Pyruvaldehyde formation

Aldolizations of trioses, with formation of hexoses, have been observed in several investigations. Meyerhof estimated that the triose-hexose equilibrium mixture from the condensation of OL-glycerose with 1,3-dihydroxy-2-propanone (in trisodium phosphate solution) contained 92% of hexose. Berl and Feazel, in their kinetic examination of this aldolization in sodium hydroxide solution, were unable to detect any triose by paper chromatography at the end of the reactions. Pyruvaldehyde formation complicates any glycerose or 1,3-dihydroxy-2-propanone reaction in alkaline medium, and this fact probably accounts for some of the disappearance of triose from these mixtures. Nevertheless, aldolizations of these short-chain sugars are side reactions to be reckoned with, whenever circumstances permit their occurrence. 

The DL-glycerose-l,3-dihydroxy-2-propanone isomerization was found to proceed at measurable rates at 50° in acidic buffers. Moreover, under these conditions, the irreversible dehydration to pyruvaldehyde appeared to be the only side-reaction of any consequence. It was actually fortunate that pyruvaldehyde formation occurred, since this permitted examination of the isomerization and dehydration reactions within a single system. 

One of the simplest examples of a dehydration in the sugar series involves the formation of pyruvaldehyde on treatment of DL-glyc-eraldehyde with mineral acid.48,48a The pyruvaldehyde is readily... 

Hayami and his coworkers have studied the mechanism of formation of acetol and pyruvic acid from D-glucose- -14C, -6-14C, and -3,4-14C2, reacting in a concentrated, phosphate buffer solution.148-151 Their data supported the supposition that the products are formed from pyruvaldehyde by way of a Cannizarro reaction. As in the formation of lactic acid, the pyruvaldehyde can be formed either from the reducing or the nonreducing end of the D-glucose molecule, and the distribution of radioactivity in the pyruvic acid and acetol... 

Komoto detected lactic acid in the mixture from reaction of D-glucose with ammonia,4 and presumed that it was produced from pyruvaldehyde formed by decomposition of D-glucose. Lactic acid has, indeed, been found as a product of the action of alkali (lime-water) on substituted D-glucose and substituted D-fructose,81,83,96 and the mechanism of its formation involves the reversible aldol reaction, followed by formation of pyruvaldehyde, and the benzilic acid rearrangement already described for saccharinic acid this is illustrated83,96 in Scheme 11. 

The characteristic aroma of wheat bread crust has been attributed to 2-acetyl-1-pyrroline, and its formation depends on the presence of bakers yeast [31]. In model systems it was demonstrated that the acetylpyrroline is formed from the reaction of proline with pyruvaldehyde or dihydroxyacetone. Other compounds with bread-like aromas formed in the reaction of proline with pyruvaldehyde include l-acetonyl-2-pyrroline and 2-acetyltetrahydropyridine (Scheme 12.5). These compounds are unstable, which explains why the characteristic aroma of freshly baked bread disappears quickly during storage. 

Kinetics of isomerization of glyceraldehyde to dihydroxyacetone—and the formation of pyruvaldehyde from both—have been studied in sub- and super-critical water.191... 

This important flavor compound was identified in the head-space volatiles of beef broth by Brinkman, et al. (43) and although it has the odor of fresh onions, it is believed to contribute to the flavor of meat. This compound can be formed quite easily from Strecker degradation products. Schutte and Koenders (49) concluded that the most probable precursors for its formation were etha-nal, methanethiol and hydrogen sulfide. As shown in Figure 5, these immediate precursors are generated from alanine, methionine and cysteine in the presence of a Strecker degradation dicarbonyl compound such as pyruvaldehyde. These same precursors could also interact under similar conditions to give dimethyl disulfide and 3,5-dimethyl-l,2,4-trithiolane previously discussed. 

Heating of dihydroxyacetone in the presence of phosphate ions in known to produce substantial amounts of pyruvaldehyde (10). Furthermore pyruvaldehyde is assumed to catalyze the formation of 1-pyrro-line by Strecker degradation of proline (11). To study the role of 1-pyrroline and pyruvaldehyde in Acp formation, three additional experiments were conducted. 

Although all of these methods suffered from deficiencies (difficulties of synthesis of starting materials, low yields, and more often than not the formation of mixtures of products requiring tedious separation procedures), they still find ample application for the preparation of many C-substituted imidazoles (e.g. 4-alkyl, 4,5-dialkyl, 2,4,5-trialkyl). The old Debus (or Radziszewski) method is still useful for preparing such compounds as 4-methyl- (132) and 2,4-dimethyl-imidazoles (133) using pyruvaldehyde (Scheme 71). However, alkaline fission of the pyruvaldehyde can result in a mixture of products. When pyruvaldehyde is treated alone with aqueous ammonia there are three main products (132), (133) and 2-acetyl-4-methylimidazole. Reversed aldol condensations cause degradation of the pyruvaldehyde, and subsequent cyclization of the fragments as in Schemes 71 and 72 accounts for the products. 

Kinetics of isomerization of glyceraldehyde to dihydroxyacetone—and the formation of pyruvaldehyde from both— have been studied in sub- and super-critical water. Formaldehyde reacts with isoeugenol [l-(3-methoxy-4-hydroxyphenyl)propene] in alkaline medium to give a 1,3-dioxane derivative via an unusual Prins-type reaction. The potential-energy surface for the equihbriimi, HCO - - HCN H2CO - - CN, has been calculated by ab initio methods. 

A dehydration of this type has actually been observed as a side reaction of a Lobry de Bruyn-Alberda van Ekenstein transformation in a very simple system. Thus, in experiments with the DL-glycerose-l,3-dihy-droxy-2-propanone isomerization in acetate, formate, and trimethylacetate buffers, pyruvaldehyde appeared in the reaction mixtures. (The formation of pyruvaldehyde from l,3-dihydroxy-2-propanone- and dl-glycerose-mineral acid mixtures had been observed much earlier.) Since these experiments in acidic buffers established that this reaction is subject to general acid and base catalysis, pyruvaldehyde must be formed in alkaline mixtures also. The results of Wohl s and Evans and Hass s experiments with DL-glycerose in alkaline solutions containing phenyl-hydrazine, in which pyruvaldehyde phenylosazone was isolated, support this view. 

Ratios of the initial rate of formation of 1,3-dihydroxy-2-propanone to that of pyruvaldehyde from glycerose were determined over a considerable range of conditions and found to be nearly constant, as shown in Table V. These data were interpreted as indicating that pyruvaldehyde and 1,3-dihydroxy-2-propanone are formed from a common intermediate. 

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