Self-condensation and rearrangement of N-alkylphenacylamines

The reaction of phenacyl bromide with benzylamine affords 1,2-dibenzyl-1,2-dihydro-2,5-diphenylpyrazine (59%) and 1,2-dibenzyl-1,2-dihydro-3,6-diphenylpyrazine (21%), contrary to previous reports. In a series of reactions of N-alkylphenacylamines thermally induced self-condensation is followed by a regiospecific 1,3-alkyl shift to a substituted carbon atom, to give in good yield 1,2-dialkyl-1,2-dihydro-2,5-diphenylpyrazines at temperatures ranging from ambient to 140°. The reaction of phenacyl bromide with methylamine gives N-methyldiphenacylamine, then the reactive 1,4-dihydro-1,4-dimethyl-2,6-diphenylpyrazine. The latter readily forms an addition product with methanol and separately forms an odd-electron species characteristic of 1,4-dialkyl-1,4-dihydropyrazines.