The general procedure for the synthesis of 5-nitro-2,3,3-trimethylindole from 2,3,3-trimethyl-3H-indole is as follows:
i) Nitration reagent was prepared by dissolving sodium nitrate (3.84 g, 45.2 mmol) in concentrated sulfuric acid (100 mL). This nitration reagent was slowly added dropwise to a solution of 2,3,3-trimethylindole (6.65 g, 41.8 mmol) in concentrated sulfuric acid (100 mL) under cooling conditions in an ice bath, keeping the reaction temperature at 0-5°C. ii) The reaction was carried out in a cooled room with a water bath. iii) The reaction was carried out in a cooled room with a water bath. iv) The reaction was carried out in a cooled room with a water bath. After dropwise addition, the reaction was continued at 0-5 °C with stirring for 90 min, followed by slow warming to room temperature and stirring for 16 hours.
ii) Upon completion of the reaction, the reaction was quenched by slowly pouring the reaction mixture into ice (200 g). Subsequently, the pH of the mixture was adjusted to 12 (tested using pH paper) with 50% aqueous sodium hydroxide solution to make it basic. The crude product was collected by filtration and washed with a large amount of water (about 1000 mL) until the washings were neutral.
iii) The resulting yellow solid was dried under vacuum and dissolved in ethyl acetate (250 mL). The organic phase was dried by adding anhydrous magnesium sulfate and filtered to remove the desiccant. The filtrate was rotary evaporated to dryness to give the crude product.
iv) The crude product was dissolved in a solvent mixture of chloroform:ethyl acetate (95:5, 30 mL) and purified by rapid column chromatography on silica gel. A dark yellow solid product of 5.12 g (25 mmol) was finally obtained in 60% yield.
Product Characterization:
- UV analysis (methanol solvent) showed a single absorption peak with λmax= 300 nm.
- Mass spectrometry (MALDI-TOF, gentisic acid matrix) measured m/z = 203.8 (calculated value C11H12N2O2 = 204.23).
- 1H NMR (δ, ppm): 1.97 (s, 6H), 2.95 (s, 3H), 8.58 (d, J = 10 Hz, 1H), 8.70 (s, 1H), 8.78 (d, J = 10 Hz, 1H).
