Step 2. Synthesis of ethyl 3-oxo-3-(2,4,5-trifluorophenyl)propionate (B). Referring to the literature method [Wierenga, W.; Skulnick, H. I. J. Org. Chem. 1979, 44, 310-311], a mixture of monoethyl malonate (0.18 mL, 1.50 mmol) with bipyridine (1 crystal) was dissolved in tetrahydrofuran (THF, 10 mL) and cooled to -75 °C under argon protection . At -75 °C, n-butyllithium (n-BuLi, 2.8 mL, 4.48 mmol, 5.9 eq.) was slowly added. The reaction mixture was warmed to -50 °C and held for about 2 minutes until the pink color no longer faded (to ensure that the amount of n-butyllithium was sufficient to form a divalent anion), and then cooled to -75 °C again. Subsequently, a THF solution (2-3 mL) of 2,4,5-trifluorobenzoyl chloride (0.75 mmol) was slowly added. The reaction mixture was gradually warmed to room temperature, diluted with ethyl acetate (50 mL) and acidified with 1N hydrochloric acid under stirring. The organic phase was washed sequentially with 5% sodium bicarbonate solution (30 mL x 2) and saturated saline (50 mL x 2), dried with anhydrous sodium sulfate and concentrated. The oil obtained was purified by silica gel column chromatography (40 g silica gel column, 20% ethyl acetate in hexane solution, 40 min gradient) to give 185 mg (89% yield) of target product B as a white solid.
