CC(C)([O-])C.[K+].[C:7]([O:11][C:12](=[O:21])[NH:13][C:14]([CH3:20])([CH3:19])[CH2:15][C:16](=O)[CH3:17])([CH3:10])([CH3:9])[CH3:8].[C:22]([O:25][CH2:26][CH3:27])(=[O:24])[CH3:23].Cl>O1CCCC1>[CH2:26]([O:25][C:22](=[O:24])/[CH:23]=[C:16](\[CH3:17])/[CH2:15][C:14]([NH:13][C:12]([O:11][C:7]([CH3:10])([CH3:9])[CH3:8])=[O:21])([CH3:19])[CH3:20])[CH3:27]Triethyl phosphonoacetate (4.7 g; 20.9 mmol) was dissolved in tetrahydrofuran (36 ml). Potassium tert-butoxide (2.3 g; 20.9 mmol) was added and the reaction mixture was stirred for 40 min at room temperature. (1,1-Dimethyl-3-oxobutyl)carbamic acid tert-butyl ester (2.5 g; 11.6 mmol) was dissolved in tetrahydrofuran (15 ml) and added dropwise to the reaction mixture which was heated to reflux for 12 h. Ethyl acetate (100 ml) and hydrochloric acid (1 N; 100 ml) were added and the phases were separated. The aqueous phase was extracted with ethyl acetate (3×50 ml). The combined organic phases were washed with an aqueous solution of sodium hydrogen carbonate (saturated; 100 ml), dried (magnesium sulfate) and evaporated in vacuo. The residue was purified by flash chromatography on silica (120 g) using ethyl acetate/heptane (1:2) as eluent to afford 2.0 g of (2E)-5-tert-butoxycarbonylamino-3,5-dimethylhex-2-enoic acid ethyl ester.
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