Isopentyl -pyrazines

Isopentyl-3,6-dimethylpyrazine 4-oxide 2-Isopentyl-5-isovaleryl-3,6-dimethylpyrazine 2-Isopentyl-6-methyl-3-(3-methylbut-l-enyl)pyrazine... 

Various Isopentyl-substituted pyrazines, such as 2-lsopentyl-3-methylpyrazine, 2-isopentyl-5-methylpyrazine, 2-isopentyl-6— methylpyrazlne, 2-i8opentyl-5,6-disiethylpyrazlne, 2-isopentyl-3,5-dimethylpyrazlne and 2-isopentyl-3,6-dimethylpyrazlne were identified from the thermal reaction of glucose and leucine (12). The formation mechanisms for these compounds may also involve the reaction of 3,6-dihydropyrazine with isovaleraldehyde, the Strecker aldehyde of leucine. Kltamura and Shibamoto (13) described 2-lsopen-tyl-5,6-dimethylpyrazlne as having a caramel-like, coffee and sweet aroma. Although isopentyl-substituted pyrazines have not yet been reported in cocoa, they could, if present, be very Important contributors to that characteristic aroma. 

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 the pyrazine series, the Mannich reaction was applied first to 2,5-dimethyl-pyrazine by Linder and Spoerri (715). Of the amines tried, positive reactions were obtained only with dime thy lamine, piperidine, and morpholine, and it was found that one, or more usually two, of the hydrogen atoms of each of the methyl groups could be substituted. Thus 2,5-dimethylpyrazine, dimethylamine hydrochloride, and formalin in refluxing isopentyl alcohol gave 2,5-bis[bis(dimethylaminomethyl)-methyl]pyrazine and 2,5-bis(j3-dimethylaminoethyl)pyrazine. Reaction of 2-methyl-pyrazine with formalin and diethylamine hydrochloride gave 2- 3-diethylamino-ethylpyrazine (716) similarly dimethylamine hydrochloride gave 2-dimethyl-aminoethylpyrazine (17) and some 2-[bis(dimethylaminomethyl)]methylpyrazine (18) (657). 

The diazotization of aminopyrazines has been described in earlier sections. Section V.IH records the preparation of 2-fluoropyrazine from 2-aminopyrazine in fluoroboric acid containing copper powder with sodium nitrite (882, 884) and Section V.ll the preparation of iodopyrazines from some aminopyrazines via isodiazotate salts (30) (887). These salts were assigned the isodiazotate structure, on the basis of their inability to couple with 0-naphthol in alkaline solution (887) and they were characterized by hydrolysis in cold 40% aqueous sulfuric acid to the hydroxypyrazine (887). Section V.I K describes the conversion of aminopyrazines to bromopyrazines (798, 800, 807, 890-892) for example, 2-amino-3-methoxy-carbonylpyrazine with hydrobromic acid, bromine, and sodium nitrite in water gave 2-bromo-3-methoxycarbonylpyrazine (798, 890). The diazotization of aminopyrazines to hydroxypyrazines has been described in Section VI. 1C, to alkoxy-pyrazines in Section V1.3C, and to oxopyrazines in Section V1.9A(5). 2-Amino-pyrazine with isopentyl nitrite in benzene gave 2-phenylpyrazine (45%) and some 2-isopentoxypyrazine and 2,2 -dipyrazinyl amino isomers (1211). 

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