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

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 mute to fluorine-substituted isoxazoles are still limited. Traditional methods include the condensation of 2-fluoro-1,3-dicarbonyl derivatives with hydroxylamine, Au-catalyzed fluoro-cyclization 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 alpha-CF3-beta-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: alpha-CF3-beta-keto ester 1 (0.2 mmol, 1.0 equiv.), HONH2 center dot HCl(46 mg, 0.66 mmol), pyridine (71 mu L, 0.88 mmol), and CH3 CN (3.0 mL) were added into an oven-dried vial equipped with a magnetic stir bat The mixture was stirred at 75 e 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.