Boron trifluoride etherate (18.5 μL, 0.15 mmol) was added to 23d (174 mg, 0.49 mmol) and (3aR,4S,7R,7aR)-7-methoxy-6,6-dimethyl-2-oxo-tetrahydro-3aH-[1.3]dioxolo[4,5-c]pyran-4-yl 2,2,2-trichloroacetimidate (621 mg, 1.71 mmol) in anhydrous CH2Cl2 (11.0 mL). After stirring at room temperature for 14 hours, triethylamine (150 μL) was added and the solvent was concentrated. The residue was purified via column chromatography (SiO2, 40:1 CH2Cl2:Acetone) to give 25d as a colorless foam (200 mg, 74%): 1H NMR (CDCl3, 500 MHz) δ 8.49 (s, 1H), 7.34-7.27 (m, 5H), 6.67 (s, 1H), 6.60 (s, 1H), 5.69 (s, 2H), 5.16 (d, J=5.3 Hz, 1H), 4.89 (t, J=7.8 Hz, 1H), 4.63 (dd, J=7.9, 2.4 Hz, 1H), 3.83 (s, 3H), 3.37 (s, 3H), 3.15 (d, J=8.0 Hz, 1H), 2.16 (s, 3H), 2.16 (s, 3H), 2.12 (s, 3H); 13C NMR (CDCl3, 125 MHz) δ 158.9, 156.0, 155.2, 154.2, 153.1 (2C), 149.4, 135.7, 128.7 (2C), 128.5, 128.2 (2C), 120.8, 117.4, 106.6, 105.4, 94.6, 94.1, 82.9, 67.4, 60.6, 60.6, 56.1, 56.0, 22.2, 22.0, 7.9; IR (film) νmax 2955, 2924, 2853, 1817, 1713, 1526, 1209, 1105, 1072, 1034, 976, 808 cm−1; HRMS (ESI+) m/z: [M+H]+ calcd for C28H29NO11, 556.1819. found, 556.1826.
