Benzaldehyde (259 mg, 2.45 mmol), Compound (2,2'-methylenebis(3,1-benzoxazine-4-one) (500 mg, 1.63 mmol), and BF3. (C2H5)2O (0.045 ml, 0.163 mmol) were heated under reflux in dry toluene (20 ml) for 20 hours. After the reaction, the solvent was removed under reduced pressure from the reaction mixture, the residue was treated with water, extracted with benzene, and the organic layer was separated, washed with water and dried over magnesium sulfate. The solvent was evaporated under reduced pressure. The residue was dissolved in the developing solvent and adsorbed on silica gel, and was purified by column chromatography to give the desired compound 52. The obtained product was crystallized from isopropylether to give the desired compound 52 in white solid in a good yield. m.p., IR, and NMR spectra of the product fully agreed with those of the one obtained in Example 19.
