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To an ice-cooled soluton of acetic anhydride (90 ml) and concentrated nitric acid (70%, 10.3 ml) was added 6-chloroindole-2-carboxylic acid (H. W. Ridon and J. C. Tweddle, J. Chem.Soc., 1955, 3499: 18.16 g, 92.84 mmol) portion wise over 20 min. The mixture was stirred for 2 h at 0° C. and the resulting precipitates collected by filtration and washed with a mixture of dichloromethane/hexane (1:1) to give 12.6 g (56%) of the title compound as a yellow solid. 1H-NMR (DMSO-d6) δ: 13.41(1H, br s), 8.04 (1H, d, J=8.8 Hz), 7.62 (1H, d, 1.8 Hz), 7.44 (1H, dd, J=1.8, 8.8 Hz)

1. customthe resulting precipitates
2. filtrationcollected by filtration
3. washwashed with a mixture of dichloromethane/hexane (1:1)