Metabolism‐Inspired Electrosynthesis

Detailed Studies on the Methoxylation and Subsequent Dealkylation of N,N-Diethylbenzenesulfonamide Using a Tailor-Made Electrosynthetic Reactor

Benzenesulfonamides are an outstandingly important family of compounds in organic and medicinal chemistry. Herein, we report detailed studies on the electrochemical mono- and dideethylation of model compound N,N-diethylbenzenesulfonamide. In this context, all parameters of the electrosynthesis were systematically investigated, with a special emphasis on solvent screening and the effect of water on the outcome of the reaction. Beside a commercially available electrochemical reactor, a custom-made device has also successfully been designed and used in these transformations. Optimization of the reaction led to a green, scaled-up synthesis of the dealkylated products. Our experiments also render the synthesis and potential in situ use of the corresponding N-methoxyalkyl intermediate, a precursor of the reactive and versatile N-sulfonyliminium cation, possible.

Electrochemistry-mass spectrometry bridging the gap between suspect and target screening of valsartan transformation products in wastewater treatment plant effluent

Degradation of xenobiotics in wastewater treatment plants may lead to the formation of transformation products with higher persistence or increased (eco-)toxic potential compared to the parent compounds. Accordingly, the identification of transformation products from wastewater treatment plant effluents has gained increasing attention. Here, we show the potential of electrochemistry hyphenated to liquid chromatography and mass spectrometry for the prediction of oxidative degradation in wastewater treatment plants using the antihypertensive drug valsartan as a model compound. This approach identifies seven electrochemical transformation products of valsartan, which are used to conduct a suspect screening in effluent of the main wastewater treatment plant in the city of Münster in Germany. Apart from the parent compound valsartan, an electrochemically predicted transformation product, the N-dealkylated ETP336, is detected in wastewater treatment plant effluent. Subsequently, a targeted liquid chromatographytandem mass spectrometry method for the detection of valsartan and its electrochemical transformation products is set up. Here, electrochemical oxidation is used to generate reference materials of the transformation products in situ by hyphenating electrochemistry online to a triple quadrupole mass spectrometer. Using this setup, multiple reaction monitoring transitions are set up without the need for laborious and costly synthesis and isolation of reference materials for the transformation products. The targeted method is then applied to extracts from wastewater treatment plant effluent and the presence of ETP336 and valsartan in the samples is verified. The presented workflow can be used to set up targeted analysis methods for previously unknown transformation products even without the need for expensive high-resolution mass spectrometers.

Elucidation of the environmental reductive metabolism of the herbicide tritosulfuron assisted by electrochemistry and mass spectrometry

The environmental impact of pesticides and other pollutants is, to a great extent, determined by degradation and accumulation processes. Consequently, degradation pathways of pesticides have to be elucidated before approval by the authorities. In this study, the environmental metabolism of the sulfonylurea-herbicide tritosulfuron was investigated using aerobic soil degradation studies, during which a previously unidentified metabolite was observed using high performance liquid chromatography and mass spectrometry. The new metabolite was formed by reductive hydrogenation of tritosulfuron but the isolated amount and purity of the substance were insufficient to fully elucidate its structure. Therefore, electrochemistry coupled to mass spectrometry was successfully applied to mimic the reductive hydrogenation of tritosulfuron. After demonstrating the general feasibility of electrochemical reduction, the electrochemical conversion was scaled up to the semi-preparative scale and 1.0 mg of the hydrogenated product was synthesized. Similar retention times and mass spectrometric fragmentation patterns proved that the same hydrogenated product was formed electrochemically and in soil studies. Using the electrochemically generated standard, the structure of the metabolite was elucidated by means of NMR spectroscopy, which shows the potential of electrochemistry and mass spectrometry in environmental fate studies.

Development of an electrochemical flow-through cell for the fast and easy generation of isotopically labeled metabolite standards

In drug development, metabolite standards of new chemical entities are required for a comprehensive safety evaluation. Stable isotope-labeled internal metabolite standards at the milligram scale, which are difficult and expensive to synthesize in common bottom-up approaches, are necessary for metabolite quantification using liquid chromatography/mass spectrometry. A preparative electrochemical flow-through cell is presented here as a powerful tool for the cheap and straightforward synthesis of milligram amounts of isotopically labeled metabolite standards. The developed cell scales up established, so-called "coulometric" electrochemical cells. Problems like electrode fouling and cross contamination between syntheses are addressed by the use of exchangeable working electrodes. The applicability of the developed cell for the synthesis of metabolite standards is demonstrated using isotopically labeled acetaminophen as a model system for the generation of a biologically relevant phase II metabolite.

Electrosynthesis of 1,2,4-Triazolium Tetrafluoroborates

The synthesis of 1,2,4-triazolium tetrafluoroborates under electrochemical conditions is reported. The reaction is performed with stoichiometric amounts of HBF₄, which converts starting materials to their corresponding cationic forms due to protonation. As a result, sufficient conductivity is gained in MeOH, CD₃OD, and EtOH, and no additional supporting electrolyte is required. Agrochemical fungicide, (±)-triticonazole (1), is transformed in this manner into 2a, an O-methylated potential intermediate involved in the metabolism of 1, in 42% yield on a gram scale.

Dialling-In New Reactivity into the Shono-Type Anodic Oxidation Reaction

This Personal Account describes the author's groups' research in the field of electrosynthetic anodic oxidation, beginning with initial trial and error attempts with the Shono oxidation. Early setbacks with complex rotameric amide mixtures, provided the ideal environment for the discovery of the Oxa-Shono reaction-O_sp ² -C_sp ³ bond cleavage of esters-providing two useful products in one-step: aldehyde selective oxidation level products and a mild de-esterification method to afford carboxylic acids in the process. The development of the Oxa-Shono reaction provided the impetus for the discovery of other electrically propelled-N_sp ² -C_sp ² and N_sp ² -C_sp ³ -bond breaking reactions in bioactive amide and sulfonamide systems. Understanding the voltammetric behaviour of the molecule under study, switching between controlled current- or controlled potential- electrolysis, and restricting electron flow (the reagent), affords exquisite control over the reaction outcomes in batch and flow. Importantly, this bio-inspired advance in electrosynthetic dealkylation chemistry mimics the metabolic outcomes observed in nature.

Voltammetric Behaviour of Drug Molecules as a Predictor of Metabolic Liabilities

Electron transfer plays a vital role in drug metabolism and underlying toxicity mechanisms. Currently, pharmaceutical research relies on pharmacokinetics (PK) and absorption, distribution, metabolism, elimination and toxicity (ADMET) measurements to understand and predict drug reactions in the body. Metabolic stability (and toxicity) prediction in the early phases of the drug discovery and development process is key in identifying a suitable lead compound for optimisation. Voltammetric methods have the potential to overcome the significant barrier of new drug failure rates, by giving insight into phase I metabolism events which can have a direct bearing on the stability and toxicity of the parent drug being dosed. Herein, we report for the first time a data-mining investigation into the voltammetric behaviour of reported drug molecules and their correlation with metabolic stability (indirectly measured via t½), as a potential predictor of drug stability/toxicity in vivo. We observed an inverse relationship between oxidation potential and drug stability. Furthermore, we selected and prepared short- (<10 min) and longer-circulation (>2 h) drug molecules to prospectively survey the relationship between oxidation potential and stability.

Charged Tags for the Identification of Oxidative Drug Metabolites Based on Electrochemistry and Mass Spectrometry

Most of the active pharmaceutical ingredients like Metoprolol are oxidatively metabolized by liver enzymes, such as Cytochrome P450 monooxygenases into oxygenates and therefore hydrophilic products. It is of utmost importance to identify the metabolites and to gain knowledge on their toxic impacts. By using electrochemistry, it is possible to mimic enzymatic transformations and to identify metabolic hot spots. By introducing charged-tags into the intermediate, it is possible to detect and isolate metabolic products. The identification and synthesis of initially oxidized metabolites are important to understand possible toxic activities. The gained knowledge about the metabolism will simplify interpretation and predictions of metabolitic pathways. The oxidized products were analyzed with high performance liquid chromatography-mass spectrometry using electrospray ionization (HPLC-ESI-MS) and nuclear magnetic resonance (NMR) spectroscopy. For proof-of-principle, we present a synthesis of one pyridinated main oxidation product of Metoprolol.