To a solution of compound 17 (380 mg, 1.0 mmol) in CH2Cl2 (50 mL) was added TFAA (252 uL, 1.2 mmol) dropwise at room temperature, followed by the addition of Et3N (404 mg, 4 mmol). After stirring for 1 h, the mixture was washed with brine, dried over Na2SO4. After concentration, the residue was purified by column chromatography on silica gel (PE: EA=15:1) to give compound 18 (300 mg, 83.6%). 1 H NMR (400 MHz, CDCl3) δ 1.40 (s, 9 H), 3.26-3.27 (m, 1 H), 3.30-3.31 (m, 1 H), 3.31-3.34 (m, 1 H), 3.43-3.47 (m, 1 H), 5.06 (bs, 1 H), 5.22 (bs, 1H), 5.50 (bs, 1H), 7.37-7.49 (m, 5H). MS (M+1): 360.
