Enantioselective Organocatalysis Using SOMO Activation

Nature-Inspired Radical Pyridoxal-Mediated C-C Bond Formation

Pyridoxal-5'-phosphate (PLP) and derivatives of this cofactor enable a plethora of reactions in both enzyme-mediated and free-in-solution transformations. With few exceptions in each category, such chemistry has predominantly involved two-electron processes. This sometimes poses a significant challenge for using PLP to build tetrasubstituted carbon centers, especially when the reaction is reversible. The ability to access radical pathways is paramount to broadening the scope of reactions catalyzed by this coenzyme. In this study, we demonstrate the ability to access a radical PLP-based intermediate and engage this radical intermediate in a number of C-C bond-forming reactions. By selection of an appropriate oxidant, single-electron oxidation of the quinonoid intermediate can be achieved, which can subsequently be applied to C-C bond-forming reactions. Through this radical reaction pathway, we synthesized a series of α-tertiary amino acids and esters to investigate the substrate scope and identify nonproductive reaction pathways. Beyond the amino acid model system, we demonstrate that other classes of amine substrates can be applied in this reaction and that a range of small molecule reagents can serve as coupling partners to the semiquinone radical. We anticipate that this versatile semiquinone radical species will be central to the development of a range of novel reactions.

Visible-Light-Promoted Enantioselective α-Amidation of Aldehydes by Harnessing Organo-Iron Dual Catalysis

The strategic integration of organocatalysis with transition-metal catalysis to achieve otherwise unattainable stereoselective transformations may serve as a powerful synthetic tool. Herein, we present a synthetically versatile α-amidation of aldehydes by leveraging dual iron and chiral enamine catalysis in an enantioselective manner (up to >99:1 er). Experimental and computational studies have led us to propose a new mechanistic platform, wherein visible-light-promoted LMCT generates [Fe(II)Cl3-], which effectively activates dioxazolones to form an iron-acylnitrenoid radical that inserts into chiral enamine intermediates.

Enantioselective SN1-type reaction via electrochemically generated chiral α-Imino carbocation intermediate

Electrochemical reactions via carbocation intermediates remain fundamental transformations that build up molecular functionality and complexity in a sustainable manner. Enantioselective control of such processes is a great challenge in a highly ionic electrolyte solution. Here, we report an anodic generation of chiral α-imino carbocation intermediates by enamine catalysis. The chiral carbocation intermediates can be intercepted by a variety of nucleophiles such as alcohols, water and thiols with high stereoselectivity. The key SN1 step proceeds via a tertiary amine-mediated proton shuttle that facilitates facial selection in reacting with carbocation.

Radical meta-C-H Halogenation of Azines via N-Benzyl Activation Strategy

Regioselective halogenation of six-membered N-heteroarenes is crucial for precise functional derivatization. We present a meta-selective halogenation method for pyridines, quinolines, and isoquinolines via electrophilic halogen radical addition utilizing an N-benzyl activation strategy. This method achieves C3- and C5-dihalogenation in pyridines, C3- and C6-dihalogenation in quinolines, and C3-monohalogenation in isoquinolines. The feasibility and potential applications of this method were validated through scale-up reactions and the bromination of quinoline derivatives with biomolecular fragments.

Navigating the intersection: Diabetes and Alzheimer's intertwined relationship

Alzheimer's disease (AD) and Diabetes mellitus (DM) exhibit comparable pathophysiological pathways. Genetic abnormalities in APP, PS-1, and PS-2 are linked to AD, with diagnostic aid from CSF and blood biomarkers. Insulin dysfunction, termed "type 3 diabetes mellitus" in AD, involves altered insulin signalling and neuronal shrinkage. Insulin influences beta-amyloid metabolism, exacerbating neurotoxicity in AD and amyloid production in DM. Both disorders display impaired glucose transporter expression, hastening cognitive decline. Mitochondrial dysfunction and Toll-like receptor 4-mediated inflammation worsen neurodegeneration in both diseases. ApoE4 raises disease risk, especially when coupled with dyslipidemia common in DM. Targeting shared pathways like insulin-degrading enzyme activation and HSP60 holds promise for therapeutic intervention. Recognizing these interconnected mechanisms underscores the imperative for developing tailored treatments addressing the overlapping pathophysiology of AD and DM, offering potential avenues for more effective management of both conditions.

Radical-Mediated α-tert-Alkylation of Aldehydes by Consecutive 1,4- and 1,3-(Benzo)thiazolyl Migrations

The direct alkylation of the α-position of aldehydes is an effective method for accessing a wide range of structurally diverse aldehydes, yet tert-alkylation has proven to be a challenging task. In this study, we present a novel radical-mediated tert-alkylation approach targeting the α-position of aldehydes, enabling the synthesis of complex aliphatic aldehydes. The transformation is initiated by the interaction between an in situ generated enamine intermediate and α-bromo sulfone, forming an electron donor-acceptor (EDA) complex, followed by consecutive 1,4- and 1,3-functional group migrations. This protocol operates under metal-free and mild photochemical conditions, delivering a broad scope of products and providing new mechanistic insights into radical rearrangement reactions.

Electrochemical Asymmetric Radical Functionalization of Aldehydes Enabled by a Redox Shuttle

Aminocatalysis is a well-established tool that enables the production of enantioenriched compounds under mild conditions. Its versatility is underscored by its seamless integration with various synthetic approaches. While the combination of aminocatalysis with metal catalysis, photochemistry, and stoichiometric oxidants has been extensively explored, its synergy with electrochemical activation remains largely unexplored. Herein, we present the successful merger of electrochemistry and aminocatalysis to perform SOMO-type transformations, expanding the toolkit for asymmetric electrochemical synthesis. The methodology harnesses electricity to drive the oxidation of catalytically generated enamines, which ultimately partake in enantioselective radical processes, leading to α-alkylated aldehydes. Crucially, mechanistic studies highlight how this electrochemical strategy is enabled by the use of a redox shuttle, 4,4'-dimethoxybiphenyl, to prevent catalyst degradation and furnishing the coveted compounds in good yield and high enantioselectivity.

An Artificial Enzyme for Asymmetric Nitrocyclopropanation of α,β-Unsaturated Aldehydes-Design and Evolution

The introduction of an abiological catalytic group into the binding pocket of a protein host allows for the expansion of enzyme chemistries. Here, we report the generation of an artificial enzyme by genetic encoding of a non-canonical amino acid that contains a secondary amine side chain. The non-canonical amino acid and the binding pocket function synergistically to catalyze the asymmetric nitrocyclopropanation of α,β-unsaturated aldehydes by the iminium activation mechanism. The designer enzyme was evolved to an optimal variant that catalyzes the reaction at high conversions with high diastereo- and enantioselectivity. This work demonstrates the application of genetic code expansion in enzyme design and expands the scope of enzyme-catalyzed abiological reactions.

Enantioselective nickel-catalyzed anodic oxidative dienylation and allylation reactions

Precision control of stereochemistry in radical reactions remains a formidable challenge due to the prevalence of incidental racemic background reactions resulting from undirected substrate oxidation in the absence of chiral induction. In this study, we devised an thoughtful approach-electricity-driven asymmetric Lewis acid catalysis-to circumvent this impediment. This methodology facilitates both asymmetric dienylation and allylation reactions, resulting in the formation of all-carbon quaternary stereocenters and demonstrating significant potential in the modular synthesis of functional and chiral benzoxazole-oxazoline (Boox) ligands. Notably, the involvement of chiral Lewis acids in both the electrochemical activation and stereoselectivity-defining radical stages offers innovative departures for designing single electron transfer-based reactions, significantly underscoring the relevance of this approach as a multifaceted and universally applicable strategy for various fields of study, including electrosynthesis, organic chemistry, and drug discovery.

1,5-Hydrogen atom transfer of α-iminyl radical cations: a new platform for relay annulation for pyridine derivatives and axially chiral heterobiaryls

The exploitation of new reactive species and novel transformation modes for their synthetic applications have significantly promoted the development of synthetic organic methodology, drug discovery, and advanced functional materials. α-Iminyl radical cations, a class of distonic ions, exhibit great synthetic potential for the synthesis of valuable molecules. For their generation, radical conjugate addition to α,β-unsaturated iminium ions represents a concise yet highly challenging route, because the in situ generated species are short-lived and highly reactive and they have a high tendency to cause radical elimination (β-scission) to regenerate the more stable iminium ions. Herein, we report a new transformation mode of the α-iminyl radical cation, that is to say, 1,5-hydrogen atom transfer (1,5-HAT). Such a strategy can generate a species bearing multiple reactive sites, which serves as a platform to realize (asymmetric) relay annulations. The present iron/secondary amine synergistic catalysis causes a modular assembly of a broad spectrum of new structurally fused pyridines including axially chiral heterobiaryls, and exhibits good functional group tolerance. A series of mechanistic experiments support the α-iminyl radical cation-induced 1,5-HAT, and the formation of several radical species in the relay annulations. Various synthetic transformations of the reaction products demonstrate the usefulness of this relay annulation protocol for the synthesis of significant molecules.

NHC-Catalyzed Enantioselective Transformations Involving α-Bromoenals

α-Haloenals, especially, α-bromoenals considered as one of the important building blocks in organic synthesis. They can participate in various (3+2)-, (3+3)-, (3+4)-, and (2+4)-annulation reactions with other organic molecules in the presence of an NHC catalyst to produce enantioenriched carbo-, and heterocyclic compounds. Herein, we have described NHC-catalyzed enantioselective transformations of α-bromoenals in the synthesis of various heterocycles, and carbocycles, as well as acyclic organic compounds.

Photocatalyzed Enantioselective Functionalization of C(sp3)-H Bonds

Owing to its diverse activation processes including single-electron transfer (SET) and hydrogen-atom transfer (HAT), visible-light photocatalysis has emerged as a sustainable and efficient platform for organic synthesis. These processes provide a powerful avenue for the direct functionalization of C(sp3)-H bonds under mild conditions. Over the past decade, there have been remarkable advances in the enantioselective functionalization of the C(sp3)-H bond via photocatalysis combined with conventional asymmetric catalysis. Herein, we summarize the advances in asymmetric C(sp3)-H functionalization involving visible-light photocatalysis and discuss two main pathways in this emerging field: (a) SET-driven carbocation intermediates are followed by stereospecific nucleophile attacks; and (b) photodriven alkyl radical intermediates are further enantioselectively captured by (i) chiral π-SOMOphile reagents, (ii) stereoselective transition-metal complexes, and (iii) another distinct stereoscopic radical species. We aim to summarize key advances in reaction design, catalyst development, and mechanistic understanding, to provide new insights into this rapidly evolving area of research.

Site-selective chemical reactions by on-water surface sequential assembly

Controlling site-selectivity and reactivity in chemical reactions continues to be a key challenge in modern synthetic chemistry. Here, we demonstrate the discovery of site-selective chemical reactions on the water surface via a sequential assembly approach. A negatively charged surfactant monolayer on the water surface guides the electrostatically driven, epitaxial, and aligned assembly of reagent amino-substituted porphyrin molecules, resulting in a well-defined J-aggregated structure. This constrained geometry of the porphyrin molecules prompts the subsequent directional alignment of the perylenetetracarboxylic dianhydride reagent, enabling the selective formation of a one-sided imide bond between porphyrin and reagent. Surface-specific in-situ spectroscopies reveal the underlying mechanism of the dynamic interface that promotes multilayer growth of the site-selective imide product. The site-selective reaction on the water surface is further demonstrated by three reversible and irreversible chemical reactions, such as imide-, imine-, and 1, 3-diazole (imidazole)- bonds involving porphyrin molecules. This unique sequential assembly approach enables site-selective chemical reactions that can bring on-water surface synthesis to the forefront of modern organic chemistry.

Electrochemical carbon-carbon coupling with enhanced activity and racemate stereoselectivity by microenvironment regulation

Enzymes are characteristic of catalytic efficiency and specificity by maneuvering multiple components in concert at a confined nanoscale space. However, achieving such a configuration in artificial catalysts remains challenging. Herein, we report a microenvironment regulation strategy by modifying carbon paper with hexadecyltrimethylammonium cations, delivering electrochemical carbon-carbon coupling of benzaldehyde with enhanced activity and racemate stereoselectivity. The modified electrode-electrolyte interface creates an optimal microenvironment for electrocatalysis-it engenders dipolar interaction with the reaction intermediate, giving a 2.2-fold higher reaction rate (from 0.13 to 0.28 mmol h-1 cm-2); Moreover, it repels interfacial water and modulates the conformational specificity of reaction intermediate by facilitating intermolecular hydrogen bonding, affording 2.5-fold higher diastereomeric ratio of racemate to mesomer (from 0.73 to 1.82). We expect that the microenvironment regulation strategy will lead to the advanced design of electrode-electrolyte interface for enhanced activity and (stereo)selectivity that mimics enzymes.

(Photoredox) Organocatalysis in the Emergence of Life: Discovery, Applications, and Molecular Evolution

ConspectusLife as we know it is built on complex and perfectly interlocking processes that have evolved over millions of years through evolutionary optimization processes. The emergence of life from nonliving matter and the evolution of such highly efficient systems therefore constitute an enormous synthetic and systems chemistry challenge. Advances in supramolecular and systems chemistry are opening new perspectives that provide insights into living and self-sustaining reaction networks as precursors for life. However, the ab initio synthesis of such a system requires the possibility of autonomous optimization of catalytic properties and, consequently, of an evolutionary system at the molecular level. In this Account, we present our discovery of the formation of substituted imidazolidine-4-thiones (photoredox) organocatalysts from simple prebiotic building blocks such as aldehydes and ketones under Strecker reaction conditions with ammonia and cyanides in the presence of hydrogen sulfide. The necessary aldehydes are formed from CO2 and hydrogen under prebiotically plausible meteoritic or volcanic iron-particle catalysis in the atmosphere of the early Earth. Remarkably, the investigated imidazolidine-4-thiones undergo spontaneous resolution by conglomerate crystallization, opening a pathway for symmetry breaking, chiral amplification, and enantioselective organocatalysis. These imidazolidine-4-thiones enable α-alkylations of aldehydes and ketones by photoredox organocatalysis. Therefore, these photoredox organocatalysts are able to modify their aldehyde building blocks, which leads in an evolutionary process to mutated second-generation and third-generation catalysts. In our experimental studies, we found that this mutation can occur not only by new formation of the imidazolidine core structure of the catalyst from modified aldehyde building blocks or by continuous supply from a pool of available building blocks but also by a dynamic exchange of the carbonyl moiety in ring position 2 of the imidazolidine moiety. Remarkably, it can be shown that by incorporating aldehyde building blocks from their environment, the imidazolidine-4-thiones are able to change and adapt to altering environmental conditions without undergoing the entire formation process. The selection of the mutated catalysts is then based on the different catalytic activities in the modification of the aldehyde building blocks and on the catalysis of subsequent processes that can lead to the formation of molecular reaction networks as progenitors for cellular processes. We were able to show that these imidazolidine-4-thiones not only enable α-alkylations but also facilitate other important transformations, such as the selective phosphorylation of nucleosides to nucleotides as a key step leading to the oligomerization to RNA and DNA. It can therefore be expected that evolutionary processes have already taken place on a small molecular level and have thus developed chemical tools that change over time, representing a hidden layer on the path to enzymatically catalyzed biochemical processes.

Iridium(III)-Catalyzed Intermolecular C(sp3 )-H Amidation for the Synthesis of Chiral 1,2-Diamines

Chiral 1,2-diamines are privileged scaffolds among bioactive natural products, active pharmaceutical ingredients, ligands for transition-metal-based asymmetric catalysis and organocatalysts. Despite this interest, the construction of chiral 1,2-diamine motifs still remains a challenge. To address this, an iridium(III)-catalyzed intermolecular C(sp3 )-H amidation reaction was developed. This method relies on the design of a new, cheap and cleavable exo-protecting/directing group derived from camphorsulfonic acid, which is directly installed from easily accessible precursors, and furnishes scalemic free 1,2-diamines upon cleavage of both nitrogen substituents. It was found applicable to both α-secondary and α-tertiary-1,2-diamines, for which a two-step protocol involving intermolecular olefin hydroamination and C(sp3 )-H amidation was developed. Kinetic and computational studies provided insights into the observed reactivity difference between pairs of diastereoisomeric substrates.

Synthesis of Allenes by Hydroalkylation of 1,3-Enynes with Ketones Enabled by Cooperative Catalysis

A method for the synthesis of allenes by the addition of ketones to 1,3-enynes by cooperative Pd(0)Senphos/B(C6F5)3/NR3 catalysis is described. A wide range of aryl- and aliphatic ketones undergo addition to various 1,3-enynes in high yields at room temperature. Mechanistic investigations revealed a rate-determining outer-sphere proton transfer mechanism, which was corroborated by DFT calculations.

Strain-Release Photocatalysis

The concept of strain in organic compounds is as old as modern organic chemistry and was initially introduced to justify the synthetic setbacks along the synthesis of small ring systems (pars construens of strain). In the last decades, chemists have developed an arsenal of strain-release reactions (pars destruens of strain) which can generate─with significant driving force─rigid aliphatic systems that can act as three-dimensional alternatives to (hetero)arenes. Photocatalysis added an additional dimension to strain-release processes by leveraging the energy of photons to create chemical complexity under mild conditions. This perspective presents the latest advancements in strain-release photocatalysis─with emphases on mechanisms, catalytic cycles, and current limitations─the unique chemical architectures that can be produced, and possible future directions.

Extended Single-Electron Transfer Model and Dynamically Associated Energy Transfer Event in a Dual-Functional Catalyst System

Organic photocatalysis has been developed flourishingly to rely on bimolecular energy transfer (EnT) or oxidative/reductive electron transfer (ET), promoting a variety of synthetic transformations. However, there are rare examples to merge EnT and ET processes rationally within one chemical system, of which the mechanistic investigation still remains in its infancy. Herein, the first mechanistic illustration and kinetic assessments of the dynamically associated EnT and ET paths were conducted for realizing the C-H functionalization in a cascade photochemical transformation of isomerization and cyclization by using the dual-functional organic photocatalyst of riboflavin. An extended single-electron transfer model of transition-state-coupled dual-nonadiabatic crossings was explored to analyze the dynamic behaviors in the proton transfer-coupled cyclization. This can also be used to clarify the dynamic correlation with the EnT-driven E → Z photoisomerization that has been kinetically evaluated by using Fermi's golden rule with the Dexter model. The present computational results of electron structures and kinetic data contribute to a fundamental basis for understanding the photocatalytic mechanism of the combined operation of EnT and ET strategies, which will guide the design and manipulation for the implementation of multiple activation modes based on a single photosensitizer.

I2-mediated coupling of quinazolinone enamines with 2-aminopyridines: a new strategy to access spiroquinazolinones

Herein, we report a novel, rapid and efficient route to the spiroquinazolinone framework via an umpolung strategy mediated by molecular iodine. A library of functionalized spiroquinazolinone iodide salts was synthesized in moderate to good yields under ambient, metal-free and mild conditions. The current methodology opens up a new efficient and concise strategy for the construction of spiroquinazolinones.

Organocatalytic synthesis of β-enaminyl radicals as single-electron donors for phenyliodine(III) dicarboxylates: direct one-pot alkylation-aminoxidation of styrenes

A direct one-pot alkylation-aminoxidation of styrene derivatives was achieved using in situ-generated alkyl and N-oxyl radicals. The corresponding O-alkylated hydroxylamine derivatives were obtained in moderate to good yields. The reaction features the generation of the alkyl radicals from phenyliodine(III) dicarboxylates via an organocatalytic process, the use of phenyliodine(III) dicarboxylates as the source of the alkyl radicals and oxidants for the generation of N-oxyl radicals, and the first generation of the β-enaminyl radicals via a HAT process and their use as single-electron donors.

Chiral Frustrated Lewis Pair@Metal-Organic Framework as a New Platform for Heterogeneous Asymmetric Hydrogenation

Asymmetric hydrogenation, a seminal strategy for the synthesis of chiral molecules, remains largely unmet in terms of activation by non-metal sites of heterogeneous catalysts. Herein, as demonstrated by combined computational and experimental studies, we present a general strategy for integrating rationally designed molecular chiral frustrated Lewis pair (CFLP) with porous metal-organic framework (MOF) to construct the catalyst CFLP@MOF that can efficiently promote the asymmetric hydrogenation in a heterogeneous manner, which for the first time extends the concept of chiral frustrated Lewis pair from homogeneous system to heterogeneous catalysis. Significantly, the developed CFLP@MOF, inherits the merits of both homogeneous and heterogeneous catalysts, with high activity/enantio-selectivity and excellent recyclability/regenerability. Our work not only advances CFLP@MOF as a new platform for heterogeneous asymmetric hydrogenation, but also opens a new avenue for the design and preparation of advanced catalysts for asymmetric catalysis.

Strategies for the synthesis of Stemona alkaloids: an update

Covering: 2009 to 2022The Stemona alkaloids, which are found in plant species from the family Stemonaceae, represent a tremendously large and structurally-diverse family of natural products. This review presents and discusses a selection of case studies, grouped by alkaloid class, that showcase the key strategies and overall progress that has been made in the synthesis of Stemona alkaloids and related compounds since 2009. Structural reassignments that have been reported over this period are also identified where necessary.

Strategies toward the Difunctionalizations of Enamide Derivatives for Synthesizing α,β-Substituted Amines

Enamide and enecarbamate derivatives containing a nucleophilic center at the β-position from their nitrogen atom as well as a latent electrophilic site at their α-position are interesting motifs in organic chemistry. This dual reactivity─analogous that of the enamines─enables difunctionalization and increased structural complexity. Furthermore, an electron-withdrawing group on nitrogen drastically increases their stability. In that respect, enamides and enecarbamates are excellent partners for multicomponent transformations, and our research primarily focuses on these compounds in particular.Difunctionalization generally occurs through the nucleophilic addition of the enecarbamate on an electrophile to form iminium, which can subsequently react with a nucleophilic species. Although potent, such an approach is highly challenging due to the low stability of the intermediate iminium, leading to undesired hydrolysis or oligomerization. Epimerization, competitivity, and compatibility issues between the reaction partners are additional hindrances to developing these methodologies. To overcome these limitations, we described many complementary strategies.To control the enantioselectivity of these transformations, chiral phosphoric acids were found to be particularly well-suited to activate multiple reactants due to the formation of a hydrogen bonds network, allowing for an organized transition state in a chiral pocket. Interestingly, when deprotonated as phosphates, they can also play the role of ligands for Lewis acidic metals.To avoid iminium oligomerization, we successfully used stabilized α-arylated enamides. However, this approach was restricted to a simple nucleophilic addition at the β-position. To achieve the difunctionalizations of α-unsubstituted derivatives, we explored reversibly linked nucleophile and electrophile to address their compatibility problem. Alternatively, we devised a sequential methodology for resolving the stability issue of the N-acyl iminium based on its intermediate trapping using a temporary nucleophile (alcohol or thiol). Interestingly, the trapping agent could further be displaced by the desired final α-substituent under Lewis acidic or photocatalytic activation. This led us to design new chiral and bifunctional phosphoric acid catalysts bearing chromophores to merge asymmetric organocatalysis and photochemistry.These photocatalysis studies incited us to focus on radical processes to manage original functionalizations that would not be feasible otherwise. β-Alkylation and β-trifluoromethylation of enecarbamates via visible-light-promoted atom transfer radical additions were successfully performed. As β-allylations remained unattainable with the precedent methods, we eventually turned our attention to cerium(IV)-mediated oxidative single electron transfers. It allowed for singly occupied molecular orbital activation of these substrates to elicit their umpolung reactivity.Thus, the functionalization of enecarbamate derivatives appears as a valid synthetic strategy for obtaining important building blocks for agrochemical, pharmaceutical, and cosmetic industries, including diamines, haloamines, aminotryptamines, and less accessible trifluoromethylated or allylic compounds.

Nickel-catalyzed switchable asymmetric electrochemical functionalization of alkenes

The development of general electrocatalytic methods for the diversity-oriented regio- and stereoselective functionalization of alkenes remains a challenge in organic synthesis. We present a switchable electrocatalytic method based on anodic oxidative activation for the controlled liberation of chiral α-keto radical species toward stereoselective organic transformations. Electrogenerated α-keto radical species capture alkene partners, allowing switchable intermolecular alkene difunctionalization and alkenylation in a highly stereoselective manner. In addition to acting as proton donors to facilitate H₂ evolution at the cathode, the unique properties of alcohol additives play an important role in determining the distinct outcomes for alkene functionalization under electrocatalytic conditions.

α-Thianthrenium Carbonyl Species: The Equivalent of an α-Carbonyl Carbocation

Here we report an α-thianthrenium carbonyl species, as the equivalent of an α-carbonyl carbocation, which is generated by the radical conjugate addition of a trifluoromethyl thianthrenium salt to Michael acceptors. The reactivity allows for the synthesis of C^α -tetrasubstituted α- and β-amino acid analogues via a Ritter reaction by addition of acetonitrile. Addition of hydroxide, methoxide, and even fluoride can afford α-heteroatom substituted α-phenylpropanoates.

Synergistic Strategies in Aminocatalysis

Synergistic catalysis offers the unique possibility of simultaneous activation of both the nucleophile and the electrophile in a reaction. A requirement for this strategy is the stability of the active species towards the reaction conditions and the two concerted catalytic cycles. Since the beginning of the century, aminocatalysis has been established as a platform for the stereoselective activation of carbonyl compounds through HOMO-raising or LUMO-lowering. The burgeoning era of aminocatalysis has been driven by a deep understanding of these activation and stereoinduction modes, thanks to the introduction of versatile and privileged chiral amines. The aim of this review is to cover recent developments in synergistic strategies involving aminocatalysis in combination with organo-, metal-, photo-, and electro-catalysis, focusing on the evolution of privileged aminocatalysts architectures.

KOtBu-catalysed α-homoallylic alkylation of acyclic amides with 1-aryl-1,3-dienes

Herein, we report a KOtBu-catalysed α-homoallylic alkylation of acyclic amides with 1-aryl-1,3-dienes. With this transition metal-free and atom-economic protocol, a series of α-homoallylic alkylated acyclic amides were synthesized in good to excellent yields. This transformation is proposed to proceed through a cation-π interaction-based C-C bond formation from the in situ-generated potassium enolate with the diene unit.

Unlocking New Reactivities in Enzymes by Iminium Catalysis

The application of biocatalysis in conquering challenging synthesis requires the constant input of new enzymes. Developing novel biocatalysts by absorbing catalysis modes from synthetic chemistry has yielded fruitful new-to-nature enzymes. Organocatalysis was originally bio-inspired and has become the third pillar of asymmetric catalysis. Transferring organocatalytic reactions back to enzyme platforms is a promising approach for biocatalyst creation. Herein, we summarize recent developments in the design of novel biocatalysts that adopt iminium catalysis, a fundamental branch in organocatalysis. By repurposing existing enzymes or constructing artificial enzymes, various biocatalysts for iminium catalysis have been created and optimized via protein engineering to promote valuable abiological transformations. Recent advances in iminium biocatalysis illustrate the power of combining chemomimetic biocatalyst design and directed evolution to generate useful new-to-nature enzymes.

A Tandem Dehydrogenation-Driven Cross-Coupling between Cyclohexanones and Primary Amines for Construction of Benzoxazoles

Herein, we report a transition metal-free, operationally simple, general method for straightforward syntheses of 2-substituted benzoxazoles from readily available cyclohexanones and aliphatic primary amines by an imine α-oxygenation-initiated cascade reaction sequence. The key to achieving high selectivity and excellent functional-group tolerance is the use of TEMPO as a mild oxidant that selectively oxidizes the reaction intermediates through its multiple reactivity modes, thus facilitating the individual steps to proceed in succession. More than 70 substrate combinations are disclosed, demonstrating the reliability of this protocol to synthesize structurally diverse products, including marketed drugs, drug candidate, and natural products that are unattainable by the existing methods.

One-Electron Oxidation of Geranyl Acetone Derivatives Using Ceric(IV) Ammonium Nitrate and Manganese(III) Acetate: Carbon–Carbon Bond Formation

Oxidation of geranyl acetone derivatives with ceric ammonium nitrate (CAN) and Mn(OAc)3 afforded tricyclic and bicyclic compounds as well as hydroxy and nitro compounds as a result of one-electron oxidation followed by carbon–carbon bond formation. This is the first example of radical cyclization (formed by one-electron oxidation) of geranyl acetone derivative 1 and its isomer 4 to give tri- and bicyclic products with carbon–carbon bond formation.

Asymmetric C-H Dehydrogenative Allylic Alkylation by Ternary Photoredox-Cobalt-Chiral Primary Amine Catalysis under Visible Light

We report herein an asymmetric C-H dehydrogenative allylic alkylation by a synergistic catalytic system involving a chiral primary amine, a photoredox catalyst, and a cobaloxime cocatalyst. The ternary catalytic system enables the coupling of β-ketocarbonyls and olefins with good yields and high enantioselectivities. Mechanism studies disclosed a cooperative radical addition process with a chiral α-imino radical and Co(II)-metalloradical wherein the chiral primary aminocatalyst and the cobaloxime catalyst work in concert to control the stereoinduction.

Organocatalyzed Cross-Nucleophile Couplings: Umpolung of Catalytic Enamines

ConspectusThe concept of umpolung, or polarity reversal, introduced by Seebach and Corey nearly half a century ago, ushered a new paradigm into synthetic chemistry. Novel connections were able to be forged among functional groups that were typically inaccessible. Conceptually, an umpolung reaction is identified only upon retrosynthetic analysis. Stoichiometric examples have served as a platform to develop and refine elegant methodologies into catalytic processes. The advent of these unconventional arrangements of canonical synthons into new points of diversity has expanded the repertoire of the synthetic toolbox. Within this context, asymmetric organocatalyzed methodologies remain rare, and there are even fewer aminocatalyzed variants.Recent years have witnessed a renaissance in α-functionalizations of aldehydes, specifically in the context of oxidative umpolung strategies. Unlike previous open-shell approaches, application of a quinone-based oxidant in conjunction with an aminocatalyst leads to a discrete, substitutionally labile quinone adduct. These have proven to be valuable building blocks toward polar reactivity─auguring the advent of new avenues to construct tetrasubstituted tertiary stereocenters through the application of conventional nucleophiles to form C-C, C-N, C-O, and C-S bonds through an organocatalyzed cross-nucleophile coupling (organo-CNC) reaction. The resulting nonepimerizable stereocenter demonstrates high optical fidelity and provides a significant advancement in many applications that suffer from racemization, such as in vivo studies.This strategy harnesses a trifunctional aminocatalyst to promote an unusual S_N2 reaction at a highly congested center. The selection of the quinone oxidant and nucleophile converges to a continuum of reactivity ranging from enantioselective oxidation to stereoselective substitution. A remarkable aspect of these developments is the identification of an asymmetric S_N2 dynamic kinetic resolution (S_N2-DKR) manifold. These organo-CNC reactions are highly modular and demonstrate complete stereocontrol from the catalyst with minimal influence from incoming chiral nucleophiles. Leveraging this facet, these technologies have been extended to peptidic bioconjugations bearing bio-orthogonoal linker molecules.This Account aims to highlight the progress, from an internal perspective, toward directing the initial result into established methodologies. Within this construct, the underlying principles of each reaction will be disseminated with specific content on inherent challenges and opportunity. Combined, these will serve as an instructive tool to stimulate applications in cross-disciplinary interfaces.

The road to fully programmable protein catalysis

The ability to design efficient enzymes from scratch would have a profound effect on chemistry, biotechnology and medicine. Rapid progress in protein engineering over the past decade makes us optimistic that this ambition is within reach. The development of artificial enzymes containing metal cofactors and noncanonical organocatalytic groups shows how protein structure can be optimized to harness the reactivity of nonproteinogenic elements. In parallel, computational methods have been used to design protein catalysts for diverse reactions on the basis of fundamental principles of transition state stabilization. Although the activities of designed catalysts have been quite low, extensive laboratory evolution has been used to generate efficient enzymes. Structural analysis of these systems has revealed the high degree of precision that will be needed to design catalysts with greater activity. To this end, emerging protein design methods, including deep learning, hold particular promise for improving model accuracy. Here we take stock of key developments in the field and highlight new opportunities for innovation that should allow us to transition beyond the current state of the art and enable the robust design of biocatalysts to address societal needs.

Heterologous Expression of Arabidopsis AtARA6 in Soybean Enhances Salt Tolerance

Salt damage is an important abiotic stress affecting the agronomic traits of soybean. Soybeans rapidly sense and transmit adverse signals when salt-damaged, inducing a set of response mechanisms to resist salt stress. AtARA6 encodes a small GTPase, which plays an important role in Arabidopsis vesicle transport and salt tolerance. In this study, we transformed the Arabidopsis gene AtARA6 into the cultivated soybean Shen Nong 9 (SN9). To investigate the salt tolerance pathways affected by AtARA6 in soybean, we performed transcriptome sequencing using transgenic soybean and wild-type (SN9) under salt treatment and water treatment. Our results suggest that AtARA6 is involved in the regulation of soybean SNARE complexes in the vesicle transport pathway, which may directly strengthen salt tolerance. In addition, we comprehensively analyzed the RNA-seq data of transgenic soybean and SN9 under different treatments and obtained 935 DEGs. GO analysis showed that these DEGs were significantly enriched in transcription factor activity, sequence-specific DNA binding, and the inositol catabolic process. Three salt-responsive negative regulator transcription factors, namely MYC2, WRKY6, and WRKY86, were found to be significantly downregulated after salt treatment in transgenic soybeans. Moreover, four genes encoding inositol oxygenase were significantly enriched in the inositol catabolic process pathway, which could improve the salt tolerance of transgenic soybeans by reducing their reactive oxygen species content. These are unique salt tolerance effects produced by transgenic soybeans. Our results provide basic insights into the function of AtARA6 in soybeans and its role in abiotic stress processes in plants.

Electrochemical Enantioselective Nucleophilic α-C(sp3)-H Alkenylation of 2-Acyl Imidazoles

Merging electrochemistry with asymmetric catalysis promises to provide an environmentally friendly and efficient strategy for the construction of nonracemic chiral molecules. However, in practice, significant challenges arise from the instability or incompatibility of the chiral catalysts under the electrochemical conditions at the interface of electrode and solution. Herein, we report a catalytic asymmetric indirect electrolysis employing the combination of a redox mediator and a chiral-at-rhodium Lewis acid, which achieves a previously elusive enantioselective nucleophilic α-C(sp³)-H alkenylation of ketones. Specifically, 2-acyl imidazoles react with potassium alkenyl trifluoroborates in high yields (up to 94%) and with exceptional enantioselectivities (27 examples with ≥99% ee) without the need for any additional stoichiometric oxidants (overall 40 examples). The new indirect electrosynthesis can be scaled to gram quantities and was applied to the straightforward synthesis of intermediates of the natural product cryptophycin A and a cathepsin K inhibitor.

Chiral stationary phases and applications in gas chromatography

Chiral compounds are ubiquitous in nature and play a pivotal role in biochemical processes, in chiroptical materials and applications, and as chiral drugs. The analysis and determination of the enantiomeric ratio (er) of chiral compounds is of enormous scientific, industrial, and economic importance. Chiral separation techniques and methods have become indispensable tools to separate chiral compounds into their enantiomers on an analytical as well on a preparative level to obtain enantiopure compounds. Chiral gas chromatography and high-performance liquid chromatography have paved the way and fostered several research areas, that is, asymmetric synthesis and catalysis in organic, medicinal, pharmaceutical, and supramolecular chemistry. The development of highly enantioselective chiral stationary phases was essential. In particular, the elucidation and understanding of the underlying enantioselective supramolecular separation mechanisms led to the design of new chiral stationary phases. This review article focuses on the development of chiral stationary phases for gas chromatography. The fundamental mechanisms of the recognition and separation of enantiomers and the selectors and chiral stationary phases used in chiral gas chromatography are presented. An overview over syntheses and applications of these chiral stationary phases is presented as a practical guidance for enantioselective separation of chiral compound classes and substances by gas chromatography.

Asymmetric Organocatalytic Syntheses of Bioactive Compounds

Background:

The total syntheses of complex natural products have evolved to include new methodologies to save time, simplifying the form to achieve these natural compounds.

Objective:

In this review, we have described the asymmetric synthesis of different natural products and biologically active compounds of the last ten years until the current day.

Results:

An asymmetric organocatalytic reaction is a key to generate stereoselectively the main structure with the required stereochemistry.

Conclusion:

Even more remarkable, the organocatalytic cascade reactions, which are carried out with high stereoselectivity, as well as a possible approximation of the organocatalysts activation with sub-strates are also described.

Photoredox-Catalyzed C-H Functionalization Reactions

The fields of C-H functionalization and photoredox catalysis have garnered enormous interest and utility in the past several decades. Many different scientific disciplines have relied on C-H functionalization and photoredox strategies including natural product synthesis, drug discovery, radiolabeling, bioconjugation, materials, and fine chemical synthesis. In this Review, we highlight the use of photoredox catalysis in C-H functionalization reactions. We separate the review into inorganic/organometallic photoredox catalysts and organic-based photoredox catalytic systems. Further subdivision by reaction class─either sp2 or sp3 C-H functionalization─lends perspective and tactical strategies for use of these methods in synthetic applications.