Pyrazine formation glyoxal

Many pyrazine and quinoxaline syntheses yield mono- or di-N-oxides (76H(4)769). The condensation of a-aminooximes with 1,2-diketones results in the direct formation of pyrazine mono-N-oxides. The a-aminooximes themselves are not easily prepared but 2-amino-2-deoxy sugars readily form the oximes, which have been condensed with glyoxal to yield the pyrazine 4-oxides (Scheme 18) (72JOC2635, 80JOC1693). 

Formation of novel free radical products at an early stage of the Maillard reaction was demonstrated by use of ESR spectrometry. Analyses of the hyperfine structures for various sugar-amino compound systems led to the conclusion that the radical products are N,N -disubstituted pyrazine cation radicals. These new pyrazine derivatives are assumed to be formed by bimolecular condensation of a two-carbon enaminol compound involving the amino reactant residue. The presence of such a two-carbon product in an early stage reaction mixture of sugar with amine was demonstrated by isolation and identification of glyoxal dialkylimine by use of TLC, GLC, NMR, MS and IR. 

The reactions of various dipeptides with glyoxal at 100° and pH 5 have been studied and, besides other products, gave a series of new l-(l proposed mechanism involved an intermediate of type (109) in the formation of the pyrazine. Pyrazines prepared from dipeptides in this manner are listed in Table 11.12 (380, 381). The study has been extended to tri- and tetrapeptides (382). Some additional data are recorded in Section VI.9A(1). 

TABLE 11.12 FORMATION OF PYRAZINES FROM DIPEPTIDES AND GLYOXAL... 

Manley et al. (1974) suggested that cyclopentapyrazines may result from the condensation of 2-hydroxy-3-methylcyclopent-2-en-l-one (D.68) with glyoxal or pyruvaldehyde and amino acids, and a mechanism of formation has also been suggested from a study on popcorn flavor by Walradt et al. (1970). Vitzthum and Werkhoff (1975) reported 17 new alkylated five-and six-membered bicyclic pyrazines in roasted coffee the mass spectra of these new constituents and particularly the fragmentation pathways of 2-methyl-(O.50) and of 6,7-dihydro-5-methyl-5//-cyclopentapyrazine (0.51) are discussed in detail. 

Walradt et al ( ) investigated the postulated mechanism for the formation of acetyl and methyl acetyl pyrazines by Wange et al (10). Walradt proposed a mechanism for the formation of 6,7-dihdyro-5H oyolopentapyrazine as a result of the interaction of glyoxal and pyruvaldehyde with amino acids and 2-hydroxy-3-methyl-2-cyolopenten-l-one, a frequently occurring product of carbohydrate degradation. 

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