[CH2:1]([O:3][C:4](=[O:17])[C:5]([C:12]([O:14][CH2:15][CH3:16])=[O:13])=[C:6]([C:8]([O:10][CH3:11])=[O:9])[CH3:7])[CH3:2].C(O[CH:21](OCC)[N:22]([CH3:24])[CH3:23])C.CN(C=O)C>C1C=CC=CC=1>[CH2:1]([O:3][C:4](=[O:17])[C:5]([C:12]([O:14][CH2:15][CH3:16])=[O:13])=[C:6]([C:8]([O:10][CH3:11])=[O:9])[CH:7]=[CH:21][N:22]([CH3:24])[CH3:23])[CH3:2]A mixture of 2-ethoxycarbonyl-3-methoxycarbonyl-but-2-enoic acid ethyl ester (15 g, 61.5 mmol), dimethylformamide diethyl acetal (11.1 mL, 64.5 mmol) and DMF (15 mL) was heated at 80° C. for 5 h. After the mixture was cooled, 200 mL of benzene was added, and the resulting mixture was washed several times with 1 M HCl and water. The organic layer was dried over MgSO4 and concentrated to give an orange solid, which was recrystallized from hexanes/CCl4 to give 8.1 g of the title compound as a bright yellow solid. 1H NMR (CDCl3, 200 MHz): δ=6.70 (d, 1H, J=13.2 Hz), 5.66 (d, 1H, J=13.4 Hz), 4.10-4.40 (m, 4H), 3.88 (s, 3H), 2.96 (br s, 6H), 1.20-1.40 (m, 6H).
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