GENERAL METHOD: 4-carboxythiazoline or 4-carboxyoxazoline (0.5 mmol) was dissolved in anhydrous DMF (1 mL), molecular sieves (4 , 100% wt) and copper(II) acetate (9.1 mg, 0.05 mmol). The reaction mixture was stirred at 120 °C for 10 to 24 h under an oxygen atmosphere (using an O2 balloon). After completion of the reaction, the reaction mixture was diluted with ethyl acetate, the organic layer was washed sequentially with water and brine, dried with anhydrous sodium sulfate, filtered, and the filtrate was concentrated under reduced pressure. The concentrated residue was purified by silica gel fast column chromatography to obtain the target product 4-carboxythiazole or 4-carboxyoxazole.
