[OH:1][C:2]1[C:9]([O:10][CH3:11])=[CH:8][C:7]([O:12][CH3:13])=[CH:6][C:3]=1[CH:4]=[O:5].C([O-])([O-])=O.[K+].[K+].[CH2:20]([O:22][CH:23]([O:26][CH2:27][CH3:28])[CH2:24]Br)[CH3:21]>CN(C=O)C>[CH2:20]([O:22][CH:23]([O:26][CH2:27][CH3:28])[CH2:24][O:1][C:2]1[C:9]([O:10][CH3:11])=[CH:8][C:7]([O:12][CH3:13])=[CH:6][C:3]=1[CH:4]=[O:5])[CH3:21]To a stirred suspension containing 38 (1.1 g, 6.0 mmol) and K2CO3 (1.0 g, 7.2 mmol) in DMF (100 mL), bromoacetaldehyde diethyl acetal (0.93 mL, 6.0 mmol) was added dropwise. The mixture was refluxed for 4 h. After cooling, the precipitate was filtered off and the solvent was evaporated in vacuo. The crude residue was adsorbed on silica gel and purified by flash chromatography. The product was eluted with hexane/EtOAc (4:1) to give 39 (1.2 g, 67%) as an oil. 1H NMR (500 MHz, CDCl3): δ: 10.50 (1H, s, CHO), 6.88 (1H, d, J4,6=2.9 Hz, H-6), 6.74 (1H, d, J4,6=2.9 Hz, H-4), 4.83 (1H, t, J4,6=5.3 Hz, CH), 4.14 (2H, d, J4,6=5.3 Hz, CH2), 3.88 (3H, s, OMe), 3.83 (3H, s, OMe), 3.77-3.71) (2H, m, CH2CH3), 3.63-3.58 (2H, m, CH2CH3), 1.24 (6H, t, J=7.1 Hz, 2×CH3).
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