1 H-Pyridazino [3, 4, 5-de] phthalazine (4) and 3-hydrazino-1 (or 9) H-pyridazino [3, 4, 5-de]- phthalazine (6) represent intramolecular hydrazones of the drugs hydralazine and dihydralazine, respectively. These novel heterocycles were synthesized by several different routes starting from 2, 6-dimethylbenzoic acid, 3-methylphthalic anhydride, or hemimellitic acid. Tetrabromination of 2, 6-dimethylbenzoic acid with bromine and carbon tetrachloride ...