Palladium hydroxide on carbon (20%) (36 mg, 0.05 mmol) was added to a solution of 18 (0.361 g, 0.71 mmol) in MeOH (8 mL). Hydrogen was bubbled through the suspension with stirring until 18 was completely consumed and converted to a polar substance (monitored by TLC:hexanes/EtOAc 2:1). The mixture was filtered through a short pad of silica gel and concentrated under vacuum. To a solution of the resulting yellow oil in dry DMF (5 mL) were added 2,2-dimethoxypropane (90.5 mg, 0.84 mmol) and camphorsulfonic acid (10 mg, 0.043 mmol). After the mixture was stirred at room temperature for 1 day, it was diluted with saturated aqueous NaHCO3 solution (15 mL) and extracted with CH2Cl2 (3×20 mL). The combined organic layer was washed sequentially with saturated aqueous NaHCO3 solution (2×20 mL), water (2×20 mL), and brine (20 mL), and was then dried (Na2SO4). The solvent was removed under vacuum, and the residue was purified by chromatography (hexanes/EtOAc 8:1 to 2:1) to give 19 (217 mg, 72%). 1H NMR δ 7.15-7.05 (m, 4H), 5.09 (s, 1H), 3.89 (d, 2H, J=11.6 Hz), 3.67 (d, 2H, J=11.6 Hz), 3.60 (t, 2H, J=6.8 Hz), 2.60-2.50 (m, 4H), 2.40 (br s, 1H), 1.96 (t, 2H, J=8.0 Hz), 1.65-1.36 (m, 21H); 13C NMR δ 154.7, 139.9, 139.0, 128.2, 128.0, 98.2, 79.1, 66.1, 62.4, 51.5, 35.5, 33.5, 32.4, 31.2, 28.4, 28.2, 27.2, 25.3, 19.7. HRMS m/z: calcd for C24H40NO5 (MH+), 422.2901; found, 422.2904.
