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Ethyl 2-(methylamino)-3-indolizinecarboxylate 6 (4.45 g, 20.4 mmol) was dissolved in CH3COOH (40 mL) and cooled to 0° C., before a solution of sodium nitrite (1.7 g, 24.0 mmol) in water (10 mL) was added. After being stirred at 0° C. for 3 h, the reaction mixture was basified with a 2 M solution of NaOH (pH=8-9) and extracted with CH2Cl2 (3×50 mL). The combined extracts were dried over anhydrous magnesium sulphate, filtered and then concentrated under vacuum. The residue was chromatographed on silica gel with an elution gradient of 1/1 to 4/1 of ethyl acetate/petroleum ether, to give, following removal of the solvents, 3.89 g of green microcrystals of ethyl 2-(methylamino)-1-nitroso-3-indolizinecarboxylate 7, m.p.: 156-157° C. 1H NMR (DMSO-d6): δ 1.40 (t, J=7.0 Hz, 3H), 3.47 (d, J=5.4 Hz, 3H), 4.38 (q, J=7.0 Hz, 2H), 7.35 (t, J=7.0 Hz, 1H), 7.23 (t, J=8.1 Hz, 1H), 7.79 (br s, 1H), 8.38 (d, J=8.1 Hz, 1H), 9.27 (br s, 1H). 13C NMR (DMSO-d6): δ 14.5, 34.5, 59.7, 98.8, 116.2, 120.0, 123.5, 128.1, 135.2, 149.4, 149.8, 160.9.

1. additionwas added
2. extractionextracted with CH2Cl2 (3×50 mL)
3. dry with materialThe combined extracts were dried over anhydrous magnesium sulphate
4. filtrationfiltered
5. concentrationconcentrated under vacuum
6. customThe residue was chromatographed on silica gel with an elution gradient of 1/1 to 4/1 of ethyl acetate/petroleum ether
7. customto give
8. customremoval of the solvents, 3.89 g of green microcrystals of ethyl 2-(methylamino)-1-nitroso-3-indolizinecarboxylate 7, m.p.: 156-157° C