glucose, melibiose, o-fructose and lactulose) are large, consistent with our understanding of the boronic acid diol interaction. Equally, the low or zero values of the observed stability constants (Afobs) for saccharides unable to form a furanose ring with a. syw-periplanar anomeric hydroxyl pair (maltose and leucrose) are also consistent with current thinking. 

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