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

Pyrolysis of [(13)C-1], [(13)C-2] and [(13)C-5]-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 alpha -aminocarbonyl compounds followed by cyclization and loss of water (Knorr pyrrole synthesis). Comparison of the label incorporation patterns of the alpha -aminocarbonyls involved in the formation of methyl and ethyl-substituted pyrroles with that of similarly substituted pyrazines, revealed their common origin. In addition, alpha -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 alpha -aminocarbonyl precursors of methyl-substituted pyrroles involved aldol addition of formaldehyde to glycolaldehyde, whereas the only pathway leading to the formation of the alpha -aminocarbonyl precursors of ethyl-substituted pyrroles involved the interaction of lanine - formed in situ - with glycolaldehyde. (C) 2001 Elsevier Science Ltd. All rights reserved.