[CH3:1][C@H:2]([CH:9]=O)[CH2:3][C:4]([O:6][CH2:7][CH3:8])=[O:5].Cl[CH:12](Cl)[B:13]1[O:17][C:16]([CH3:19])([CH3:18])[C:15]([CH3:21])([CH3:20])[O:14]1.[Li+].[I-]>C1COCC1>[CH3:1][C@H:2](/[CH:9]=[CH:12]/[B:13]1[O:17][C:16]([CH3:19])([CH3:18])[C:15]([CH3:21])([CH3:20])[O:14]1)[CH2:3][C:4]([O:6][CH2:7][CH3:8])=[O:5]Following the same procedure for the preparation of pinacol boronates, the reaction of ethyl (S)-3-methyl-4-oxobutanoate (S)-3 (0.057 g, 0.40 mmol) with CrCl2 (0.39 g, 3.18 mmol), 2-(dichloromethyl)-4,4,5,5-tetramethyl-1,3,2-dioxaborolane 5 (0.168 g, 0.80 mmol) and LiI (0.21 g, 1.59 mmol) in THF (6 mL) afforded, after purification by column chromatography (silica gel, 97:3 hexane/ethyl acetate), 69 mg (65%) of a colourless oil identified as ethyl (3S,4E)-3-methyl-5-(4,4,5,5-tetramethyl-1,3,2-dioxaborolan-2-yl)pent-4-enoate (S)-4.
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