Aldose complex

The C n.m.r. of aldoses complexed to molybdate shows that ribose, talose, and allose behave as tridentate donors using hydroxy-groups at positions 2, 3,... 

The n.m.r. spectra of aldoses complexed to molybdate show that D-ribose, D-talose, and D-allose behave as tridentate donors via hydroxy-groups at C-2, C-3, and C-4, whereas D-lyxose and D-mannose are donors via C-1, C-2, and C-3 hydroxy-groups. ... 

The hydrated, acyclic form of an aldose complexed by its four adjacent hydroxyl groups, HO-1, HO-2, HO-3, and HO-4, in the tetradentate binuclear molybdate complex 8 undergoes delocalization to give the transient state 9. At this point both the C-2-C-3 and the C-l-C-3 bond, the former one disrupting whilst the latter one is newly formed, are equivalent. Moreover, the process is... 

In the presence of lime water more complex reactions occur, leading to the formation of aldoses and hexoses (iv). This particular reaction is of interest to the biochemist as it is now generally held that optically active plant carbohydrates are obtained from carbon dioxide and water via formaldehyde. 

The complex thioamide lolrestat (8) is an inhibitor of aldose reductase. This enzyme catalyzes the reduction of glucose to sorbitol. The enzyme is not very active, but in diabetic individuals where blood glucose levels can. spike to quite high levels in tissues where insulin is not required for glucose uptake (nerve, kidney, retina and lens) sorbitol is formed by the action of aldose reductase and contributes to diabetic complications very prominent among which are eye problems (diabetic retinopathy). Tolrestat is intended for oral administration to prevent this. One of its syntheses proceeds by conversion of 6-methoxy-5-(trifluoroniethyl)naphthalene-l-carboxyl-ic acid (6) to its acid chloride followed by carboxamide formation (7) with methyl N-methyl sarcosinate. Reaction of amide 7 with phosphorous pentasulfide produces the methyl ester thioamide which, on treatment with KOH, hydrolyzes to tolrestat (8) 2[. 

Figure 5.7 Examples of (a) elimination, (b) isomerization (aldose/ketose) and (c) a complex rearrangement of the pinacol-pinacolone type found in the biosynthesis of valine and isoleucine.

The fact that only Cr(V)-disaccharide species are formed in the Cr(VI)/disaccha-ride reaction at pH 3-7 indicates that the disaccharides are better chelating agents for Cr(V) than are aldoses. The latter are poorer complexation agents than the corresponding aldonic acids so that oxo-Cr(V)-aldose-aldonic acid and oxo-Cr(V)-(aldonic acid)2 bis-chelates are the major species observed in the EPR spectra of the Cr(VI)/aldose reaction mixtures. 

One of the differences between ketoses-and aldoses is that the ketoses have a CH2OH group bound to the reducing carbon atom instead of a hydrogen atom. In the absence of further experimental results on the action of boric acid on ketoses it seems best to defer allocations and to emphasize only that ketoses form much greater quantities of the boric acid complex than do aldoses. This is apparently due to the fact that... 

The interconversion of aldoses and the respective 2-ketoses in alkaline solution may be somewhat more complex than originally supposed, as it has been reported that a partial transfer of hydrogen from C-2 of the aldose to C-l of the corresponding ketose occurs during the reaction.29 This observation is not inconsistent with isomerizations that involve 1,2-enediol intermediates. The transfer could occur as a result of a rapid conversion in which some of the protons originally at C-2 of the aldose molecules are retained by the solvent cage that surrounds the intermediate 1,2-enediol, and are, therefore, available for addition to C-l of the resulting ketose. It should be noted that other interpretations, such as hydride-transfer mechanisms, are also possible. 

Wilson DK, Tarle I, Petrash JM, Quiocho FA. Refined 1.8 A structure of human aldose reductase complexed with the potent inhibitor zopolrestat. Proc Natl Acad Sci USA 1993 90 9847-9851. 

Diols and polyols can participate in equilibria with boric acid in aqueous solution. The stability of polyolborates is determined by the number of OH groups in cis positions. Complexes with polyols are more stable than with diols, and 1,2-diol complexes are more stable than their 1,3-diol counterparts (Table 10) since the resulting five-membered chelate ring is unstrained.75120 In the case of 1,3,5-triols stable cage-like structures (5) and (6) are favored. Open-chain or five-membered cyclic polyols form more stable chelate complexes than their six-membered counterparts.120 Thus, chelates from alditols and ketohexoses are more stable than the corresponding aldose chelates (Table 10). Many polyols allow quantitative titrimetric determination of boric acid. Of these, mannitol remains the most widely used reagent on the basis of availability, cost and ease of handling.75... 

A minor, but significant, fraction of each isolated complex is an alkali metal hydroxide adduct. All complexes were treated as pure alcoholate in the calculations of the combining ratio. Except where indicated by superscript c, the ratios are corrected, for ethanol content, only in those cases where the solvent content is known (ethanol of solvation, if known, is reported as molecules per cation in parentheses next to the combining ratio). Based on the dimeric form of the aldose. The complex is actually composed of approximately four aldose residues. Estimated on the basis of a posable molecule of solvation (ethanol) per cation. 

The Mn(II)-catalysed oxidation of glucose by peroxodisulfate ions occurs via a radical-chain mechanism.26 Kinetics of oxidation of thiodiglycollic acid by (trans-cyclohexane-l,2-diaminc-/V, N, N, /V -tetraacetatolmanganateilJI) have been investigated.27 Oxidations of ketoses and aldoses by manganese(IV) in sulfuric acid media have a first-order dependence on sugar and fractional-order dependence on oxidant.28 A mechanism has been proposed for the oxidation of L-malic acid by Mn(III) pyrophosphate in aqueous acid, involving complex formation and radicals.29... 

In perchloric acid, hexoses and pentoses are oxidized by Ce(IV) via formation of two complex intermediates. The first is partly oxidized following Michaelis-Menten kinetics and partly dissociated to the second, which is oxidized more slowly than the former.180 The first step in the oxidation of aldoses by Tl(III) in the same medium involves the C-l-C-2 cleavage of the aldehydo form of the sugar. Thus, D-glucose gives D-arabinose and formic acid. With an excess of oxidant the final product is carbon dioxide.181 In the presence of a catalytic amount of sulfuric acid in acetic acid, Tl(III) oxidizes maltose and lactose to the corresponding disaccharide aldonic acids. The reaction showed activation enthalpies and enthropies characteristic of second-order reactions.182... 

Decarbonylation of aldoses.2 Although this rhodium complex has been known since 1968 to effect decarbonylation of aldehydes, it has been used for decarbonylation of sugars only recently, probably for lack of a compatible solvent. Actually, this reaction when carried out in N-methyl-2-pyrrolidinone (NMP) at 110-130° is extremely useful in the case of simple aldoses, which are converted to the lower alditol with formation of carbonylchlorobis(triphenylphosphine)rhodium(I). The yields are 75-95%. This method of degradation has the further advantage that protecting groups are not necessary. Deoxyaldoses, particularly 2-deoxyaldoses, are decar-bonylated in 75-99% yield. A disadvantage of this reaction is that a full equivalent of the complex is required. 

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