摘要/Abstract

β,γ-不饱和酯和γ-酮酯是重要的合成中间体, 从丰富的氨基酸衍生的Katritzky吡啶盐出发, 在可见光照射下, 合成了一系列的β,γ-不饱和酯和γ-酮酯, 该方法具有反应条件温和, 操作简单等优点, 且有良好的官能团兼容性, 所得的产物可进一步转化.
关键词: Katritzky盐, β,γ-不饱和酯, γ-酮酯, 自由基反应, 光催化, C—N键活化
β,γ-Unsaturated ester and γ-ketone ester are important synthons, which can be used to convert into various heterocyclic compounds, natural products and pharmaceuticals. The development of efficient methods for the synthesis of β,γ-unsaturated ester and γ-ketone ester compounds has attracted much attention from synthetic chemists. By using Katritzky pyridinium salts as radical precursors, commercially available Ru(bpy)₃Cl₂?6H₂O as photocatalyst, K₂CO₃ as base, and dichloromethane (DCM) as solvent, we developed a simple and efficient method for the synthesis of a series of β,γ-unsaturated esters and γ-ketone esters by C—N bond activation. Bench-stable and easily handled redox-active Katritzky pyridinium salts derived from abundant amino acids were used as radical precursors for the alkylation of 1,1-diarylethylene and aryl enol silyl ether species upon irradiation with household blue LEDs. The reaction displays an excellent functional group tolerance and a potential utility for amino acids functionalization, allowing to access desired products in moderate to good yields. Moreover, under air conditions, the reaction has moderate compatibility. Scaling up the reaction in grams, the yield was higher and the target product was obtained with 91% yield. Control experiments demonstrated that the photocatalyst and visible light were both essential for the success of the reaction. Performing the reaction in the presence of radical scavenger TEMPO, did lead to no desired product 3a formation. Moreover, a TEMPO-trapped product was determined by MS analysis and NMR, indicating a radical-type mechanism of this reaction. It is of note that this protocol could offer a powerful complementary strategy for the use of amino acids that were also employed in photoredox-catalyzed decarboxylative reactions. A representative procedure for this reaction is as following: A 10 mL oven-dried Schlenk-tube was charged with 1a (111.5 mg, 0.20 mmol), Ru(bpy)₃Cl₂?6H₂O (3.0 mg, 2 mol%), K₂CO₃ (55.2 mg, 0.40 mmol) and a magnetic stirring bar. The tube was evacuated and back-filled three times with argon. A solution of 2a (53 μL, 0.30 mmol) in DCM (2 mL) was injected into the tube by syringe. The resulting mixture was stirred at room temperature upon irradiation with blue LEDs (22 W) and monitored by thin-layer chromatography (TLC). After completion, the solvent was then removed under reduced pressure and the residue was purified by flash column chromatography on silica gel to give 3a as an off-white solid (42.7 mg, 65% yield).
Key words: Katritzky salt, β,γ-unsaturated ester, γ-ketone ester, free radical reaction, photocatalysis, C—N bond activation;


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