A mixture of (S)-4-(3-hydroxypyrrolidin-1-yl)-9-((2-(trimethylsilyl)ethoxy)methyl)-9H-dipyrido[2,3-b;4′,3′-d]pyrrole-6-carbonitrile (140 mg, 0.34 mmol) and N-chlorosuccinimide (137 mg, 1.02 mmol) in acetonitrile (1.3 mL) and isopropyl alcohol (0.4 mL) was stirred at 35° C. for 5 hours. The cooled reaction mixture was quenched with saturated aqueous sodium thiosulfate (1 mL), diluted with ethyl acetate (50 mL), and washed with water (20 mL). The organic layer was separated, dried over sodium sulfate, filtered, concentrated in vacuo, and purified by flash chromatography (silica, 4 g, ISCO, 0-50% ethyl acetate in heptane) to afford the title compound as a pale yellow solid, which was used in the next step without any further purification (100 mg).
