Formation of alpha -hydroxycarbonyl and alpha -dicarbonyl compounds during degradation of monosaccharides.

摘要

The formation of alpha -hydroxycarbonyl and alpha -dicarbonyl compounds from monosaccharides (glucose, fructose, arabinose, glyceraldehyde, and 1,3-dihydroxyacetone) was studied in 3 different model systems comprising an aqueous and alkaline solution of potassium peroxodisulfate (K2S2O8), and a solution of sodium hydroxide, respectively. In total, 6 alpha -hydroxycarbonyl (in the form of O-ethyloximes) and 6 alpha -dicarbonyl compounds (as quinoxaline derivatives) were identified by GC/MS and quantified. Acetol, glycolaldehyde, 1,3-dihydroxyacetone, methylglyoxal, and glyoxal were the most abundant low molecular weight carbonyls. Within the model systems studied, the yield of alpha -hydroxycarbonyl and alpha -dicarbonyl compounds was 0.32-4.90% (n) and 0.35-9.81% (n), respectively. The yield of alpha -dicarbonyls was higher than that of alpha -hydroxycarbonyls only in aqueous solution of K2S2O8 and in the other 2 model systems an inverse ratio of these 2 carbonyl types was found. For the first time, ethylglyoxal was identified as a sugar degradation product and several mechanisms explaining its formation were proposed. The achieved data indicated that low molecular weight alpha -hydroxycarbonyl and alpha -dicarbonyl compounds are predominantly formed by a direct retroaldol reaction and alpha - and beta -dicarbonyl cleavage. It was evident that some compounds were produced from the sugar fragmentation products. Thus, isomerisation, reduction of dicarbonyls by formaldehyde (cross-Cannizzaro reaction), and mutual disproportionation are possible reaction pathways participating in the formation of alpha -hydroxycarbonyl compounds. Oxidation and disproportionation of alpha -hydroxycarbonyl precursors as well as the aldol condensation of low molecular weight carbonyl species (followed by subsequent reactions) play an important role in the formation of several alpha -dicarbonyl compounds..