Methoxypyrazine

A cyano group produces practical reactivity (methanolic CH30, 66°, < 3hr) by its presence in 2-chloro-3-cyano-6-methylpyridine, opposing the deactivating effect of a methyl group, and on other 2-chloropyridines (see references 3-6 in ref. 140). The cyano group activates 3-cyano-6,6-diphenyl-2-methoxypyrazine (168) (pre-... 

Minuscule quantities of naturally-occurring pyrazines have been found in some foodstuffs and are largely responsible for their flavor and aroma. For example, 3-isopropyl-2-methoxypyrazine is isolated from green peas and wine and a seasoned wine connoisseur can identify a ppt quantity. In addition, 2-methyl-6-vinylpyrazine exists in coffee. 

When disturbed or molested, these insects release small droplets of hemo-lymph from the tibio-femoral joints of their legs, and it is now well established that the deterrency exhibited by many species of coccinellids towards potential predators results from the presence of repellent and bitter alkaloids in that fluid [ 12,13]. In ladybirds, this unpalatability is associated with a bright aposematic coloration and a characteristic smell due to 3-alkyl-2-methoxypyrazines [14, 15]. The beetles use these molecules not only to reinforce the visual alerting signal on an olfactory level, but also as aggregation pheromones [16]. 

Three 3-alkyl-2-methoxypyrazines (24b, 24c, and 24d) are detected as odor components in the monarch butterfly, Danaus plexippus (Table III). The wide variability in pyrazine content observed with this insect is correlated with similar variability in the larval food plants, Asclepias sp. It seems possible that the pyrazines may be one of the factors implicated in the food choice mechanism (69). 

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). 

The edible portion of broccoli Brassica oleracea var. italica) is the inflorescence, and it is normally eaten cooked, with the main meal. Over 40 volatile compounds have been identified from raw or cooked broccoli. The most influential aroma compounds found in broccoli are sulfides, isothiocyanates, aliphatic aldehydes, alcohols and aromatic compounds [35, 166-169]. Broccoli is mainly characterised by sulfurous aroma compounds, which are formed from gluco-sinolates and amino acid precursors (Sects. 7.2.2, 7.2.3) [170-173]. The strong off-odours produced by broccoli have mainly been associated with volatile sulfur compounds, such as methanethiol, hydrogen sulfide, dimethyl disulfide and trimethyl disulfide [169,171, 174, 175]. Other volatile compounds that also have been reported as important to broccoli aroma and odour are dimethyl sulfide, hexanal, (Z)-3-hexen-l-ol, nonanal, ethanol, methyl thiocyanate, butyl isothiocyanate, 2-methylbutyl isothiocyanate and 3-isopropyl-2-methoxypyrazine... 

Raw potato possesses little aroma. Approximately 50 compounds have been reported to contribute to raw potato aroma. Raw potatoes have a high content of LOX, which catalyses the oxidation of unsaturated fatty acids into volatile degradation products (Scheme 7.2) [187]. These reactions occur as the cells are disrupted, e.g. during peeling or cutting. Freshly cut, raw potatoes contain ( ,Z)-2,4-decadienal, ( ,Z)-2,6-nonadienal, ( )-2-octenal and hexanal, which are all products of LOX-initiated reactions of unsaturated fatty acids [188,189]. It is reported that two compounds represent typical potato aroma in raw potato methional and ( ,Z)-2,6-nonadienal [189]. Other important volatiles in raw potatoes produced via the LOX pathway are l-penten-3-one, heptanal, 2-pen-tyl furan, 1-pentanol and ( , )-2,4-heptadienal [189]. Pyrazines such as 3-iso-propyl-2-methoxypyrazine could be responsible for the earthy aroma of potato [35]. Some of the most important character-impact compounds of raw potatoes are summarised in Table 7.8. Aroma compounds from cooked, fried and baked potatoes have previously been reviewed [35]. 

Parsley (Petroselinum crispum) is a member of the Apiaceae family. The fresh leaves of parsley and the dried herb are widely used as flavouring. More than 80 compounds have been identified in the volatile fraction, and the aromatic volatiles of parsley are mainly monoterpenes and the aromatics myristicin and api-ole. It is suggested that the characteristic odour of parsley is due to the presence ofp-mentha-l,3,8-triene, myrcene, 3-sec-butyl-2-methoxypyrazine, myristicin, linalool, (Z)-6-decenal and (Z)-3-hexenal [227, 228]. Furthermore, /3-phellan-drene, 4-isopropenyl-l-methylbenzene and terpinolene contribute significantly... 

The root of parsnip Pastinaca sativa) is eaten boiled or baked. The major classes of compounds identified in raw and cooked parsnip are monoterpenoids, aliphatic sulfur compounds, and 3-alkyl-2-methoxypyrazines [35]. To the best of our knowledge, no investigations have been performed to elucidate the character-impact compounds in parsnip by modern GC-O techniques however, it has been suggested that volatile compounds such as terpinolene, myristicin and 3-sec-butyl-2-methoxypyrazine maybe important contributors to the flavour of parsnip owing to either their high concentrations or their low threshold values, or both [35]. 

Methoxypyrazines 3-isobutyl-2-methoxy-pyrazine, 3-isopropyl-2-methoxypyrazine Strong X No ... 

The presence of an N-oxide function in pyrazine is not sufficient in itself to promote electrophilic halogenation, and even the presence of other weak donor groups may fail to promote reaction unless they can act in concert. Although 2-methoxypyrazine 1-oxide could not be bromi-nated, the 3-methoxy isomer [and 3-aminopyrazine 1-oxide (54)] was bro-minated in the 2- and 6-positions (ortho to oxide and ortho or para to the amino group cf. Scheme 47). Similarly, 2-aminopyrazine was brominated more readily than its Af-oxide (55 R = H) where the two groups are in opposition [83JOC1064 84H(22) 1195]. 

Ethyl-5-methoxy pyrazine (5-Ethyl-2-methoxypyrazine) C2H5 och3... 

Isopropyl-5-methoxy pyrazine (5-Isopropyl-2-methoxypyrazine) i-C3H7 OCH3... 

Hexyl-3-methoxypyrazine (3-Hexy 1-2-methoxypyrazine) -"C6H13 och3 ... 

Methoxypyrazine (142) gave 2-(a-cyanobenzyl)pyrazine (143) (PhCH2CN, NaH, THF, reflux, 30 min then substrate j, reflux, N2, TLC monitored 46%).309 other carbanions seem to have been less successful.309... 

---