Elucidation of the mechanism of pyrrole formation during thermal degradation of 13C-labeled l-serines

Pyrolysis of [13C-1], [13C-2] and [13C-3]-labelled l-serines generated mono-substituted methyl and ethyl derivatives of pyrroles and pyrazines among other compounds. Analyses of label incorporation into the pyrroles have indicated their formation through aldol condensation of acetaldehyde with different α-aminocarbonyl compounds followed by cyclization and loss of water (Knorr pyrrole synthesis). Comparison of the label incorporation patterns of the α-aminocarbonyls involved in the formation of methyl and ethyl-substituted pyrroles with that of similarly substituted pyrazines, revealed their common origin. In addition, α-aminocarbonyls involved in the formation of 2- and 3-substituted pyrroles had identical label distribution patterns, indicating their formation through the same carbonyl precursors. Furthermore, the major pathway (55%) leading to the formation of the α-aminocarbonyl precursors of methyl-substituted pyrroles involved aldol addition of formaldehyde to glycolaldehyde, whereas the only pathway leading to the formation of the α-aminocarbonyl precursors of ethyl-substituted pyrroles involved the interaction of alanine — formed in situ — with glycolaldehyde.