Benzamide (1.0 g, 8.3 mmol) was dissolved in 1,2-dichloroethane (5 mL), oxalyl dichloride (1.2 g, 9.1 mmol) was added thereto, and the mixture was stirred at 90° C. for 30 hours. The reaction solution was concentrated under reduced pressure. Toluene (4 mL) was added thereto and dissolved. Hexahydro-1,1-diphenyl-3H-oxazolo[3,4-a]pyrazin-3-one (0.20 g, 0.68 mmol) was added thereto, and the mixture was stirred at 60° C. for 6 hours. The reaction solution was concentrated under reduced pressure to a half its original volume, and the precipitated crystals were filtered off. The residue was purified with silica gel column chromatography (hexane:ethyl acetate=1:1), and toluene (4 mL) was added thereto. The precipitated crystals were filtered off, and the filtrate was concentrated. The residue was further purified with silica gel column chromatography (hexane:ethyl acetate=1:1) and crystallized from diisopropyl ether to obtain the title compound (70 mg, yield 23%).
