6,8-Dimethylpyrimido[4,5-c]pyridazine-5,7(6H,8H)-dione: new heterocyclizations based on SNH-methodology. Unexpected formation of the first iso-π-electronic analogue of the still unknown dibenzo[a,o]pycene

N(2)-Oxide and 3-amino derivatives of 6,8-dimethylpyrimido[4,5-c]pyridazine-5,7(6H,8H)-dione have been shown to react with primary alkylamines in the presence of an oxidant to produce condensed imidazolines or imidazoles. Both conversions based on SNH-strategy represent a new approach to imidazoline–(imidazole-)ring annulation. Heterocyclic analogues of the still unknown dibenzo[a,o]pycene were obtained as a by-products of the above transformations upon using of cyclohexylamine.