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

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-2 H -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]-2 H -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 (2 R )-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-1 H -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]-2 H -furan-3-one; (2 R )-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-1 H -pyrrole-1-acetic acid