To a 10 mL round bottom flask was added sodium triacetoxyborohydride (104 mg, 0.493 mmol), (R)-2-amino-2-cyclopentylacetic acid (47.1 mg, 0.329 mmol) and DCM (2 mL). The reaction mixture was stirred at room temperature for 30 min after which was added 1-tert-butyl 3-methyl 4-formyl-1H-pyrrolo[2,3-b]pyridine-1,3-dicarboxylate (50 mg, 0.164 mmol) in DCM (2 mL). The reaction was stirred at room temperature for 2 h and then quenched with MeOH (3 drops). The mixture was concentrated to afford the title compound, which was used in the next step without further purification. [M+H] calc'd for C22H29N3O6, 432; found, 432.4.
