Reaction Steps
To a solution of 2-((3S,7R,8R,9S)-3-(tert-butoxycarbonylamino)-8-(isobutyryloxy)-9-methyl-2,6-dioxo-1,5-dioxonan-7-yl)acetic acid (500 mg, 1.2 mmol) in anhydrous THF (7 mL) at −15° C. was added N-methylmorpholine (127 μL, 1.2 mmol) and isobutylchloroformate (152 μL, 1.2 mmol). The mixture was stirred for 5 min and then treated with a solution of 2-mercaptopyridine N-oxide (177 mg, 1.4 mmol) and TEA (162 μL, 1.2 mmol) in THF (2 mL). The reaction was stirred at −15° C. in the dark (covered with aluminum foil) for 1 h. The reaction was then filtered, concentrated, and the resulting bright yellow residue dissolved in CH2Cl2 (5 mL) and irradiated for 1 hr. The crude reaction mixture was concentrated and purified by flash chromatography (SiO2, 10% EtOAc/hexanes) to furnish the title compound (169 mg, 29%) as a fluffy white solid: mp 54-58° C.; IR (neat) 3370, 2980, 2939, 2878, 1753, 1718 cm−1; 1H NMR (400 MHz, CDCl3) δ 8.35 (d, J=4.9 Hz, 1H), 7.48-7.41 (m, 1H), 7.11 (d, J=8.0 Hz, 1H), 7.00-6.94 (m, 1H), 5.32 (bs, J=12.9 Hz, 1H), 5.21-5.07 (m, 2H), 4.87 (dt, J=16.1, 6.4 Hz, 2H), 3.66-3.60 (m, 1H), 3.59 (bs, 1H), 3.15-3.00 (m, 2H), 2.68 (dq, J=14.2, 7.1 Hz, 1H), 1.43 (s, 9H), 1.33-1.25 (m, 9H); 13C NMR (101 MHz, CDCl3) δ 176.11, 171.91, 171.33, 157.67, 155.07, 149.73, 136.35, 122.89, 120.10, 81.00, 75.34, 74.76, 66.22, 51.66, 51.26, 34.68, 28.74, 28.64 (3), 19.38, 19.23, 18.14; ESIMS m/z 519 ([M+Na]+).
