To a solution of 2a (13.6 mg, 0.05 mmol, 1.0 equiv.), phenol (5.65 mg, 0.06 mmol, 1.2 mmol) and triphenylphosphine (15.7 mg, 0.06 mmol, 1.2 equiv.) in THF (0.1 mL) was added diethyl diazocarboxylate solution (2.2 M in toluene, 27 µL, 0.06 mol) dropwise over 14 min at 0 °C. After stirring for 48 h at the room temperature, the reaction mixture was concentrated. To this residue were added diethyl ether (1 mL) and hexane (4 mL). The resulting suspension was filtered through a pad of Celite and the filter cake was washed with a mixed solvent of diethyl ether and hexane (1:4, 5 mL). To the combined filtrates were partitioned between diethyl ether and aqueous sodium hydroxide (1 M). The aqueous phase was extracted twice with diethyl ether. The combined organic phases were dried over sodium sulfate, filtered and concentrated. The residue was purified by flash column chromatography on silica gel eluting with 10-20% ethyl acetate in hexane to give 4 (10.12 mg, 58%) as a white solid.