Pyrazine chemistry. Part IV. Thermal [1,4] eliminations from 3,6-di-hydropyrazines

Abstract

Reaction of piperazine-2,5-diones with an excess of triethyloxonium fluoroborate produced a cis–trans mixture of 2,5-diethoxy-3.6-dihydropyrazines which could be oxidised to the corresponding pyrazines in high yield by dichlorodicyanobenzoquinone. cis-3,6-Dibenzylpiperazine-2,5-dione reacted with the Meerwein salt to afford a mixture of the cis- and trans-dihydropyrazines. On pyrolysis, the trans-isomer gave 3-benzyl-2,5-diethoxypyrazine in high yield, by elimination of the elements of toluene. Pyrolysis of the cis-isomer under the same conditions gave the same products after an initial isomerisation to the trans-dihydropyrazine.