[CH3:1][O:2][C:3](=[O:12])/[C:4](/[CH2:10]Br)=[CH:5]/[C:6]([O:8][CH3:9])=[O:7].[CH3:13][Mg]Br.Cl.[Cl-].[NH4+]>C(OCC)C.CN(C)P(=O)(N(C)C)N(C)C>[CH3:1][O:2][C:3](=[O:12])[CH:4]([CH3:10])[C:5](=[CH2:13])[C:6]([O:8][CH3:9])=[O:7]To a solution of (Z)-2-bromomethyl-2-butenedioic acid dimethyl ester (19.3 g) and hexamethylphosphoric acid triamide (70 ml) in diethyl ether (200 ml) was added dropwise methyl magnesium bromide (1.12 M solution in tetrahydrofuran, 100 ml) over 1 hour at −20° C. and stirred for additional 1 hour. To this reaction mixture were added a 6M aqueous solution of hydrochloric acid (18 ml) and a saturated aqueous solution of ammonium chloride (100 ml), and then the mixture was warmed to room temperature and extracted with diethyl ether (200 ml). The resulting organic layer was washed with water twice, followed by a saturated aqueous solution of sodium chloride once, dried over anhydrous sodium sulfate, and concentrated under reduced pressure. The resulting residue was purified by silica gel column chromatography (eluent: n-hexane/chloroform=1/1 to 0/1) to give the titled compound (9.92 g).
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