Identification of Novel Colored Compounds Containing Pyrrole and Pyrrolinone Structures Formed by Maillard Reactions of Pentoses and Primary Amino Acids

Abstract

Heating of pentoses with alanine in a ratio of 10:1 in aqueous solution at pH 7.0 generated the yellow 2-[(2-furyl)methylidene]-4-hydroxy-5-methyl-2H-furan-3-one (1), being well in line with data reported in the literature. Decreasing the relative concentrations of the pentose produced further colored nitrogen-containing compounds, among which (S)-4-hydroxy-5-methyl-2-[N-(1‘-carboxyethyl)pyrrolyl-2-methylidene]-2H-furan-3-one (2) could be identified by spectroscopic and synthetic experiments. On the other hand, thermal treatment of an aqueous solution of pentose and l-alanine in the presence of furan-2-carboxaldehyde led to the formation of the novel red (2R)-4-oxo-3,5-bis[(2-furyl)methylidene] tetrahydropyrrolo[1,2-c]-5(S)-(2-furyl)oxazolidine and its 5(R)-(2-furyl)oxazolidine diasteromer (3a/3b), which were characterized by several 1D- and 2D-NMR techniques, LC/MS, and UV−vis spectroscopy as well as by synthesis of the chromophoric substructure. In addition, the red compounds (S)-4-[(E)-1-formyl-2-(2-furyl)ethenyl]-5-(2-furyl)-2-[(E)-(2-furyl)methylidene]-2,3-dihydo-α-amino-3-oxo-1H-pyrrole-1-acetic acid (4b) and the corresponding 2-[(Z)-(2-furyl)methylidene] isomer (4b) were identified in this Maillard mixture. Quantitative studies on the formation of these colorants clearly demonstrates the key role of 3-deoxypentos-2-ulose as an intermediate in the formation of 4a/4b. Reaction pathways leading to the colorants 2, 3a/3b, and 4a/4b from pentoses and alanine are discussed. Keywords: Maillard reaction; colored compounds; (S)-4-hydroxy-5-methyl-2-[N-(1‘-carboxyethyl)pyrrolyl-2-methylidene]-2H-furan-3-one; (2R)-4-oxo-3,5-bis[(2-furyl)methylidene]tetrahydropyrrolo[1,2-c]-5(S)-(2-furyl)oxazolidine; (S)-4-[(E)-1-formyl-2-(2-furyl)ethenyl]-5-(2-furyl)-2-[(E)-(2-furyl)methylidene]-2,3-dihydo-α-amino-3-oxo-1H-pyrrole-1-acetic acid