103855-01-2

53995-82-7

Step 3: Preparation of the intermediate ethyl 4,6-dichloro-1H-indole-2-carboxylate (I-ic) Ethyl 2-[2-(3,5-dichlorophenyl)hydrazin-1-ylidene]propanoate (26 g, 94.50 mmol in two batches) was reacted with polyphosphoric acid (260 g, each batch) at 110 °C with stirring for 3 hours. Upon completion of the reaction, the reaction mixture was slowly poured into crushed ice, stirred thoroughly and the precipitate was collected by filtration. The resulting solid was alkalized with saturated aqueous NaHCO3 (adjusted pH to 10) and subsequently extracted with ethyl acetate (3 x 150 mL). The organic layers were combined, washed with brine, dried over anhydrous Na2SO4 and concentrated. The crude product was purified by silica gel column chromatography using a hexane solution of 30% ethyl acetate as eluent to afford ethyl 4,6-dichloro-1H-indole-2-carboxylate (I-ic) 36 g in 73.8% yield as an off-white solid. 1H NMR (400 MHz, DMSO-d6): δ 12.46 (s, 1H), 7.46 (s, 1H), 7.30 (s, 1H), 7.12 (s, 1H), 4.36 (q, J = 7.2 Hz, 2H), 1.35 (t, J = 7.2 Hz, 3H). ESI MS: m/z 255.8 (M-H).