[CH2:1]([O:8][C@@H:9]([CH3:17])[CH2:10][C:11](N(OC)C)=[O:12])[C:2]1[CH:7]=[CH:6][CH:5]=[CH:4][CH:3]=1.[CH3:18][Mg]Br.C(OCC)C.Cl>C1COCC1>[CH2:1]([O:8][C@@H:9]([CH3:17])[CH2:10][C:11](=[O:12])[CH3:18])[C:2]1[CH:3]=[CH:4][CH:5]=[CH:6][CH:7]=1Under a nitrogen atmosphere, to a solution of the above (3S)-3-(benzyloxy)-N-methoxy-N-methylbutanamide (196 g) in THF (1.5 L), a 3M methyl magnesium bromide/diethyl ether solution (0.38 mL, 1.1 mol) was gradually added under ice-cooling, and the mixture was stirred at the same temperature for 3 hours. After the reaction, the reaction mixture was poured to 2N hydrochloric acid, and after THF was distilled off under reduced pressure, it was extracted with diethyl ether. After it was successively washed with water, a saturated aqueous sodium hydrogencarbonate solution and saturated brine, it was dried with sodium sulfate and the solvent was distilled off under reduced pressure to give a crude product. This was purified by reduced pressure distillation to give a title compound (44 g) as an oil.
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