Reaction Steps
Under an argon atmosphere, 5-(4-acetoxyphenyl)-2-amino-3-benzylpyrazine (c-19) (140 mg, 438 μmol) was dissolved in pyridine (2 mL) and to this was added 4-(dimethylamino)pyridine (6.0 mg, 49 μmol). To this was added p-nitrobenzenesulfonyl chloride (293 mg, 1.32 mmol) while stirring at room temperature and stirred for 1.5 h at the same temperature. To this was further added p-nitrobenzenesulfonyl chloride (195 mg, 880 μmol) and stirred for 17 h. To this was added saturated aqueous solution of sodium bicarbonate to stop the reaction and the product was extracted twice with dichloromethane. The organic layer was washed once with water and once with saturated aqueous solution of sodium sulfate and then dried over anhydrous sodium sulfate. After removing anhydrous sodium sulfate by filtration, the filtrate was concentrated under reduced pressure. The residue was purified by column chromatography (24 g, n-hexane/ethyl acetate=4/1→3/1→7/3→ethyl acetate alone) to give 5-(4-acetoxyphenyl)-3-benzyl-2-bis(4-nitrophenylsulfonyl)aminopyrazine (c-40) as a dark brown solid (157 mg, 52.1%). Rf=0.62 (n-hexane/ethyl acetate=3/2). Mp. 227-229.5° C. (dec.). UV (MeOH, 0.3% DMSO) λmax (log ε) 309 (3.99), 258 (4.21). 1H NMR (400 MHz, DMSO-d6) δ 2.30 (s, 3H), 4.22 (s, 2H), 7.19-7.27 (m, 1H), 7.29-7.35 (m, 6H), 8.05-8.10 (AA′BB′, 4H), 8.14-8.19 (AA′BB′, 2H), 8.39-8.44 (AA′BB′, 4H), 9.13 (s, 1H). 13C NMR. (75.5 MHz, DMSO-d6) δ 20.9, 38.0, 122.8 (2C), 124.8 (2C×2), 126.6, 128.3 (2C), 128.7 (2C), 129.6 (2C), 130.7 (2C×2), 131.9, 137.3, 139.8, 140.3, 142.1 (1C×2), 151.1 (1C×2), 151.9, 152.7, 156.2, 169.0. I,R (KBr, cm−1) 548, 598, 617, 687, 737, 775, 810, 854, 880, 897, 916, 935, 1013, 1165, 1180, 1206, 1315, 1348, 1368, 1385, 1404, 1418, 1435, 1530, 1603, 1755, 3107. HRMS (FAR) m/z 690.0975 (M+H, C31H24N5O10S2 requires 690.0965).
