3-Alkyl-2-methoxy pyrazines

The other results in Table III are those of data sets not having noncolinear physical-chemical properties. Log P was highly correlated with these data sets as well. Ethylesters threshold data in air weis linearly related to log P (eq. 4) while 3-alkyl-2-methoxy pyrazines had threshold odor intensity which was parabolically related to log P (eq. 7). The pyrazine data indicates that 3-alkyl-2-methoxy pyrazines having a log P value of 2.43 would have the most intense odor of the series. 

The use of log P and HB parameters as a tool for predicting odor intensity seems promising. Although many excellent correlations were obtained as presented in Tables I-V further studies are needed to investigate several unresolved areas. The question on whether log P is linearly or parabolically related to odor intensity for a specific medium needs to be resolved. Six equations in Tables 1-V linearly related log P to odor intensity, while five parabolic relationships were observed which had an optimum hydrophobicity (log P) associated with maximum odor intensity. Log Po values observed were 3.17 and 2.90 for alcohols (threshold-aic). Alkanes had a log Po value of 5.33 (threshold-air). In aqueous media alcohols had a log Po value of 3.98 while 3-alkyl-2-methoxy pyrazines had a value of 2A3. The animal data indicates that rats had log Po values of 5.90 for acetates and 7.91 for alcohols. 

Alkyl methoxy pyrazines are an important class of aroma compounds exhibiting intense green bean/green pea aroma notes. Reineccius s group at the University of Minnesota show that mutant strains of Pseudomonas perolens produce the isopropyl isomer to a final level of 15 mg/L of culture. Since the threshold is 2xl0 ° ppm, the reported yield is substantial. 

Warning tubatancet (odors). W. s. are used by animals, similar to warning colors, to intimidate opponents. Particularly strong-smelling, alkylated methoxy-pyrazines have been discussed as the W. s. of certain insects see also alarm substances, defensive secretions. 

Alkyl-2-methoxypyrazines exhibit a base peak at miz 124 in the mass spectrum. The peak corresponds to a molecular ion in 2-methoxy-3-methylpyrazine (24a) and to a fragment ion, resulting from a McLafferty rearrangement of an alkyl group, in 3-isopropyl-2-methoxypyrazine (24b) and 3-5cc-butyl-2-methoxy pyrazine (24d) (97). 

Ring aUylation and propenylation of methylpyrazine has been described (634) acetonylpyrazine with phenyllithium gives 2-acetonyl-6-phenylpyrazine (639) and 2,5-dimethylpyrazine with isopentylUthium gave 3-isopentyl-2,S-dimethylpyrazine (70). Aldehydes and ketones in the presence of a solution of an alkali or alkaline earth metal in liquid ammonia, or a suspension of these metals in other solvents, can be used to alkylate the pyrazine ring in moderate to good yields (614, 640, 641). This alkylation has been successfully applied to alkyl- and dialkyl(amino- and methoxy)pyrazines, and a mechanism has been proposed for the reaction (614). For example, the reaction of potassium with methylpyrazine and ethyl methyl ketone, catalyzed by sodamide (0.25 mol) gave 88% of 2 -butyl-6-methylpyrazine. 

In addition to determining which GC peaks have an odor, GC/0 work can aid in compound identifications. An experienced chemist can provide a great deal of information about a chemical by sniffing it as it exits the GC column. The chemist can typically give molecular weight and functional group or compound class estimates. The chemist may be able to identify a chemical simply based on its aroma and GC retention properties. For example, it is relatively easy to distinguish between an alkyl pyrazine and a methoxy pyrazine by odor. 

More than 100 different pyrazines have been identified in various food products. The sensory properties of the pyrazines are quite diverse. The alkyl pyrazines (Figure 5.5a) generally possess roasted, nut-like notes while methoxypyrazines (Figure 5.5b) often possess earthy, vegetable notes [47]. The 2-isobutyl-3-methoxy pyrazine has a freshly cut green pepper flavor with a sensory threshold of 0.002 ppb in water. The acetyl pyrazines typically have a popcorn character, and 2-acetonyl pyrazine has a... 

As can be seen in Table 9.5, nutrient source had a strong influence on the production of the methoxy pyrazines by a selected mutant of Pseudomonas perolens. The end result is that an organism that initially produced a few ppb of the methoxy alkyl pyrazines was converted into an organism that produced ppm levels of the target compounds. While one may not think that ppm levels of an aroma compound is significant, these pyrazines have sensory thresholds in the pptr levels. 

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]. 

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. 

Methylation (666, 912) of 2-methoxypyrazine with methyl iodide in dimethyl sulfoxide at room temperature gave 3-methoxy-l-methylpyrazinium iodide with a rate of methylation relative to pyrazine of 1.05 (666). 2-Methoxypyrazine with tetracyanoethylene oxide gave a small yield of 3 ethoxypyrazinium dicyano-methylide (53) (1094). Alkylation of 2-methoxypyrazine with ethyl methyl ketone in the presence of sodium in liquid ammonia to give 2-s-butyl-6-methoxypyrazine (17%) has been described (614). The reactions of 3-hydroxy-2,5-dimethylpyrazine and alkylhalides have been examined (1095). 

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. ...

The Production of Selected Methoxy Alkyl Pyrazines by Selected P. perolens Mutant on Different Growth Media... 

Mclver, R.C., Factors Influencing the Synthesis of Methoxy Alkyl Pyrazines by Pseudomonas Perolens, Ph.D. thesis, University of Minnesota, St. Paul, p. 88, 1986. Kuila, R.K., B. Ranganathan, Ultraviolet light-induced mutants of Streptococcus lactis subspecies diacetylactis with enhanced acid- or flavor-producing ahihties, J. Dairy ScL, 61, 4, p. 379, 1978. 

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