Abstract 5-Substituted 2, 3-dihydro-1H-pyrimido [1, 2-a] quinoxaline 6-oxides 1a-f were synthesized by ring closure of N-acyl-N′-(o-nitroaryl)-1, 3-propanediamines 2 with ethyl polyphosphate (PPE) or trimethylsilyl polyphosphate (PPSE) to the corresponding 2- substituted 1-(o-nitroaryl)-1, 4, 5, 6-tetrahydropyrimidines 3, followed by spontaneous heterocyclization. The method was extended to the synthesis of the homologous 6-aryl-1, ...