Pyrazines, alkyl

The following example illustrates how pronounced the specificity of the sense of smell can be in cyclic compounds. The relationship between stmcture and odor activity was tested with 80 alkyl pyrazines. A part of the results is shown in Fig. 5.38. 

Barnard, J., Cunningham, D.G. A procedure for the sensory analysis of gas chromatographic effluents. Food Chem. 14, 273 (1984) 

Teranishi, R. (Eds.) Flavor science - Sensible principles and techniques. Am. Chem. Soc., Washington DC, 1993 

Structure-activity relationships in human chemoreception. Applied Science Publ. London. 1978 

Berger, R.G. Microbial flavors. In Flavor chemistry. Thirty years of progress. (Eds. R. Teranishi, E.L. Wick, I. Hornstein) p. 229, Kluwer Aca-demic/Plenum Publ., 1999 

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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. 

The number of simple alkylated pyrazines and the food products with which they are associated are enormous and a few examples are listed in Table 5. Fuller reviews on this subject are available (73MI21400). 

Alkyl pyrazines lose RCN from their molecular ions when the alkyl group is attached to the carbon adjacent to the nitrogen atom. 

The results obtained with this procedure are similar to those previously reported in the literature by Hangartner, Hagenguth et al and Zeman et al (4, 2, 5, 6). The major exception, being the complete lack of alkylated pyrazines and alkylated thiophenes. These odour-intensive compounds arise from the thermal decomposition of proteins and sulphur containing amino acids. They also develop in the thermally induced reaction of proteins with carbohydrates (Maillard reaction). The authors indicated above have demonstrated that these compounds are invariably produced during the thermal treatment of sludge, a process which is not used in this Authority. 

Methoxy-3-isobutylpyrazine 176 (Structure 4.53) is found in galbanum oil obtained from Ferula galbaniflua. 2,4-disubstituted pyridines 177, N,N-dimeth-ylated amino compounds 178, alkyl pyrazines 179, quinoline 180 and methyl quinolines 181 were isolated from fig leaf absolute [64]. 

Amino acids may also undergo thermal degradation, which is almost always coupled with some other food components, particular sugars. The major types of volatile compounds formed from amino-sugar interactions include Strecker degradation aldehydes, alkyl pyrazines, alkyl thiazolines and thiazoles and other heterocycles [35, 36]. As the subject has mainly relevance for baked and roasted vegetable food products, this subject will not be discussed in further detail. 

Pyrazines are mainly used in roasted, peanut and chocolate flavours. Alkyl pyrazines can be obtained by the reaction in an autoclave at high temperature (120-160 °C) of reducing sugars like rhamnose and ammonia. 

Musty or potato-like flavor and aroma have been observed as a defect in milk (Hammer and Babel 1957) and Gruyere de Comte cheese (Dumont et al. 1975). This off-flavor results from the production of nitrogenous cyclic compounds by Pseudomonas taetrolens and P. perolens (Morgan 1976). Musty-flavored compounds produced by these organisms include 2,5-dimethylpyrazine and 2-methoxy-3-isopropyl-pyrazine. The Gruyere de Comte with potato off-flavor contained 3-methoxy-2-propyl pyridine, as well as alkyl pyrazine compounds (Dumont et al. 1975). Murray and Whitfield (1975) postulated that alkyl pyrazines are formed in vegetables by condensation of amino acids such as valine, isoleucine, and leucine with a 2-carbon compound. Details of the synthetic mechanism in pseudomonads are unknown. 

Hara in 1981 (8) reported on volatiles produced by roasting L-theanine and glucose at about 150°C for one hour. l-Ethyl-3,4-dehydropyrrolidone was the main product. Five pyrroles, three alkyl pyrazines and four furans which were identified by GC-MS and NMR are shown in Table VII. With the exception of l-ethyl-3,4-dehydropyrro-lidone these products are quite different than found from L-threonine alone. Surprisingly, l-ethyl-3,4-dehydropyrrolidone has never been found in tea aroma. 

Found in processed truffles but absent in fresh truffles were two furans (compound 4 and 7) and a pyrazine (compound 24). Furans are well known to result from the thermal degradation of carbohydrates while alkyl pyrazines result from the reaction of amino acids and alpha-diketones. A number of methyl ketones were also observed in the canned product which were absent in the fresh product. Methyl ketones are known to result from the thermal decomposition of beta-keto acids (17-19) -... 

Concentrations of thermally generated meat flavor components are diminished by protein adsorption when soy extenders are added to fresh meat products before heating. The amounts of individual alkyl pyrazines, thermally generated by heating beef diffusate, decreased linearly as the amount of whole soy, soy 7S or soy 11S proteins were increased in a model system. Similar recoveries were obtained when pyrazines were mixed with soy either as chemical standards or from diffusate. Stoichiometry and energetics of interaction were determined for methyl pyrazine congeners with soy proteins at 120° and 145°C. Results of this study suggest that flavorants can be added in readily determined amounts to compensate for losses due to adsorption in meat-soy products. 

Table I. Alkyl Pyrazines Thermally Generated in Matrix with Soy Proteins (Percentage of Control)...

The stoichiometry and energetics of alkyl pyrazines binding to soy proteins were determined from the free, [L], and bound, u, ligand concentration and the amount of protein (12). A double reciprocal plot of 1/u vs 1/[L], a Klotz plot, yielded a straight line with the equation ... 

Table III. Stoichiometry and Energetics of Alkyl Pyrazines with Soy Proteins...

Free energies of interaction of the alkyl pyrazines with soy proteins are listed in Table IV. 

The problem of developing desirable meat flavor in the presence of vegetable protein has been clearly demonstrated in the literature. Physical measurements after heating a meat model system with soy proteins have shown a dramatic reduction in the concentration of alkyl pyrazine compounds due to interaction with the soy proteins. These interactions have been defined in terms of stoichiometry and binding energies from measurements on pure standards of the methyl pyrazines. 

Citral (123), formic acid (124), and n-undecane (125) are among a host of other compounds identified as formicid alarm pheromones. Recently, Wheeler and Blum (126) reported that alkyl-pyrazines were secreted by Odontomachus spp. in response to foreign stimuli. Some species produced 2,5-dimethyl-3-isopentylpyrazine (XXIX) whereas 2,5-dimethyl-3-pentylpyrazine (XXX) and related compounds were produced by others. Although these compounds are attractants that release attack behavior in Odontomachus workers, ponerine species that form small colonies utilize one of the alkylpyrazines to release escape behavior (127). 

Chemists produce synthetic flavourings such as smoky bacon and even chocolate . Meaty flavours come from simple heterocycles such as alkyl pyrazines (present in coffee as well as roast meat) and furonol, originally found in pineapples. Compounds such as corylone and maltol give caramel and meaty flavours. Mixtures of these and other synthetic compounds can be tuned to taste like many roasted foods from fresh bread to coffee and barbecued meat. 

The conversion of one (unsubstituted-alkyl) pyrazine into another such pyrazine by extranuclear alkylation has been covered in Section 3.2.1.4. The similar formation of (substituted-alkyl)pyrazines is illustrated here. 

The rates for deuteration of the 2-methyl groups of 2,3-dimethylpyrazinium chloride (370) and 1,2,3-trimethylpyrazinium chloride (371) have been determined in DC1/D20 the quaternary substrate (371) reacted 30 times faster a similar factor applied to other such pairs of alkylated pyrazines.563 ... 

The mass spectrometric behavior of alkylated pyrazines resembles that of a-alkyl-substituted pyridines, which also have the side chain attached to a ring carbon adjacent to a nitrogen (Biemann,10 pp. 184, 132 Biemann in12, p. 534). A prominent cleavage reaction in such compounds occurs at a C-2—C-3 bond in the side chain (cf. (65)- -[66]), probably because the positive charge at C-2 of the chain is stabilized in the same manner as in [68a]. 

Phthalimid N-[2-(Imidazol-4-yl)-ethoxy]- E16a, 238 (O-Alkyl.) Pyrazin 3-Amino-5-benzoyl-2-methoxycarbonyl- E9b/2, 324 (H - CO-Ar)... 

Dianil from formaldehyde (and other aldehydes) and ethylenediamine at 350-520° over Al-Cr catalysts Pyrazine (and 2,3-alkyl pyrazines) 522b... 

Nuclear C-alkylation of alkyl pyrazines has been extensively studied methyl-pyrazines react with organolithium reagents to form products resulting from ring alkylation (or arylation) and side chain metalation (635). 

CIC The flavour of freshly roasted filberts, is a balanced composition of alkyl pyrazines and aldehydes, derived from the Maillard reaction in combination with 4-methyl-5-hepten-2-one. 

There are many reports of Pseudomonas cultures producing musty, earthy, and potato-like odors ( l-6). The work of Morgan et al. (T) established 2-methoxy-3-isopropyl pyrazine to be partially responsible for these odors. Subsequently, 2-methyoxy-3-isopropyl pyrazine was found in bell peppers (8), a similar compound 2-methoxy-3-secbutyl pyrazine was identified in galbanum oil (9), and several 2-methoxy-3-alkyl pyrazines were identified in various raw botanicals (10). The odor threshold exhibited by 2-methoxy-3-isobutyl pyrazine (1 part in 10 indicates flavor significance for these compounds even at the exceptionally low concentrations in which they occur in foods and other natural products Producing these compounds from microbial fermentations could be an economical source of flavor for the food industry. 

The work of MacDonald (11, 12) on the synthesis of aspergillic acid from valine and isoleucine suggests that 2-methoxy-3-alkyl pyrazine, as one of a group of substituted pyrazines and diketopi-... 

Because the potential for producing "natural" 2-methoxy-3-alkyl pyrazines via fermentation might be improved by optimizing media composition, we studied the influence of media components on the synthesis 2-methoxy-3-isopropyl pyrazine by cultures of Pseudomonas perolens and selected mutant strains. 

Table V. The production of 2-methoxy-3-alkyl pyrazines by selected mutant strains of P. perolens. ...

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