Of furanose sugars

Borate complexing appears to involve the generation of a substantial proportion of furanose sugars in solution,85 although virtually none of the furanose forms may be present before the addition of borate. 

An extension of the foregoing to include acid-catalyzed transacetalation re-su ted in the formation of furanose sugars. By treating the protected xyloside 115... 

FIGURE 12.40. The conformations of furanose sugars, (a) Conformations of five-membered rings. When a line is drawn through C4 -01 -C1, the pucker is described by which atom in the five-membered ring is most out of this plane of the five-membered ring, (b) CZ -endo with C3 most out of the plane on the C5 side, (c) C2 -exo with C2 most out of the plane on the opposite side from C5, (d) C2 -endo with C2 most out of the plane on the C5 side, and (e) C3 -exo with C3 most out of the plane on the opposite side from C5. ... 

Fraser-Reid, B, Sun, K M, Tam, T F, Carbohydrate derivatives in the asymmetric synthesis of natural products some applications of furanose sugars. Bull. Soc. Chim. Fr., 238-246, 1981. 

The Altona pseudorotation description has been an extremely significant contribution, allowing an intuitively comfortable description of the conformational continuum in between limiting twist and envelope conformations. However, when used as a basis for the computational modeling of furanose sugars, several limitations become apparent ... 

Each of these compounds, 53-56, was shown to be a very effective competitive inhibitor of the enzyme with respect to the fructose 1,6-diphosphate, whereas several other analogs, including acyclic structures, had no effect. These and other results suggest that the furanose form of the sugar diphosphate is the active form in the enzymatic reaction (105). More recent studies using rapid quenching techniques and C-nmr measurements have confirmed this hypothesis and indicate that the enzyme uses the a anomer 52 much more rapidly than the 3 anomer 50 and probably uses the a anomer exclusively (106). 

There are two forms of carba-sugar carba-pyranoses and -furanoses. The former, especially the carba-hexopyranoses, have been extensively studied during the past two decades, ever since their derivatives were found in Nature as components of important antibiotics. However, very little is known about carba-furanoses, except for 4a-carba-)3-L-arabinofuranose ... 

As already shown in Sect. 4.4, ribose-2,4-diphosphate is obtained in a base-catalysed condensation of glycolaldehyde phosphate in the presence of formaldehyde (Muller et al., 1990). The phosphate group in the 4 position of the sugar prevents the formation of a 5-membered furanose ring, but a 6-membered pyranose structure can be formed. 

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... 

A number of examples involving the stereochemistry of five membered rings are met in furanose sugars. An interesting example is that of 2, 5 dimethylcyclopentane 1, 1 dicarboxylic acid. This acid can exist in two geometrically isomeric forms which can be distinguished by decarboxylation. The cis xxvii isomer forms two monocarboxylic acids which are meso because they possess a vertical plane of symmetry. The trans isomer xxviii forms only one monocarboxylic acid and since it possesses no elements of symmetry, therefore, exists in optically active forms and a meso variety. 

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