To a tetrahydrofuran solution of oxazole F-1 (800 mg, 11.58 mmol) was slowly added a hexane solution of 1.6 M n-butyllithium (7.2 mL, 11.58 mmol) at -78 °C. After the reaction mixture was stirred at this temperature for 1 h, a tetrahydrofuran (8 mL) solution of N,N-dimethylformamide (0.89 mmol, 11.58 mmol) was added. Subsequently, the reaction system was gradually warmed to room temperature and stirring was continued overnight. Upon completion of the reaction, the reaction was quenched with methanol, filtered through Dowex 50W x 8-200 resin (H+ type, 1:1 mass ratio of resin to oxazole F-1) and washed with methanol. The filtrates were combined and concentrated to give the crude products oxazole-2-carboxaldehyde F-2a and F-1a. The crude product was purified by column chromatography (using 100-200 mesh silica gel, eluent 2% ether/dichloromethane) to give purified oxazole-2-carboxaldehyde (900 mg, 80% yield) as a light brown liquid (Rf = 0.7, 2% ether/dichloromethane system). The product could be used directly in the subsequent reaction without further characterization.
