A solution containing crude 49 (229 mg, 0.47 mmol) in DCM (5 mL) was cooled to 0° C. TEA (73 mg, 0.72 mmol) was added followed by ethyl chloroformate (73 mg, 0.78 mmol). After 30 min, the reaction mixture was diluted with DCM and washed successively with 1M HCl and brine, dried over anhydrous Na2SO4, filtered, and concentrated. The crude product was purified by flash silica gel chromatography (2:1 hexanes/EtOAc to 1:1 hexanes/EtOAc) to afford 226 mg (88%) of 50 as an off-white-colored foam. 1H NMR (300 MHz, CDCl3): δ8.25-8.14 (m, 1H), 8.10 (s, 1H), 7.03-6.97 (m, 1H), 6.95-6.83 (m, 1H), 5.36-5.32 (m, 1H), 4.63 (dd, J=5.4, 19.8 Hz, 1H), 4.39-4.34 (m, 1H), 4.26-4.22 (m, 1H), 4.09-4.04 (m, 2H), 3.95 (app t, J=5.7 Hz, 1H), 3.80 (s, 3H), 3.53-3.48 (m, 2H), 2.49-2.43 (m, 0.5H), 2.31-2.24 (m, 0.5H), 1.94-1.88 (m, 1H), 1.77-1.65 (m, 6H), 1.44 (s, 9H), 1.29-1.10 (m, 5H) ppm. Mass spectrum, m/z [543.4] (M+H)+.
