Potassium carbonate (1.40 g, 9.68 mmol) was added to a solution of compound 4 (303 mg, 1.21 mmol) in 4 mL of anhydrous DMF at 0° C. After stifling the reaction mixture for 20 min, a solution of tert-butyl (2-bromoethyl)carbamate (268 mg, 1.21 mmol) in anhydrous DMF (2 mL) was added and then the reaction mixture was stirred at room temperature for 3 h. The reaction was quenched by adding water and then extracted with ethyl acetate. The ethyl acetate layer was washed with 10% NH4Cl aqueous solution followed by water and dried over anhydrous Na2SO4, filtered, concentrated and purified by silica column chromatography (20% ethyl acetate in hexanes) to afford compound 5 in 53.1% (253 mg) as a colorless oil.
