2-Isobutyl-5-methoxypyrazine

Other compounds, such as 2,4,6-trichloroanisole (TCA), 2-isopropyl-3-methoxypyrazine and 2-isobutyl-3-methoxypyrazine, were found downstream of a pulp mill effluent, and were considered as off-flavours. These compounds are by-products of chlorination, or can be produced by actinomycetes or other biota [74]. 

Aposematic species from two different families produce pungent pyrazines, presumably as an additional warning to potential predators of their toxicity. These include Oncopeltus fasciatus (Lygaeidae, seed bug family) that secretes 2-isobutyl-3-methoxypyrazine [28], and the stink bug Murgantia histrionica (Pentatomidae), that oozes froth containing 2-isobutyl- and 2-sec-butyl-3-methoxypyrazine when molested [39]. 

Solid phase microextraction (SPME) is an ideal approach to monitor volatile flavor components. This approach has been used to identify the volatile compounds in the headspace of fresh fruit during maturation [92], Using SPME fibers and GC/MS, the key flavor components are hexanal, 2-isobutyl-3-methoxypyrazine, 2,3-butanedione, 3-carene, trans-2-hexenal, and linalool (Fig. 8.1). In this study, the principal aroma compounds whose abundance varied during fruit development were specifically identified. 

Upstream migrating glass eels also swim toward compounds with earthy and green (vegetable) odors. These are pyrazines, thiazoles, and alcohols of the cyclohexanol type (Sola, 1995). One of these compounds, 2-isobutyl-3-methoxypyrazine, is common in rivers and lakes (Hwang eta/., 1984). 

The mucus of the olfactory epithelimn contains odorant-hinding proteins that are specific to various odorants, such as anisole, camphor, benzalde-hyde (cherry-almond odor), 2-isobutyl-3-methoxypyrazine (green bell pepper odor), and 5o -androst-16-en-3-one (urine odor) (summarized by Leffingwell, 2001). 

Pyrazines are the major volatile flavor chemicals produced in Maillard reactions. The discovery of this role of pyrazines was one of the most significant advances in flavor chemistry and two comprehensive reviews of pyrazines have appeared (25, 26). In the 1970 s, pyrazines were well-characterized as the compounds which directly contribute to roasted or smoky flavors. Some pyrazines possess an extremely low odor threshold (25, 29). For example, odor threshold of 2-isobutyl-3-methoxypyrazine in water is 0.002 ppb. 

The GOBPs mainly found in the female moth antennae seem to carry odorant molecules found in plant volatiles (Steinbrecht et al., 1995). In the honeybee, Apis mellifera, ASP2 (AmelOBP2) expressed in the antennae of workers and drones can bind odorant molecules of floral scents such as 1,8-cineol, 2-isobutyl-3-methoxypyrazine, 2-heptanone and isoamyl-acetate but do not bind pheromone components (Danty et al., 1997, 1999 Briand et al., 2001). 

Van Ruth et al. (1995) described the flavour components in bell peppers. Out of 47 compounds identified, 12 could be detected by assessors at a sniffing port on a gas chromatograph. 3(2220) 2-Methylpropanal,9 2-methylbutanal, 10 3-methylbutanal, 13(2370) 2,3-butanedione, 15(3382) l-penten-3-one, 19(2557) hexanal, 25(2540) heptanal, 29 P-ocimene, 35 frans-3-hepten-2-one, 39 dimethyltrisulphide, 45(3132) 2-isobutyl-3-methoxypyrazine and 47(3639) P-cyclocitral were the major compounds. 

Other flavouring compounds found in reasonable quantities are 14(3098) pen-tanal, 21 l-methyl-l/f-pyrrole, 23(3584) 1-penten-3-ol, 26(2633) (B)-(+)-limonene, 36 cis-2-heplenal, 40(2782) nonanal, 42(2805) l-octen-3-ol and 44(2362) decanal. The compound described as the character-imparting compound in bell peppers is 2-isobutyl-3-methoxypyrazine. This very powerful odour-ant has a threshold in water of 1 part in 10A12 (Govindarajan and Sathyanarayana, 1986). 

Isobutyl-3-methoxypyrazine (123, R = H) gave 2-(l-hydroxy-2-methylpropyl)-3-methoxypyrazine (123, R = OH) (PiTNLi, Et20—C6H14, N2, -78 20°C,... 

Isobutyl-3-methoxy-5-methylpyrazine 2-Isobutyl-3-methoxy-5-methylpyrazine 2-Isobutyl-6-methoxy-5-methylpyrazine 4-oxide 2-Isobutyl-3-methoxypyrazine... 

Three extremely odorous pyrazines, 3-isopropyl-2-methoxypyrazine, 2-s-butyl-3-methoxypyrazine, and 2-isobutyl-3-methoxypyrazine have been shown to be present in green peas, and are likely to be of major significance in the flavor of peas (59). The volatile oil of green bell peppers has been found to contain 2-isobutyl-3-methoxypyrazine as a major component (60,61). The alkylpyrazines in potato chips (62,63) and roasted peanuts (63) have been examined. 2-Isopropyl-3-methoxypyrazine has been characterized in the vacuum steam volatile oil of potatoes (64), 2-ethyl-3-methoxypyrazine in cooked potato (65), and 3-ethyl-2,5-dimethylpyrazine and 2-ethyl-3,5-dimethylpyTazine as the components important to the aroma of baked potato (66). A variety of alkylpyrazines have been identified in roasted sesame seeds (67) and 21 pyrazines have been identified in the aroma components isolated from roasted green tea (68). 

Recent work has shown the presence of pyrazine and 2,6-dimethylpyrazine in leek (75), pyrazine and alkylpyrazines in the volatile constituents of tamarind (76), five alkylpyrazines in soong-neung (extract of cooked and roasted rice) (77) and in shoyu (soy sauce) (78), and alkylpyrazines in white bread (79). Murray and Whitfield (80) have examined vegetable tissue for 2-isopropyl-, 2is-butyl-, and 2-isobutyl-3-methoxypyrazines and observed at least one of these compounds in 23 of the 27 samples studied. 2-Methylpyrazine and 2,5- and 2,6-dimethylpyrazines have been determined in black tobacco and in the smoke of nonfilter cigarettes made from these tobaccos (81). 

Demethylation of 5-(2 -hydroxy-2 -methylpropyl)-2-isobutyl-3-methoxypyrazine occurred on refluxing with 10% hydrochloric acid to give 3-hydroxy-5-(2 -hydroxy-2 -methylpropyl)-2-isobutylpyrazine but 2-isobutyl-3-methoxy-5-(2 -methylprop-l -enyl)pyrazine and 3-hydroxy-24sobutyl-5-(2 -methylprop-l -enyl)pyrazine were also produced (113b). 

A large number of 2-alkyl-3-methoxypyrazines has been isolated from raw vegetables (59, 60, 64, 65, 69, 80, 1099), bell peppers (60,61) and gambanum oil (49). 2-Isobutyl-3-methoxypyrazine is already finding commercial use as a flavoring material (368) and 2-ethoxy-3-methylpyrazine may be used for pineapple flavor (981). The odor characteristics (367, 368, 976, 977) and structure (977) of some alkoxy alkylpyrazines have been examined. 3-Guanidino-6-hydroxymethyl-l-methyl-2-oxo-l,2-dihydropyrazine has been isolated from seeds of Stizolobium hassjoo and on mild alkaline hydrolysis gave 3-hydroxy-6-hydroxymethyl-l -methyl-2-oxo-l, 2-dihydropyrazine (1060). 

These often have very powerful odours (Boelens and Gemert, 1994) for example, 2-isobutyl-3-methoxypyrazine from galbanum oil has a powerful green note and l-para-menthene-8-thiol, a very powerful natural material, has a strong grapefruit odour at concentrations in the p.p.b. range. 

Simple pyridine, pyrazine, and pyrrolo derivatives have been described from many different types of arthropods. Nicotine (177), nicotinamide, nicotinic acid, 2-pyrrolidone, N-methylpyrrolididone (178), 8-hydroxyquinoline (179), and 2-isobutyl-3-methoxypyrazine (180) have been detected in the defensive secretion and blood of larvae of the moth Lymantria dispar The nicotine isomer anabasine (181) and its congener anabaseine (182) have been identified as venom components in several ant species,whereas... 

FIGU RE 17.19 Proposed precursors of (Z)-l,5-octadien-3-one and 2-isobutyl-3-methoxypyrazine. 

Using odor units, Tressl (1989) found that some of the compounds that contribute the most are furfurylthiol (Section 5,1.128, sulfury), 4-hydroxy-2,5-dimethyl-3(2//)-furanone (Furaneol", Section 5, 1.100, caramel), 2-isobutyl-3-methoxypyrazine (Section 5,0.77, green), 2- and 3-methylbutanal (Section 5,C.12 and 5,C.13, malty, burnt), guaiacol (Section 5,H.33), and 4-vinylguaiacol (Section 5,H.36, smoky burnt). 

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