[CH3:1][P:2](=[O:7])([O:5][CH3:6])[O:3][CH3:4].[Li]CCCC.[C:13]([O:17][C:18]([NH:20][C@@H:21]([CH2:26][CH:27]=[CH2:28])[C:22](OC)=[O:23])=[O:19])([CH3:16])([CH3:15])[CH3:14].O>C1COCC1.CCOC(C)=O>[CH3:4][O:3][P:2]([CH2:1][C:22](=[O:23])[C@@H:21]([NH:20][C:18](=[O:19])[O:17][C:13]([CH3:15])([CH3:14])[CH3:16])[CH2:26][CH:27]=[CH2:28])([O:5][CH3:6])=[O:7]To a solution of dimethyl methylphosphonate (13.98 mL, 131 mmol) in THF (87 mL) at −78° C. was added n-BuLi (82 mL, 131 mmol) slowly. After completion of addition, the reaction was stirred for 40 min and then a solution of (5)-methyl 2-(tert-butoxycarbonylamino)pent-4-enoate (6.0 g, 26.2 mmol) in THF (30 mL) was added slowly. Stirring was continued for another 40 min at −78° C. The reaction mixture was then quenched by adding H2O (2.357 mL, 131 mmol). The reaction mixture was diluted with EtOAc (100 mL) and the layers were separated. The organic layer was washed with 1M HCl, saturated NaHCO3 solution followed by brine. The organic phase was then dried over MgSO4, filtered and concentrated to give a clear oil. The crude product was finally purified using silica gel chromatography to give the desired product (7.46 g, 89%) as colorless oil. MS (ESI) m/z: 343.9 (M+Na)+. 1H NMR (500 MHz, CDCl3) δ 5.63-5.76 (1H, m), 5.08-5.17 (2H, m), 4.33-4.43 (1H, m), 3.80 (3H, d, J=2.20 Hz), 3.77 (3H, d, J=2.20 Hz), 3.28-3.37 (1H, m), 3.05-3.16 (1H, m), 2.58-2.69 (1H, m), 2.42 (1H, dt, J=14.58, 7.29 Hz), 1.43 (9H, s) ppm.
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