Pyrazolines and pyrazoles are common and important motifs of pharmaceutical agents and agrochemicals. Herein, the first electrochemical approach for their direct synthesis from easily accessible hydrazones and dipolarophiles up to decagram scale is presented. The application of a biphasic system (aqueous/organic) even allows for the conversion of highly sensitive alkenes, wherein inexpensive sodium iodide is employed in a dual role as supporting electrolyte and mediator. In addition, mechanistic insight into the reaction is given by the isolation of key step intermediates. The relevance of the presented reaction is underlined by the synthesis of commercial herbicide safener mefenpyr‐diethyl in good yields.
Novel lupane‐type triterpenoid‐isoxazole conjugates were designed by direct placing of isoxazole linker at C(17) of triterpenoid. The suggested synthetic sequence demonstrates successful combination of electro‐organic synthesis and conventional approaches. TEMPO‐mediated electrooxidation of betulin to betulinal was developed and optimized at boron‐doped diamond anodes with potassium acetate as inexpensive supporting electrolyte. Betulinal‐derived oxime was further selectively electro‐oxidized at a graphite anode to nitrile oxide, which proved to be stable and isolable species. The same reaction sequence was performed with 3β‐lupane‐3,28‐diol. Nitrile oxides were characterized by 15N NMR and X‐ray crystallography. The isolable nitrile oxides allowed creation of isoxazole library by 1,3‐dipolar cycloaddition reactions with various alkynes. Some of the title conjugates exhibit cytostatic properties against breast cancer cell line MCF7, glioblastoma multiform cell line U‐87 MG and lung carcinoma cell line A549 with growth inhibition (GI50) concentrations up to 11 μm, while being harmless to immortalized human fibroblasts hTERT (GI50 >100 μm).
Primary and secondary alcohols are oxidized to corresponding aldehydes and ketones, respectively, with nitroxyl radicals. The stable radical 2,2,6,6‐tetramethylpiperidine‐1‐oxyl (TEMPO) is used as a mediator for selective oxidation of anisyl alcohol to anisaldehyde. This reaction is operated in different continuous microreactors either in single‐phase or in multiphase applications like double emulsions. The latter are used for a simple separation of the hydrophilic coproduct 1‐hydroxy‐2,2,6,6‐tetramethylpiperidine (TEMPO‐H) and the lipophilic product anisaldehyde. In addition, cyclic voltammetry is applied to determine conditions and parameters for electrochemical recycling of TEMPO‐H. TEMPO‐H is reactivated in a continuous electrolysis cell by anodic oxidation for reuse as mediator.
To demonstrate the technical application potential of electrochemical reactions, we developed a synthetic method to access commercially available herbicide safener mefenpyr-diethyl. In a simple undivided electrolysis cell with non-hazardous aqueous...
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