m-Chloroperbenzoic acid → tert-Butyl [(3R,4R,5S)-4-hydroxy-5-methyl-1-(3-nitro-1-oxido-6,7-dihydro-5H-cyclopenta[b]pyridin-4-yl)piperidin-3-yl]carbamate

mCPBA (0.420 g, 1.87 mmol) was slowly added to a solution of tert-butyl [(3R,4R,5S)-4-hydroxy-5-methyl-1-(3-nitro-6,7-dihydro-5H-cyclopenta[b]pyridin-4-yl)piperidin-3-yl]carbamate (0.200 g, 0.510 mmol) in DCM (3.0 mL) at room temperature. The reaction mixture was stirred at room temperature for 1 h. The mixture was then washed with aq. Na2S2O3, followed by 1 M NaOH. The organic layer was separated, dried and concentrated under reduced pressure. The resulting residue was further purified by silica gel column chromatography (eluting with 0-30% MeOH in EtOAc) to give the sub-title compound (90 mg, 43%) as a light orange powder. LCMS calc. for C19H29N4O6 (M+H)+: m/z=409.2. Found: 409.2.