Ausgeprägte Methyleffekte in der Wirkstoff-Forschung und der Bedarf an neuen C-H-Methylierungsreaktionen

Cyclic Sulfoximines as Methyl and Perdeuteromethyl Transfer Agents and Their Applications in Photoredox Catalysis

Benzo[1,3,2]dithiazole-1,1,3-trioxides are bench-stable and easy-to-use reagents. In photoredox catalysis, they generate methyl and perdeuteromethyl radicals which can add to a variety of radical acceptors, including olefins, acrylamides, quinoxalinones, isocyanides, enol silanes, and N-Ts acrylamide. As byproduct, a salt is formed which can be regenerated to the original methylating agent. Flow chemistry provides an option for reaction scale-up further underscoring the synthetic usefulness of these methylation reagents. Mechanistic investigations suggest a single-electron transfer (SET) pathway induced by photoredox catalysis.

Trifluoromethyl Rhodium-Carbynoid in [2+1+2] Cycloadditions

Trifluoromethyl cationic carbyne (CF3 C+ :) possessing dual carbene-carbocation behavior emulated as trifluoromethyl metal-carbynoid (CF3 C+ =M) has not been explored yet, and its reaction characteristics are unknown. Herein, a novel α-diazotrifluoroethyl sulfonium salt was prepared and used in Rh-catalyzed three-component [2+1+2] cycloadditions for the first time with commercially available N-fused heteroarenes and nitriles, yielding a series of imidazo[1,5-a] N-heterocycles that are of interest in medicinal chemistry, in which the insertion of trifluoromethyl Rh-carbynoid (CF3 C+ =Rh) into C=N bonds of N-fused heteroarenes was involved. This strategy demonstrates synthetic applications in late-stage modification of pharmaceuticals, construction of CD3 -containing N-heterocycles, gram-scale experiments, and synthesis of phosphodiesterase 10A inhibitor analog. These highly valuable and modifiable imidazo[1,5-a] N-heterocycles exhibit good antitumor activity in vitro, thus demonstrating their potential applications in medicinal chemistry.

Ruthenium(II)-Catalyzed Hydrogenation and Tandem (De)Hydrogenation via Metal-Ligand Cooperation: Base- and Solvent-Assisted Switchable Selectivity

A versatile, selective, solvent (methanol vs ethanol)- and base (potassium vs lithium carbonate)-assisted switchable synthesis of saturated ketone and α-methyl saturated ketone from α,β-unsaturated ketone is developed. Mechanistic aspects, evaluated from spectroscopic studies, in situ monitoring of the reaction progress, control studies, and labeling studies, further indicate the involvement of a tandem dehydrogenation-condensation-hydrogenation sequence in the reaction, in which the interconvertible coordination mode (imino N → Ru and amido N-Ru) of coordinated imidazole with Ru(II)-para-cymene is crucial, without which the efficiency and selectivity of the catalyst are completely lost. The catalyst demonstrates good efficiency, selectivity, and functional group tolerance and displays a broad scope (69 examples) for monomethylation and hydrogenation of unsaturated chalcones, double methylation of ketones, and N-methylation of amines.

Photo-Induced C-H Methylation Reactions

Direct C-H methylation is a highly valuable approach for introducing methyl groups into organic molecules, particularly in pharmaceutical chemistry. Among the various methodologies available, photo-induced methylation stands out as an exceptional choice due to its mild reaction conditions, energy efficiency, and compatibility with functional groups. This article offers a comprehensive review of photochemical strategies employed for the direct and selective methylation of C(sp3 )-H, C(sp2 )-H, and C(sp)-H bonds in various organic molecules. The discussed methodologies encompass transition-metal-based photocatalysis, organophotocatalysis, as well as other metal-free approaches, including electron donor-acceptor (EDA)-enabled transformations. Importantly, a wide range of easily accessible agents such as tert-butyl peroxide, methanol, DMSO, methyl tert-butyl ether, TsOMe, N-(acetoxy)phthalimide, acetic acid, methyl halides, and even methane can serve as effective methylating reagents for modifying diverse targets. These advancements in photochemical C-H methylation are anticipated to drive further progress in the fields of organic synthesis, photocatalysis, and pharmaceutical development, opening up exciting avenues for creating novel organic molecules and discovering new drug compounds.

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.

Nutlin-3a-aa: Improving the Bioactivity of a p53/MDM2 Interaction Inhibitor by Introducing a Solvent-Exposed Methylene Group

Nutlin-3a is a reversible inhibitor of the p53/MDM2 interaction. We have synthesized the derivative Nutlin-3a-aa bearing an additional exocyclic methylene group in the piperazinone moiety. Nutlin-3a-aa is more active than Nutlin-3a against purified wild-type MDM2, and is more effective at increasing p53 levels and releasing transcription of p53 target genes from MDM2-induced repression. X-ray analysis of wild-type MDM2-bound Nutlin-3a-aa indicated that the orientation of its modified piperazinone ring was altered in comparison to the piperazinone ring of MDM2-bound Nutlin-3a, with the exocyclic methylene group of Nutlin-3a-aa pointing away from the protein surface. Our data point to the introduction of exocyclic methylene groups as a useful approach by which to tailor the conformation of bioactive molecules for improved biological activity.

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.

Manganese-Catalyzed Chemoselective Coupling of Secondary Alcohols, Primary Alcohols and Methanol

Herein, we report a manganese-catalyzed three-component coupling of secondary alcohols, primary alcohols and methanol for the synthesis of β,β-methylated/alkylated secondary alcohols. Using our method, a series of 1-arylethanol, benzyl alcohol derivatives, and methanol undergo sequential coupling efficiently to construct assembled alcohols with high chemoselectivity in moderate to good yields. Mechanistic studies suggest that the reaction proceeds via methylation of a benzylated secondary alcohol intermediate to generate the final product.

Methyltransferases: Functions and Applications

In this review the current state-of-the-art of S-adenosylmethionine (SAM)-dependent methyltransferases and SAM are evaluated. Their structural classification and diversity is introduced and key mechanistic aspects presented which are then detailed further. Then, catalytic SAM as a target for drugs, and approaches to utilise SAM as a cofactor in synthesis are introduced with different supply and regeneration approaches evaluated. The use of SAM analogues are also described. Finally O-, N-, C- and S-MTs, their synthetic applications and potential for compound diversification is given.

A High-Throughput Continuous Spectroscopic Assay to Measure the Activity of Natural Product Methyltransferases

Natural product methyltransferases (NPMTs) represent an emerging class of enzymes that can be of great use for the structural and functional diversification of bioactive compounds, such as the strategic modification of C-, N-, O- and S-moieties. To assess the activity and the substrate scope of the ever-expanding repertoire of NPMTs, a simple, fast, and robust assay is needed. Here, we report a continuous spectroscopic assay, in which S-adenosyl-L-methionine-dependent methylation is linked to NADH oxidation through the coupled activities of S-adenosyl-L-homocysteine (SAH) deaminase and glutamate dehydrogenase. The assay is highly suitable for a high-throughput evaluation of small molecule methylation and for determining the catalytic parameters of NPMTs under conditions that remove the potent inhibition by SAH. Through the modular design, the assay can be extended to match the needs of different aspects of methyltransferase cascade reactions and respective applications.

Dual Nickel/Photoredox-Catalyzed Site-Selective Cross-Coupling of 1,2-Bis-Boronic Esters Enabled by 1,2-Boron Shifts

Site-selective transition-metal-catalyzed mono-deboronative cross-couplings of 1,2-bis-boronic esters are valuable methods for the synthesis of functionalized organoboron compounds. However, such cross-couplings are limited to reaction of the sterically less hindered primary boronic ester. Herein, we report a nickel/photoredox-catalyzed mono-deboronative arylation of 1,2-bis-boronic esters that is selective for coupling of the more sterically hindered secondary/tertiary position. This is achieved by taking advantage of a 1,2-boron shift of primary β-boryl radicals to the thermodynamically favored secondary/tertiary radicals, which are subsequently intercepted by the nickel catalyst to enable arylation. The mild conditions are amenable to a broad range of aryl halides to give β-aryl boronic ester products in good yields and with high regioselectivity. This method also allows stereodivergent coupling of cyclic cis-1,2-bis-boronic esters to give trans-substituted products.

Covalent Allosteric Inhibitors of Akt Generated Using a Click Fragment Approach

Akt is a protein kinase that has been implicated in the progression of cancerous tumours. A number of covalent allosteric Akt inhibitors are known, and based on these scaffolds, a small library of novel potential covalent allosteric imidazopyridine-based inhibitors was designed. The envisaged compounds were synthesised, with click chemistry enabling a modular approach to a number of the target compounds. The binding modes, potencies and antiproliferative activities of these synthesised compounds were explored, thereby furthering the structure activity relationship knowledge of this class of Akt inhibitors. Three novel covalent inhibitors were identified, exhibiting moderate activity against Akt1 and various cancer cell lines, potentially paving the way for future covalent allosteric inhibitors with improved properties.

Synthesis of 20-Membered Macrocyclic Pseudo-Natural Products Yields Inducers of LC3 Lipidation

Design and synthesis of pseudo-natural products (PNPs) through recombination of natural product (NP) fragments in unprecedented arrangements enables the discovery of novel biologically relevant chemical matter. With a view to wider coverage of NP-inspired chemical and biological space, we describe the combination of this principle with macrocycle formation. PNP-macrocycles were synthesized efficiently in a stereoselective one-pot procedure including the 1,3-dipolar cycloadditions of different dipolarophiles with dimeric cinchona alkaloid-derived azomethine ylides formed in situ. The 20-membered bis-cycloadducts embody 18 stereocenters and an additional fragment-sized NP-structure. After further functionalization, a collection of 163 macrocyclic PNPs was obtained. Biological investigation revealed potent inducers of the lipidation of the microtubule associated protein 1 light chain 3 (LC3) protein, which plays a prominent role in various autophagy-related processes.

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.

Design, synthesis, and anticancer activity of cinnamoylated barbituric acid derivatives

Background As reactive and biomimetic pharmacophores, heterocycles have found a potent impact in the domain of medicinal chemistry. Aim This work deals with the design and synthesis of 18 barbituric acid derivatives bearing 1,3-dimethylbarbituric acid and cinnamic acid scaffolds to find potent anticancer agents. Methodology The target molecules were obtained through Knoevenagel condensation and acylation reaction. The cytotoxicity was assessed by the MTT assay. Flowcytometry was performed to determined the cell cycle arrest, apoptosis, ROS levels and the loss of MMP. The ratios of GSH/GSSG and the MDA levels were determined by using UV spectrophotometry. Results The results revealed that introducing substitutions (-CF3, -OCF3, -F) on the meta- of the benzyl ring of barbituric acid derivatives led to a considerable increase in the antiproliferative activities compared with that of corresponding ortho- and para-substituted barbituric acid derivatives. Mechanism investigation implied that the 1c could increase the ROS and MDA level, decrease the ratio of GSH/GSSG and MMP, and lead to cell cycle arrest. Conclusion Further research is needed for structural optimization to enhance hydrophilicity, thereby improve the biological activity of these compounds.

Biocatalytic C3-Indole Methylation-A Useful Tool for the Natural-Product-Inspired Stereoselective Synthesis of Pyrroloindoles

Enantioselective synthesis of bioactive compounds bearing a pyrroloindole framework is often laborious. In contrast, there are several S-adenosyl methionine (SAM)-dependent methyl transferases known for stereo- and regioselective methylation at the C3 position of various indoles, directly leading to the formation of the desired pyrroloindole moiety. Herein, the SAM-dependent methyl transferase PsmD from Streptomyces griseofuscus, a key enzyme in the biosynthesis of physostigmine, is characterized in detail. The biochemical properties of PsmD and its substrate scope were demonstrated. Preparative scale enzymatic methylation including SAM regeneration was achieved for three selected substrates after a design-of-experiment optimization.

Four-Selective Pyridine Alkylation via Wittig Olefination of Dearomatized Pyridylphosphonium Ylides

Methods to synthesize alkylated pyridines are valuable because these structures are prevalent in pharmaceuticals and agrochemicals. We have developed a distinct approach to construct 4-alkylpyridines using dearomatized pyridylphosphonium ylide intermediates in a Wittig olefination-rearomatization sequence. Pyridine N-activation is key to this strategy, and N-triazinylpyridinium salts enable coupling between a wide variety of substituted pyridines and aldehydes. The alkylation protocol is viable for late-stage functionalization, including methylation of pyridine-containing drugs. This approach represents an alternative to metal-catalyzed sp² -sp³ cross-coupling reactions and Minisci-type processes.

From Natural Methylation to Versatile Alkylations Using Halide Methyltransferases

Halide methyltransferases (HMTs) enable the enzymatic synthesis of S-adenosyl-l-methionine (SAM) from S-adenosyl-l-homocysteine (SAH) and methyl iodide. Characterisation of a range of naturally occurring HMTs and subsequent protein engineering led to HMT variants capable of synthesising ethyl, propyl, and allyl analogues of SAM. Notably, HMTs do not depend on chemical synthesis of methionine analogues, as required by methionine adenosyltransferases (MATs). However, at the moment MATs have a much broader substrate scope than the HMTs. Herein we provide an overview of the discovery and engineering of promiscuous HMTs and how these strategies will pave the way towards a toolbox of HMT variants for versatile chemo- and regioselective biocatalytic alkylations.

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.

Enzymkatalysierte späte Modifizierungen: Besser spät als nie

Die Enzymkatalyse gewinnt zunehmend an Bedeutung in der Synthesechemie. Die durch Bioinformatik und Enzym‐Engineering stetig wachsende Zahl von Biokatalysatoren eröffnet eine große Vielfalt selektiver Reaktionen. Insbesondere für späte Funktionalisierungsreaktionen ist die Biokatalyse ein geeignetes Werkzeug, das oftmals der konventionellen De‐novo‐Synthese überlegen ist. Enzyme haben sich als nützlich erwiesen, um funktionelle Gruppen direkt in komplexe Molekülgerüste einzuführen sowie für die rasche Diversifizierung von Substanzbibliotheken. Biokatalytische Oxyfunktionalisierungen, Halogenierungen, Methylierungen, Reduktionen und Amidierungen sind von besonderem Interesse, da diese Strukturmotive häufig in Pharmazeutika vertreten sind. Dieser Aufsatz gibt einen Überblick über die Stärken und Schwächen der enzymkatalysierten späten Modifizierungen durch native und optimierte Enzyme in der Synthesechemie. Ebenso werden wichtige Beispiele in der Wirkstoffentwicklung hervorgehoben.

Enzymatic Late-Stage Modifications: Better Late Than Never

Enzyme catalysis is gaining increasing importance in synthetic chemistry. Nowadays, the growing number of biocatalysts accessible by means of bioinformatics and enzyme engineering opens up an immense variety of selective reactions. Biocatalysis especially provides excellent opportunities for late-stage modification often superior to conventional de novo synthesis. Enzymes have proven to be useful for direct introduction of functional groups into complex scaffolds, as well as for rapid diversification of compound libraries. Particularly important and highly topical are enzyme-catalysed oxyfunctionalisations, halogenations, methylations, reductions, and amide bond formations due to the high prevalence of these motifs in pharmaceuticals. This Review gives an overview of the strengths and limitations of enzymatic late-stage modifications using native and engineered enzymes in synthesis while focusing on important examples in drug development.

Site-Specific Alkene Hydromethylation via Protonolysis of Titanacyclobutanes

Methyl groups are ubiquitous in biologically active molecules. Thus, new tactics to introduce this alkyl fragment into polyfunctional structures are of significant interest. With this goal in mind, a direct method for the Markovnikov hydromethylation of alkenes is reported. This method exploits the degenerate metathesis reaction between the titanium methylidene unveiled from Cp2 Ti(μ-Cl)(μ-CH2 )AlMe2 (Tebbe's reagent) and unactivated alkenes. Protonolysis of the resulting titanacyclobutanes in situ effects hydromethylation in a chemo-, regio-, and site-selective manner. The broad utility of this method is demonstrated across a series of mono- and di-substituted alkenes containing pendant alcohols, ethers, amides, carbamates, and basic amines.

A Chiral Pentafluorinated Isopropyl Group via Iodine(I)/(III) Catalysis

An I(I)/(III) catalysis strategy to construct an enantioenriched fluorinated isostere of the i Pr group is reported. The difluorination of readily accessible α-CF3 -styrenes is enabled by the in situ generation of a chiral ArIF2 species to forge a stereocentre with the substituents F, CH2 F and CF3 (up to 95 %, >20:1 vicinal:geminal difluorination). The replacement of the metabolically labile benzylic proton results in a highly preorganised scaffold as was determined by X-ray crystallography (π→σ* and stereoelectronic gauche σ→σ* interactions). A process of catalyst editing is disclosed in which preliminary validation of enantioselectivity is placed on a structural foundation.

Mechanochemical Solvent-Free Catalytic C-H Methylation

The mechanochemical, solvent-free, highly regioselective, rhodium-catalyzed C-H methylation of (hetero)arenes is reported. The reaction shows excellent functional-group compatibility and is demonstrated to work for the late-stage C-H methylation of biologically active compounds. The method requires no external heating and benefits from considerably shorter reaction times than previous solution-based C-H methylation protocols. Additionally, the mechanochemical approach is shown to enable the efficient synthesis of organometallic complexes that are difficult to generate conventionally.

Into the Fray! A Beginner's Guide to Medicinal Chemistry

Modern medicinal chemistry is a complex, multidimensional discipline that operates at the interface of the chemical and biological sciences. The medicinal chemistry contribution to drug discovery is typically described in the context of the well-recited linear progression of the drug discovery pipeline. However, compound optimization is idiosyncratic to each project, and clear definitions of hit and lead molecules and the subsequent progress along the pipeline becomes easily blurred. In addition, this description lacks insight into the entangled relationship between chemical and pharmacological properties, and thus provides limited guidance on how innovative medicinal chemistry strategies can be applied to solve optimization problems, regardless of the stage in the pipeline. Through discussion and illustrative examples, this article seeks to provide insights into the finesse of medicinal chemistry and the subtlety of balancing chemical properties pharmacology. In so doing, it aims to serve as an accessible and simple-to-digest guide for anyone who wishes to learn about the underlying principles of medicinal chemistry, in a context that has been decoupled from the pipeline description.

Synthesis, DNA-binding and antiproliferative properties of diarylquinolizinium derivatives

A series of ten 2,7- and 2,8-diarylquinolizinium derivatives was synthesized and their DNA-binding and cytotoxic properties were investigated. Except for one nitro-substituted derivative all tested diarylquinolizinium ions bind to DNA with sufficient affinity (2 × 10⁴ M^-1-2 × 10⁵ M^-1). It was shown with photometric, fluorimetric and polarimetric titrations as well as with flow-LD analysis that the ligands bind mainly by intercalation to duplex DNA, however, depending on the ligand-DNA ratio, groove binding and backbone association were also observed with some derivatives. The biological activity was further investigated with tests of cytotoxicity and antiproliferative properties towards non-tumor cells and selected cancer cells, along with cell cycle analysis and an annexin-V assay. Notably, substrates that carry donor-functionalities in the 4-position of the phenyl substituents revealed a strong, and in some cases selective, antiproliferative activity as quantified by the growth inhibition, GI₅₀, at very low micromolar and even submicromolar level both in leukemia and solid tumors.

Directed Evolution of a Halide Methyltransferase Enables Biocatalytic Synthesis of Diverse SAM Analogs

Biocatalytic alkylations are important reactions to obtain chemo-, regio- and stereoselectively alkylated compounds. This can be achieved using S-adenosyl-l-methionine (SAM)-dependent methyltransferases and SAM analogs. It was recently shown that a halide methyltransferase (HMT) from Chloracidobacterium thermophilum can synthesize SAM from SAH and methyl iodide. We developed an iodide-based assay for the directed evolution of an HMT from Arabidopsis thaliana and used it to identify a V140T variant that can also accept ethyl-, propyl-, and allyl iodide to produce the corresponding SAM analogs (90, 50, and 70 % conversion of 15 mg SAH). The V140T AtHMT was used in one-pot cascades with O-methyltransferases (IeOMT or COMT) to achieve the regioselective ethylation of luteolin and allylation of 3,4-dihydroxybenzaldehyde. While a cascade for the propylation of 3,4-dihydroxybenzaldehyde gave low conversion, the propyl-SAH intermediate could be confirmed by NMR spectroscopy.