The Shono-type electroorganic oxidation of unfunctionalised amides. Carbon-carbon bond formation via electrogenerated N-acyliminium ions

Computational Predictive and Electrochemical Detection of Metabolites (CP-EDM) of Piperine

In this article, we introduce a proof-of-concept strategy, Computational Predictive and Electrochemical Detection of Metabolites (CP-EDM), to expedite the discovery of drug metabolites. The use of a bioactive natural product, piperine, that has a well-curated metabolite profile but an unpredictable computational metabolism (Biotransformer v3.0) was selected. We developed an electrochemical reaction to oxidize piperine into a range of metabolites, which were detected by LC-MS. A series of chemically plausible metabolites were predicted based on ion fragmentation patterns. These metabolites were docked into the active site of CYP3A4 using Autodock4.2. From the clustered low-energy profile of piperine in the active site, it can be inferred that the most likely metabolic position of piperine (based on intermolecular distances to the Fe-oxo active site) is the benzo[d][1,3]dioxole motif. The metabolic profile was confirmed by comparison with the literature, and the electrochemical reaction delivered plausible metabolites, vide infra, thus, demonstrating the power of the hyphenated technique of tandem electrochemical detection and computational evaluation of binding poses. Taken together, we outline a novel approach where diverse data sources are combined to predict and confirm a metabolic outcome for a bioactive structure.

Recent advances in catalytic asymmetric synthesis

Asymmetric catalysis stands at the forefront of modern chemistry, serving as a cornerstone for the efficient creation of enantiopure chiral molecules characterized by their high selectivity. In this review, we delve into the realm of asymmetric catalytic reactions, which spans various methodologies, each contributing to the broader landscape of the enantioselective synthesis of chiral molecules. Transition metals play a central role as catalysts for a wide range of transformations with chiral ligands such as phosphines, N-heterocyclic carbenes (NHCs), etc., facilitating the formation of chiral C-C and C-X bonds, enabling precise control over stereochemistry. Enantioselective photocatalytic reactions leverage the power of light as a driving force for the synthesis of chiral molecules. Asymmetric electrocatalysis has emerged as a sustainable approach, being both atom-efficient and environmentally friendly, while offering a versatile toolkit for enantioselective reductions and oxidations. Biocatalysis relies on nature's most efficient catalysts, i.e., enzymes, to provide exquisite selectivity, as well as a high tolerance for diverse functional groups under mild conditions. Thus, enzymatic optical resolution, kinetic resolution and dynamic kinetic resolution have revolutionized the production of enantiopure compounds. Enantioselective organocatalysis uses metal-free organocatalysts, consisting of modular chiral phosphorus, sulfur and nitrogen components, facilitating remarkably efficient and diverse enantioselective transformations. Additionally, unlocking traditionally unreactive C-H bonds through selective functionalization has expanded the arsenal of catalytic asymmetric synthesis, enabling the efficient and atom-economical construction of enantiopure chiral molecules. Incorporating flow chemistry into asymmetric catalysis has been transformative, as continuous flow systems provide precise control over reaction conditions, enhancing the efficiency and facilitating optimization. Researchers are increasingly adopting hybrid approaches that combine multiple strategies synergistically to tackle complex synthetic challenges. This convergence holds great promise, propelling the field of asymmetric catalysis forward and facilitating the efficient construction of complex molecules in enantiopure form. As these methodologies evolve and complement one another, they push the boundaries of what can be accomplished in catalytic asymmetric synthesis, leading to the discovery of novel, highly selective transformations which may lead to groundbreaking applications across various industries.

A General Method to Access Underexplored Ylideneamino Sulfates as Interrupted Beckmann-Type Rearrangement Intermediates

The Beckmann rearrangement of ketoximes to their corresponding amides, using a Brønsted acid-mediated fragmentation and migration sequence, has found wide-spread industrial application. We postulated that the development of a methodology to access ylideneamino sulfates using tributylsulfoammonium betaine (TBSAB) would afford isolable Beckmann-type intermediates and competent partners for subsequent rearrangement cascades. The ylideneamino sulfates generated, isolated as their tributylammonium salts, are sufficiently activated to undergo Beckmann rearrangement without additional reagent activation. The generation of sulfuric acid in situ from the ylideneamino sulfate giving rise to a routine Beckmann rearrangement and additional amide bond cleavage to the corresponding aniline was detrimental to reaction success. The screening of bases revealed inexpensive sodium bicarbonate to be an effective additive to prevent classic Brønsted acid-mediated fragmentation and achieve optimal conversions of up to 99%.

Electrochemical Vicinal C-H Difunctionalization of Saturated Azaheterocycles

A method to functionalize two vicinal C-H bonds of saturated azaheterocycles is described. The procedure involves subjecting the substrate to a mixture of hydrochloric acid, acetic acid, and acetic anhydride in an undivided electrochemical cell at a constant current, resulting in stereoselective conversion to the corresponding α-acetoxy-β-chloro derivative. The α-position can be readily substituted with a range of other groups, including alkyl, aryl, allyl, alkynyl, alkoxy, or azido functionalities. Furthermore, we demonstrate that the β-chloro position can be engaged in Suzuki cross-coupling. This protocol thus enables the rapid diversification of simple five-, six-, and seven-membered saturated azaheterocycles at two adjacent positions.

Nitrogen Heterocycle Synthesis through Hydride Abstraction of Acyclic Carbamates and Related Species: Scope, Mechanism, Stereoselectivity, and Product Conformation Studies

Acyliminium ions and related species are potent electrophiles that can be quite valuable in the synthesis of nitrogen-containing molecules. This manuscript describes a protocol to form these intermediates through hydride abstractions of easily accessible allylic carbamates, amides, and sulfonamides that avoids the reversibility that is possible in classical condensation-based routes. These intermediates are used in the preparation of a range of nitrogen-containing heterocycles, and in many cases high levels of stereocontrol are observed. Specifically areas of investigation include the impact of chemical structure on oxidation efficiency, the geometry of the intermediate iminium ions, the impact of a substrate stereocenter on stereocontrol, and an examination of transition state geometry.

Electrochemical Late-Stage Functionalization

Late-stage functionalization (LSF) constitutes a powerful strategy for the assembly or diversification of novel molecular entities with improved physicochemical or biological activities. LSF can thus greatly accelerate the development of medicinally relevant compounds, crop protecting agents, and functional materials. Electrochemical molecular synthesis has emerged as an environmentally friendly platform for the transformation of organic compounds. Over the past decade, electrochemical late-stage functionalization (eLSF) has gained major momentum, which is summarized herein up to February 2023.

Pairing Electrocarboxylation of Unsaturated Bonds with Oxidative Transformation of Alcohol and Amine

A parallel paired electrosynthetic method, coupling electrocarboxylation incorporating CO2 into ketone, imine, and alkene with alcohol oxidation or oxidative cyanation of amine, was developed for the first time. Various carboxylic acids as well as aldehyde/ketone or α-nitrile amine were prepared at the cathode and anode respectively in a divided cell. Its utility and merits on simultaneously achieving high atom-economic CO2 utilization, elevated faradaic efficiency (FE, total FE of up to 166 %), and broad substrate scope were demonstrated. The preparation of pharmaceutical intermediates for Naproxen and Ibuprofen via this approach proved its potential application in green organic electrosynthesis.

Copper catalyzed Shono-type oxidation of proline residues in peptide

Since the initial report in 1975, the Shono oxidation has become a powerful tool to functionalize the α position of amines, including proline derivatives, by electrochemical oxidation. However, the application of electrochemical Shono oxidations is restricted to the preparation of simple building blocks and homogeneous Shono-type oxidation of proline derivatives remains challenging. The late-stage functionalization at proline residues embedded within peptides is highly important as substitutions about the proline ring are known to affect biological and pharmacological activities. Here, we show that homogenous copper-catalyzed oxidation conditions complement the Shono oxidation and this general protocol can be applied to a series of formal C-C coupling reactions with a variety of nucleophiles using a one-pot procedure. This protocol shows good tolerance toward 19 proteinogenic amino acids and was used to functionalize several representative bioactive peptides, including captopril, enalapril, Smac, and endomorphin-2. Last, peptide cyclization can also be achieved by using an appropriately positioned side-chain hydroxyl moiety.

Electrochemically Enabled Total Syntheses of Natural Products

Electrochemical techniques have helped to enable the total synthesis of natural products since the pioneering work of Kolbe in the mid 1800's. The electrochemical toolset grows every day and these new possibilities change the way chemists look at and think about natural products. This review provides a perspective on total syntheses wherein electrochemical techniques enabled the carbon─carbon bond formations in the skeletal assembly of important natural products, discussion of mechanistic details, and representative examples of the bond formations enabled over the last several decades. These bond formations are often distinctly different from those possible with conventional chemistries and allow assemblies complementary to other techniques.

C(sp3)-H Aerobic Alkenylation of Tetrahydroisoquinolines via Organic Electrosynthesis

A method for the C(sp³)-H alkenylation of N-aryl-tetrahydroisoquinoline (THIQ) has been developed by the combination of electrooxidation and a copper catalyst. The corresponding products were obtained with good to excellent yields under mild conditions. Besides, the addition of TEMPO as an electron mediator is crucial to this transformation, since the oxidative reaction could proceed under a low electrode potential. In addition, the catalytic asymmetric variant has also been demonstrated with good enantioselectivity.

C(sp3)-H Alkenylation of Tetrahydroisoquinolines via Merging Electrochemistry and Organocatalysis

C(sp³)-H alkenylation of tetrahydroisoquinoline by merging Shono oxidation and the Morita-Baylis-Hillman reaction is developed, employing 4-dimethylaminopyridine as an organocatalyst and TEMPO/NaBr as an electrocatalyst. The reaction proceeds via the interception of an iminium cation intermediate, which is generated in situ from anodic oxidation, leading to aza-Morita-Baylis-Hillman reaction products. Additionally, the use of TEMPO and NaBr as mediators is crucial to avoid the decomposition of products by lowering the oxidation potential of the reaction.

Tunable Electrochemical Peptide Modifications: Unlocking New Levels of Orthogonality for Side-Chain Functionalization

Electrochemical transformations provide enticing opportunities for programmable, residue-specific peptide modifications. Herein, we harness the potential of amidic side-chains as underutilized handles for late-stage modification through the development of an electroauxiliary-assisted oxidation of glutamine residues within unprotected peptides. Glutamine building blocks bearing electroactive side-chain N,S-acetals are incorporated into peptides using standard Fmoc-SPPS. Anodic oxidation of the electroauxiliary in the presence of diverse alcohol nucleophiles enables the installation of high-value N,O-acetal functionalities. Proof-of-principle for an electrochemical peptide stapling protocol, as well as the functionalization of dynorphin B, an endogenous opioid peptide, demonstrates the applicability of the method to intricate peptide systems. Finally, the site-selective and tunable electrochemical modification of a peptide bearing two discretely oxidizable sites is achieved.

Recent Advances in the Electrochemical Functionalization of Isocyanides

Isocyanides are well-known as efficient CO surrogates and C1 synthons in modern organic synthesis. Although tremendous efforts have been devoted to fully exploiting the reactivity of isocyanides, these transformations are primarily limited by their utilization of stoichiometric toxic chemical oxidants. With the recent resurgence of organic electrochemistry, which has considerably laid dormant over the past several decades, electrolysis has been identified as a green and powerful tool to enrich structural diversity by solely utilizing electric current as clean and inherently safe redox equivalents of stoichiometric chemical oxidants. In this regard, the unique reactivity of isocyanides has been studied in numerous electrochemical transformations. This review comprehensively highlights the most relevant progress in electrochemical strategies towards the functionalization of isocyanides up until June of 2022, with a focus on reaction outcomes and mechanisms.

A flow electrochemistry-enabled synthesis of 2-substituted N-(methyl-d)piperidines

A synthesis of N-monodeuteriomethyl-2-substituted piperidines is described. An efficient and readily scalable anodic methoxylation of N-formylpiperidine in an undivided microfluidic electrolysis cell delivers methoxylated piperidine 3, which is a precursor to a N-formyliminium ion and enables C-nucleophiles to be introduced at the 2-position. The isotopically labelled N-deuteriomethyl group is installed using the Eschweiler-Clarke reaction with formic acid-d₂ and unlabelled formaldehyde. Monodeuterated N-methyl groups in these molecular systems possess small isotropic proton chemical shift differences important in the investigation of molecules that are able to support long-lived nuclear spin states in solution nuclear magnetic resonance.

One-Step Regio- and Stereoselective Electrochemical Synthesis of Orexin Receptor Antagonist Oxidative Metabolites

Synthesis of drug metabolites, which often have complex structures, is an integral step in the evaluation of drug candidate metabolism, pharmacokinetic (PK) properties, and safety profiles. Frequently, such synthetic endeavors entail arduous, multiple-step de novo synthetic routes. Herein, we present the one-step Shono-type electrochemical synthesis of milligrams of chiral α-hydroxyl amide metabolites of two orexin receptor antagonists, MK-8133 and MK-6096, as revealed by a small-scale (pico- to nano-mole level) reaction screening using a lab-built online electrochemistry (EC)/mass spectrometry (MS) (EC/MS) platform. The electrochemical oxidation of MK-8133 and MK-6096 was conducted in aqueous media and found to produce the corresponding α-piperidinols with exclusive regio- and stereoselectivity, as confirmed by high-resolution nuclear magnetic resonance (NMR) characterization of products. Based on density functional theory (DFT) calculations, the exceptional regio- and stereoselectivity for this electrochemical oxidation are governed by more favorable energetics of the transition state, leading to the preferred secondary carbon radical α to the amide group and subsequent steric hindrance associated with the U-shaped conformation of the cation derived from the secondary α-carbon radical, respectively.

Exploring Electrosynthesis: Bulk Electrolysis and Cyclic Voltammetry Analysis of the Shono Oxidation

As electrochemistry continues to gain broader acceptance and use within the organic chemistry community, it is important that advanced undergraduate students are exposed to fundamental and practical knowledge of electrochemical applications for chemical synthesis. Herein, we describe the development of an undergraduate laboratory experience that introduces synthetic and analytical electrochemistry concepts to an advanced organic chemistry class. Experiments focus on the electrooxidative α-functionalization of carbamates, more generally known as the Shono oxidation, and include cyclic voltammetry analysis of two cyclic carbamates and a constant current bulk electrolysis reaction. The exercise offers students an authentic experience in organic electrochemistry, lays a practical and theoretical foundation for future engagement with concepts in electrochemistry and redox chemistry, and strengthens fundamental organic chemistry skills.

Electrochemical Friedel–Crafts-type amidomethylation of arenes by a novel electrochemical oxidation system using a quasi-divided cell and trialkylammonium tetrafluoroborate

Electrochemical Friedel–Crafts-type amidomethylation was successfully carried out by a novel electrochemical oxidation system using a quasi-divided cell and trialkylammonium tetrafluoroborates, such as iPr₂NHEtBF₄. Constant current electrolysis of 1,3,5-trimethoxybenzene or indoles in DMA containing 0.1 M iPr₂NHEtBF₄ using an undivided cell equipped with a Pt plate cathode and a Pt wire anode (a quasi-divided cell) resulted in selective formation of N-acyliminium ions of DMA at the anode, which reacted with arenes to give the corresponding amidomethylated products in good to high yields.

Electrosynthesis of 2-Substituted Benzoxazoles via Intramolecular Shono-Type Oxidative Coupling of Glycine Derivatives

Herein, an atom-economical and eco-friendly electrochemical oxidation/cyclization of glycine derivatives through intramolecular Shono-type oxidative coupling is disclosed, leading to a variety of 2-substituted benzoxazoles in 51-85% yields. This oxidative cyclization proceeded in transition metal- and oxidant-free conditions and generated H₂ as only a byproduct. Additionally, gram-scale reactions and a broad substrate scope demonstrated the synthetic usefulness of this protocol.

AgNTf2 catalyzed cycloaddition of N-acyliminium ions with alkynes for the synthesis of the 3,4-dihydro-1,3-oxazin-2-one skeleton

A catalyzed process for the synthesis of the 4,6-substituted 3,4-dihydro-1,3-oxazin-2-one skeleton has been developed through cycloaddition of in situ generated acyliminium intermediates with alkynes. A range of chain N,O-acetals and terminal alkynes were amenable for this mild transformation. As a result, a series of desired cycloaddition products were obtained in moderate to good yields.

N ‐Terminal Selective C−H Azidation of Proline‐Containing Peptides: a Platform for Late‐Stage Diversification

A methodology for the C−H azidation of N‐terminal proline‐containing peptides was developed employing only commercially available reagents. Peptides bearing a broad range of functionalities and containing up to 6 amino acids were selectively azidated at the carbamate‐protected N‐terminal residue in presence of the numerous other functional groups present on the molecules. Post‐functionalizations of the obtained aminal compounds were achieved: cycloaddition reactions or C−C bond formations via a sequence of imine formation/nucleophilic addition were performed, offering an easy access to diversified peptides.

Photons or Electrons? A Critical Comparison of Electrochemistry and Photoredox Catalysis for Organic Synthesis

Redox processes are at the heart of synthetic methods that rely on either electrochemistry or photoredox catalysis, but how do electrochemistry and photoredox catalysis compare? Both approaches provide access to high energy intermediates (e.g., radicals) that enable bond formations not constrained by the rules of ionic or 2 electron (e) mechanisms. Instead, they enable 1e mechanisms capable of bypassing electronic or steric limitations and protecting group requirements, thus enabling synthetic chemists to disconnect molecules in new and different ways. However, while providing access to similar intermediates, electrochemistry and photoredox catalysis differ in several physical chemistry principles. Understanding those differences can be key to designing new transformations and forging new bond disconnections. This review aims to highlight these differences and similarities between electrochemistry and photoredox catalysis by comparing their underlying physical chemistry principles and describing their impact on electrochemical and photochemical methods.

Exploring Electrochemical C(sp3)-H Oxidation for the Late-Stage Methylation of Complex Molecules

The "magic methyl" effect, a dramatic boost in the potency of biologically active compounds from the incorporation of a single methyl group, provides a simple yet powerful strategy employed by medicinal chemists in the drug discovery process. Despite significant advances, methodologies that enable the selective C(sp3)-H methylation of structurally complex medicinal agents remain very limited. In this work, we disclose a modular, efficient, and selective strategy for the α-methylation of protected amines (i.e., amides, carbamates, and sulfonamides) by means of electrochemical oxidation. Mechanistic analysis guided our development of an improved electrochemical protocol on the basis of the classic Shono oxidation reaction, which features broad reaction scope, high functional group compatibility, and operational simplicity. Importantly, this reaction system is amenable to the late-stage functionalization of complex targets containing basic nitrogen groups that are prevalent in medicinally active agents. When combined with organozinc-mediated C-C bond formation, our protocol enabled the direct methylation of a myriad of amine derivatives including those that have previously been explored for the "magic methyl" effect. This synthesis strategy thus circumvents multistep de novo synthesis that is currently necessary to access such compounds and has the potential to accelerate drug discovery efforts.

Continuous flow technology-a tool for safer oxidation chemistry

The advantages and benefits of continuous flow technology for oxidation chemistry have been illustrated in tube reactors, micro-channel reactors, tube-in-tube reactors and micro-packed bed reactors in the presence of various oxidants.

Electrochemical Generation of a Nonstabilized Azomethine Ylide: Access to Substituted N-Heterocycles

Azomethine ylides are fascinating 1,3-dipoles for [3 + 2] cycloaddition reactions toward the construction of N-heterocycles. Herein, an efficient and environmentally benign electrochemical approach for the generation of a nonstabilized azomethine ylide has been established under metal-free and external oxidant-free conditions. The resulting 1,3-dipole undergoes a [3 + 2] cycloaddition reaction with olefins. This electrosynthetic methodology indulges a straightforward and facile approach for the construction of substituted pyrrolidines.

Capitalizing on Mediated Electrolyses for the Construction of Complex, Addressable Molecular Surfaces

Synthetic organic chemists are beginning to exploit electrochemical methods in increasingly creative ways. This is leading to a surge in productivity that is only now starting to take advantage of the full-potential of electrochemistry for accessing new structures in novel, more efficient ways. In this perspective, we provide insight into the potential of electrochemistry as a synthetic tool gained through studies of both direct anodic oxidation reactions and more recent indirect methods, and highlight how the development of new electrochemical methods can expand the nature of synthetic problems our community can tackle.

TEMPO-Enabled Electrochemical Enantioselective Oxidative Coupling of Secondary Acyclic Amines with Ketones

An electrochemical asymmetric coupling of secondary acyclic amines with ketones via a Shono-type oxidation has been described, affording the corresponding amino acid derivatives with good to excellent diastereoselectivity and enantioselectivity. The addition of an N-oxyl radical as a redox mediator could selectively oxidize the substrate rather than the product, although their oxidation potential difference is subtle (about 13 mV). This electrochemical transformation proceeds in the absence of stoichiometric additives, including metals, oxidants, and electrolytes, which gives it good functional group compatibility. Mechanistic studies suggest that proton-mediated racemization of the product is prevented by the reduction of protons at the cathode.

C-H Bond Functionalization of Amines: A Graphical Overview of Diverse Methods

This Graphical Review provides a concise overview of the manifold and mechanistically diverse methods that enable the functionalization of sp3 C-H bonds in amines and their derivatives.

Shono-Type Oxidation for Functionalization of N-Heterocycles

The development of facile synthetic methods for stereodefined aliphatic cyclic amines is an important research field in synthetic organic chemistry since such scaffolds constitute a variety of natural products and biologically active compounds. N-Acyl cyclic N,O-acetals which prepared by electrochemical oxidation of the corresponding cyclic amines have proven to be useful and versatile precursors for the synthesis of such skeletons. In this Personal Account, we introduce our efforts toward the development of synthetic strategies for the diastereo- and/or enantioselective synthesis of cyclic amines by using electrochemically prepared cyclic N,O-acetals. In addition, the investigation of the "memory of chirality" in the electrooxidative methoxylation of N-acyl amino acid derivatives, the strategy for the synthesis of chiral azabicyclic compounds by utilizing electrochemical oxidation, and halogen cation-mediated synthesis of nitrogen-containing heterocycles are also described.

Electrochemical access to benzimidazolone and quinazolinone derivatives via in situ generation of isocyanates

Isocyanates are the key intermediates for several organic transformations towards the synthesis of diverse pharmaceutical targets. Herein, we report the development of an oxidant-free protocol for electrochemical in situ generation of isocyanates. This strategy highlights expedient access to benzimidazolones and quinazolinones and eliminates the need for exogenous oxidants. Furthermore, detailed mechanistic studies provide strong support towards our hypothesis of in situ isocyanate generation.

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.

Electrochemical asymmetric synthesis of biologically active substances

Electrically driven oxidation and reduction reactions are well-established methods for synthesis even in the chemical industry, but asymmetric versions are still few. The mild conditions used, atom efficiency and low cost make these reactions a very attractive alternative to other methods of synthesis. Very fine tuning can be achieved based on minute changes in potentials, allowing only one functional group in a molecule to react in the presence of several others, which is ideal for applications in total synthesis. In this review, the literature in the field of asymmetric synthesis of biologically active substances over the last 10 years is surveyed.

Anodic Oxidation as an Enabling Tool for the Synthesis of Natural Products

Electrochemistry provides a valuable toolbox for organic synthesis and offers an appealing, environmentally benign alternative to the use of stoichiometric quantities of chemical oxidants or reductants. Its potential to control current efficiency along with providing alternative reaction conditions in a classical sense makes electrochemistry a suitable method for large-scale industrial transformations as well as for laboratory applications in the synthesis of complex molecular architectures. Even though research in this field has intensified over the recent decades, many synthetic chemists still hesitate to add electroorganic reactions to their standard repertoire, and hence, the full potential of preparative organic electrochemistry has not yet been unleashed. This short review highlights the versatility of anodic transformations by summarizing their application in natural product synthesis.

1 Introduction

2 Shono-Type Oxidation

3 C–N/N–N Bond Formation

4 Aryl–Alkene/Aryl–Aryl Coupling

5 Cycloadditions Triggered by Oxidation of Electron-Rich Arenes

6 Spirocycles

7 Miscellaneous Transformations

8 Future Prospects

Synthesis of Functionalized Isoindolinones via Calcium Catalyzed Generation and Trapping of N-Acyliminium Ions

Herein we report our full investigation into the calcium catalyzed generation and trapping of N-acyliminium ions from readily available 3-hydroxyisoindolinones. We have successfully employed a range of traditional nucleophiles including carbon, nitrogen, and sulfur containing reactive partners. The reaction is tolerant to a wide range of functionalities and provides high value scaffolds in good to excellent yields.

Anodic Oxidation for the Stereoselective Synthesis of Heterocycles

Stereodefined aliphatic heterocycles are one of the fundamental structural motifs observed in natural products and biologically active compounds. Various strategies for the synthesis of these building blocks based on transition metal catalysis, organocatalysis, and noncatalytic conditions have been developed. Although electrosynthesis has also been utilized for the functionalization of aliphatic heterocycles, stereoselective transformations under electrochemical conditions are still a challenging field in electroorganic chemistry. This Account consists of four main topics related to our recent efforts on the diastereo- and/or enantioselective synthesis of aliphatic heterocycles, especially N-heterocycles, using anodic oxidations as key steps. The first topic is the development of stereoselective synthetic methods for multisubstituted piperidines and pyrrolidines from anodically prepared α-methoxy cyclic amines. Our strategies were based primarily on N-acyliminium ion chemistry, and the key electrochemical transformations were diastereoselective anodic methoxylation, diastereoselective arylation, and anodic deallylative methoxylation. Furthermore, we found a unique property of the N-cyano protecting group that enabled the electrochemical α-methoxylation of α-substituted cyclic amines. The second topic of investigation is memory of chirality in electrochemical decarboxylative methoxylation. We observed that the electrochemical decarboxylative methoxylation of oxazolidine and thiazolidine derivatives with the appropriate N-protecting group occurred in a stereospecific manner even though the reaction proceeded through an sp² planar carbon center. Our findings demonstrated the first example of memory of chirality in N-acyliminium ion chemistry. The third topic is the synthesis of chiral azabicyclo-N-oxyls and their application to chiral organocatalysis in the electrochemical oxidative kinetic resolution of secondary alcohols. The final topic is stereoselective transformations utilizing anodically generated halogen cations. We investigated the oxidative kinetic resolution of amino alcohol derivatives using anodically generated bromo cations. We also developed an intramolecular C-C bond formation of keto amides, a diastereoselective bromoiminolactonization of α-allyl malonamides, and an oxidative ring expansion reaction of allyl alcohols. It is noteworthy that most of the electrochemical reactions were performed in undivided cells under constant-current conditions, which avoided a complicated reaction setup and was beneficial for a large-scale reaction. In addition, we developed some enantioselective electrochemical transformations that are still challenges in electroorganic chemistry. We hope that our research will contribute to the further development of diastereo- and/or enantioselective transformations and the construction of valuable heterocyclic compounds using an electrochemical approach.

A Survival Guide for the "Electro-curious"

The appeal and promise of synthetic organic electrochemistry have been appreciated over the past century. In terms of redox chemistry, which is frequently encountered when forging new bonds, it is difficult to conceive of a more economical way to add or remove electrons than electrochemistry. Indeed, many of the largest industrial synthetic chemical processes are achieved in a practical way using electrons as a reagent. Why then, after so many years of the documented benefits of electrochemistry, is it not more widely embraced by mainstream practitioners? Erroneous perceptions that electrochemistry is a "black box" combined with a lack of intuitive and inexpensive standardized equipment likely contributed to this stagnation in interest within the synthetic organic community. This barrier to entry is magnified by the fact that many redox processes can already be accomplished using simple chemical reagents even if they are less atom-economic. Time has proven that sustainability and economics are not strong enough driving forces for the adoption of electrochemical techniques within the broader community. Indeed, like many synthetic organic chemists that have dabbled in this age-old technique, our first foray into this area was not by choice but rather through sheer necessity. The unique reactivity benefits of this old redox-modulating technique must therefore be highlighted and leveraged in order to draw organic chemists into the field. Enabling new bonds to be forged with higher levels of chemo- and regioselectivity will likely accomplish this goal. In doing so, it is envisioned that widespread adoption of electrochemistry will go beyond supplanting unsustainable reagents in mundane redox reactions to the development of exciting reactivity paradigms that enable heretofore unimagined retrosynthetic pathways. Whereas the rigorous physical organic chemical principles of electroorganic synthesis have been reviewed elsewhere, it is often the case that such summaries leave out the pragmatic aspects of designing, optimizing, and scaling up preparative electrochemical reactions. Taken together, the task of setting up an electrochemical reaction, much less inventing a new one, can be vexing for even seasoned organic chemists. This Account therefore features a unique format that focuses on addressing this exact issue within the context of our own studies. The graphically rich presentation style pinpoints basic concepts, typical challenges, and key insights for those "electro-curious" chemists who seek to rapidly explore the power of electrochemistry in their research.

Fragment-oriented synthesis: β-elaboration of cyclic amine fragments using enecarbamates as platform intermediates

Selective β-sp³ functionalisation of cyclic amines is achieved through enecarbamate formation and photoredox-catalysed derivatisation with functionalised alkyl halides.

Sulfonamide Synthesis through Electrochemical Oxidative Coupling of Amines and Thiols

Sulfonamides are key motifs in pharmaceuticals and agrochemicals, spurring the continuous development of novel and efficient synthetic methods to access these functional groups. Herein, we report an environmentally benign electrochemical method which enables the oxidative coupling between thiols and amines, two readily available and inexpensive commodity chemicals. The transformation is completely driven by electricity, does not require any sacrificial reagent or additional catalysts and can be carried out in only 5 min. Hydrogen is formed as a benign byproduct at the counter electrode. Owing to the mild reaction conditions, the reaction displays a broad substrate scope and functional group compatibility.

Recent synthetic applications of α-amido sulfones as precursors of N-acylimino derivatives

α-Amido sulfones can be directly used as N-acylimine or N-acyliminium ion precursors in several synthetic processes aimed at the preparation of nitrogen containing compounds. This review collects the most relevant and practical utilizations of α-amido sulfones appeared in the literature after 2005.

Electrochemical strategies for C-H functionalization and C-N bond formation

Conventional methods for carrying out carbon-hydrogen functionalization and carbon-nitrogen bond formation are typically conducted at elevated temperatures, and rely on expensive catalysts as well as the use of stoichiometric, and perhaps toxic, oxidants. In this regard, electrochemical synthesis has recently been recognized as a sustainable and scalable strategy for the construction of challenging carbon-carbon and carbon-heteroatom bonds. Here, electrosynthesis has proven to be an environmentally benign, highly effective and versatile platform for achieving a wide range of nonclassical bond disconnections via generation of radical intermediates under mild reaction conditions. This review provides an overview on the use of anodic electrochemical methods for expediting the development of carbon-hydrogen functionalization and carbon-nitrogen bond formation strategies. Emphasis is placed on methodology development and mechanistic insight and aims to provide inspiration for future synthetic applications in the field of electrosynthesis.