[C:1]([O:5][C:6](=[O:15])[NH:7][C:8]([CH3:14])([CH3:13])[CH2:9][C:10](=O)[CH3:11])([CH3:4])([CH3:3])[CH3:2].[C:16]([O:19][CH2:20][CH3:21])(=[O:18])[CH3:17].Cl>O1CCCC1>[CH2:20]([O:19][C:16](=[O:18])/[CH:17]=[C:10](\[CH3:11])/[CH2:9][C:8]([NH:7][C:6]([O:5][C:1]([CH3:4])([CH3:3])[CH3:2])=[O:15])([CH3:13])[CH3:14])[CH3:21](1,1-dimethyl-3-oxobutyl)carbamic acid tert-butylester (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 chromatographed on silica (3×40 cm) using ethyl acetate/heptane (1:2) as eluent to afford 2.0 g of (E)-5-tert-Butoxycarbonylamino-3,5-dimethylhex-2-enoic acid ethyl ester.
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