2.3- Dimethyl-5,6-dihydropyrazine alkylation

Instead of alanine and valine, several other chiral auxiliaries have been used, such as tert-leucine13, leucine14 and isoleucine15. In some cases diastereomeric excesses may be higher with the dihydropyrazines 5 and 6, derived from 0,0-dimethyl-alkylation with 3-bromo-propyne gives a de of 60% with (2S)-2,5-dihydro-2-isopropyl-3,6-dimethoxypyrazine (3), in contrast to 85% de with 516 and >95% de with 613. 

Lipid-Protein-Carbohydrate Interactions. Evidence for such complex interaction was recently reported by Huang et al (36) who observed that the addition of corn lipids to zein and corn carbohydrates enhanced the formation of alkylpyrazines, indicating that lipid-derived free radicals may accelerate the rate of Maillard reactions. Two of the alkylpyrazines, identified in such mixtures after heating for 30 minutes at 180°C, have 5-carbon alkyl substitution at the pyrazine ring and could only be explained by interaction of lipid or lipid decomposition products. These authors suggested that condensation of amino ketones, formed by protein-carbohydrate interaction, may yield 3,6-dihydropyrazine which would in turn react with pentanal, a lipid oxidation product, to form 2,5-dimethyl-3-pentylpyrazine. 

An excellent source of readily available chiral auxiliaries is to be found in amino-acids, and Schollkopf and co-workers have once again used them to prepare lithiated dihydropyrazines (11). In particular, they have used L-valine, tert-leucine, and (5)-0,0-dimethyl-a-methyldopa in the synthesis of (i )-a-amino-acids from, for example, glycine. "" The same authors have also used the oxazinones (12) to give chiral (S )-a-alkyl-a-phenylglycines from DL-phenyl-glycines however, in this case 2-hydroxyalkanoic acids are used as the chiral auxiliaries. In a quite different approach, protected and lithiated glycine is converted into alanine by the methyl sulphate (13) in enantiomeric excesses approaching 40... 

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