Novel routes to pyrazino[2,3-b]quinoxalines

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Abstract
Several pyrazino[2,3-b]quinoxalines were prepared by the condensation of substituted 2,3-diaminoquinoxaline with 1,2-dicarbonyl compounds. In addition, three novel routes to this system were devised, namely the reaction of 2,3-diaminoquinoxaline with α-hydroximinophenylacetic acid, 4-methyl-2-trifluoromethyloxazol-5(4H)-one, and diethyl acetylenedicarboxylate. The condensation of diethyl acetylenedicarboxylate with 2,3-diaminoquinoxaline gave a pyrazino[2,3-b]-quinoxaline which has the isomeric structure 2-ethoxycarbonylmethylene-3-oxo-1,2,3,4-tetrahydropyrazino[2,3-b]quinoxaline in the solid state. N.m.r. studies showed that in solution this compound existed as a tautomeric mixture of the unsaturated and the saturated ester.

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