Application of Site Specific 13C Enrichment and 13C NMR Spectroscopy for the Elucidation of the Formation Pathway Leading to a Red 1H-Pyrrol-3(2H)-one during the Maillard Reaction of Furan-2-carboxaldehyde and l-Alanine

The intensely red compounds (S)-4[(E)-1-formyl-2-(2-furyl)ethenyl)]-5-(2-furyl)-2-[(E)-(2-furyl)methylidene]-2,3-dihydro-α-methyl-3-oxo-1H-pyrrole-1-acetic acid (1a) and its 2-[(Z)-(2-furyl)methylene] isomer (2b) have recently been identified as the main colored compounds formed upon thermal treatment of an aqueous solution of furan-2-carboxaldehyde and l-alanine. For clarification of their formation pathways, a labeling experiment with synthetic site specific 13C-enriched furan-2-carboxaldehyde was performed. The labeled carbon positions in 1a were unequivocally assigned by 1H broad band decoupled 13C NMR spectroscopy. In contrast to conventional labeling experiments, prior to the reaction with l-alanine, the 13C-labeled furan-2-carboxaldehyde was diluted with the natural 13C abundant analogue to suppress the spectrum complexity due to homonuclear 13C−13C spin couplings. This powerful technique has been successfully used for the first time to clarify the formation route of a colored Maillard reaction product. Keywords: Maillard reaction; furan-2-carboxaldehyde; alanine; 1H-pyrrol-3(2H)-one; furan-2-[13C]carboxaldehyde; 13C-labeling experiment