Formation of α-hydroxycarbonyl and α-dicarbonyl compounds during degradation of monosaccharides

The formation of α-hydroxycarbonyl and α-dicarbonyl compounds from monosaccharides (glucose, fructose, arabinose, glyceraldehyde, and 1,3-dihydroxyacetone) was studied in three different model systems comprising an aqueous and alkaline solution of potassium peroxodisulfate (K₂S₂O₈), and a solution of sodium hydroxide, respectively. In total, six α-hydroxycarbonyl (in the form of O-ethyloximes) and six α-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 α-hydroxycarbonyl and α-dicarbonyl compounds was 0.32−4.90% (n/n) and 0.35−9.81% (n/n), respectively. The yield of α-dicarbonyls was higher than that of α-hydroxycarbonyls only in aqueous solution of K₂S₂O₈ and in the other two model systems an inverse ratio of these two 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 α-hydroxycarbonyl and α-dicarbonyl compounds are predominantly formed by a direct retro-aldol reaction and α- and β-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 α-hydroxycarbonyl compounds. Oxidation and disproportionation of α-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 α-dicarbonyl compounds.