2- pyrazine, alkylation

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. 

IR spectral data for most known 1,2,4-triazines have been published. The absorption of the 1,2,4-triazines in the IR region are those expected for this system. The IR spectrum of the parent compound (1) shows three absorption bands for the C—H stretching vibrations at 3090, 3060 and 3030 cm-1, five bands for C=N and C=C stretching vibrations at 1560, 1529,1435,1380 and 1295 cm-1, three for the C—H in-plane deformations at 1163,1135 and 1113 cm-1, two for the characteristic ring skeleton vibrations at 1050 and 995 cm-1 and three bands for the C—H out-of-plane deformation vibrations at 851,768 and 713 cm-1 (68CB3952). These values are in good agreement with similar bands for pyridine, pyridazine, pyrimidine and pyrazine. Alkyl and aryl derivatives of 1,2,4-triazine show similar bands in their IR spectra, with additional bands from the substituents. 

Classic A/-heterocychc ligands, eg, bipyridyl (bipy), terpyridyl, imidazole, pyrazine, phenanthroline, piperazine (including alkyl- and aryl-substituted derivatives), and polypyrazol-l-yl-borates (bis, tris, and tetra), have all been found to coordinate Th(IV) chlorides, perchlorates, and nitrates. The tripodal hydrotris(pyrazolyl)borates, HBPz, have been used to stabilize organometaHic complexes (31). Bis-porphyrin Th(IV) "sandwich" complexes have been... 

Inductive and resonance stabilization of carbanions derived by proton abstraction from alkyl substituents a to the ring nitrogen in pyrazines and quinoxalines confers a degree of stability on these species comparable with that observed with enolate anions. The resultant carbanions undergo typical condensation reactions with a variety of electrophilic reagents such as aldehydes, ketones, nitriles, diazonium salts, etc., which makes them of considerable preparative importance. 

Although most of the reactions of preparative importance involving the a-alkyl carbanions are usually carried out under controlled conditions with NHa /NHs being used as the base, a number of reactions using less severe conditions are known, both in the pyrazine and quinoxaline series. In the case of alkylquinoxalines, where an increased number of resonance possibilities exist, mildly basic conditions are usually employed in condensation reactions. 

Alkyl side chains in both pyrazines and quinoxalines are susceptible to halogenation by elemental halogens (28JCS1960, 68TL5931) and under radical conditions with NBS (72JOC511). Thus, bromination of 2-methylquinoxaline with bromine in the presence of sodium acetate... 

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 ease of oxidation varies considerably with the nature and number of ring substituents thus, although simple alkyl derivatives of pyrazine, quinoxaline and phenazine are easily oxidized by peracetic acid generated in situ from hydrogen peroxide and acetic acid, some difficulties are encountered. With unsymmetrical substrates there is inevitably the selectivity problem. Thus, methylpyrazine on oxidation with peracetic acid yields mixtures of the 1-and 4-oxides (42) and (43) (59YZ1275). In favourable circumstances, such product mixtures may be separated by fractional crystallization. Simple alkyl derivatives of quinoxalines are... 

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). 

Despite the fact that one of the first pteridine syntheses was based on an intramolecular Hofmann carboxamide degradation of pyrazine-2,3-dicarboxamide by action of potassium hypobromite and leads to lumazine (equation 104), (07CB4857), pyrazine derivatives in general have not often been used because of availability problems. The reaction of alkyl... 

Pyrido[2,3-b]pyrazin-8-one-7-carboxylic acids H NMR, 3, 249 Pyrido[2,3-b]pyrazinones alkylation, 3, 250... 

This reaction gives good results for a variety of activated and unactivated alkyl halides [112] Oxidation of 2-bromoketones with iV-phenyltriflamide was used m a one pot synthesis of pyrazines by the sequence of reactions shown m equation 57 [II3] The procedure was successfully applied to the synthesis of deoxyaspergilhc acid [II4 ... 

Pyrazin-2-one (124) has been shown to exist predominantly as such by comparison of its ultraviolet spectrum with those of the fixed alkylated derivatives and by its infrared spectrum. The pK measurements support this conclusion but cannot yield quantitative results since cations of a common type are not formed. ... 

Pyrimidine-2-thiones (e.g., 206) have been shown to exist as such by comparison of their ultraviolet spectra with those of both alkylated forms.The ultraviolet spectra of pyrimidine-4-thiones are different from those of 4-alkylthiopyrimidines, therefore the former compounds exist as 207 and/or 208, the predominant form not having been determined. Infrared spectral evidence suggests that quinazoline-4-thione exists as 209 and/or 210 and has been used recently to demonstrate the thione formulation for pyrimidine-2- and -4-thione, pyrazine-2-thione, and quinoxaline-2-thione. In view of this work, the report that X-ray crystallographic evidence supports the mer-... 

Pyrazine-2-thione (213) and quinoxaline-2-thione (214) probably exist in the thione form since their ultraviolet spectra are different from those of the 2-methylthio analogs. The basicity of quinoxaline-2-thione is 1.4 pK units less than that of 2-methylthio-quinoxaline, and the ultraviolet spectra of the cations are dissimilar. Presumably quinoxaline-2-thione and its 2-methylthio derivative do not form similar cations (215, P = alkyl, H), and it would appear that either the thione gives the cation 216 or the 2-thioether gives the cation 217. Similar considerations apply to pyrazine-2-thione. 

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