The general procedure for the synthesis of 7-azaindole-5-carbaldehyde from N,N-dimethylformamide and 5-bromo-7-azaindole was as follows: to an anhydrous THF solution (50 mL) of 5-bromo-1H-pyrrolo[2,3-b]pyridine (1.0 g, 5.0 mmol, 1.0 eq.) was slowly added, under protection of argon, at -78 °C a hexane solution of 1.6 M n-butyllithium (6.7 mL, 10.7 mmol, 2.1 eq.). The reaction mixture was stirred at -78 °C for 40 min. Subsequently, 1 mL of anhydrous DMF was added to the resulting suspension and the reaction mixture was continued to be stirred overnight at room temperature. Upon completion of the reaction, the reaction mixture was quenched with saturated NH4Cl solution. The organic layer was separated and the aqueous phase was extracted with EtOAc. The organic layers were combined, washed with brine, dried over Na2SO4, filtered and concentrated to afford 1H-pyrrolo[2,3-b]pyridine-5-carbaldehyde (Intermediate 3a) as a yellow solid (1.0 g; yield: 69%; UPLC purity: 98%).
