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Under an argon atmosphere, 2-amino-3-benzyl-5-(4-hydroxyphenyl)pyrazine (coelenteramine) (c-5) (prepared by the method described in Adamczyk, M. et al., Org. Prep. Proced. Int., 33, 477-485 (2001)) (471 mg, 1.70 mmol) was dissolved in pyridine (3.6 mL) and chloroform (9 mL) and cooled to 0° C. To this was added acetyl chloride (910 μL, 12.8 mmol) and stirred for an hour after warming to room temperature. To this was added saturated aqueous solution of sodium bicarbonate to stop the reaction and extracted 3 times with dichloromethane. The organic layer was washed with aqueous solution of saturated sodium sulfate and dried over anhydrous sodium sulfate. After removing anhydrous sodium sulfate by filtration, the filtrate was concentrated under reduced pressure with a rotary evaporator. The residue was purified by silica gel column chromatography (45 g, n-hexane/ethyl acetate=1/1→1/3) to give 5-(4-acetoxyphenyl)-2-acetylamino-3-benzylpyrazine (c-33) as a pale yellow solid (355 mg, 57.8%). Rf=0.17 (n-hexane/ethyl acetate=1/1). Mp. 215.5-217° C. (dec.). UV (MeOH) λmax (log ε) 317 (4.09), 290 (4.08), 259 (4.15). UV (pH 7.4 PB) λmax (log ε) 315 (4.07), 292 (3.98), 253.5 (4.08). 1H NMR (400 MHz, DMSO-d6) δ 2.05 (s, 3H), 2.29 (s, 3H), 4.18 (s, 2H), 7.15-7.22 (m, 3H), 7.23-7.29 (m, 4H), 8.09-8.14 (AA′BB′, 2H), 8.94 (s, 1H), 10.31 (s, 1H). 13C NMR (75.5 MHz, DMSO-d6) δ 20.9, 23.0, 39.9, 122.5 (2C), 126.3, 127.7 (2C), 128.3 (2C), 129.0 (2C), 133.2, 137.8, 138.2, 144.7, 147.3, 150.5, 151.6, 169.1, 169.2. IR (KBr, cm−1) 706, 916, 1016, 1159, 1223, 1260, 1369, 1450, 1497, 1545, 1578, 1670, 1751, 3279. HRMS (FAB+) m/z 362.1509 (M+H, C21H20N3O3 requires 362.1505).
