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

Pyrolysis of [C-13-1], [C-13-2] and [C-13-5]-labelled L-serines generated m ono-substituted methyl and ethyl derivatives of pyrroles and pyrazines amon g other compounds. Analyses of label incorporation into the pyrroles have i ndicated their formation through aldol condensation of acetaldehyde with di fferent alpha -aminocarbonyl compounds followed by cyclization and loss of water (Knorr pyrrole synthesis). Comparison of the label incorporation patt erns of the alpha -aminocarbonyls involved in the formation of methyl and e thyl-substituted pyrroles with that of similarly substituted pyrazines, rev ealed their common origin. In addition, alpha -aminocarbonyls involved in t he formation of 2- and 3-substituted pyrroles had identical label distribut ion patterns, indicating their formation through the same carbonyl precurso rs. Furthermore, the major pathway (55%) leading to the formation of the al pha -aminocarbonyl precursors of methyl-substituted pyrroles involved aldol addition of formaldehyde to glycolaldehyde, whereas the only pathway leadi ng to the formation of the alpha -aminocarbonyl precursors of ethyl-substit uted pyrroles involved the interaction of lanine - formed in situ - with gl ycolaldehyde. (C) 2001 Elsevier Science Ltd. All rights reserved.