Diethyl azodicarboxylate (250 μL, 1.87 mmol, 6.43 eq) is added at room temperature to a stirred solution of 4-[3-hydroxy-2-(4-nitro-pyrazol-1-yl)-propyl]-6-methoxy-quinolin-3-ol (100 mg, 0.29 mmol, 1.0 eq) and triphenylphosphine (122 mg, 0.46 mmol, 1.6 eq) in N,N-dimethylformamide (38 mL). After 3 hours stiffing at room temperature, solvent is evaporated and the residue is extracted with ethyl acetate (3×20 mL) and water (20 mL). The combined organic layers are dried over sodium sulfate, filtered and concentrated to give a crude product that is purified by column chromatography (silica gel, eluent: petroleum ether:ethyl acetate, 1:1, v/v) to afford 6-methoxy-3-(4-nitro-pyrazol-1-yl)-3,4-dihydro-2H-1-oxa-9-aza-phenanthrene as a light yellow solid (60 mg, 63% yield).
