Acetylformoin - A chemical switch in the formation of colored Maillard reaction products from hexoses and primary and secondary amino acids

Thermal treatment of a methanolic solution of N-(1-deoxy-D-fructos-1-yl)-L-proline and furan-2-carboxaldehyde led to the rapid formation of colored compounds, among which 4-hydroxy-2-methoxy-2-methyl-5-[(E)-(2-furyl)methylidene]methyl-2H-furan-3-one (1) could be identified as one of the main colored compounds by NMR, LC/MS, and UV-vis spectroscopy. Studies on its formation revealed that condensation of the hexose dehydration product acetylformoin (2a/2b) with furan-2-carboxaldehyde formed the labile 2,4-hydroxy-2-methyl-5-[(E)-(2-furyl)methylidene]methyl-2H-furan-3-one (3a/3b), which then reacts with methanol to the more stable colorant 1. Reacting furan-2-carboxaldehyde with N-(1-deoxy-D-fructos-1-yl)-L-alanine, however, completely stalled the formation of acetylformoin and, consequently, also of colorant 1. Further model studies revealed acetylformoin as a chemical snitch in the Maillard reaction determining different reaction pathways in the presence of primary and secondary amino acids; for example, the reaction with glycine methyl ester and glycine revealed 2,4-dihydroxy-2,5-dimethyl-1-methoxycarbonylmethyl- (4) and 2,4-dihydroxy-2,5-dimethyl-1-carboxymethyl -3-oxo-2H-pyrrole (5), respectively, whereas the reaction with L-proline resulted in the formation of pyrrolidino- (6) and bispyrrolidino-hexose-reductone (7). Dry heating of these amino reductones in the presence of furan-2-carboxaldehyde produced the colorants 2,4-dihydroxy-2-methyl-5-[(E)-(2-furyl)methylidene]methyl-1-methoxycarbonylmethyl-3-oxo-2H-pyrrole (8) from 4 and 3-hydroxy-4-[(E)-(2-furyl)methylidene] methyl-3-cyclopentene-1,2-dione (9) from 6 and 7. Quantitative studies revealed 3-hydroxy-4-methyl-3-cyclopentene-1,2-dione (10) and methylene-reductinic acid (11) as the key intermediates in the formation of colorant 9.