Glucose with enolic compounds

Experiments attempting the interaction of D-glucose with ethyl O-pro-pionylacetoacetate have proved fruitless. A positive result for this compound, in which the enolic structure has been fixed by substitution on the oxygen, would have been favorable to theories based on involvement of the enolic form of the ketonic compound. 

Figure 2.9. Glucose can enolize and reduce transition metals thereby generating superoxide free radicals (02" ), hydroxyl radicals ( OH), hydrogen peroxide (H202) and reactive dicarbonyl compounds. Adapted with permission from Wolff, S. P. (1996). Free radicals and glycation theory. In The Maillard Reaction. Consequences for the Chemical and Life Sciences, Ikan, R., ed., John Wiley Sons, Chichester, UK, 73-88.

Although there is no positive evidence in support of it, a plausible alternative to the above reaction sequence is enolization of the aldose before condensation with the amine. In such a sequence, the Y-substituted glycosylamine would not be a precursor of the aminodeoxyketose. This postulate merits investigation because, in spite of attempts to isolate Y, Y-dibenzyl-D-glucosylamine, this compound could not be found in the reaction product of D-glucose with dibenzylamine only D-glucose or the Amadori rearrangement product was isolated. ... 

These compounds exist normally under the enol form, as shown by IR spectroscopy. Gianturco et al. (1963). Gianturco and Friedel (1963) published the identification and synthesis of some of these cyclic diketones. Nishimura and Mihara (1990) found ten other compounds of the series (identified also when heating glucose with alkali) in analysis of a roasted Columbian arabica coffee steam distillation, extraction, separation of the concentrate in basic, acidic, weakly acidic and neutral fractions, the weakly acidic being further fractionated by TLC. 

In the first stage of peptidoglycan synthesis, UDP-N-acetylmuramic acid is synthesized, and then a pentapeptide chain becomes attached to the carboxyl group of N-acetylmuramic acid (see Scheme 2). Strom-inger found that the reaction of enolpyruvate phosphate with UDP-2-acetamido-2-deoxy-D-glucose results in the formation of a compound that appeared to be a UDP-2-acetamido-2-deoxy-D-glucose-pyruvate enol ether, and he predicted that its reduction would aflFord UDP-N-acetylmuramic acid. The occurrence of these reactions has now been confirmed, and the intermediate product has been characterized in greater detail. ... 

Table II indicates those compounds which, on reaction with D-glucose, have not yielded any crystalline product. A large enol content in the ketonic...

In order to carry out asymmetric cycloadditions of nitrosoalkenes, Reissig et al. have introduced chiral enol ethers derived from terpenes [378] and from the glucose derivative 4-46 [379]. Using these compounds, considerable asymmetric induction has been obtained thus, the 5,6-dihydro-4H-l,2-oxazine 4-45 was formed by hetero Diels-Alder reaction of 4-34 with chiral 4-44 in good diastereo-selectivity (Fig. 4-10) [379]. 

The enol ether (23) has been obtained from 2-0-methyl-D-glucose and from 2,3-di-O-methyl-D-glucose. Klemer, Lukowski, and Zerhusen isolated a crystalline form of (23) with C ]d + 16° no mutarotation was mentioned. They assumed their compound to be pyranoid by analogy with (24). The nuclear magnetic resonance spectrum of the reaction mixture from 2,3-di-O-methyl-D-glucose showed the presence of equimolar amounts... 

All of the above compounds have cis double bonds it is not known whether trans forms, which must be acyclic, exist. The low yields of enol ethers obtained by the action of alkali on some 2-0-methylaldoses may be due to the formation of 3,6-anhydroaldoses. For example, 3,6-anhydro-2-deoxy-D-ami no-hexose is formed reversibly from 2-deoxy-o-arabino-hexose ( 2-deoxy-n-glucose ) by the action of alkali. Another possibility is the formation of 5-(l,2-dihydroxyethyl)-3-methoxyfuran by analogy with the reaction of (53) to give (54) (see Section VII, la p. 202). 

Both aldoses and ketoses are oxidized to aldonic acids by Tollens reagent (Ag, NH3, HO ), so that reagent cannot be used to distinguish between aldoses and ketoses. Recall from Section 20.3, however, that Tollens reagent oxidizes aldehydes but not ketones. Why, then, are ketoses oxidized by Tollens reagent, while ketones are not Ketoses are oxidized because the reaction is carried out under basic conditions, and in a basic solution, ketoses are converted into aldoses by enolization (Section 19.2). For example, the ketose D-fructose is in equilibrium with its enol. However, the enol of D-fructose is also the enol of D-glucose, as well as the enol of D-maimose. Therefore, when the enol reketonizes, all three carbonyl compounds are formed. 

Autoxidation of D-fructosamine residues in glycated polylysine in the presence of iron(III) can be promoted by sunlight, as shown by monitoring formation of the main oxidation product, A -carboxymethyl-L-lysine (CML, 65). The proposed oxidation mechanism includes 2,3-enolization of the carbohydrate and formation of the enolate complex with Fe (Scheme 34). Ferricyanide anion, Fe(CN)6, is readily reduced by D-fructosamines and has traditionally been employed as a selective indicator for Amadori compounds in the presence of parent o-glucose or lactose,... 

Isotopic exchange of H-5 occurs in the enzymic oxidation of UDP-D-glucose to the D-glucuronic acid analogue, which is explained by enolization of an intermediate aldehydo-species. Study of the acid-catalysed decarboxylation of penturonic, hexuronic, and hepturonic acids showed that the first two gave one equivalent of carbon dioxide nearly qualitatively, which was not the case with the hepturonic compounds. Derivatives of such hepturonic acids have been prepared by application of the Ivanov reaction as outlined in Scheme 4. ... 

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