[CH3:1]COCC.C[Mg]Br.[O:9]=[C:10]1[CH2:14][O:13][CH2:12][CH:11]1[NH:15][C:16](=[O:25])[O:17][CH2:18][C:19]1[CH:24]=[CH:23][CH:22]=[CH:21][CH:20]=1.C>O1CCCC1>[OH:9][C:10]1([CH3:1])[CH2:14][O:13][CH2:12][CH:11]1[NH:15][C:16](=[O:25])[O:17][CH2:18][C:19]1[CH:20]=[CH:21][CH:22]=[CH:23][CH:24]=1A 3 M ether solution of methylmagnesium bromide (4.07 mL, 12.20 mmol) was added dropwise to a solution of benzyl 4-oxotetrahydrofuran-3-ylcarbamate (1.305 g, 5.55 mmol, from Step 3) in tetrahydrofuran (50 mL) at 0° C. Gas evolution was observed during the addition, indicating formation of methane. After 1 h at 0° C., the mixture was quenched with saturated ammonium chloride (75 mL) and extracted with ethyl acetate three times. The combined extracts were dried (MgSO4) and concentrated. Silica gel chromatography, eluting with 20-70% ethyl acetate in hexanes, gave impure material. 1H NMR showed presence of the desired product. The impure material was combined with the material from another run (from 2.18 g of ketone starting material) and taken to the next reaction without purification.
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