Crafting mono- and novel bis-methylated pyrroloquinoxaline derivatives from a shared precursor and its application in the total synthesis of marinoquinoline A

The synthesis of mono- and novel bis-methylated pyrrolo[1,2-a]quinoxalines through the addition of unstable methyl radicals to aryl isocyanides is described contingent upon the reaction conditions employed. The strategy has been effectively employed in the total synthesis of the natural product marinoquinoline A.

One-Pot Formal Carboradiofluorination of Alkenes: A Toolkit for Positron Emission Tomography Imaging Probe Development

We report the first one-pot formal alkene carboradiofluorination reaction employing easily accessible alkenes as both prosthetic group precursors and coupling partners. The methodology features rapid sequential Markovnikov-selective iodofluorination and photoinduced Pd(0/I/II)-catalyzed alkyl Heck reaction as a mild and robust fluorine-18 (18F) radiochemical approach for positron emission tomography (PET) imaging probe development. A new class of prosthetic groups for PET imaging probe synthesis was isolated as iodofluorinated intermediates in moderate to excellent yields. The one-pot formal alkenylfluorination reaction was carried out to produce over 30 analogues of a wide range of bioactive molecules. Further application of the Pd(0/I/II) manifold in PET probe development was illustrated by the direct carbo(radio)fluorination of electron-rich alkenes. The methods were successfully translated to radiolabel a broad scope of medicinally relevant small molecules in generally good radiochemical conversion. The protocol was further optimized to accommodate no-carrier-added conditions with similar efficiency for future (pre)clinical translation. Moreover, the radiosynthesis of prosthetic groups was automated in a radiochemistry module to facilitate its practical use in multistep radiochemical reactions.

The Magic Methyl and Its Tricks in Drug Discovery and Development

One of the key scientific aspects of small-molecule drug discovery and development is the analysis of the relationship between its chemical structure and biological activity. Understanding the effects that lead to significant changes in biological activity is of paramount importance for the rational design and optimization of bioactive molecules. The "methylation effect", or the "magic methyl" effect, is a factor that stands out due to the number of examples that demonstrate profound changes in either pharmacodynamic or pharmacokinetic properties. In many cases, this has been carried out rationally, but in others it has been the product of serendipitous observations. This paper summarizes recent examples that provide an overview of the current state of the art and contribute to a better understanding of the methylation effect in bioactive small-molecule drug candidates.

Robust Synthesis of Terpenoid Scaffolds under Mn(I)-Catalysis

The 6/6/5-fused tricyclic scaffold is a central feature of structurally complex terpenoid natural products. A step-economical cascade transformation that leads to a complex molecular skeleton is regarded as a sustainable methodology. Therefore, we report the first Mn(I)-catalyzed C(sp²)-H chemoselective in situ dienylation and diastereoselective intramolecular Diels-Alder reaction using iso-pentadienyl carbonate to access 6/6/5-fused tricyclic scaffolds. To the best of our knowledge, there is no such report thus far to utilize iso-pentadienyl carbonate as a substrate in C-H activation catalysis. Extensive mechanistic studies, such as the isolation of catalytically active organo-manganese(I) complexes, 1,3-dienyl-intermediates, and isotopic labeling experiments have supported the proposed mechanism of this cascade reaction.

Cu(II)-Catalyzed Cascade of N-Phenyl-o-phenylenediamine with Benzaldehyde: One-Step Direct Construction of 2-(1-Phenyl-1H-benzo[d]imidazol-2-yl)phenols

A practical protocol for the construction of hydroxylated 2-(1-phenyl-1H-benzo[d]imidazol-2-yl)phenols (PBIs) from N-phenyl-o-phenylenediamine with benzaldehydes was developed. The cascade reaction was enabled by heating a mixture of the two substrates in the presence of air as an oxidant and anhydrous Cu(OAc)₂ as a catalyst in dimethyl sulfoxide, and a diverse series of PBIs were synthesized in moderate to good yields (69-81%). Furthermore, the synthesis of the PBIs was enabled via a one-pot cascade reaction that proceeded through subsequent dehydration condensation, intramolecular cyclization, and aromatic C-H hydroxylation. This protocol can be used for the synthesis of hydroxylated PBI via a one-pot annulation C-H hydroxylation reaction rather than through a series of multistep reactions, which provides the possibility of further modification.

Transition-Metal-Catalyzed C-H Bond Activation for the Formation of C-C Bonds in Complex Molecules

Site-predictable and chemoselective C-H bond functionalization reactions offer synthetically powerful strategies for the step-economic diversification of both feedstock and fine chemicals. Many transition-metal-catalyzed methods have emerged for the selective activation and functionalization of C-H bonds. However, challenges of regio- and chemoselectivity have emerged with application to highly complex molecules bearing significant functional group density and diversity. As molecular complexity increases within molecular structures the risks of catalyst intolerance and limited applicability grow with the number of functional groups and potentially Lewis basic heteroatoms. Given the abundance of C-H bonds within highly complex and already diversified molecules such as pharmaceuticals, natural products, and materials, design and selection of reaction conditions and tolerant catalysts has proved critical for successful direct functionalization. As such, innovations within transition-metal-catalyzed C-H bond functionalization for the direct formation of carbon-carbon bonds have been discovered and developed to overcome these challenges and limitations. This review highlights progress made for the direct metal-catalyzed C-C bond forming reactions including alkylation, methylation, arylation, and olefination of C-H bonds within complex targets.

An update on late-stage functionalization in today's drug discovery

INTRODUCTION

Late-stage functionalization (LSF) allows for the introduction of new chemical groups toward the end of a synthetic sequence, which means new molecules can be rapidly accessed without laborious de novo chemical synthesis. Over the last decade, medicinal chemists have begun to implement LSF strategies into their drug discovery programs, affording benefits such as efficient access to diverse libraries to explore structure-activity relationships and the improvement of physicochemical and pharmacokinetic properties.

AREAS COVERED

An overview of the key advancements in LSF methodology development from 2019 to 2022 and their applicability to drug discovery is provided. In addition, several examples from both academia and industry where LSF methodologies have been applied by medicinal chemists to their drug discovery programs are presented.

EXPERT OPINION

Utilization of LSF by medicinal chemists is on the rise, both in academia and in industry. The maturation of the LSF field to produce methodologies bearing increased regioselectivity, scope, and functional group tolerance is envisaged to narrow the gap between methodology development and medicinal chemistry research. The authors predict that the sheer versatility of these techniques in facilitating challenging chemical transformations of bioactive molecules will continue to increase the efficiency of the drug discovery process.

Iridium catalysed C2 site-selective methylation of indoles using a pivaloyl directing group through weak chelation-assistance

Here we present an iridium catalysed C2-selective methylation of indoles using methyltrifluoroborate as a source of methyl group. The iridium catalyst selectively discriminates the indole C2 and C4 C-H bonds by coordination with a pivaloyl directing group.

"Magic Chloro": Profound Effects of the Chlorine Atom in Drug Discovery

Chlorine is one of the most common atoms present in small-molecule drugs beyond carbon, hydrogen, nitrogen, and oxygen. There are currently more than 250 FDA-approved chlorine-containing drugs, yet the beneficial effect of the chloro substituent has not yet been reviewed. The seemingly simple substitution of a hydrogen atom (R = H) with a chlorine atom (R = Cl) can result in remarkable improvements in potency of up to 100,000-fold and can lead to profound effects on pharmacokinetic parameters including clearance, half-life, and drug exposure in vivo. Following the literature terminology of the "magic methyl effect" in drugs, the term "magic chloro effect" has been coined herein. Although reports of 500-fold or 1000-fold potency improvements are often serendipitous discoveries that can be considered "magical" rather than planned, hypotheses made to explain the magic chloro effect can lead to lessons that accelerate the cycle of drug discovery.

Fe-Catalyzed Aliphatic C-H Methylation of Glycine Derivatives and Peptides

Direct and selective C-H methylation is a powerful tool with which to install methyl groups into organic molecules, and is particularly useful in pharmaceutical chemistry. However, practical methods for such modification of biologically interesting targets have been rarely developed. We here report an iron-catalyzed C(sp³ )-H methylation reaction of glycine derivatives, peptides and drug-like molecules in an alcohol in the presence of di-tert-butyl peroxide. A readily available iron catalyst plays multiple roles in the transformation, which accelerates oxidation of C-N bonds to C=N double bonds, activates imine intermediates as Lewis acids by bidentate chelation, and at the same time facilitates cleavage of the peroxide to generate methyl radicals. A variety of methylated N-aryl glycine derivatives and peptides were obtained in good yield and with excellent chemo- and site-selectivity. This reaction is scalable, easily managed, and can be completed within 1-2 h. It features an economic, bio-friendly catalyst, a green solvent and low toxic reagents, and will provide effective access to precise C-H modification of biomolecules and natural products.

Three Component syn-1,2-Arylmethylation of Internal Alkynes

A Pd-catalyzed three-component syn-1,2-arylmethylation of internal alkynes (ynamides/yne-acetates/alkynes) is described. The readily available and bench stable coupling partners iodo-arenes, and methyl boronic acid are successfully used in this coupling strategy to access the methyl-containing tetra-substituted olefins; the scope is broad showing excellent functional-group tolerance. Notably, the transformation is regio- as well as stereoselective. The biologically relevant motifs (BRM) bearing iodo-arenes and ynamides are also used for the late-stage syn-1,2-arylmethylation of alkynes. Aryl-alkylation, aryl-trideuteriomethylation, alkynyl-methylation, and alkenyl-methylation of ynamides are also presented. The Me-substituted alkenes are further transformed into synthetically important β-amino-indenones and α-fluoro-α'-methyl ketones.

Functionalized Cycloolefin Ligand as a Solution to Ortho-Constraint in the Catellani-Type Reaction

The Catellani reaction, i.e., the Pd/norbornene (NBE) catalysis, has been evolved into a versatile approach to multisubstituted arenes via the ortho-functionalization/ipso-termination process of a haloarene. Despite significant advances over the past 25 years, this reaction still suffered from an intrinsic limitation in the substitution pattern of haloarene, referred to as "ortho-constraint". When an ortho substituent is absent, the substrate often fails to undergo an effective mono ortho-functionalization process, and either ortho-difunctionalization products or NBE-embedded byproducts predominate. To tackle this challenge, structurally modified NBEs (smNBEs) have been developed, which were proved effective for the mono ortho-aminative, -acylative, and -arylative Catellani reactions of ortho-unsubstituted haloarenes. However, this strategy is incompetent for solving the ortho-constraint in Catellani reactions with ortho-alkylation, and to date there lacks a general solution to this challenging but synthetically useful transformation. Recently, our group developed the Pd/olefin catalysis, in which an unstrained cycloolefin ligand served as a covalent catalytic module to enable the ortho-alkylative Catellani reaction without NBE. In this work, we show that this chemistry could afford a new solution to ortho-constraint in the Catellani reaction. A functionalized cycloolefin ligand bearing an amide group as the internal base was designed, which allowed for mono ortho-alkylative Catellani reaction of iodoarenes suffering from ortho-constraint before. Mechanistic study revealed that this ligand is capable of both accelerating the C-H activation and inhibiting side reactions, which accounts for its superior performance. The present work showcased the uniqueness of the Pd/olefin catalysis as well as the power of rational ligand design in metal catalysis.

Late-Stage C(sp3)-H Methylation of Drug Molecules

Methyl groups are well understood to play a critical role in pharmaceutical molecules, especially those bearing saturated heterocyclic cores. Accordingly, methods that install methyl groups onto complex molecules are highly coveted. Late-stage C-H functionalization is a particularly attractive approach, allowing chemists to bypass lengthy syntheses and facilitating the expedited synthesis of drug analogues. Herein, we disclose the direct introduction of methyl groups via C(sp3)-H functionalization of a broad array of saturated heterocycles, enabled by the merger of decatungstate photocatalysis and a unique nickel-mediated SH2 bond formation. To further demonstrate its synthetic utility as a tool for late-stage functionalization, this method was applied to a range of drug molecules en route to an array of methylated drug analogues.

Metal-Free Carbon-Based Covalent Organic Frameworks with Heteroatom-Free Units Boost Efficient Oxygen Reduction

Accurate identification of carbon-based metal-free electrocatalyst (CMFE) activity and enhancing their catalytic efficiency for O₂ conversion is an urgent and challenging task. This study reports a promising strategy to simultaneously develop a series of covalent organic frameworks (COFs) with well-defined heterocyclic-free biphenyl or fluorenyl units. Unlike heteroatom doping, the developed method not only supplies methyl-induced molecular configuration to promote activity, but also provides a direct opportunity to identify heteroatom-free carbon active centers. The introduction of methyl groups (MGs) with reversible valence bonds into a pristine biphenyl-based COF results in an excellent performance with a half-wave potential of 0.74 V versus the reversible hydrogen electrode (RHE), which is among the highest values for CMFE-COFs as oxygen reduction reaction (ORR) electrocatalysts. Combined with in situ Raman spectra and theoretical calculations, the MG-bound skeleton (DAF-COF) is found to produce ortho activation, confirming the ortho carbon (site-5) adjacent to MGs as active centers. This may be attributed to the opening and binding of MGs, which effectively regulate the molecular configuration and charge redistribution, as well as improve charge transfer and reduce the energy barrier. This study provides insight into the design of highly efficient metal-free organic electrocatalysts via the regulation of valence bonds.

Photocatalytic Late-Stage C-H Functionalization

The emergence of modern photocatalysis, characterized by mildness and selectivity, has significantly spurred innovative late-stage C-H functionalization approaches that make use of low energy photons as a controllable energy source. Compared to traditional late-stage functionalization strategies, photocatalysis paves the way toward complementary and/or previously unattainable regio- and chemoselectivities. Merging the compelling benefits of photocatalysis with the late-stage functionalization workflow offers a potentially unmatched arsenal to tackle drug development campaigns and beyond. This Review highlights the photocatalytic late-stage C-H functionalization strategies of small-molecule drugs, agrochemicals, and natural products, classified according to the targeted C-H bond and the newly formed one. Emphasis is devoted to identifying, describing, and comparing the main mechanistic scenarios. The Review draws a critical comparison between established ionic chemistry and photocatalyzed radical-based manifolds. The Review aims to establish the current state-of-the-art and illustrate the key unsolved challenges to be addressed in the future. The authors aim to introduce the general readership to the main approaches toward photocatalytic late-stage C-H functionalization, and specialist practitioners to the critical evaluation of the current methodologies, potential for improvement, and future uncharted directions.

Late-stage functionalization of 5-nitrofurans derivatives and their antibacterial activities

Structure modification of drugs is a reliable way to optimize lead compounds, among which the most striking and direct method is late-stage functionalization (LSF). Here, we employed the Cu-catalyzed C-H LSF to modify 5-nitrofuran drugs. A series of modifications have been carried out including hydroxylation, methylation, azidination, cyanation, arylation, etc. Antibacterial activities of all compounds in vitro were measured. The results showed that compound 1 and compound 18 were the most active among all compounds. Meanwhile, the cell cytotoxicity assays of potent compounds 1, 3, 4, 5 & 18 and the parent drug FZD were conducted.

Structural Alterations of the "Address" Moiety of NAN Leading to the Discovery of a Novel Opioid Receptor Modulator with Reduced hERG Toxicity

The search for selective opioid ligands with desired pharmacological potency and improved safety profile has always been an area of interest. Our previous effort yielded a potent opioid modulator, NAN, a 6α-N-7'-indolyl-substituted naltrexamine derivative, which exhibited promising pharmacological activities both in vitro and in vivo. However, significant human ether-a-go-go-related gene (hERG) liability limited its further development. Therefore, a systematic structural modification on NAN was conducted in order to alleviate hERG toxicity while preserving pharmacological properties, which led to the discovery of 2'-methylindolyl derivative compound 21. Compared to NAN, compound 21 manifested overall improved pharmacological profiles. Follow-up hERG channel inhibition evaluation revealed a seven-fold decreased potency of compound 21 compared to NAN. Furthermore, several fundamental drug-like property evaluations suggested a reasonable ADME profile of 21. Collectively, compound 21 appeared to be a promising opioid modulator for further development as a novel therapeutic agent toward opioid use disorder treatments.

Ruthenium-Catalyzed Monoselective C-H Methylation and d 3-Methylation of Arenes

Site-selective installation of C-Me bonds remains a powerful and sought-after tool to alter the chemical and pharmacological properties of a molecule. Direct C-H functionalization provides an attractive means of achieving this transformation. Such protocols, however, typically utilize harsh conditions and hazardous methylating agents with poor applicability toward late-stage functionalization. Furthermore, highly monoselective methylation protocols remain scarce. Herein, we report an efficient monoselective, directed ortho-methylation of arenes using N,N,N-trimethylanilinium salts as noncarcinogenic, bench-stable methylating agents. We extend this protocol to d ₃-methylation in addition to the late-stage functionalization of pharmaceutically active compounds. Detailed kinetic studies indicate the rate-limiting in situ formation of MeI is integral to the observed reactivity.

Theoretical Study for Evaluating and Discovering Organic Hydride Compounds as Potential Novel Methylation Reagents

Methylation reaction is a fundamental chemical reaction that plays an important role in the modification of drug molecules, DNA, as well as proteins. This work focuses on seeking potential novel methylation reagents through a systematic investigation of the thermodynamics and reactivity of methyl-substituted organic hydride radical cations (XH^•+s). In this work, 45 classical and important XH^•+s were designed to investigate the relationship between their structure and reactivity, to find excellent or potential methylation reagents. The Gibbs free energy and activation free energy of XH^•+ to release the methyl radical in MeCN at 298.15 and 355 K are calculated with the density functional theory (DFT) method to quantitatively measure the reactivity of XH^•+ as a methylation reagent in this work. The relationships between structures and reactivities on XH^•+s as methylation reagents are well examined. Since we have calculated the Gibbs free energy and activation free energy of trifluoromethyl-substituted organic hydride compound radical cations (X'H^•+) releasing trifluoromethyl radicals in MeCN with the DFT method in our previous work, accordingly, the relationship of thermodynamics and reactivity between X'H^•+ releasing trifluoromethyl radical and XH^•+ releasing methyl radical is discussed in detail. Excitingly, 4 XH^•+s (1H^•+, 3H^•+∼4H^•+, and 44H^•+) are found to be excellent methyl radical reagents, while 9 XH^•+s (5H^•+, 6H^•+, 9H^•+, 10H^•+, 12H^•+, 13H^•+, 15H^•+, 43H^•+, and 45H^•+) are found to be potential methyl radical reagents in chemical synthesis. The molecular library and reactivity database of novel methylation reagents could be established for synthetic chemists to query and use. Our work may offer a theoretical basis and reference experience for screening different substituted organic hydride compounds (YRHs) as alkylation reagents.

Monoselective N-Methylation of Amides, Indoles, and Related Structures Using Quaternary Ammonium Salts as Solid Methylating Agents

We herein report the use of phenyl trimethylammonium iodide (PhMe3NI) as a safe, nontoxic, and easy-to-handle reagent for an absolutely monoselective N-methylation of amides and related compounds as well as for the N-methylation of indoles. In addition, we expanded the method to N-ethylation using PhEt3NI. The ease of operational setup, high yields of ≤99%, high functional group tolerance, and especially the excellent monoselectivity for amides make this method attractive for late-stage methylation of bioactive compounds.

On drug discovery against infectious diseases and academic medicinal chemistry contributions

This perspective is an attempt to document the problems that medicinal chemists are facing in drug discovery. It is also trying to identify relevant/possible, research areas in which academics can have an impact and should thus be the subject of grant calls. Accordingly, it describes how hit discovery happens, how compounds to be screened are selected from available chemicals and the possible reasons for the recurrent paucity of useful/exploitable results reported. This is followed by the successful hit to lead stories leading to recent and original antibacterials which are, or about to be, used in human medicine. Then, illustrated considerations and suggestions are made on the possible inputs of academic medicinal chemists. This starts with the observation that discovering a "good" hit in the course of a screening campaign still rely on a lot of luck - which is within the reach of academics -, that the hit to lead process requires a lot of chemistry and that if public-private partnerships can be important throughout these stages, they are absolute requirements for clinical trials. Concerning suggestions to improve the current hit success rate, one academic input in organic chemistry would be to identify new and pertinent chemical space, design synthetic accesses to reach these and prepare the corresponding chemical libraries. Concerning hit to lead programs on a given target, if no new hits are available, previously reported leads along with new structural data can be pertinent starting points to design, prepare and assay original analogues. In conclusion, this text is an actual plea illustrating that, in many countries, academic research in medicinal chemistry should be more funded, especially in the therapeutic area neglected by the industry. At the least, such funds would provide the intensive to secure series of hopefully relevant chemical entities which appears to often lack when considering the results of academic as well as industrial screening campaigns.

A late-stage functionalization tool: sulfonyl fluoride mediated deoxymethylation of phenols

The late-stage functionalization of drugs and natural products has been identified as a promising approach to accelerate the discovery of new bioactive compounds. Due to the presence of the "Magic Methyl Effect", the direct deoxymethylation of phenolic hydroxyl groups, which are widespread in natural molecules, is a challenging task. A mild and rapid strategy for direct phenol deoxymethylation under metal catalysis using SO₂F₂ is described in this paper, while good functional group tolerance and high chemoselectivity allow this strategy to be one of the powerful tools for LSF. The power of this new platform is showcased through gram-scale and orthogonal experiments.

Mechanism and Origin of Remote Stereocontrol in the Organocatalytic Enantioselective Formal C(sp2)–H Alkylation Using Nitroalkanes as Alkylating Agents

Experimental ¹³C kinetic isotope effects (KIEs) and density functional theory (DFT) calculations are used to evaluate the mechanism and origin of enantioselectivity in the formal C(sp²)-H alkylative desymmetrization of cyclopentene-1,3-diones using nitroalkanes as the alkylating agent. An unusual combination of an inverse (∼0.980) and a normal (∼1.033) KIE is observed on the bond-forming carbon atoms of the cyclopentene-1,3-dione and nitroalkane, respectively. These data provide strong support for a mechanism involving reversible carbon-carbon bond formation followed by rate- and enantioselectivity-determining nitro group elimination. The theoretical free-energy profile and the predicted KIEs indicate that this elimination event occurs via an E1cB pathway. The origin of remote stereocontrol is evaluated by distortion-interaction and SAPT0 analyses of the E1cB transition states leading to both enantiomers.

Applying a Bioisosteric Replacement Strategy in the Discovery and Optimization of Mesoionic Pyrido[1,2-a]pyrimidinone Insecticides: A Review

Mesoionic pyrido[1,2-a]pyrimidinones are a unique class of heterocyclic compounds. Compounds from this class with a n-propyl group substituted at the 1 position of the mesoionic core were discovered with interesting insecticidal activity in our screen. In this overview, we showcase how a bioisosteric replacement strategy was applied during the discovery and optimization of this class of compounds. Through exploring various substituents at the 1 position, evaluating a variety of mesoionic bicyclic ring scaffolds, and examining substituents on the phenyl group at the 3 position of the mesoionic core as well as substituents on the mesoionic ring skeleton, many compounds were discovered with excellent hopper activity or potent activity against a wide range of Lepidoptera. Ultimately, dicloromezotiaz was identified for commercial development to control a broad spectrum of lepidopteran pests.

Scalable (Enantioselective) Syntheses of Novel 3-Methylated Analogs of Pazinaclone, (S)-PD172938 and Related Biologically Relevant Isoindolinones

Herein, we report the application of an efficient and practical K₂CO₃ promoted cascade reaction of 2-acetylbenzonitrile in the synthesis of novel 3-methylated analogs of Pazinaclone and PD172938, belonging to isoindolinones heterocyclic class bearing a tetrasubstituted stereocenter. Organocatalytic asymmetric synthesis of the key intermediate and its transformation into highly enantioenriched 3-methylated analog of (S)-PD172938 was also developed. These achievements can be of particular interest also for medicinal chemistry, since the methyl group is a very useful structural modification in the rational design of new and more effective bioactive compounds.

Metal-Free Photochemical Imino-Alkylation of Alkenes with Bifunctional Oxime Esters

The concurrent installation of C-C and C-N bonds across alkene frameworks represents a powerful tool to prepare motifs that are ubiquitous in pharmaceuticals and bioactive compounds. To construct such prevalent bonds, most alkene difunctionalization methods demand the use of precious metals or activated alkenes. We report a metal-free, photochemically mediated imino-alkylation of electronically diverse alkenes to install both alkyl and iminyl groups in a highly efficient manner. The exceptionally mild reaction conditions, broad substrate scope, excellent functional group tolerance, and facile one-pot reaction protocol highlight the utility of this method to prepare privileged motifs from readily available alkene and acid feedstocks. One key and striking feature of this transformation is that an electrophilic trifluoromethyl radical is equally efficient with both electron-deficient and electron-rich alkenes. Additionally, dispersion-corrected density functional theory (DFT) and empirical investigations provide detailed mechanistic insight into this reaction.

Structural Basis for Control of Methylation Extent in Polyketide Synthase Metal-Dependent C-Methyltransferases

Installation of methyl groups can significantly improve the binding of small-molecule drugs to protein targets; however, site-selective methylation often presents a significant synthetic challenge. Metal- and S-adenosyl-methionine (SAM)-dependent methyltransferases (MTs) in natural-product biosynthetic pathways are powerful enzymatic tools for selective or chemically challenging C-methylation reactions. Each of these MTs selectively catalyzes one or two methyl transfer reactions. Crystal structures and biochemical assays of the Mn2+-dependent monomethyltransferase from the saxitoxin biosynthetic pathway (SxtA MT) revealed the structural basis for control of methylation extent. The SxtA monomethyltransferase was converted to a dimethyltransferase by modification of the metal binding site, addition of an active site base, and an amino acid substitution to provide space in the substrate pocket for two methyl substituents. A reciprocal change converted a related dimethyltransferase into a monomethyltransferase, supporting our hypothesis that steric hindrance can prevent a second methylation event. A novel understanding of MTs will accelerate the development of MT-based catalysts and MT engineering for use in small-molecule synthesis.

The quest for magic: recent advances in C(sp3)–H methylation

Frequently referred to as the “magic methyl” effect, the introduction of a methyl group into a biologically active molecule has the potential to drastically alter its physical and biological properties and significantly increase potency. This effect is most pronounced when the methyl group is added at the α-position of an aliphatic heterocycle or ortho to a large rotatable group on an aromatic ring. Although seminal developments in C–H activation strategies offered solutions to the latter, until recent years there had been no selective and functional-group-tolerant method for C(sp3)–H methylation at late stages of synthesis. For many years, the lack of a generally applicable methylation strategy necessitated arduous de novo synthesis approaches to access methylated drug candidates, and discouraged further investigation and understandings of the magic methyl effect. This review will provide a summary of the most recent advances that enabled non-directed late-stage C(sp3)–H methylation, including through hydride transfer, chemical or anodic oxidation, and photocatalytic hydrogen atom transfer.

Facilitating Rh-Catalyzed C-H Alkylation of (Hetero)arenes and 6-Arylpurine Nucleosides (Nucleotides) with Electrochemistry

An electrochemical approach to promote the ortho-C-H alkylation of (hetero)arenes via rhodium catalysis under mild conditions is described. This approach features mild conditions with high levels of regio- and monoselectivity that tolerate a variety of aromatic and heteroaromatic groups and offers a widely applicable method for late-stage diversification of complex molecular architectures including tryptophan, estrone, diazepam, nucleosides, and nucleotides. Alkyl boronic acids and esters and alkyl trifluoroborates are demonstrated as suitable coupling partners. The isolation of key rhodium intermediates and mechanistic studies provided strong support for a rhodium(III/IV or V) regime.

Selective α-Methylation of Aryl Ketones Using Quaternary Ammonium Salts as Solid Methylating Agents

We describe the use of phenyl trimethylammonium iodide (PhMe3NI) as an alternative methylating agent for introducing a CH3 group in α-position to a carbonyl group. Compared to conventional methylating agents, quaternary ammonium salts have the advantages of being nonvolatile, noncancerogenic, and easy-to-handle solids. This regioselective method is characterized by ease of operational setup, use of anisole as green solvent, and yields up to 85%.

Modifications of the Triaminoaryl Metabophore of Flupirtine and Retigabine Aimed at Avoiding Quinone Diimine Formation

The potassium channel opening drugs flupirtine and retigabine have been withdrawn from the market due to occasional drug-induced liver injury (DILI) and tissue discoloration, respectively. While the mechanism underlying DILI after prolonged flupirtine use is not entirely understood, evidence indicates that both drugs are metabolized in an initial step to reactive ortho- and/or para-azaquinone diimines or ortho- and/or para-quinone diimines, respectively. Aiming to develop safer alternatives for the treatment of pain and epilepsy, we have attempted to separate activity from toxicity by employing a drug design strategy of avoiding the detrimental oxidation of the central aromatic ring by shifting oxidation toward the formation of benign metabolites. In the present investigation, an alternative retrometabolic design strategy was followed. The nitrogen atom, which could be involved in the formation of both ortho- or para-quinone diimines of the lead structures, was shifted away from the central ring, yielding a substitution pattern with nitrogen substituents in the meta position only. Evaluation of K_V7.2/3 opening activity of the 11 new specially designed derivatives revealed surprisingly steep structure-activity relationship data with inactive compounds and an activity cliff that led to the identification of an apparent "magic methyl" effect in the case of N-(4-fluorobenzyl)-6-[(4-fluorobenzyl)amino]-2-methoxy-4-methylnicotinamide. This flupirtine analogue showed potent K_V7.2/3 opening activity, being six times as active as flupirtine itself, and by design is devoid of the potential for azaquinone diimine formation.

Rhodium-Catalyzed C-H Methylation and Alkylation Reactions by Carbene-Transfer Reactions

In this combined computational and experimental study, the C-H functionalization of 2-phenyl pyridine with diazoalkanes was investigated. Initial evaluation by computational methods allowed the evaluation of different metal catalysts and diazoalkanes and their compatibility in this C-H functionalization reaction. With these findings, suitable reaction conditions for the C-H methylation reactions were quickly identified by using highly reactive TMS diazomethane and C-H alkylation reactions with donor/acceptor diazoalkanes, which is applied to a broad scope on alkylation reactions of 2-aryl pyridines with TMS diazomethane and donor/acceptor diazoalkane (51 examples, up to 98 % yield).

Introduction to Spatial Anion Control for Direct C–H Arylation

C–H activation of functionally rich molecules without the need for directing groups promises shorter organic syntheses and late-stage diversification of molecules for drug discovery. We highlight recent examples of palladium-catalyzed nondirected functionalization of C–H bonds in arenes as limiting substrates with a focus on the development of the concept of spatial anion control for direct C–H arylation.

1 C–H Activation and the CMD Mechanism

2 Nondirected C–H Functionalizations of Arenes as Limiting Substrates

3 Nondirected C–H Arylation

4 Spatial Anion Control for Direct C–H Arylation

5 Coordination Chemistry with Spatial Anion Control

6 Conclusion

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.

Using Data Science To Guide Aryl Bromide Substrate Scope Analysis in a Ni/Photoredox-Catalyzed Cross-Coupling with Acetals as Alcohol-Derived Radical Sources

Ni/photoredox catalysis has emerged as a powerful platform for C(sp2)-C(sp3) bond formation. While many of these methods typically employ aryl bromides as the C(sp2) coupling partner, a variety of aliphatic radical sources have been investigated. In principle, these reactions enable access to the same product scaffolds, but it can be hard to discern which method to employ because nonstandardized sets of aryl bromides are used in scope evaluation. Herein, we report a Ni/photoredox-catalyzed (deutero)methylation and alkylation of aryl halides where benzaldehyde di(alkyl) acetals serve as alcohol-derived radical sources. Reaction development, mechanistic studies, and late-stage derivatization of a biologically relevant aryl chloride, fenofibrate, are presented. Then, we describe the integration of data science techniques, including DFT featurization, dimensionality reduction, and hierarchical clustering, to delineate a diverse and succinct collection of aryl bromides that is representative of the chemical space of the substrate class. By superimposing scope examples from published Ni/photoredox methods on this same chemical space, we identify areas of sparse coverage and high versus low average yields, enabling comparisons between prior art and this new method. Additionally, we demonstrate that the systematically selected scope of aryl bromides can be used to quantify population-wide reactivity trends and reveal sources of possible functional group incompatibility with supervised machine learning.

Rhoda-Electrocatalyzed C-H Methylation and Paired Electrocatalyzed C-H Ethylation and Propylation

The use of electricity over traditional stoichiometric oxidants is a promising strategy for sustainable molecular assembly. Herein, we describe the rhoda-electrocatalyzed C-H activation/alkylation of several N-heteroarenes. This catalytic approach has been successfully applied to several arenes, including biologically relevant purines, diazepam, and amino acids. The versatile C-H alkylation featured water as a co-solvent and user-friendly trifluoroborates as alkylating agents. Finally, the rhoda-electrocatalysis with unsaturated organotrifluoroborates proceeded by paired electrolysis.

Palladium-catalyzed nucleomethylation of alkynes for synthesis of methylated heteroaromatic compounds

Herein, we disclosed a novel and efficient palladium-catalyzed nucleomethylation of alkynes for the simultaneous construction of the heteroaromatic ring and methyl group. The 3-methylindoles, 3-methylbenzofurans and 4-methylisoquinolines were obtained in moderate to excellent yields. Notably, this methodology was employed as a key step for synthesis of a pregnane X receptor antagonist, zindoxifene, bazedoxifene and AFN-1252. The kinetic studies revealed that reductive elimination might be the rate-determining step.

A novel palladium-catalyzed nucleomethylation of alkynes is developed, affording 3-methylindoles, 3-methylbenzofurans and 4-methylisoquinolines in moderate to excellent yields.

Towards better syntheses of partially methylated carbohydrates?

We give an overview of the reported synthetic strategies towards partially methylated glycosides and discuss how better protocols could stem from catalytic site-selective transformations of carbohydrates and cleaner methylation reagents.

Metal-Free Alkylation of Quinoxalinones with Aryl Alkyl ketones

The first metal-free method for alkylation of quinoxalinones using cheap and stable aryl alkyl ketones as nucleophilic alkylation reagents is reported. This strategy greatly broadens the application channels of aryl...