Triethylamine → Benzo[1,3]dioxol-5-ylmethyl-(4-chloro-7-nitro-phthalazin-1-yl)-amine

3.5 g of the 1,4-dichloro-6-nitrophthalazine prepared in the step (a) was dissolved in 100 ml of ethanol, followed by the addition thereto of 2.17 g of piperonylamine and 3 ml of triethylamine. The resultant mixture was heated under reflux for 12 hours. The reaction mixture was distilled under reduced pressure to remove the solvent. Water was added to the residue thus obtained to give a solution. The obtained aqueous solution was extracted with methylene chloride. The organic phase was dried over magnesium sulfate and then subjected to vacuum concentration to remove the solvent. The residue was purified by silica gel column chromatography [n-hexane/ethyl acetate (2: 1 to 1: 1)]. Thus, 1.8 g of 1-chloro-4-(3,4-methylenedioxybenzyl)amino-6-nitrophthalazine was obtained from less polar fractions as a yellow solid and 1.2 g of 1-chloro-4-(3,4-methylene-dioxybenzyl)amino-7-nitrophthalazine was obtained from more polar fractions as a yellow solid.