Of reductones

Knowledge about the chemical structure of the antioxidative MRP is very limited. Only a few attempts have been made to characterize them. Evans, et al. (12) demonstrated that pure reductones produced by the reaction between hexoses and secondary amines were effective in inhibiting oxidation of vegetable oils. The importance of reductones formed from amino acids and reducing sugars is, however, still obscure. Eichner (6) suggested that reductone-like compounds, 1,2-enaminols, formed from Amadori rearrangement products could be responsible for the antioxidative effect of MRP. The mechanism was claimed to involve inactivation of lipid hydroperoxides. 

Figure 10.2 Typical plots of degree of reducton with time at different temperatures...

The formation of reductones and rearrangements of the carbon chain are possible side reactions which should be considered. 

Lipides, 238, 239, 241-243, 261 Lipidosis, 239, 242, 243 Lobry de Bruyn-Alberda van Ekenstein transformation, 63, 291 acid catalysis of, 79 aldolization in, 77 base catalysis of, 79-81 catalysis of, by metal ions, 81 dealdolization in, 77 dehydration reactions in, 73 enzyme-catalyzed, 66, 70 formation of reductones in, 79 of or-hydroxy aldehydes, 71 mechanism of, 84 of noncarbohydrate a-ketols, 71 non-enzymic, 66, 67, 83 in paper chromatography, 81 rearrangement of carbon chain, 79 scope of, 65 of steroids, 72 use of, for synthesis, 82 Lyxonic acid, 3-deoxy-D-, 300 Lyxose, D-, condensation of, with urea, 218... 

Quite similar results have been described for the action of ultrasound on D-fructose solutions. After two hours, an absorption maximum was observed at 283 mu, probably showing the formation of reductone, as mentioned by Phillips. An effect on the specific rotation was also noted, but this was much less than that observed during some irradiation studies. An x-ray investigation proved that at least 15% of the parent sugar is changed to other products, but isolation of the products has not yet been attempted. ... 

Recent developments in the chemistry of the 1,2-unsaturated cyclic compounds, namely, the glycals and the 2-hydroxyglycals, are included in order to supplement the earlier Chapters on these topics. There follows a discussion of other cyclic and acyclic sugars which possess, at various positions in the carbon chain, alkenyl, enolic, or enamine systems. The scope has been arbitrarily restricted by the exclusion of the ends themselves [and, therefore, of reductones and compoimds related directly to L-ascorbic acid (1)], of such enones as the pyrone derivative (2), and of dienes or dienones [for example, kojic acid (3)]. Cyclohexene derivatives... 

Conditions for formation of reductones by pyrolytic degradation of cellulose, dextrin, maltose, and glucose have been reported. The photoreactions of (7) with R,R CO (R,R = Ph or Me) in deoxygenated solution under nitrogen... 

As shown in Table III no radicals could be detected clearly demonstrating that the CROSSPY formation was still in the induction period. In order to check the influence of reductones on radical formation, this ftiermally pre-treated mixture was incubated in the presence of ascorbic acid at room temperature. Analysis of the mixture by EPR spectroscopy revealed that instanftmeously after reductone addition the radical cation was generated (Table III). To investigate the effectivity of carbohydrate-derived reductones in CROSSPY formation, in comparative experiments, acetylformoin as well as methylene reductinic acid, both well-known to be formed during thermal treatment of hexoses (19), were added to the thermally pre-treated mixture. Both the Maillard reaction products were found to rapidly induce radical formation, however, in somewhat lower effectivity when compared to ascorbic acid (Table III). 

Table III. Influence of Reductones on the Generation of Radical Cations in the Induction Period of a Thermally Treated Glucose/ Na-acetyl-L-lysine Mixture...

Highly specific even in presence of reductone and other -SH compounds... 

Table 8.8. Some dicarbonyl group molecules involved in suifur dioxide combinations (hydroxypropanedial is a tautomer form of reductone) (Guillou-Largeteau, 1996)...

Mapson did not find any pH rai at which formaldehyde condenses with ascorbic acid and does not condense with reductones. However, at pH 2.0 ascorbic acid condenses fairly rapidly with formaldehyde and the reaction of reductones with formaldehyde is slower and proceeds at a linear rate. Mapson (14), therefore, proposed that the indophenol titration be carried out at pH 2.0 after formaldehyde treatment to estimate ascorbic acid in the presence of reductones. In this procedure advantage is taken of the fact that reductones react with formaldehyde at a linear rate. To measure the reductones accurately a series of titrations is carried out at 10 minute intervals after the addition of formaldehyde a graph is constructed and the reductone titration value is obtained by extrapolation of the curve... 

Compounds of types 69 and 70 are classified as reductones. Nonaromatic reductones, such as 71 and 72, exist entirely in the lactam or lactone form, but an appreciable proportion of aromatic reductones is in the y-oxo form (cf. 73) at equilibrium. These conclusions are largely based on studies of 3,4-dihydroxythiacouma-rinio7-io8a a nd 3,4-dihydroxycoumarin (70, Z = S and O, respec-... 

On the basis of experiments with myoinositol (50), it has been suggested that triose reductone should be oxidized by periodate to yield two molar equivalents of formic acid and one molar equivalent of carbon dioxide. However, it has been reported by two groups (1,29) that crystalline triose reductone is oxidized by two moles of periodic acid to give formic acid and glyoxylic acid, free iodine being liberated during the... 

The appearance of free iodine during the periodate oxidation of compounds having an active hydrogen atom (27) or an ene-diol structure (1,39) has frequently been observed, and this implies that further reduction of iodate, formed from periodate during the main reaction, takes place. It has, in fact, been shown that, in acid solution, iodate is fairly readily reduced by such compounds as triose reductone (27), dihydfoxy-fumaric (39), and tartronic (32) acids. 

We examined the reaction of triose reductone with both periodate and iodate (55,56), and found that, whereas iodine was invariably set free from both sodium periodate and sodium iodate if the concentration of the reductone were greater than 10 3M, no iodine was liberated at lower concentrations (e.g. 6 x 10 4M) of substrate, even in the presence of relatively large amounts of the oxidants. 

The reaction of iodate with triose reductone is not only a function of the concentration of the reagents, it is also dependent on the pH of the solution. In solutions of triose reductone more dilute than 10"3M, iodine is set free from iodate, if the pH of the solution is lower than about 3 (55). Dihydroxyfumaric and L-ascorbic acids (26), which also have free ene-diol structures, behave similarly. 

Hesse and Mix (29) oxidized a relatively concentrated solution of triose reductone using limited amounts of free periodic acid. In these conditions, the iodic acid formed by the initial reduction of periodic acid could be further reduced and the reduction product could then, in turn, react with the remaining periodic acid and liberate iodine. Thus glyoxylic acid could be isolated from the oxidation mixture, as no periodate was available for its oxidation. 

We therefore carried out periodate oxidation of triose reductone in dilute solutions using sodium metaperiodate as the oxidizing agent (55,56), Triose reductone could react with periodate according to the following reaction sequence ... 

Mesoxalic dialdehyde can be reasonably expected (16,28,50) to undergo normal glycol cleavage and give one mole of formic acid and one mole of glyoxylic acid in fact, when a second molar equivalent of periodate was added to the above solution, two molar equivalents of titratable acid were formed. If an excess of periodate is now added, two molar equivalents of titratable acid remain, but in addition, one molar equivalent of carbon dioxide can be expelled from the solution. Thus, in the overall reaction, one mole of triose reductone is oxidized by three moles of periodate to give two moles of formic acid and one mole of carbon dioxide ... 

It must be stressed again, that the reaction sequence given here proceeds quantitatively, without formation of free iodine, only if the concentration of triose reductone is less than 10 3M and if the pH is above 3. 

Thus, if triose reductone is, in fact, the first intermediate in the periodate oxidation of malonaldehyde, the total consumption of periodate per mole of malonaldehyde should be four molar equivalents two moles of formic acid and one mole of carbon dioxide should be formed, in accordance with the sequence proposed by Fleury and his collaborators (22). As in the case of the periodate oxidation of malonic acid (32) the rate determining step should be the hydroxylation step. 

However, when we oxidized malonaldehyde (56) in the conditions just described for triose reductone, although formic acid and carbon dioxide were produced in high yields, the periodate consumption was erratic. Similar results were obtained with deoxy sugars. This discrepancy may be caused by the incomplete enolization of the first intermediate, hydroxy malonaldehyde —i.e. tartronic dialdehyde (5,22,32), to triose reductone, or may concern the hydroxylation step itself. 

Crystalline triose reductone has been shown (56) by titration with strong base and with iodine, to exist in solution, for practical purposes, entirely as the enol form. In addition, the fact that it reduces exactly three molar equivalents of periodate to give quantitative yields of formic acid and of carbon dioxide indicates that it is also oxidized entirely in this form. However, nothing is known of the rate of enolization of tartronic dialdehyde and the possibility therefore remains that part of it may be oxidized in the dialdehydo form. If this were the case, the results of periodate oxidations would be dependent on the ratio of the rate of enolization of tartronic dialdehyde to the rate of its oxidation by periodate, since the oxidation of triose reductone is, again, for practical purposes, instantaneous. 

An apparent anomaly is encountered when a reductone structure, —C=C—C=0, is oxidized. The first equivalent of oxidant produces a... 

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