(S)-methyl 2-(tert-butoxycarbonyl)-3-(2-nitro-N-(pent-4-enyl)phenylsulfonamido)-propanoate iv (500 mg, 1.06 mmol) was dissolved in the mixed solvent system: THF (4 mL), methanol (1 mL), and water (2 mL). Powdered lithium hydroxide (250 mg, 10.4 mmol) was added. The light yellow slurry was stirred at rt for 15 h, and then concentrated in vacuo. The residue was partitioned between ether and water. The ether phase was discarded, and the aqueous phase was treated with 1 N HCl until the pH was 4. This acidic solution was extracted with ethyl acetate four times. The combined ethyl acetate extracts were dried (MgSO4) and concentrated in vacuo to give 430 mg (89%) of (S)-2-(tert-butoxycarbonyl)-3-(2-nitro-N-(pent-4-enyl)phenylsulfonamido)propanoic acid v as a yellow oil. 1H NMR (CD3OD, 300 MHz) δ 1.38 (s, 9H), 1.51–1.60 (m, 2H), 1.89–1.98 (m, 2H), 3.28–3.32 (m, 2H), 3.59–3.64 (dd, J=14.95, 9.46 Hz, 1H), 3.71–3.74 (m, 1H), 4.33 (dd, J=9.61, 4.43 Hz, 1H), 4.87–4.94 (m, 2H), 5.63–5.72 (m, 1H), 7.71–7.77 (m, 3H), 8.01 (dd, J=7.48, 1.37 Hz, 1H). LC-MS (Method D, retention time: 2.04 min), MS m/z 358 (M++1-Boc).
