Dimerization of azomethine ylides: an alternate route to pyrazine formation in the Maillard reaction.

Recently, azomethine ylides have been implicated as reactive intermediates in the Maillard reaction. They are known to undergo 1,3-cycloaddition reactions with dipolarophiles to form pyrroles, and, more importantly, they can undergo dimerization reaction leading to the formation of a piperazine moiety. Although the reactivity of azomethine ylides toward dipolarophiles in Maillard model systems has been studied, their role as precursors of pyrazines remains unexplored. To study this possibility, a simple model system such as glyoxylic acid/glycine that is unable to generate α-dicarbonyl compounds but is able to form azomethine ylides was used to demonstrate pyrazine formation. The specific piperazine-2,5-dicarboxylic acid that is expected to form in this particular system can undergo oxidative decarboxylation to generate dihydropyrazine moieties similar to that of the dimerization product of the α-amino carbonyl compounds generated through the Strecker reaction. The model system when reacted under pyrolytic conditions at 200 °C indeed generated most of the theoretically expected pyrazines as major products, the structures of which were confirmed by comparison of their retention times with commercial standards and through NIST library matches in addition to isotope labeling data generated from labeled precursors such as [(13)C-1]glycine, [(13)C-2]glycine, and [(15)N]glycine.