碳-氟键断裂用于构建氟代异噁唑化合物

Facile Synthesis of Fluorinated Isoxazoles via Consecutive Double C—F Bond Cleavage

含氟异噁唑骨架是许多药物及农药分子中的重要结构单元,其中单氟代异噁唑类化合物的合成方法还比较有限.从易得的α-三氟甲基-β-羰基酯出发,发展了基于碱促进的碳氟键断裂策略的氟代异噁唑化合物合成方法,该反应条件温和,操作简便,官能团兼容性良好,可以中等到较好的收率得到一系列3-氟代异噁唑化合物,且所得产物可进一步转化为生物活性分子的类似物.

Fluorinated heterocycles represent a ubiquitous structural motif found in numerous pharmaceuticals, agrochemicals, and functional materials. This is especially true for fluorine-containing five-membered heteroaromatic compounds that have been widely investigated in various fields for a long time. In this context, fluorinated isoxazoles have emerged as valuable scaffolds owing to their diverse biological properties. Among various approaches that have been developed for the synthesis and functionalization of isoxazoles, efficient and modular route to fluorine-substituted isoxazoles are still limited. Traditional methods include the condensation of 2-fluoro-1,3-dicarbonyl derivatives with hydroxylamine, Au-catalyzed fluorocyclization of 2-alkyne O-methyloximes, and direct fluorination of isoxazoles. However, the wide applicability of these approaches often suffers from low chemical yields, harsh reaction conditions, and limited substrate scope. Herein, we describe a one-pot protocol for the construction of fluorinated isoxazoles from CF3-containing precursors with hydroxylammonium chloride. Typical features of this reaction include mild conditions, simple operations, and good functional group compatibility. This method provides facile access to a series of 3-F-5-aryl-isoxazoles in moderate to good yields from easily available α-CF3-β-keto esters. Moreover, further synthetic transformations of obtained isoxazoles to important bio-active molecular derivatives have also been demonstrated. A representative procedure for this reaction is as following: α-CF3-β-keto ester 1 (0.2 mmol, 1.0 equiv.), HONH2·HCl (46 mg, 0.66 mmol), pyridine (71 μL, 0.88 mmol), and CH3CN (3.0 mL) were added into an oven-dried vial equipped with a magnetic stir bar. The mixture was stirred at 75 ℃ for 12 h and monitored by thin-layer chromatography (TLC). After completion, 10 mL of water was added and the mixture was extracted with EtOAc for three times. The combined organic layers were washed with saturated NaCl and dried over Na2SO4. The mixture was evaporated under reduced pressure and residue was purified by flash chromatography on silica gel eluting with petroleum ether/ethyl acetate (V:V=30:1) to afford the 3-F-5-aryl-isoxazole 2.