ClC(O[CH2:5][CH:6](C)C)=O.[C:9]([O:13][C:14]([NH:16][CH2:17][P:18]([CH2:21][NH:22][C:23]([O:25][C:26]([CH3:29])([CH3:28])[CH3:27])=[O:24])(=[O:20])[OH:19])=[O:15])([CH3:12])([CH3:11])[CH3:10].C(O)C.C(N(CC)CC)C>C(Cl)Cl>[C:9]([O:13][C:14]([NH:16][CH2:17][P:18]([CH2:21][NH:22][C:23]([O:25][C:26]([CH3:29])([CH3:28])[CH3:27])=[O:24])(=[O:19])[O:20][CH2:5][CH3:6])=[O:15])([CH3:12])([CH3:11])[CH3:10]Isobutyl chloroformate (1.50 ml, 11.48 mmol) is added dropwise with stirring at 0° C. to a solution of the phosphinic acid 1 (3.200 g, 9.88 mmol) in a mixture of methylene chloride (50 ml), ethanol (3.00 ml, 51.59 mmol), triethylamine (1.40 ml, 10.06 mmol) and N,N-dimethylpyridine (10 mg). After 90 minutes, the reaction solution is extracted with saturated aqueous ammonium chloride solution (30 ml) and saturated aqueous KHCO3 solution (30 ml). The organic phase is dried over Na2SO4 and concentrated under reduced pressure. After purification by column chromatography on silica gel (ethyl acetate/heptane=3:2), the phosphinic acid ester 3 (1.840 g, 53% of theory) is obtained. See above spectroscopic data.
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