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5-Amino-2-nitro-benzoic acid methyl ester (compound A′) (800 mg) was dissolved in dry THF (15 ml). Propionaldehyde (compound D) (870 μl) dissolved in a mixed liquid composed of 3 M sulfuric acid (4 ml) and THF (1 ml) was added to the solution at room temperature. Subsequently, sodium borohydride (231 mg) was added thereto at 0° C., and the mixture was stirred at room temperature for 3 hr. Thereafter, distilled water was added thereto, and the mixture was subjected to separatory extraction with chloroform. The organic layer was washed with saturated brine, was dried over sodium sulfate and was then concentrated. The residue was purified by column chromatography using a hexane-acetone system to give 2-nitro-5-propylamino-benzoic acid methyl ester as a useful intermediate (608 mg, yield 63%).

1. additionwas added to the solution at room temperature
2. distillationThereafter, distilled water
3. additionwas added
4. extractionthe mixture was subjected to separatory extraction with chloroform
5. washThe organic layer was washed with saturated brine
6. dry with materialwas dried over sodium sulfate
7. concentrationwas then concentrated
8. customThe residue was purified by column chromatography