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Abstract: In this paper, we provide a systematic analysis of glyoxal (1) formation from a range of monosaccharides and related compounds, to determine their potential role as sources of this α-oxoaldehyde in vivo. Substrates were reacted with the Fenton reagent (Fe2+/EDTA/H2O2) and the mixtures were analyzed by HPLC using the 6-hydroxy-2,4,5-triaminopyrimidine fluorimetric assay. The rank order of hexoses and their derivatives as glyoxal sources was found to be fructose>glucose=mannose=galactose>glucose-6-phosphate>mannitol. Within the pentose group, arabinose and ribose gave the higher yields of 1 followed by deoxyribose and its adenine N-glycosides and ribulose. Among the tested substrates, three-carbon compounds, that is, trioses and glycerol, but not glyceraldehyde-3-phosphate, were by far the most effective sources of 1. The effects of H2O2 and Fe2+/EDTA concentrations as well as of other metal ions were also investigated. [Copyright &y& Elsevier]

Abstract: The reactions of 5-S-cysteinyldopa (1), l-α-methyldopa (2) and dl-m-tyrosine (3) with d-glucose were investigated at 90°C in phosphate buffer at pH ranging from 5.0 to 9.0. Whereas 1 gave mainly the double Maillard condensation product N,N′-bis(1″-deoxy-d-fructos-1″-yl)-5-S-cysteinyldopa, as an inseparable mixture of β-d-fructopyranosyl and α,β-d-fructofuranosyl derivatives, 2 and 3 gave both Maillard and Pictet–Spengler products, although to different extents and with different regio- and stereochemistry. A peculiar pattern of reactivity was displayed by 2 which gave, besides the Maillard product and the expected 6,7-dihydroxy-1,2,3,4-tetrahydroisoquinoline C-1 diastereoisomeric pairs, the unprecedented 7,8-dihydroxy-1,2,3,4-tetrahydroisoquinoline derivative via the ortho cyclization pathway. Pictet–Spengler cyclization of 2 and 3 proceeded with Felkin–Anh-type asymmetric induction, favouring the 1R isomer throughout the pH range 5.0–9.0. These results, which highlight the first example of carbohydrate-derived 7,8-dihydroxytetrahydroisoquinoline, provide new insights into the factors governing competition between Maillard and Pictet–Spengler condensation pathways. [Copyright &y& Elsevier]