Mechanism of imidazole and oxazole formation in [13C-2]-labelled glycine and alanine model systems

Studies with 13C-2-labelled glycine and alanine in model systems containing 2,3-butanedione, glyceraldehyde or glyoxal have indicated that imidazoles and oxazoles can be formed from α-dicarbonyl compounds through Strecker reaction and subsequent formation of α-amino carbonyl reactive intermediates. These intermediates can react with any aldehyde in the reaction mixture to form an imine which in turn can either cyclize to form oxazoles or react with an amino compound and then cyclize to form imidazole after an oxidation step. On the other hand, Amadori products, formed in α-hydroxycarbonyl containing systems, can undergo decarboxylation followed by reaction with ammonia to form an amino imine intermediate which, after reaction with any aldehyde followed by cyclization, can form imidazoles after a dehydration step. This latter mechanism fixes the C-2 atom of glycine as an N-alkyl substituent in imidazoles. In addition, model studies with α-dicarbonyl compounds, using ammonium carbonate as a source of ammonia and paraformaldehyde as a source of formaldehyde, also produced imidazoles and oxazoles.