Ketoses, molybdate complexes

Based on the results of studies with isotopically substituted D-fructoses, and following the stereochemical rules of molybdate complexes, the mechanism of the molybdic acid catalyzed mutual interconversion of 2-ketoses and 2-C-(hydroxymethyl)aldoses (referred to as the primary process) [54,55] is shown in Scheme 11. 

The tungstate and molybdate complexes of D-gtycero-D-marmo-heptitol, d-gfycero-D-gu/o-heptonate, and a number of aldoses and ketoses containing the fyxo or marmo configuration have been characterized using C- and NMR methods. 

Similar suggestions have been made for ketoses. By virtue of the equilibrium and the free rotation around the C-1 to C-2 bond, unsubstituted hydroxyl groups on C-1 and C-2 and an equatorial hydroxyl group on C-3 of the pyranose form of 2-ketoses can form structures approximating that of (13). The same applies for the furanose form of any 2-ketose having unsubstituted hydroxyl groups on C-1, C-2, and C-3. Thus, compounds not possessing such structural features show little or no tendency to form complexes with molybdate and wolframate. 

The influence of cross-linking and bead size on the resolution of mannitol/sorbitol and arabinitol/xylitol mixtures on seven Ca -form cation exchange resins has been studied. Alditols with at least four vicinal hydroxy-groups are retained more strongly than aldoses and ketoses when applied to anion exchange resins as their complexes with hexaammonium heptamolybdate. While the free sugars can be eluted with water, the alditols require 0.1 M aq. ammonia. Oxalic, citric and a-hydroxycarboxylic acids afford stable complexes with molybdate ions, so that they need to be removed from mixtures, if the separation of aldoses and alditols is to be achieved." ... 

Addition of four equivalents of boric acid to the starting ketose improves the original 6.5% yield of D-hamamelose up to 20% [55]. Thus, the product of the molybdic acid catalyzed rearrangement, D-hamamelose, is apparently being removed from its thermodynamic equilibrium with D-fructose by a competitive complex formation with boric acid. 

The mechanism of the secondary process (Scheme 12) that accompanies the molybdic acid catalyzed mutual interconversion of 2-ketoses and 2-C-(hydroxy-methyl)aldoses is illustrated as the interconversion of D-fructopyranose (25) and D-sorbopyranose (28). The most probable structure of the active complex that catalyzes the secondary process is a dimolybdate of a joint-cornered dioctahedron structure such as, for example, K2 [Mo02(C204)H20]20 [24]. It has been postulated [54] that, in such complexes, the 2-ketoses are involved as the bidentate hexopyran-2-ulose ligands exploiting their trans-arranged C-3-OH and C-4-OH hydroxyl groups (25, 28). 

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