The Deuterated "Magic Methyl" Group: A Guide to Site-Selective Trideuteromethyl Incorporation and Labeling by Using CD3 Reagents

Leveraging long-lived arenium ions in superacid for meta-selective methylation

Electrophilic aromatic substitution is one of the most mechanistically studied reactions in organic chemistry. However, precluded by innate substituent effects, the access to certain substitution patterns remains elusive. While selective C-H alkylation of biorelevant molecules is eagerly awaited, especially for the insertion of a methyl group whose magic effect can boost lead molecules potency, one of the most obvious strategies would rely on electrophilic aromatic substitution. Yet, the historical Friedel-Crafts methylation remains to date poorly selective and limited to activated simple aromatics. Here, we report the development of a selective electrophilic methylation enabling the direct access to highly desirable 1,3-disubstituted arenes. This study demonstrates that this reaction is driven by the generation of long-lived arenium intermediates generated by protonation in superacid and can be applied to a large variety of functionalized (hetero)aromatics going from standard building blocks to active pharmaceutical ingredients.

Property and Activity Refinement of Dihydroquinazolinone-3-carboxamides as Orally Efficacious Antimalarials that Target PfATP4

To contribute to the global effort to develop new antimalarial therapies, we previously disclosed initial findings on the optimization of the dihydroquinazolinone-3-carboxamide class that targets PfATP4. Here we report on refining the aqueous solubility and metabolic stability to improve the pharmacokinetic profile and consequently in vivo efficacy. We show that the incorporation of heterocycle systems in the 8-position of the scaffold was found to provide the greatest attainable balance between parasite activity, aqueous solubility, and metabolic stability. Optimized analogs, including the frontrunner compound S-WJM992, were shown to inhibit PfATP4-associated Na+-ATPase activity, gave rise to a metabolic signature consistent with PfATP4 inhibition, and displayed altered activities against parasites with mutations in PfATP4. Finally, S-WJM992 showed appreciable efficacy in a malaria mouse model and blocked gamete development preventing transmission to mosquitoes. Importantly, further optimization of the dihydroquinazolinone class is required to deliver a candidate with improved pharmacokinetic and risk of resistance profiles.

The role of the methoxy group in approved drugs

The methoxy substituent is prevalent in natural products and, consequently, is present in many natural product-derived drugs. It has also been installed in modern drug molecules with no remnant of natural product features because medicinal chemists have been taking advantage of the benefits that this small functional group can bestow on ligand-target binding, physicochemical properties, and ADME parameters. Herein, over 230 methoxy-containing small-molecule drugs, as well as several fluoromethoxy-containing drugs, are presented from the vantage point of the methoxy group. Biochemical mechanisms of action, medicinal chemistry SAR studies, and numerous X-ray cocrystal structures are analyzed to identify the precise role of the methoxy group for many of the drugs and drug classes. Although the methoxy substituent can be considered as the hybridization of a hydroxy and a methyl group, the combination of these functionalities often results in unique effects that can amount to more than the sum of the individual parts.

A Modified Arbuzov-Michalis Reaction for Selective Alkylation of Nucleophiles

The alkylation of nucleophiles is among the most fundamental and well-developed transformations in chemistry. However, to achieve selective alkylation of complex substrates remains a nontrivial task. We report herein a general and selective alkylation method without using strong acids, bases, or metals. In this method, the readily available phosphinites/phosphites, in combination with ethyl acrylate, function as effective alkylating agents. Various nucleophilic groups, including alcohols, phenols, carboxylic acids, imides, and thiols can be alkylated. This method can be applied in the late-stage alkylation of natural products and pharmaceutical agents, achieving chemo- and site-selective modification of complex substrates. Experimental studies indicate the relative reactivity of a nucleophile depends on its acidity and its steric environment. Mechanistic studies suggest the reaction pathway resembles that of the Arbuzov-Michalis reaction.

Synthesis of Deuterated Endochin-Like Quinolones

Malaria continues to be a serious and debilitating disease. The emergence and spread of high-level resistance to multiple antimalarial drugs by Plasmodium falciparum has brought about an urgent need for new treatments that will be active against multidrug resistant malaria infections. One such treatment, ELQ-331 (MMV-167), an alkoxy carbonate prodrug of 4(1H)-quinolone ELQ-300, is currently in preclinical development with the Medicines for Malaria Venture. Clinical development of ELQ-331 or similar compounds will require the availability of isotopically labeled analogs. Unfortunately, a suitable method for the deuteration of these important compounds was not found in the literature. Here, we describe a facile and scalable method for the deuteration of 4(1H)-quinolone ELQ-300, its alkoxycarbonate prodrug ELQ-331, and their respective N-oxides using deuterated acetic acid.

Electro-oxidative Methylation of 2-Isocyanobiaryls Using N,N-dimethylformamide (DMF) as Carbon Source: Synthesis of 6-Methylphenanthridines

A benign electrochemical method to access 6-methylphenanthridines from 2-isocyanobiaryls using N,N-dimethylformamide (DMF) as a methyl source is reported. The protocol operates at ambient temperature without the need for harmful methylating reagents. Mechanistic studies suggested that DMF delivered a methylene synthon, followed by reduction at the cathode and tautomerization. The method offers environmental benefits by avoiding metal-based reagents and harsh conditions.

Deoxygenative Transformation of Alcohols via Phosphoranyl Radical from Exogenous Radical Addition

A general approach to the direct deoxygenative transformation of primary, secondary, and tertiary alcohols has been developed. It undergoes through phosphoranyl radical intermediates generated by the addition of exogenous iodine radical to trivalent alkoxylphosphanes. Since these alkoxylphosphanes are readily in situ obtained from alcohols and commercially available, inexpensive chlorodiphenylphosphine, a diverse range of alcohols with various functional groups can be utilized to proceed deoxygenative cross-couplings with alkenes or aryl iodides. The selective transformation of polyhydroxy substrates and the rapid synthesis of complex organic molecules are also demonstrated with this method.

A Bimolecular Homolytic Substitution-Enabled Platform for Multicomponent Cross-Coupling of Unactivated Alkenes

The construction of C(sp3)-C(sp3) bonds remains one of the most difficult challenges in cross-coupling chemistry. Here, we report a photoredox/nickel dual catalytic approach that enables the simultaneous formation of two C(sp3)-C(sp3) linkages via trimolecular cross-coupling of alkenes with alkyl halides and hypervalent iodine-based reagents. The reaction harnesses a bimolecular homolytic substitution (SH2) mechanism and chemoselective halogen-atom transfer (XAT) to orchestrate the regioselective addition of electrophilic and nucleophilic alkyl radicals across unactivated alkenes without the need for a directing auxiliary. Utility is highlighted through late-stage (fluoro)alkylation and (trideutero)methylation of C═C bonds bearing different substitution patterns, offering straightforward access to drug-like molecules comprising sp3-hybridized carbon scaffolds.

Improved Pharmacokinetic and Pharmacodynamic Profile of Deuterium-Reinforced Tricyclic Antidepressants Doxepin, Dosulepin, and Clomipramine in Animal Models

BACKGROUND AND OBJECTIVES

Doxepin, dosulepin, and clomipramine are tricyclic antidepressants (TCAs) that act as serotonin and noradrenaline reuptake inhibitors. The metabolites formed by N-dealkylation of these tricyclic antidepressants contribute to overall poor pharmacokinetics and efficacy. Deuteration of the methyl groups at metabolically active sites has been reported to be a useful strategy for developing more selective and potent antidepressants. This isotopic deuteration can lead to better bioavailability and overall effectiveness. The objective is to study the effect of site-selective deuteration of TCAs on their pharmacokinetic and pharmacodynamic profile by comparison with their nondeuterated counterparts.

METHODS

In the current study, the pharmacokinetic profile and antidepressant behavior of deuterated TCAs were evaluated using the forced swim test (FST) and tail suspension test (TST), using male Wistar rats and male Swiss albino mice, respectively; additionally, a synaptosomal reuptake study was carried out.

RESULTS

Compared with the nondeuterated parent drugs, deuterated forms showed improved efficacy in the behavior paradigm, indicating improved pharmacological activity. The pharmacokinetic parameters indicated increased maximum concentration in the plasma (Cmax), elimination half-life (t1/2), and area under the concentration-time curve (AUC)  in deuterated compounds. This can have a positive clinical impact on antidepressant treatment. Synaptosomal reuptake studies indicated marked inhibition of the reuptake mechanism of serotonin (5-HT) and norepinephrine.

CONCLUSIONS

Deuterated TCAs can prove to be potentially better molecules in the treatment of neuropsychiatric disorders as compared with nondeuterated compounds. In addition, we have demonstrated a concept that metabolically active, site-selective deuteration can be beneficial for improving the pharmacokinetic and pharmacodynamic profiles of TCAs. A further toxicological study of these compounds is needed to validate their future clinical use.

Formation of Trideuteromethylated Artifacts of Pyrrole-Containing Natural Products

Pyrrole-containing natural products form a large group of structurally diverse compounds that occur in both terrestrial and marine organisms. In the present study the formation of trideuteromethylated artifacts of pyrrole-containing natural products was investigated, focusing on the discorhabdins. Three deuterated discorhabdins, 1, 3, and 5, were identified to be isolation procedure artifacts caused by the presence of DMSO-d6 during NMR sample preparation and handling. Three additional semisynthetic derivatives, 7-9, were made during the investigation of the mechanism of formation, which was shown to be driven by trideuteromethyl radicals in the presence of water, methanol, TFA, and traces of iron in the deuterated solvent. Generation of trideuteromethylated artifacts was also confirmed for other classes of pyrrole-containing metabolites, namely, makaluvamines, tambjamines, and dibromotryptamines, which had also been dissolved in DMSO-d6 during the structure elucidation process. Semisynthetic discorhabdins were assessed for antiproliferative activity against a panel of human tumor cell lines, and 14-trideuteromethyldiscorhabdin L (3) averaged low micromolar potency.

Three-Component Cross-Electrophile Coupling: Regioselective Electrochemical Dialkylation of Alkenes

The cross-electrophile dialkylation of alkenes enables the formation of two C(sp3)-C(sp3) bonds from readily available starting materials in a single transformation, thereby providing a modular and expedient approach to building structural complexity in organic synthesis. Herein, we exploit the disparate electronic and steric properties of alkyl halides with varying degrees of substitution to accomplish their selective activation and addition to alkenes under electrochemical conditions. This method enables regioselective dialkylation of alkenes without the use of a transition-metal catalyst and provides access to a diverse range of synthetically useful compounds.

B(C6F5)3-Catalyzed Intramolecular Hydroalkoxylation Deuteration Reactions of Unactivated Alkynyl Alcohols

Using D2O as a deuterium source, a method for the deuteration of intra- and extra-cyclic methylene has been developed for cyclic ethers with moderate yield and excellent deuterium incorporation. This transformation features superb functional group tolerance in a wide range of alkynols. Notably, the critical factor to achieve high deuterium incorporation is determined by the hydrogen isotope exchange reaction of an unstable oxonium ion. This novel methodology provides an efficient and concise synthetic route to a number of valuable deuterated cyclic ethers that are often difficult to prepare with other methods.

Photo-Induced C1 Substitution Using Methanol as a C1 Source

The development of sustainable and efficient C1 substitution methods is of central interest for organic synthesis and pharmaceuticals production, the methylation motifs bound to a carbon, nitrogen, or oxygen atom widely exist in natural products and top-selling drugs. In the past decades, a number of methods involving green and inexpensive methanol have already been disclosed to replace industrial hazardous and waste-generating C1 source. Among the various efforts, photochemical strategy is considered as a "renewable" alternative that shows great potential to selectively activate methanol to achieve a series of C1 substitutions at mild conditions, typically C/N-methylation, methoxylation, hydroxymethylation, and formylation. Herein the recent advances in selective transformation of methanol to various C1 functional groups via well-designed photochemical systems involving different types of catalysts or not is systematically reviewed. Both the mechanism and corresponding photocatalytic system were discussed and classified on specific methanol activation models. Finally, the major challenges and perspectives are proposed.

Facile access to S-methyl dithiocarbamates with sulfonium or sulfoxonium iodide as a methylation reagent

Efficient access to S-methyl dithiocarbamates was achieved with sulfonium or sulfoxonium iodide as a methylation reagent. This method is reliable for the synthesis of dithiocarbamates from primary or secondary amines, with sulfoxonium iodide demonstrating more robust methylation capability than sulfonium iodide. Moreover, it also enables facile access to S-trideuteromethyl dithiocarbamates via sulfoxonium metathesis between sulfoxonium iodide and DMSO-d6 with high yields.

Deuteration Degree-Controllable Methylation via a Cascade Assembly Strategy using Methylamine-Water as Methyl Source

We present a novel and effective photocatalytic method for the methylation of β-diketones with controllable degrees of deuterium incorporation via development of new methyl sources. By utilizing a methylamine-water system as the methyl precursor and a cascade assembly strategy for deuteration degree control, we synthesized methylated compounds with varying degrees of deuterium incorporation, showcasing the versatility of this approach. We examined a range of β-diketone substrates and synthesized key intermediates for drug and bioactive compounds with varying degrees of deuterium incorporation, ranging from 0 to 3. We also investigated and discussed the postulated reaction pathway. This work demonstrates the utility of readily available reagents, methylamines and water, as a new methyl source, and provides a simple and efficient strategy for the synthesis of degree-controllable deuterium-labelled compounds.

Deuterium in drug discovery: progress, opportunities and challenges

Substitution of a hydrogen atom with its heavy isotope deuterium entails the addition of one neutron to a molecule. Despite being a subtle change, this structural modification, known as deuteration, may improve the pharmacokinetic and/or toxicity profile of drugs, potentially translating into improvements in efficacy and safety compared with the non-deuterated counterparts. Initially, efforts to exploit this potential primarily led to the development of deuterated analogues of marketed drugs through a 'deuterium switch' approach, such as deutetrabenazine, which became the first deuterated drug to receive FDA approval in 2017. In the past few years, the focus has shifted to applying deuteration in novel drug discovery, and the FDA approved the pioneering de novo deuterated drug deucravacitinib in 2022. In this Review, we highlight key milestones in the field of deuteration in drug discovery and development, emphasizing recent and instructive medicinal chemistry programmes and discussing the opportunities and hurdles for drug developers, as well as the questions that remain to be addressed.

Direct Trideuteromethylation of Sulfenate Anions Generated In Situ from β-Sulfinyl Esters: An Access to Trideuteromethyl Sulfoxides

Deuterated organic molecules have immense value in the pharmaceutical industry. Here, we present a synthetic strategy for direct trideuteromethylation of sulfenate ions derived in situ from β-sulfinyl esters in the presence of a base utilizing inexpensive and abundant CD₃OTs as the electrophilic trideuteromethylating agent. This protocol provides straightforward access to an array of trideuteromethyl sulfoxides in yields of 75-92% with a high degree of deuteration. The ensuing trideuteromethyl sulfoxide can be readily modified into trideuteromethyl sulfone and sulfoximine.

Photocatalytic Deuterium Atom Transfer Deuteration of Electron-Deficient Alkenes with High Functional Group Tolerance

Due to its mild reaction conditions and unique chemoselectivity, hydrogen atom transfer (HAT) hydrogenation represents an indispensable method for the synthesis of complex molecules. Its analog using deuterium, deuterium atom transfer (DAT) deuteration, is expected to enable access to complex deuterium-labeled compounds. However, DAT deuteration has been scarcely studied for synthetic purposes, and a method that possesses the favorable characteristics of HAT hydrogenations has remained elusive. Herein, we report a protocol for the photocatalytic DAT deuteration of electron-deficient alkenes. In contrast to the previous DAT deuteration, this method tolerates a variety of synthetically useful functional groups including haloarenes. The late-stage deuteration also allows access to deuterated amino acids as well as donepezil-d₂ . Thus, this work demonstrates the potential of DAT chemistry to become the alternative method of choice for preparing deuterium-containing molecules.

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.

Rhodium(I)-catalyzed directed trideuteromethylation of (hetero)arene C-H bonds with CD3CO2D

A Rh(I)-catalyzed trideuteromethylation of heteroarenes with inexpensive and readily available deuterated acetic acid (CD₃CO₂D) with the aid of a N-containing directing groups is developed. The oxidant-free reaction is applicable to a wide range of heteroarene substrates, including 2-pyridones, indoles, aryl rings, pyrroles and carbazoles. It allows installation of CD₃ groups under straightforward reaction conditions. It is expected that the salient and practical features of this trideuteromethylation protocol will be of use to academic and industrial researchers.

Enantioselective alkylative cross-coupling of unactivated aromatic C-O electrophiles

Nonpolar alkyl moieties, especially methyl group, are frequently used to modify bioactive molecules during lead optimization in medicinal chemistry. Thus transition-metal catalyzed alkylative cross-coupling reactions by using readily available and environmentally benign C–O electrophiles have been established as powerful tools to install alkyl groups, however, the C(sp³)–C(sp²) cross-coupling via asymmetric activation of aromatic C–O bond for the synthesis of alkylated chiral compounds remains elusive. Here, we unlock a C(sp³)–C(sp²) cross-coupling via enantioselective activation of aromatic C–O bond for the efficient synthesis of versatile axially chiral 2-alkyl-2’-hydroxyl-biaryl compounds. By employing a unique chiral N-heterocyclic carbene ligand, this transformation is accomplished via nickel catalysis with good enantiocontrol. Mechanistic studies indicate that bis-ligated nickel complexes might be formed as catalytically active species in the enantioselective alkylative cross-coupling. Moreover, further derivation experiments suggest this developed methodology holds great promise for complex molecule synthesis and asymmetric catalysis.

Transition-metal catalyzed alkylative cross-couplings are established, powerful tools for the installation of alkyl groups. Here, the authors unlock a C(sp3)–C(sp2) cross-coupling via the asymmetric activation of the aromatic C–O bond by bis-ligated nickel complexes for the synthesis of alkylated, axially chiral biaryl compounds.

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.

Recent Developments for the Deuterium and Tritium Labeling of Organic Molecules

Organic compounds labeled with hydrogen isotopes play a crucial role in numerous areas, from materials science to medicinal chemistry. Indeed, while the replacement of hydrogen by deuterium gives rise to improved absorption, distribution, metabolism, and excretion (ADME) properties in drugs and enables the preparation of internal standards for analytical mass spectrometry, the use of tritium-labeled compounds is a key technique all along drug discovery and development in the pharmaceutical industry. For these reasons, the interest in new methodologies for the isotopic enrichment of organic molecules and the extent of their applications are equally rising. In this regard, this Review intends to comprehensively discuss the new developments in this area over the last years (2017-2021). Notably, besides the fundamental hydrogen isotope exchange (HIE) reactions and the use of isotopically labeled analogues of common organic reagents, a plethora of reductive and dehalogenative deuteration techniques and other transformations with isotope incorporation are emerging and are now part of the labeling toolkit.

V. K, Reddy VS. Regioselective Allenylation and Propargylation of Various para-Quinone Methides Using Alkynyl Azaarenes as Pronucleophile

We have developed a Brønsted base-mediated regioselective allenylation and propargylation of various para-quinone methides using unfunctionalized 2-alkynyl azaarenes as the pronucleophile. The appropriate choice of a base provides an opportunity...

Platinum on Carbon-Catalysed Site-Selective H-D Exchange Reaction of Allylic Alcohols Using Alkyl Amines as a Hydrogen Source

We developed platinum on carbon-catalysed deuteration reaction of tert-allylic alcohols using deuterium oxide as a deuterium source. Amylamine was dehydrogenated by platinum on carbon to generate an appropriate amount of...

Broadening of horizons in the synthesis of CD3-labeled molecules

In the light of the recent potentials of deuterated molecules as pharmaceuticals or even in mechanistic understanding, efficient methods for their synthesis are continually desired. CD₃-containing molecules are prominent amongst these motifs due to the parallel of the "magic methyl effect": introducing a methyl group into pharmaceuticals could positively affect biological activities. The trideuteromethyl group is bound to molecules either by C, N, O, or S atom. For a long time, the preparation methods of such labeled compounds were underestimated and involved multi-step syntheses. More recently, specific approaches dealing with the direct incorporation of the CD₃ group have been developed. This Review gives an overview of the methods for the preparation of CD₃-labeled molecules from conventional functional group interconversion techniques to catalytic approaches and include radical strategy. Detailed reaction mechanisms are also discussed.

The Deuterated "Magic Methyl" Group: A Guide to Site-Selective Trideuteromethyl Incorporation and Labeling by Using CD3 Reagents

In the field of medicinal chemistry, the precise installation of a trideuteromethyl group is gaining ever-increasing attention. Site-selective incorporation of the deuterated "magic methyl" group can provide profound pharmacological benefits and can be considered an important tool for drug optimization and development. This review provides a structured overview, according to trideuteromethylation reagent, of currently established methods for site-selective trideuteromethylation of carbon atoms. In addition to CD3 , the selective introduction of CD2 H and CDH2 groups is also considered. For all methods, the corresponding mechanism and scope are discussed whenever reported. As such, this review can be a starting point for synthetic chemists to further advance trideuteromethylation methodologies. At the same time, this review aims to be a guide for medicinal chemists, offering them the available C-CD3 formation strategies for the preparation of new or modified drugs.