[CH3:1][P:2](=[O:7])([O:5][CH3:6])[O:3][CH3:4].C([Li])CCC.O=C1CCC(=O)N1[O:20][C:21](=O)[CH2:22][CH2:23][CH2:24][CH2:25][CH2:26][NH:27][C:28]([O:30][C:31]([CH3:34])([CH3:33])[CH3:32])=[O:29]>C1COCC1>[CH3:4][O:3][P:2]([CH2:1][C:21](=[O:20])[CH2:22][CH2:23][CH2:24][CH2:25][CH2:26][NH:27][C:28]([O:30][C:31]([CH3:33])([CH3:32])[CH3:34])=[O:29])(=[O:7])[O:5][CH3:6]A solution of dimethyl methylphosphonate (9.308 g, 75.01 mmol) in THF (30 mL) was cooled to −78 ° C. and treated with n-butyllithium (29.5 mL of 2.5 M). The reaction mixture was stirred at −78 ° C. for 10 min, followed by addition of 6-tert-butoxycarbonylamino-hexanoic acid 2,5-dioxo-pyrrolidin-1-yl ester (24-1) (5.060 g, 15.41 mmol) in THF (30 mL). After 15 min the reaction was quenched with aqueous NH4Cl and allowed to warm to room temperature overnight. The water was extracted three times with ethyl acetate and the combined organics washed with brine and dried (Na2SO4). The solvent was removed in vacuo to afford the title compound 24-2 as a pale yellow oil (5.5 g).
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