Pyranoses stability

It IS not possible to tell by inspection whether the a or p pyranose form of a par ticular carbohydrate predominates at equilibrium As just described the p pyranose form IS the major species present m an aqueous solution of d glucose whereas the a pyranose form predominates m a solution of d mannose (Problem 25 8) The relative abundance of a and p pyranose forms m solution depends on two factors The first is solvation of the anomeric hydroxyl group An equatorial OH is less crowded and better solvated by water than an axial one This effect stabilizes the p pyranose form m aqueous solution The other factor called the anomeric effect, involves an electronic interaction between the nng oxygen and the anomeric substituent and preferentially stabilizes the axial OH of the a pyranose form Because the two effects operate m different directions but are com parable m magnitude m aqueous solution the a pyranose form is more abundant for some carbohydrates and the p pyranose form for others... 

It is interesting to note that the formation of the dioxane ring stabilizes the anhydrides as to their pyranose or furanose structures, difructose anhydride I as 2,1 l,2 -di-D-fructofuranose and dihetero-levulosan as 2,1 l,2 -di-D-fructopyranose. 

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. 

This enzyme oxidizes a- and P-anomers of D-glucose to the same extent and shows excellent stability and sensitivity about twice that for the methods with immobilized glucose oxidase. In this approach, pyranose oxidase is immobilized on tresylate-poly(vinylalcohol) beads and packed into a stainless column and peroxidase is immobilized on tresylate-hydrophylic vinyl polymer beads and packed into a transparent FITE tube that is used as the CL flow cell. The H202... 

On account of the more favorable anomeric effect,12 as well as of the larger nonbonding interactions of vicinal, synclinal substituents in furanoses as compared to pyranoses, jS-D-glucofuranose,13 /3-D-glucofuranurono-6,3-lactone, and their derivatives are of higher thermodynamic stability than their respective a-D anomers. 

As conformational anomeric effects represent only a fraction (—1 to —3 kcal/mol) of the global enthalpic anomeric effect or gem-dioxy stabilizing effect (—6 to —17 kcal/mol), additional factors have to be taken into account. Depending on substitution, steric factors can affect the relative stability of acetal conformers. Dubois et al.2S have demonstrated that in furanose pyranose derivatives a bulky substituent at the furanose... 

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... 

Spherical polar coordinates are used for conformational representation of pyranose rings in the C-P system. Unlike the free pseudorotation of cyclopentane, the stable conformations of cyclohexane conformers are in deeper energy wells. Even simong the (less stable) equatorial (6 = 90 ) forms, pseudorotation is somewhat hindered. Substitutions of heteroatoms in the ring and additions of hydroxylic or other exocyclic substituents further stabilize or destabilize other conformers compared to cyclohexane. A conformational analysis of an iduronate ring has been reported based on variation of < ) and 0 (28), and a study of the glucopyranose ring... 

The transition state for the substitution step requires that, for maximum stability, the incoming nucleophile be coplanar with the three-membered oxide ring, and, for an epoxide on a pyranose ring (for which a half-chair conformation is expected, by analogy with... 

It appears that the conditions favoring the stability of the pyranose form are precisely those under which the equilibrium is usually observed aqueous solutions, ambient temperature, free hydroxyl groups. 

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,... 

---