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To a solution of 2,6-dimethylbenzoic acid (20 g) in conc. sulfuric acid (100 ml) was dropwise added under ice-cooling a mixture of 70% nitric acid and conc. sulfuric acid (21.6 ml), which was prepared by dropwise adding conc. sulfuric acid (10.8 ml) to 70% nitric acid (15.1 ml) under ice-cooling, and the mixture was stirred for 1 hour at the same temperature. Ice-water was added to the reaction mixture, and the resulting precipitates were filtered off. The filtrate was concentrated, and the residue was purified by flash chromatography (dichloromethane:methanol=20:1 including 1% acetic acid) to give 2,6-dimethyl-3-nitrobenzoic acid (7.0 g) as a colorless crystal.

1. temperaturecooling
2. temperaturecooling
3. customthe resulting precipitates
4. filtrationwere filtered off
5. concentrationThe filtrate was concentrated
6. customthe residue was purified by flash chromatography (dichloromethane:methanol=20:1 including 1% acetic acid)