To solution of tert-butyl N-tert-butoxycarbonyl-N-[(3S,7S,8R,9S)-7,8-dihydroxy-9-methyl-2-oxo-1,5-dioxonan-3-yl]carbamate (0.220 g, 0.543 mmol) and Na2SO4 (500 mg) in CH2Cl2 (5.4 mL) at 0° C. was added Proton-Sponge™ (1.63 g, 7.60 mmol) and trimethyloxonium tetrafluoroborate (0.562 g, 3.80 mmol). The reaction mixture was slowly warmed to room temperature and stirred overnight. The reaction mixture was diluted with EtOAc and filtered. The organic layer was washed successively with H2O, 1 M aqueous NaHSO4, and saturated aqueous NaHCO3. The organic layer was dried over MgSO4, filtered, and the filtrate was concentrated and purified by flash chromatography (SiO2, hexanes/EtOAc gradient) to yield the title compound as a colorless oil (133 mg, 57%): IR (neat) 2980, 2935, 1746, 1707, 1367, 1254, 1106 cm−1; 1H NMR (400 MHz, CDCl3) δ 5.19 (dd, J=8.9, 4.2 Hz, 1H), 4.75 (dq, J=9.4, 6.3 Hz, 1H), 4.23 (dd, J=12.1, 4.2 Hz, 1H), 3.89 (dd, J=12.1, 9.0 Hz, 1H), 3.80 (d, J=10.6 Hz, 1H), 3.56-3.48 (m, 1H), 3.52 (s, 3H), 3.42 (s, 3H), 3.17-3.11 (m, 1H), 3.06 (t, J=9.0 Hz, 1H), 1.48 (s, 18H), 1.40 (d, J=6.3 Hz, 3H); 13C NMR (101 MHz, CDCl3) δ 169.10, 152.73, 86.77, 85.12, 83.13, 75.07, 72.76, 72.52, 61.21, 58.43, 57.94, 28.03, 18.65; HRMS-ESI (m/z) [M]+ calcd for C20H35NO9, 433.2312; found, 433.2317.
