5-Alkylpyrazines

Methylpyrazine reacts with sodamide in liquid ammonia to generate the anion, which may be alkylated to give higher alkylpyrazines (Scheme 10) (61JOC3379). The alkylpyrazines have found extensive use as fiavouring and aroma agents (see Section 2.14.4). Condensation reactions with esters, aldehydes and ketones are common, e.g. methyl benzoate yields phenacylpyrazine in 95% yield, and reactions of this type are summarized in Scheme 11. 

Direct oxidation of ethylpyrazines to the corresponding acetylpyrazines may also be carried out in favourable circumstances using hot chromic acid (75JOC1178). Treatment of 2-ethyl-3-alkylpyrazines with chromic acid yields the corresponding 2-acetyl-3-alkyl-pyrazines in yields of 50-70%. In the absence of the 3-alkyl substituent the yields fall dramatically to less than 10%. Acetylpyrazines are more generally prepared by the inverse addition of a Grignard reagent to a cyanopyrazine. 

Alkylpyrazines are prepared by two main procedures, (1) self-condensation of a-amino carbonyl compounds and (2) alkylation (or acylation) of pyrazines at nuclear or side chain carbons. Condensation of aminoacetone hydrochloride (491) in the presence of the corresponding aldehyde gave the 3- -alkyl-2,5-dimethylpyrazines 20e and 20g (Scheme 61) 36). 2,5-Dimethylpyrazine 20a was prepared from hydroxyiminoacetone (493) by reduction with tin and hydrochloric acid (Scheme 61) 145). 3-Ethyl-2,6-dimethylpyrazine (21b) was prepared along with 20a and 495 by condensation of aminoacetone hydrochloride (491) with 2-aminopentan-3-one hydrochloride (494) in the presence of sodium ethoxide (Scheme 61) 36). 

Alkylpyrazines produced by bacterial spoilage of heat-treated and gamma-irradiated coconut. Chem Ind (London) 1987 1987(16) 567-568. 

The use of headspace solid-phase microextraction (HS-SPME) combined with GC was effective for isolation and determination of alkylpyrazines in cocoa liquor <2004JBS267, 2004MI291>, and similarly methoxypyrazine in wine was analyzed by HS-SPME/2-D GC <2005MI1075>. 

Photolysis of 3-phenyl-5(4//)-isoxazolone 152 gives 2,5-diphenylpyrazine 154 in 67% yield (Scheme 41) <1997JIC648>, probably through diradical intermediate 153. Radical chain reactions of a-azido ketones 155 with tributyltin hydride lead to symmetrical alkylpyrazines 156, albeit in low to moderate yields (Scheme 42) <2002T3485>. 

Lipid-Protein-Carbohydrate Interactions. Evidence for such complex interaction was recently reported by Huang et al (36) who observed that the addition of corn lipids to zein and corn carbohydrates enhanced the formation of alkylpyrazines, indicating that lipid-derived free radicals may accelerate the rate of Maillard reactions. Two of the alkylpyrazines, identified in such mixtures after heating for 30 minutes at 180°C, have 5-carbon alkyl substitution at the pyrazine ring and could only be explained by interaction of lipid or lipid decomposition products. These authors suggested that condensation of amino ketones, formed by protein-carbohydrate interaction, may yield 3,6-dihydropyrazine which would in turn react with pentanal, a lipid oxidation product, to form 2,5-dimethyl-3-pentylpyrazine. 

Alkylpyrazines have been recognized as important trace flavor components of a large number of cooked, roasted, toasted and deep-fat fried foods (3). As a rule, alkylpyrazines have a roasted nut-like odor and flavor. Formation pathways for alkylpyrazines have been proposed by numerous researchers (4, 5, 6). Model studies suggest that they are minor products of the Maillard reaction. 

Peptide or Amino Acids Total NPD Area/1000 Alkylpyrazines (X of NPD) ... 

The importance of Maillard reaction products to the flavor of foods has received considerable attention. One group of Maillard products, the alkylpyrazines, are thought to contribute roasted, toasted and nutty flavor notes to a variety of foods. Several reviews have detailed the presence of pyrazines in a wide variety of foods (1-7). Considerable work has previously focused on mechanisms of formation and the effects of various parameters on pyrazine formation (8-17). Part one of this study reported on the effects of type of amino acid and type of sugar on the kinetics and distribution pattern of pyrazines formed (18). The current study investigates the effect of pH and water activity on the kinetics of alkylpyrazines formation. 

Activation energies for alkylpyrazine formation were calculated from the slope of Arrhenius plots, ranging from 33 to 45 kcal/mole (see Table II). Activation energies for pyrazine and... 

Most classes of heterocycles found are also commonly observed in other cooked food flavors. Pyridines, pyrroles, thiophenes and alkylthiazoles were minor components. As main constituents, ten methyl- and ethyl-substituted alkylpyrazines were identified. The pyrazines occurred markedly in all samples and seemed to be one of the main contributors to the roasted-nut like flavor of cooked small shrimp. 

The contents of trithiolanes and dithiazines were larger than that of pyrazines in raw lucens and A japonicus, both of which belong to the same taxonomical group. Conversely, in raw Antarctic krill, alkylpyrazines comprised the predominant part of the volatiles with few cyclic polysulfides also present. The composition of precooked krill corresponded to that of lucens, and that of fermented A. japonicus to raw krill. 

Other furans included methyl- 2-furfural [4], 2-acetylfuran [5], 2-propionylfuran [6], l-(2-furanyl)-2-propanone [7] and l-(-2-furanyl)-3-butanone [8]. Several alkylpyrazines were also found, with the 2,5-dimethyl [9] and 2-methy 1-5-ethyl [10] the most abundant derivatives, and some 2-acyl-l-methylpyrroles of which only the formyl [11] and acetyl [12] derivatives were conclusively identified. 

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