A synthetic method for the so far unknown dipyridazino[4,3-b:3',4'-e][1,4]-diazepinone system (7a) was elaborated. Cyclisation of 6-chloro-N-(3,6-dichloropyridazin-4-yl)-3-(4-methoxybenzylamino)-N-propylpyr idazine-4-carboxamide (5) under basic conditions was found to yield in addition to the expected compound (7a) an isomeric dipyridazino[3,4-b:3',4'-e][1,4]diazepinone (7b) resulting from Smiles rearrangement. The 7a7b ratio was found to be influenced by the nature of the base and the solvent.