AcetylformoinA 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-2 H -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-2 H -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 switch 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-2 H -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-2 H -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 .