Citric Acid → N-cyclopropyl-4-fluoro-3-nitrobenzenesulfonamide

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To a solution of fuming nitric acid (8 mL) and conc. H2SO4 (16 mL), was added 4-fluorobenzenesulfonyl chloride (1.3 g, 6.7 mmol) in small portions. The mixture was stirred at RT for 2 h. and then poured onto crushed ice (60 g) and extracted with dichloromethane (2×). The organic layer was washed with brine, dried over Na2SO4, filtered and concentrated. The residue was dissolved in CH2Cl2 (30 mL) cooled to −40° C. and cyclopropylamine (388 mg, 6.8 mmol), followed by DIPEA (8 mmol) were added. After stirring at −40° C. for 2 h, the mixture was poured on 5% citric acid solution and extracted with dichloromethane. The organic layer was separated, and passed through a short pad of silica gel and concentrated in vacuo to afford N-cyclopropyl-4-fluoro-3-nitrobenzenesulfonamide (1.6 g, 92%) as a yellow solid.

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