Reaction Steps
A solution containing (S)-5-(4-benzyloxy-benzyl)-2,2-dimethyl-[1,3]dioxolan-4-one (S)-3-(4-benzyloxy-phenyl)-2-hydroxy-propionic acid (2.0 g, 7.34 mmol), 2,2-dimethoxypropane (18.63 g, 0.179 mol) and pyrididium p-toluene sulfonate (0.92 g, 3.66 mmol) in chloroform (80 mL) was heated to reflux for 40 minutes under N2. The reaction was cooled, diluted with water and extracted with CH2Cl2. The organic layer was dried (Na2SO4) and the solvent was removed in vacuo to give crude product which was purified by a flash chromatography using 10:1 hexanes:acetone to afford about 2.01 g (88%) of (S)-5-(4-benzyloxy-benzyl)-2,2-dimethyl-[1,3]dioxolan-4-one. 1H NMR (400 MHz, CDCl3) δ 7.43–7.29 (m, 5H), 7.17 (d, 2H, J=8.80 Hz), 6.91 (d, 2H, J=8.80 Hz), 5.04 (s, 2H), 4.61 (dd, 1H, J=6.36 Hz, J=4.40 Hz), 3.12 (dd, 1H, J=14.67 Hz, J=4.40 Hz), 2.99 (dd, 1H, J=14.67 Hz, J=6.36 Hz), 1.49 (s, 3H), 1.36 (s, 3H). MS (ES+) m/z exact mass calculated for C19H20O4 (M+NH4) 330. Found m/z 330. Step 2: (S)-3-(4-benzyloxy-phenyl)-2-isopropoxy-propionic acid A −78° C. solution of (S)-5-(4-benzyloxy-benzyl)-2,2-dimethyl-[1,3]dioxolan-4-one (0.20 g, 0.64 mmol) and triethylsilane (0.223 g, 1.92 mmol) in CH2Cl2 (8 mL) was treated dropwise with a 1 molar solution of TiCl4 in CH2Cl2 (0.64 mL, 0.64 mmol) under N2. The solution was stirred at −78° C. for 15 minutes and then quenched with water. The mixture was extracted with EtOAc, and the organic layer was dried (Na2SO2). The organic layer was filtered, and the solvent was removed in vacuo to give crude product which was purified by a flash chromatography using 5:1 hexanes:acetone to afford 0.171 g (85%) (S)-3-(4-benzyloxy-phenyl)-2-isopropoxy-propionic acid. 1H NMR (400 MHz, DMSO-d6) δ 12.50 (bs, 1H), 7.42–7.27 (m, 5H), 7.12 (d, 2H, J=8.80 Hz), 6.88 (d, 2H, J=8.80 Hz), 5.02 (s, 2H), 3.96 (dd, 1H, J=7.83 Hz, J=4.89 Hz), 3.47 (hp, 1H, J=5.87 Hz), 2.83 (dd, 1H, J=13.69 Hz, J=4.89 Hz), 2.71 (dd, 1H, J=13.69 Hz, J=8.31 Hz), 1.02 (d, 2H, J=5.87 Hz), 0.86 (d, 2H, J=5.87 Hz). IR (KBr) 3012, 2977, 2931, 1767, 1722, 1610, 1511, 1380, 1240, 1116, 1020. HRMS (TOF ES−) m/z exact mass calculated for C19H21O4 (M−1) 13.1440. Found m/z 313.1434.
