Furanose stability in solution

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... 

Until the discovery of antibiotic substances, apiose and hamamelose were the only branched-chain sugars that had been found in Nature. The composition of both sugars in aqueous solution has been determined, and it is typical of the effect of branching on the stability of pyranose and furanose forms,... 

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)... 

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-... 

For silicon, the metal-coordinating properties, as found in AnEiyt, should, in principle, be shared by each of the monosaccharides. A particular monosaccharide is expected to act as a good ligand if its cA-furanose form is of considerable stability, so that the stability constant of the complex is not charged with the isomerization energy of the ligand. In fact, almost all of the monosaccharides enrich alkaline aqueous silicate solutions with five- and six-coordinate silicon species, to some extent. However, the determination of the compositions and stmctures of the involved species is largely complicated by the mere number of the species in equilibrium. This has recently been demonstrated for D-ribose solutions by Kinrade et al., who detected a vast amount of various five- and six-coordinate species in such solutions [94]. For monosaccharides other than D-ribose, the situation is not any better. 

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