Carbonyl compounds, addition reactions substituted imine formation

Pyrazines are formed from transamination reactions, in addition to carbon dioxide and formaldehyde. A requirement is that the carbonyl compound contains a dione and the amino group is alpha to the carboxyl group (16). If the hydrogen on the ct-carbon oI the amino acid is substituted, a ketone is produced. Newell (17) initially proposed a pyrazine formation mechanism between sugar and amino acid precursors. (See Figure 3). The Schiff base cation is formed by addition of the amino acid to the anomeric portion of the aldo-hexose, with subsequent losses of vater and a hydroxyl ion. Decarboxylation forms an imine which can hydrolyze to an aldehyde and a dienamine. Enolization yields a ketoamine, vhich dissociates to amino acetone and glyceraldehyde. 2,5-Dimethylpyrazine is formed by the condensation of the tvo molecules of amino acetone. 

Like acetal formation, the reaction of aldehydes and ketones with primary amines— compounds of the type RNH2 and ArNH2—is a two-stage process. Its first stage is nucleophilic addition of the amine to the carbonyl group to give a carbinolamine. The second stage is the dehydration of the carbinolamine to yield the isolated product of the reaction, an A-alkyl or A-aryl-substituted imine. 

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