Treatment of 2,3-dihydro-2-methyl-3-phenyl-1H-pyrazino[3,4,5-cd]indolizine 2-oxides (5) with trifluoroacetic anhydride gave new heterocyclic six-membered betaines (6) which underwent 1,3-dipolar cycloaddition reaction with dimethyl acetylenedicarboxylate and maleimides in hot toluene to yield the corresponding cycloadducts (7) and (8). Treatment of the maleimide adducts (8) with p-toluenesulfonic acid in boiling acetic acid gave the [2.3.4]cyclazines (9).