Under a nitrogen atmosphere, a solution of N-methylacetamide (116 μl, 1.52 mmol) in tetrahydrofuran (2 ml) and then hexamethylphosphoric triamide (555 μl, 3.19 mmol) were dropwise added at -78° C. to a solution comprising 2.0M lithium diisopropylamide--tetrahydrofuran (1.6 ml, 3.19 mmol) and tetrahydrofuran (10 ml). The mixture was stirred at -78° C. for 30 minutes. Then, a solution of (2S,4R)-N-tert-butoxycarbonyl-4-tert-butyldimethylsiloxyprolinal (500 mg, 1.52 mmol) in tetrahydrofuran (2 ml) was dropwise added. The reaction mixture was stirred at room temperature for 2 h. Water was added to the reaction mixture. Then, the reaction mixture was poured into a mixture of ethyl acetate-water. The organic layer was washed with saturated aqueous sodium chloride, dried over anhydrous sodium sulfate, and concentrated in vacuo. The obtained oily residue was subjected to silica gel column chromatography (chloroform--methanol=20:1) to give the title compound (158 mg, yield: 25.9%).
