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General procedure: Dimethylaminoacetaldehyde diethyl acetal (5 mmol) was added to a solution of 7 (0.5 mmol) in AcOH (5mL) at room temperature, and the mixture was heated at 100 °C. After 1 h, the mixture was gradually cooled to room temperature. The mixture was diluted with AcOEt (100 mL), washed with saturated NaHCO3 solution and brine, and dried over MgSO4. The solvent was removed, and the residue was separated by silica gel column chromatography with hexane/AcOEt (5:1) to give 3, 4 and 8.
General procedure: Dimethylaminoacetaldehyde diethyl acetal (5 mmol) was added to a solution of 7 (0.5 mmol) in AcOH (5mL) at room temperature, and the mixture was heated at 100 °C. After 1 h, the mixture was gradually cooled to room temperature. The mixture was diluted with AcOEt (100 mL), washed with saturated NaHCO3 solution and brine, and dried over MgSO4. The solvent was removed, and the residue was separated by silica gel column chromatography with hexane/AcOEt (5:1) to give 3, 4 and 8.