Reaction Steps
To a solution of tert-butyl N-tert-butoxycarbonyl-N-[(3S,7S,8R,9S)-8-hydroxy-9-methyl-7-(1-naphthylmethyl)-2-oxo-1,5-dioxonan-3-yl]carbamate (290 mg, 0.548 mmol) and DMAP (201 mg, 1.64 mmol) in CH2Cl2 (2.7 mL) was added isobutyryl chloride (86 μL, 0.821 mmol) slowly to the flask and the resulting solution was stirred at room temperature for 20 h. The reaction mixture was dissolved in CH2Cl2 (4 mL), treated with Celite®, the solvent evaporated, and the adsorbed crude material purified by flash chromatography (SiO2, 0→15% EtOAc/hexanes) to provide [(3S,6S,7R,8S)-3-[bis(tert-butoxycarbonyl)amino]-6-methyl-8-(1-naphthylmethyl)-4-oxo-1,5-dioxonan-7-yl]2-methylpropanoate (270 mg, 82%) as a white powder: mp 130-132° C.; 1H NMR (400 MHz, CDCl3) δ 8.01-7.93 (m, 1H), 7.84 (dd, J=8.0, 1.5 Hz, 1H), 7.72 (d, J=8.1 Hz, 1H), 7.54-7.41 (m, 2H), 7.37 (dd, J=8.2, 7.0 Hz, 1H), 7.28 (dd, J=7.0, 1.3 Hz, 1H), 5.20 (dd, J=8.7, 5.7 Hz, 1H), 5.11 (app t, J=9.3 Hz, 1H), 5.08-4.97 (m, 1H), 4.06 (dd, J=11.8, 5.7 Hz, 1H), 3.86 (dd, J=11.9, 8.7 Hz, 1H), 3.66 (d, J=11.0 Hz, 1H), 3.50 (dd, J=10.8, 6.4 Hz, 1H), 3.23 (dd, J=14.0, 3.1 Hz, 1H), 2.74-2.55 (m, 2H), 2.37-2.20 (m, 1H), 1.42 (s, 18H), 1.37 (d, J=6.2 Hz, 3H), 1.26 (app dd, J=7.0, 4.4 Hz, 6H); ESIMS m/z 622 ([M+Na]+).
