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General procedure for the synthesis of 7-chloro-1H-pyrrolo[3,2-b]pyridine from 4-azaindole-4-oxide: 1H-pyrrolo[3,2-b]pyridine-4-oxide (33 g, 246.27 mmol, 1.00 equiv.) was added to a 1000 mL round bottom flask. To the mixture was added phosphorus trichloride (POCl3, 165.3 g). The resulting solution was stirred in an oil bath at 80 °C for overnight reaction. Upon completion of the reaction, the reaction mixture was quenched by slowly adding 400 mL of water. The pH of the reaction mixture was adjusted to 7-8 with sodium hydroxide (NaOH) solution. the aqueous phase was extracted three times with ethyl acetate (EtOAc, 400 mL), the organic layers were combined and dried over anhydrous sodium sulfate (Na2SO4). After filtration, the organic phase was concentrated under reduced pressure using a rotary evaporator to give 33 g (88% yield) of 7-chloro-1H-pyrrolo[3,2-b]pyridine as a yellow solid.

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