Fluorine and Fluorinated Motifs in the Design and Application of Bioisosteres for Drug Design

Synthetic approaches and application of representative clinically approved fluorine-enriched anti-cancer medications

Fluorine possesses distinctive chemical characteristics, such as its strong electron-withdrawing ability and small atomic size, which render it an invaluable asset in the design and optimization of pharmaceuticals. The utilization of fluorine-enriched medications for combating cancer has emerged as a prominent approach in medicinal chemistry and drug discovery, offering improved clinical outcomes and enhanced pharmacological properties. This comprehensive review explores the synthetic approaches and clinical applications of approved 22 representative fluorinated anti-cancer drugs from 2019 to present, shedding light on their historical development, brand names, drug target activity, mechanism of action, preclinical pharmacodynamics, clinical efficacy, and toxicity. Additionally, the review provides an extensive analysis of the representative synthetic techniques employed. Overall, this review emphasizes the significance of incorporating fluorine chemistry into anti-cancer drug research while highlighting promising future prospects for exploring compounds enriched with fluorine in the battle against cancer.

Visible-light-induced photocatalytic four-component fluoroalkylation-dithiocarbamylation via difunctionalization of styrenes

Herein we demonstrate that a visible-light-induced photocatalytic four-component fluoroalkylation-dithiocarbamylation is a unified method for the fluoroalkylation of diverse activated fluoroalkyl halides, including monofluoroalkyl bromides, difluoroalkyl bromides, trifluoromethyl iodide, and perfluoroalkyl iodides. The synthetic value of this method has been demonstrated by the transformations of various substrates containing drug/natural product skeletons, gram scale reactions, and further derivatizations of the fluorodithiocarbamate products. This work features an atom economical protocol that is simple to operate, does not require any additives or strong bases, and can be carried out under mild conditions.

Synthesis of Z-gem-Cl,CF3-Substituted Alkenes by Stereoselective Cross-Metathesis and the Role of Disubstituted Mo Alkylidenes

Stereochemically defined organofluorine compounds are central to drug discovery and development. Here, we present a catalytic cross-metathesis method for the synthesis of Z-trisubstituted olefins that contain a Cl- and a CF3-bound carbon terminus. Notably, the process is stereoselective, which is in contrast to the existing stereoretentive strategies that also involve a trisubstituted olefin as starting material. Reactions are catalyzed by a Mo monoaryloxide pyrrolide alkylidene, involve a trisubstituted alkene and gem-Cl,CF3-substituted alkene, and are fully Z-selective. Catalytic cross-coupling can be used to convert the C-Cl bond of the trisubstituted olefin to C-B, C-D, and different C-C bonds. We elucidate the role of Cl,CF3-disubstituted Mo alkylidenes. Experimental and computational (DFT) data show that in some instances a disubstituted alkylidene is formed and then transformed to a more active complex. In other cases, the Cl,CF3-disubstituted alkylidene is a direct participant in a catalytic cycle. The studies described shed new light on the chemistry of high oxidation-state disubstituted alkylidenes-scarcely investigated entities likely to be pivotal to approaches for stereocontrolled synthesis of tetrasubstituted alkenes through olefin metathesis.

Cu-Electrocatalysis Enables Vicinal Bis(difluoromethylation) of Alkenes: Unraveling Dichotomous Role of Zn(CF2H)2(DMPU)2 as Both Radical and Anion Source

The difluoromethyl group (CF2H) serves as an essential bioisostere in drug discovery campaigns according to Lipinski's Rule of 5 due to its advantageous combination of lipophilicity and hydrogen bonding ability, thereby improving the ADME properties. However, despite the high prevalence and importance of vicinal hydrogen bond donors in pharmaceutical agents, a general synthetic method for doubly difluoromethylated compounds in the vicinal position is absent. Here we describe a copper-electrocatalyzed strategy that enables the vicinal bis(difluoromethylation) of alkenes. By leveraging electrochemistry to oxidize Zn(CF2H)2(DMPU)2-a conventionally utilized anionic transmetalating source, we paved a way to utilize it as a CF2H radical source to deliver the CF2H group in the terminal position of alkenes. Mechanistic studies revealed that the interception of the resultant secondary radical by a copper catalyst and subsequent reductive elimination is facilitated by invoking the Cu(III) intermediate, enabling the second installation of the CF2H group in the internal position. The utility of this electrocatalytic 1,2-bis(difluoromethylation) strategy has been highlighted through the late-stage bioisosteric replacement of pharmaceutical agents such as sotalol and dipivefrine.

General Defluoroalkylation of Trifluoromethylarenes with Both Electron-Donating and -Withdrawing Alkenes

The incorporation of gem-difluoromethylene units into organic molecules remains a formidable challenge. Conventional methodologies for constructing aryldifluoromethyl derivatives relied on the use of high-functional fluorinating regents under harsh conditions. Herein, we report general and efficient photoredox catalytic systems for defluoroalkylation of readily available trifluoromethylarenes through selective C-F cleavage to deliver gem-difluoromethyl radicals which proceed through reductive addition to both electron-donating and withdrawing alkenes under transition-metal free conditions. Mechanistic studies reveal that thiol serves as both photocatalyst and HAT reagent under visible light irradiation. This synergistic photocatalysis and HAT catalysis protocol exhibits ample and salient features such as high chemo- and regioselectivity, broad substrate scope, amenable gram-scale synthesis and late-stage modification of bioactive molecules.

gem-Difluoroalkylation of Cyclic Ethers Enabled by Cobalt-Catalyzed C(sp3)-H Oxidation under Mild Conditions

Fluorinated rings have emerged as privileged structural modules in the fields of drug discovery and materials research. The incorporation of fluorine atoms into aromatic rings or heterocycles can lead to significant improvements in the physicochemical and biological properties of small molecules, making them valuable components in the design of new drugs and functional materials. Herein, we presented a cobalt-catalyzed C-H oxidation/gem-difluorination cascade reaction of readily available cyclic ethers with difluoroenoxysilanes, affording a series of gem-difluorinated analogues with moderate to high yields. The obtained products as versatile fluoroalkyl building blocks were showcased through divergent-oriented transformations.

Metal- versus ligand-centered reactivity of a cobalt-phenylenediamide complex with electrophiles

A new series of [CoIII-CF3]n+ complexes supported by a bidentate redox-active ligand is presented. The cationic [Co-CF3]+ complex was first obtained by reacting [CpCo(tBuUreaopda)] (Cp = cyclopentadienyl, opda = o-phenylenediamide) with an electrophilic trifluoromethyl source, for which the redox-active phenylenediamide ligand serves as a 2e- reservoir to generate [CpCp(tBuUreabqdi)(CF3)]+ (bqdi = benzoquinonediimine). Electrochemical studies of [Co-CF3]+ revealed two reversible 1e- reductions. Chemical reduction with 1 or 2 equiv. reducing agent enabled isolation of the neutral and anionic complexes, respectively, where the [CoIII-CF3] bond remains intact in all three oxidation states (n = +1, 0, -1). Structural analysis shows systematic changes to the redox-active ligand backbone upon reduction, consistent with sequential ligand-centered electron transfer in the series [bqdi]0 to [s-bqdi]˙- to [opda]2-. In contrast, the reaction of [CpCo(tBuUreaopda)] with alkyl triflates resulted in ligand-centered alkylation at the ureayl groups instead of the targeted Co-alkyl bond formation, suggesting less favorable bond formation at cobalt and greater nucleophilic accessibility of the ligand compared to the metal center.

Ribosomal translation of fluorinated non-canonical amino acids for de novo biologically active fluorinated macrocyclic peptides

Fluorination has emerged as a promising strategy in medicinal chemistry to improve the pharmacological profiles of drug candidates. Similarly, incorporating fluorinated non-canonical amino acids into macrocyclic peptides expands chemical diversity and enhances their pharmacological properties, from improved metabolic stability to enhanced cell permeability and target interactions. However, only a limited number of fluorinated non-canonical amino acids, which are canonical amino acid analogs, have been incorporated into macrocyclic peptides by ribosomes for de novo construction and target-based screening of fluorinated macrocyclic peptides. In this study, we report the ribosomal translation of a series of distinct fluorinated non-canonical amino acids, including mono-to tri-fluorinated variants, as well as fluorinated l-amino acids, d-amino acids, β-amino acids, etc. This enabled the de novo discovery of fluorinated macrocyclic peptides with high affinity for EphA2, and particularly the identification of those exhibiting broad-spectrum activity against Gram-negative bacteria by targeting the BAM complex. This study not only expands the scope of ribosomally translatable fluorinated amino acids but also underscores the versatility of fluorinated macrocyclic peptides as potent therapeutic agents.

Photoinduced Metal-Free Radical Addition/Cyclization of 2-Cyanoaryl Acrylamides to Prepare gem-Difluorinated Naphthyridinone Scaffolds

Direct construction of gem-difluorinated heterocycles represents a long-standing challenge in organic chemistry. Herein, we developed a transition-metal-free photocatalytic radical addition/cyclization of BrCF2COR with 2-cyanoaryl acrylamides to give gem-difluorinated naphthyridinone scaffolds in moderate to good yields. Furthermore, some natural products were found to be suitable in the reaction system. The easily available substrates, mild reaction conditions, simple operation, and wide functionality tolerance show practical and environmental advantages in this method.

Rhodium(III)-Catalyzed Coupling of Quinolin-8-carboxaldehydes with CF3-Imidoyl Sulfoxonium Ylides by Chelation-Assisted C(sp2)-H Bond Activation for the Synthesis of Trifluoromethyl-Substituted Enaminones

A rhodium(III)-catalyzed aldehydic C(sp2)-H imidoylmethylation of quinolin-8-carboxaldehydes with CF3-imidoyl sulfoxonium ylides (TFISYs) has been developed for the generation of α-imino ketones, which could be readily tautomerized to enaminones in moderate to excellent yields. In the transformation, TFISYs act as a kind of masked alkenylating reagents for the aldehyde moiety, and the obtained CF3-enaminone products have been successfully converted into other useful trifluoromethyl-substituted heterocycles.

Recent Progress in Synthesis of Alkyl Fluorinated Compounds with Multiple Contiguous Stereogenic Centers

Organic fluorides are widely used in pharmaceuticals, agrochemicals, material sciences, and other fields due to the special physical and chemical properties of fluorine atoms. The synthesis of alkyl fluorinated compounds bearing multiple contiguous stereogenic centers is the most challenging research area in synthetic chemistry and has received extensive attention from chemists. This review summarized the important research progress in the field over the past decade, including asymmetric electrophilic fluorination and the asymmetric elaboration of fluorinated substrates (such as allylic alkylation reactions, hydrofunctionalization reactions, Mannich addition reactions, Michael addition reactions, aldol addition reactions, and miscellaneous reactions), with an emphasis on synthetic methodologies, substrate scopes, and reaction mechanisms.

Photocatalytic Synthesis of γ,γ-Difluoroallylic Ketones and δ,δ-Difluoroallylic Ketones via a Desulfurative/Defluorinative Alkylation Process

The gem-difluoroalkene moiety is frequently found in medicinal chemistry. From α-keton sulfides and thioic acids, we were able to develop a universal approach for the synthesis of γ,γ-difluoroallylic ketones and δ,δ-difluoroallylic ketones via a desulfurative/defluorinative alkylation process. We expect that this mild and efficient method will be complementary to other known strategies.

Halogen Atom Transfer-Induced Homolysis of C-F Bonds by the Excited-State Boryl Radical

A novel reactivity toward C-F bond functionalization has been developed, which could be designated as fluorine atom transfer (FAT). A photoexcited state of an N-heterocyclic carbene-ligated boryl radical exhibits a transcendent reactivity, capable of activating chemically inert carbon-fluorine bonds through homolysis. Combined experimental and computational studies suggest that the ligated boryl radical species directly abstracts a fluorine atom from the organofluoride substrates to provide valuable carbon-centered radicals.

Visible-Light-Induced Domino Perfluoroalkylation/Cyclization to Access Perfluoroalkylated Quinazolinones by an EDA Complex

The electron donor-acceptor (EDA) complexes have been extensively studied, which formed an electronically excited state, obviating the need for an exogenous photocatalyst. Herein, we report a mild and efficient strategy for photoinduced radical domino perfluoroalkylation/cyclization using N,N,N',N'-tetramethylethane-1,2-diamine (TMEDA) as an electron donor. This protocol could be well expanded to access various polycyclic quinazolinones containing perfluoroalkyl groups, exhibiting photocatalyst-free, good functional group tolerance, and environmentally friendly features.

Sequential Modification of Pyrrole Ring with up to Three Different Nucleophiles

An umpolung strategy was used for the preparation of highly functionalized 3-pyrrolin-2-ones. This approach involves dearomative double chlorination of 1H-pyrroles to form highly reactive dichloro-substituted 2H-pyrroles. The resulting intermediate reacts selectively with wet alcohols to form the corresponding alkoxy-substituted 3-pyrrolin-2-ones via double nucleophilic substitution in up to 99% yield. The subsequent reaction with different N-, O-, and S-nucleophiles opens access to highly functionalized pyrrolinones bearing additional functionality. The overall outcome of the reported sequence is step-by-step nucleophilic modification of pyrroles with three different nucleophiles. All steps were found to be highly efficient and 100% regioselective. This transformation proceeds under mild conditions and does not require any catalyst to give final products in very high yields. The obtained experimental results are in perfect agreement with the data obtained by theoretical investigation of these reactions.

1,3-Hydro-di/monofluoromethylation of N,N'-Cyclic Azomethine Imines with HCF2SO2Na/H2CFSO2Na via Photocatalytic Radical Addition

The radical 1,3-hydro-di/monofluoromethylation of N,N'-cyclic azomethine imines with HCF2SO2Na/H2CFSO2Na via photoredox catalysis is described. This reaction exhibits broad functional group compatibility, providing the desired products in good yields. However, CF3SO2Na failed to produce the trifluoromethyl product. DFT calculations revealed that the transition state activation energy for radical trifluoromethylation was significantly higher and the isotropic charge repulsion makes it difficult for the CF3 radical to transfer electrons.

Nickel-catalyzed regiodivergent hydrosilylation of α-(fluoroalkyl)styrenes without defluorination

The fluoroalkyl-containing organic molecules are widely used in drug discovery and material science. Herein, we report ligand regulated nickel(0)-catalyzed regiodivergent hydrosilylation of α-(fluoroalkyl)styrenes without defluorination, providing an atom- and step-economical synthesis route of two types of fluoroalkyl substituted silanes with exclusive regioselectivity. The anti-Markovnikov addition products (β-fluoroalkyl substituted silanes) are formed with monodentate phosphine ligand. Noteworthy, the bidentate phosphine ligand promote the generation of the more challenging Markovnikov products (α-fluoroalkyl substituted silanes) with tetrasubstituted saturated carbon centers. This protocol features with easy available starting materials and commercially available nickel catalysis, a wide range of substrates and excellent regioselectivity. The structure divergent products undergo a variety of transformations. Comprehensive mechanistic studies including the inverse kinetic isotope effects demonstrate the regioselectivity controlled by ligand structure through α-CF3 nickel intermediate. DFT calculations reveal a distinctive mechanism involving an open-shell singlet state, which is crucial for generating intricate tetra-substituted Markovnikov products.

Regioselective Synthesis of Highly Functionalized 2H-Pyrroles via Dearomative Chlorination of 1H-Pyrroles

An efficient protocol was developed for the synthesis of highly functionalized 2H-pyrroles. This synthetic approach involves the in situ generation of highly reactive 2,5-dichloro-substituted 2H-pyrroles through dearomative chlorination of the corresponding 1H-pyrroles. The resulting reaction mixture is then treated with various amines, leading to the formation of 2,5-diaminated 2H-pyrroles. Subsequent nucleophilic substitution of fluorine with different N-, O-, and S-nucleophiles allows us to introduce additional functionality into a 2H-pyrrole core. The overall outcome of this reaction sequence is the triple nucleophilic modification of pyrroles. All steps of the sequence were found to be highly efficient, regioselective in the preparation of desired di- and trisubstituted derivatives in up to 96% overall yield. In addition, the computational study of this reaction sequence was carried out using density functional theory (DFT). The results of calculations are in perfect agreement with experimental observations.

Asymmetric photoenzymatic incorporation of fluorinated motifs into olefins

Enzymes capable of assimilating fluorinated feedstocks are scarce. This situation poses a challenge for the biosynthesis of fluorinated compounds used in pharmaceuticals, agrochemicals, and materials. We developed a photoenzymatic hydrofluoroalkylation that integrates fluorinated motifs into olefins. The photoinduced promiscuity of flavin-dependent ene-reductases enables the generation of carbon-centered radicals from iodinated fluoroalkanes, which are directed by the photoenzyme to engage enantioselectively with olefins. This approach facilitates stereocontrol through interaction between a singular fluorinated unit and the enzyme, securing high enantioselectivity at β, γ, or δ positions of fluorinated groups through enzymatic hydrogen atom transfer-a process that is notably challenging with conventional chemocatalysis. This work advances enzymatic strategies for integrating fluorinated chemical feedstocks and opens avenues for asymmetric synthesis of fluorinated compounds.

eFluorination for the Rapid Synthesis of Carbamoyl Fluorides from Oxamic Acids

In this letter, we disclose the anodic oxidation of oxamic acids in the presence of Et3N·3HF as a practical, scalable, and robust method to rapidly access carbamoyl fluorides from readily available and stable precursors. The simplicity of this method also led us to develop the first flow electrochemical preparation of carbamoyl fluorides, demonstrating scale-up feasibility as a proof of concept.

Fluoride-Catalyzed Cross-Coupling of Silylamides to CF3-Acrylates: Access to Highly Functionalized Trifluoromethylated and Quaternary Fluorinated Molecular Architectures

A direct and simple catalytic coupling of nonfluorinated and fluorinated silylbutenamides with β-CF3 α,β-unsaturated esters mediated by fluoride ion was carried out. The transformation proceeded with excellent yields to afford new, highly functionalized trifluoromethylated and quaternary fluorinated products.

Synthesis of α-Trifluoromethylated Ketones from α,β-Unsaturated Ketones via Catecholboron Enolates

Herein, we report a protocol for the synthesis of α-trifluoromethylated ketones through trifluoromethylation of the corresponding catecholboron enolates of α,β-unsaturated ketones. The reaction of the 1,4-hydroborated product of enones with the Togni II reagent affords the α-trifluoromethylated ketones without any catalyst or additive. Moreover, the protocol has been employed on a series of α,β-unsaturated ketones, including chalcones, substrate-bearing heterocycles, and bioactive molecules. Mechanistic studies suggest the involvement of a trifluoromethyl radical during the reaction.

Visible light-mediated difluoromethylation/cyclization in batch and flow: scalable synthesis of CHF2-containing benzimidazo- and indolo[2,1-a]isoquinolin-6(5H)-ones

We report here a practical and cost-effective method for the synthesis of CHF2-containing benzimidazo- and indolo[2,1,a]-isoquinolin-6(5H)-ones through a visible light-mediated difluoromethylation/cyclization cascade. The method, which affords functionalized multifused N-heterocyclic scaffolds in moderate to high yields under mild reaction conditions, is also easily scalable using low-cost 3D printed photoflow reactors.

An Update on the Nitrogen Heterocycle Compositions and Properties of U.S. FDA-Approved Pharmaceuticals (2013-2023)

This Perspective is a continuation of our analysis of U.S. FDA-approved small-molecule drugs (1938-2012) containing nitrogen heterocycles. In this study we report drug structure and property analyses of 321 unique new small-molecule drugs approved from January 2013 to December 2023 as well as information about frequency of important heteroatoms such as sulfur and fluorine and key small nitrogen substituents (CN and NO2). The most notable change is an incredible increase in drugs containing at least one nitrogen heterocycle─82%, compared to 59% from preceding decades─as well as a significant increase in the number of nitrogen heterocycles per drug. Pyridine has claimed the #1 high-frequency nitrogen heterocycle occurrence spot from piperidine (#2), with pyrimidine (#5), pyrazole (#6), and morpholine (#9) being the big top 10 climbers. Also notable is high number of fused nitrogen heterocycles, apparently driven largely by newly approved cancer drugs.

Chemoenzymatic Synthesis of Fluorinated Mycocyclosin Enabled by the Engineered Cytochrome P450-Catalyzed Biaryl Coupling Reaction

Installing fluorine atoms onto natural products holds great promise for the generation of fluorinated molecules with improved or novel pharmacological properties. The enzymatic oxidative carbon-carbon coupling reaction represents a straightforward strategy for synthesizing biaryl architectures, but the exploration of this method for producing fluorine-substituted derivatives of natural products remains elusive. Here, in this study, we report the protein engineering of cytochrome P450 from Mycobacterium tuberculosis (MtCYP121) for the construction of a series of new-to-nature fluorine-substituted Mycocyclosin derivatives. This protocol takes advantage of a "hybrid" chemoenzymatic procedure consisting of tyrosine phenol lyase-catalyzed fluorotyrosine preparation from commercially available fluorophenols, intermolecular chemical condensation to give cyclodityrosines, and an engineered MtCYP121-catalyzed intramolecular biphenol coupling reaction to complete the strained macrocyclic structure. Computational mechanistic studies reveal that MtCYP121 employs Cpd I to abstract a hydrogen atom from the proximal phenolic hydroxyl group of the substrate to trigger the reaction. Then, conformational change makes the two phenolic hydroxyl groups close enough to undergo intramolecular hydrogen atom transfer with the assistance of a pocket water molecule. The final diradical coupling process completes the intramolecular C-C bond formation. The efficiency of the biaryl coupling reaction was found to be influenced by various fluorine substitutions, primarily due to the presence of distinct binding conformations.

Modular synthesis of cyclic β-difluoroamines

Fluorine-containing saturated nitrogen heterocycles are very attractive structures in medicinal and biological chemistry because fluorine can be used to tune conformation as well as key properties such as basicity and bioavailability. At present cyclic fluorinated amines are accessed using hazardous reagents such as DAST or by lengthy synthesis routes. Here we report a modular two-step synthesis of cyclic β-fluoroalkyl amines using a photoredox-catalysed cyclisation/hydrogen atom transfer reaction of bromodifluoroethylamines.

Hydroazidation of trifluoromethyl alkenes with trimethylsilyl azide enabled by organic photoredox catalysis

Herein, we report an organic photoredox-catalyzed hydroazidation of trifluoromethyl alkenes with user-friendly trimethylsilyl azide (TMSN3), enabling a direct access to a broad range of valuable β-CF3-azides with exclusive selectivity under mild reaction conditions. The synthetic utility of this reaction was demonstrated by the late-stage modification of complex drug derivatives, scale-up of the reaction and diverse further derivatizations of the β-CF3-azide product.

Electrochemical deoxygenative amination of stabilized alkyl radicals from activated alcohols

Alkylamine structures represent one of the most functional and widely used in organic synthesis and drug design. However, the general methods for the functionalization of the shielded and deshielded alkyl radicals remain elusive. Here, we report a general deoxygenative amination protocol using alcohol-derived carbazates and nitrobenzene under electrochemical conditions. A range of primary, secondary, and tertiary alkylamines are obtained. This practical procedure can be scaled up through electrochemical continuous flow technique.

Photoredox-Catalyzed Sequential Decarboxylative/Defluorinative Aminoalkylation of CF3-Alkenes with N-Arylglycines

A photoredox-catalyzed sequential decarboxylative/defluorinative aminoalkylation of CF3-alkenes with N-arylglycines is described. This metal-free and redox-neutral protocol provided efficient access to the monofluoroalkenyl-1,5-diamines in good yields with excellent functional group compatibility. Mechanistic studies revealed that the reaction proceeds via a radical pathway with the gem-difluoroalkenyl amine as an intermediate.

Synthesis of CF3-Isoquinolinones and Imidazole-Fused CF3-Isoquinolinones Based on C-H Activation-Initiated Cascade Reactions of 2-Aryloxazolines

Presented herein are novel syntheses of CF3-isoquinolinones and imidazole fused CF3-isoquinolinones based on the cascade reactions of 2-aryloxazolines with trifluoromethyl imidoyl sulfoxonium ylides. The formation of CF3-isoquinolinone involves an intriguing cascade process including oxazolinyl group-assisted aryl alkylation through C(sp2)-H bond metalation, carbene formation, migratory insertion, and proto-demetalation followed by intramolecular condensation and water-promoted oxazolinyl ring-scission. With this method, the isoquinolinone scaffold tethered with valuable functional groups was effectively constructed. By taking advantage of the functional groups embedded therein, the products thus obtained could be readily transformed into imidazole-fused CF3-isoquinolinones or coupled with some clinical drugs to furnish hybrid compounds with potential applications in drug development. In general, the developed protocols feature expeditious and convenient formation of valuable CF3-heterocyclic skeletons, broad substrate scope, and ready scalability. In addition, studies on the activity of selected products against some human cancer cell lines demonstrated their potential as lead compounds for the development of novel anticancer drugs.

Making Full Use of TMSCF3: Deoxygenative Trifluoromethylation/Silylation of Amides

As one of the most powerful trifluoromethylation reagents, (trifluoromethyl)trimethylsilane (TMSCF3) has been widely used for the synthesis of fluorine-containing molecules. However, to the best of our knowledge, the simultaneous incorporation of both TMS- and CF3- groups of this reagent onto the same carbon of the products has not been realized. Herein, we report an unprecedented SmI2/Sm promoted deoxygenative difunctionalization of amides with TMSCF3, in which both silyl and trifluoromethyl groups are incorporated into the final product, yielding α-silyl-α-trifluoromethyl amines with high efficiency. Notably, the silyl group could be further transformed into other functional groups, providing a new method for the synthesis of α-quaternary α-CF3-amines.

Catalytic Asymmetric Construction of Chiral Polysubstituted 3-Azabicyclo[3.1.1]heptanes by Copper-Catalyzed Stereoselective Formal [4π+2σ] Cycloaddition

The direct construction of bioisosteric compounds enriched in Csp3 content represents an attractive and dependable approach to imbuing biologically active molecules with enhanced three-dimensional characteristics, finding wide utility across the synthetic and medicinal chemistry community. Despite recent advancements in the synthesis of (aza)-bicyclo[3.1.1]heptanes (BCHeps and aza-BCHeps), which serve as meta-substituted (aza)-arene bioisosteres, the enantioselective assembly of chiral 3-aza-BCHeps remains a coveted goal yet to be achieved. Here, we disclose an unprecedented copper-catalyzed asymmetric formal [4π+2σ] cycloaddition of bicyclo[1.1.0]butanes (BCBs) and azomethine ylides, furnishing a diverse array of enantioenriched 3-aza-BCHeps with exceptional levels of diastereo- and enantioselectivites (51 examples, all >20:1 dr, mostly 97-99% ee). Both mono- and disubstituted BCBs are well compatible with this protocol, offering an enticing route for the efficient synthesis of challenging tetrasubstituted bicyclic products bearing two quaternary centers. The synthetic significance of this methodology is further demonstrated by the successful preparation of several piperidine drug analogues.

Clickable tryptophan modification for late-stage diversification of native peptides

As the least abundant residue in proteins, tryptophan widely exists in peptide drugs and bioactive natural products and contributes to drug-target interactions in multiple ways. We report here a clickable tryptophan modification for late-stage diversification of native peptides, via catalyst-free C2-sulfenylation with 8-quinoline thiosulfonate reagents in trifluoroacetic acid (TFA). A wide range of groups including trifluoromethylthio (SCF3), difluoromethylthio (SCF2H), (ethoxycarbonyl)difluoromethylthio (SCF2CO2Et), alkylthio, and arylthio were readily incorporated. The rapid reaction kinetics of Trp modification and full tolerance with other 19 proteinogenic amino acids, as well as the super dissolving capability of TFA, render this method suitable for all kinds of Trp-containing peptides without limitations from sequences, hydrophobicity, and aggregation propensity. The late-stage modification of 15 therapeutic peptides (1.0 to 7.6 kilodaltons) and the improved bioactivity and serum stability of SCF3- and SCF2H-modified melittin analogs illustrated the effectiveness of this method and its potential in pharmacokinetic property improvement.

Divergent functionalization of alkenes enabled by photoredox activation of CDFA and α-halo carboxylic acids

Herein we present our studies on the solvent-controlled difunctionalization of alkenes utilizing chlorodifluoroacetic acid (CDFA) and α-halo carboxylic acids for the synthesis of γ-lactones, γ-lactams and α,α-difluoroesters. Mechanistic insights revealed that photocatalytic reductive mesolytic cleavage of the C-X bond delivers elusive α-carboxyl alkyl radicals. In the presence of an olefin molecule, this species acts as a unique bifunctional intermediate allowing for stipulated formation of C-O, C-N and C-H bonds on Giese-type adducts via single electron transfer (SET) or hydrogen atom transfer (HAT) events. These protocols exhibit great efficiency across a broad spectrum of readily available α-halo carboxylic acids and are amenable to scalability in both batch and flow. To demonstrate the versatility of this concept, the synthesis of (±)-boivinianin A, its fluorinated analog and eupomatilone-6 natural products was successfully accomplished.

Benzylic C(sp3)-H fluorination

The selective fluorination of C(sp3)-H bonds is an attractive target, particularly for pharmaceutical and agrochemical applications. Consequently, over recent years much attention has been focused on C(sp3)-H fluorination, and several methods that are selective for benzylic C-H bonds have been reported. These protocols operate via several distinct mechanistic pathways and involve a variety of fluorine sources with distinct reactivity profiles. This review aims to give context to these transformations and strategies, highlighting the different tactics to achieve fluorination of benzylic C-H bonds.

Regioselective fluorination of allenes enabled by I(I)/I(III) catalysis

The prominence and versatility of propargylic fluorides in medicinal chemistry, coupled with the potency of F/H and F/OH bioisosterism, has created a powerful impetus to develop efficient methods to facilitate their construction. Motivated by the well-established conversion of propargylic alcohols to allenes, an operationally simple, organocatalysis-based strategy to process these abundant unsaturated precursors to propargylic fluorides would be highly enabling: this would consolidate the bioisosteric relationship that connects propargylic alcohols and fluorides. Herein, we describe a highly regioselective fluorination of unactivated allenes based on I(I)/I(III) catalysis in the presence of an inexpensive HF source that serves a dual role as both nucleophile and Brønsted acid activator. This strategy enables a variety of secondary and tertiary propargylic fluorides to be prepared: these motifs are prevalent across the bioactive small molecule spectrum. Facile product derivatisation, concise synthesis of multi-vicinal fluorinated products together with preliminary validation of enantioselective catalysis are disclosed. The expansive potential of this platform is also demonstrated through the highly regioselective organocatalytic oxidation, chlorination and arylation of allenes. It is envisaged that the transformation will find application in molecular design and accelerate the exploration of organofluorine chemical space.

Reasons why life on Earth rarely makes fluorine-containing compounds and their implications for the search for life beyond Earth

Life on Earth is known to rarely make fluorinated carbon compounds, as compared to other halocarbons. We quantify this rarity, based on our exhaustive natural products database curated from available literature. We build on explanations for the scarcity of fluorine chemistry in life on Earth, namely that the exclusion of the C-F bond stems from the unique physico-chemical properties of fluorine, predominantly its extreme electronegativity and strong hydration shell. We further show that the C-F bond is very hard to synthesize and when it is made by life its potential biological functions can be readily provided by alternative functional groups that are much less costly to incorporate into existing biochemistry. As a result, the overall evolutionary cost-to-benefit balance of incorporation of the C-F bond into the chemical repertoire of life is not favorable. We argue that the limitations of organofluorine chemistry are likely universal in that they do not exclusively apply to specifics of Earth's biochemistry. C-F bonds, therefore, will be rare in life beyond Earth no matter its chemical makeup.

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.

Effects of pH on opioid receptor activation and implications for drug design

G-protein-coupled receptors are integral membrane proteins that transduce chemical signals from the extracellular matrix into the cell. Traditional drug design has considered ligand-receptor interactions only under normal conditions. However, studies on opioids indicate that such interactions are very different in diseased tissues. In such microenvironments, protons play an important role in structural and functional alterations of both ligands and receptors. The pertinent literature strongly suggests that future drug design should take these aspects into account in order to reduce adverse side effects while preserving desired effects of novel compounds.

Silver-Catalyzed Carbofluorination of Olefins and α-Fluoroolefins with Carbamoyl Radicals

The reactivity of carbamoyl radicals, generated in situ from sodium oxamate salts, has been investigated in the context of radical carbofluorination reactions of olefins and α-fluoroolefins, respectively. Both transformations are catalyzed by silver salts and required the presence of potassium persulfate (K2S2O8) and SelectfluorTM as a radicophilic fluorine source. The reported methods provide a direct access to β-fluoroamides and β,β-difluoroamides.

Site-selective S-gem-difluoroallylation of unprotected peptides with 3,3-difluoroallyl sulfonium salts

Bench-stable 3,3-difluoroallyl sulfonium salts (DFASs), featuring tunable activity and their editable C-β and gem-difluoroallyl group, proved to be versatile fluoroalkylating reagents for site-selective S-gem-difluoroallylation of cysteine residues in unprotected peptides. The reaction proceeds with high efficiency under mild conditions (ambient temperature and aqueous and weak basic conditions). Various protected/unprotected peptides, especially bioactive peptides, are site-selectively S-gem-difluoroallylated. The newly added gem-difluoroallyl group and other functional groups derived from C-β of DFASs are poised for ligation with bio-functional groups through click and radical chemistry. This stepwise "doubly orthogonal" modification of peptides enables the construction of bioconjugates with enhanced complexity and functionality. This proof of principle is successfully applied to construct a peptide-saccharide-biotin chimeric bioconjugate, indicating its great potential application in medicinal chemistry and chemical biology.

Design, Synthesis, and Antifungal Activities of Novel Potent Fluoroalkenyl Succinate Dehydrogenase Inhibitors

The development of new fungicide molecules is a crucial task for agricultural chemists to enhance the effectiveness of fungicides in agricultural production. In this study, a series of novel fluoroalkenyl modified succinate dehydrogenase inhibitors were synthesized and evaluated for their antifungal activities against eight fungi. The results from the in vitro antifungal assay demonstrated that compound 34 exhibited superior activity against Rhizoctonia solani with an EC50 value of 0.04 μM, outperforming commercial fluxapyroxad (EC50 = 0.18 μM) and boscalid (EC50 = 3.07 μM). Furthermore, compound 34 showed similar effects to fluxapyroxad on other pathogenic fungi such as Sclerotinia sclerotiorum (EC50 = 1.13 μM), Monilinia fructicola (EC50 = 1.61 μM), Botrytis cinerea (EC50 = 1.21 μM), and also demonstrated protective and curative efficacies in vivo on rapeseed leaves and tomato fruits. Enzyme activity experiments and protein-ligand interaction analysis by surface plasmon resonance revealed that compound 34 had a stronger inhibitory effect on succinate dehydrogenase compared to fluxapyroxad. Additionally, molecular docking and DFT calculation confirmed that the fluoroalkenyl unit in compound 34 could enhance its binding capacity with the target protein through p-π conjugation and hydrogen bond interactions.

The rat acute oral toxicity of trifluoromethyl compounds (TFMs): a computational toxicology study combining the 2D-QSTR, read-across and consensus modeling methods

All areas of the modern society are affected by fluorine chemistry. In particular, fluorine plays an important role in medical, pharmaceutical and agrochemical sciences. Amongst various fluoro-organic compounds, trifluoromethyl (CF3) group is valuable in applications such as pharmaceuticals, agrochemicals and industrial chemicals. In the present study, following the strict OECD modelling principles, a quantitative structure-toxicity relationship (QSTR) modelling for the rat acute oral toxicity of trifluoromethyl compounds (TFMs) was established by genetic algorithm-multiple linear regression (GA-MLR) approach. All developed models were evaluated by various state-of-the-art validation metrics and the OECD principles. The best QSTR model included nine easily interpretable 2D molecular descriptors with clear physical and chemical significance. The mechanistic interpretation showed that the atom-type electro-topological state indices, molecular connectivity, ionization potential, lipophilicity and some autocorrelation coefficients are the main factors contributing to the acute oral toxicity of TFMs against rats. To validate that the selected 2D descriptors can effectively characterize the toxicity, we performed the chemical read-across analysis. We also compared the best QSTR model with public OPERA tool to demonstrate the reliability of the predictions. To further improve the prediction range of the QSTR model, we performed the consensus modelling. Finally, the optimum QSTR model was utilized to predict a true external set containing many untested/unknown TFMs for the first time. Overall, the developed model contributes to a more comprehensive safety assessment approach for novel CF3-containing pharmaceuticals or chemicals, reducing unnecessary chemical synthesis whilst saving the development cost of new drugs.

Silver-Catalyzed Coupling of Unreactive Carboxylates: Synthesis of α-Fluorinated O-Aryl Esters

α-Fluorinated aryl esters pose a challenge in synthesis via O-arylation of α-fluorinated carboxylates owing to their low reactivities. This limitation has been addressed by combining a silver catalyst with aryl(trimethoxyphenyl)iodonium tosylates to access α-fluorinated aryl esters. We envision that the catalytic system involves high-valent aryl silver species generated via the oxidation of silver(I) salt. The present method provided a synthetic protocol for various α-fluorinated aryl esters including fluorinated analogs of drug derivatives.

Halo-perfluoroalkoxylation of gem-difluoroalkenes with short-lived alkali metal perfluoroalkoxides in triglyme

Alkali metal alkoxides play a pivotal role in nucleophilic alkoxylation reactions, offering pathways for the synthesis of ethers, including the increasingly sought-after trifluoromethyl ethers. However, the synthesis of long-chain perfluoroalkyl ethers remains a substantial challenge in this field. Through the innovative use of triglyme to encapsulate potassium ions, we enhanced the stability of short-lived, longer-chain perfluoroalkoxy anions, thereby facilitating efficient nucleophilic perfluoroalkoxylation reactions. This method provides a new precedent for the halo-perfluoroalkoxylation of gem-difluoroalkenes and offers a versatile tool for the design of perfluoroalkyl ethers, including those containing complex moieties of heterocycles and drug molecules. We also demonstrated the utility of the resulting halo-perfluoroalkoxyl adducts through various chemical transformations to valuable diverse perfluoroalkyl ethers.

The 18F-Difluoromethyl Group: Challenges, Impact and Outlook

The difluoromethyl functionality has proven useful in drug discovery, as it can modulate the properties of bioactive molecules. For PET imaging, this structural motif has been largely underexploited in (pre)clinical radiotracers due to a lack of user-friendly radiosynthetic routes. This Minireview provides an overview of the challenges facing radiochemists and summarises the efforts made to date to access 18F-difluoromethyl-containing radiotracers. Two distinct approaches have prevailed, the first of which relies on 18F-fluorination. A second approach consists of a 18F-difluoromethylation process, which uses 18F-labelled reagents capable of releasing key reactive intermediates such as the [18F]CF2H radical or [18F]difluorocarbene. Finally, we provide an outlook for future directions in the radiosynthesis of [18F]CF2H compounds and their application in tracer radiosynthesis.

Organocatalyzed Asymmetric Michael Addition of 3-Fluorooxindole to Vinylidene Bisphosphonates

Both the 3-fluorooxindole and germinal bisphosphonate structural motifs are prevalent in bioactive molecules because of their associated biological activities. We describe an approach to accessing 3,3-disubstituted 3-fluorooxindoles bearing a geminal bisphosphate fragment through a highly enantioselective Michael addition reaction between 3-fluorooxindoles and vinylidene bisphosphonates. These reactions are catalyzed by a commercially available cinchona alkaloid catalyst, have a broad substrate scope concerning 3-fluorooxindoles, and provide the corresponding addition products in a yield of up to 95% with an enantiomeric excess of up to 95%. A reasonable reaction pathway to explain the observed stereochemistry is also proposed.

Conformational Analysis of 1,3-Difluorinated Alkanes

Fluorine substitution can have a profound impact on molecular conformation. Here, we present a detailed conformational analysis of how the 1,3-difluoropropylene motif (-CHF-CH2-CHF-) determines the conformational profiles of 1,3-difluoropropane, anti- and syn-2,4-difluoropentane, and anti- and syn-3,5-difluoroheptane. It is shown that the 1,3-difluoropropylene motif strongly influences alkane chain conformation, with a significant dependence on the polarity of the medium. The conformational effect of 1,3-fluorination is magnified upon chain extension, which contrasts with vicinal difluorination. Experimental evidence was obtained from NMR analysis, where polynomial complexity scaling simulation algorithms were necessary to enable J-coupling extraction from the strong second-order spectra, particularly for the large 16-spin systems of the difluorinated heptanes. These results improve our understanding of the conformational control toolkit for aliphatic chains, yield simple rules for conformation population analysis, and demonstrate quantum mechanical time-domain NMR simulations for liquid state systems with large numbers of strongly coupled spins.

2,2-Difluoroethylation of Heteroatom Nucleophiles via a Hypervalent Iodine Strategy

The 2,2-difluoroethyl group is an important lipophilic hydrogen bond donor in medicinal chemistry, but its incorporation into small molecules is often challenging. Herein, we demonstrate electrophilic 2,2-difluoroethylation of thiol, amine and alcohol nucleophiles with a hypervalent iodine reagent, (2,2-difluoro-ethyl)(aryl)iodonium triflate, via a proposed ligand coupling mechanism. This transformation offers a complementary strategy to existing 2,2-difluoroethylation methods and allows access to a wide range of 2,2-difluoroethylated nucleophiles, including the drugs Captopril, Normorphine and Mefloquine.

Expedient access to polysubstituted acrylamides via strain-release-driven dual phosphine and palladium catalysis

Polysubstituted acrylamides are ubiquitous in bioactive molecules and natural products. However, synthetic methods for the assembly of these important motifs remain underdeveloped. Herein, we report the expedient synthesis of structurally diverse and synthetically challenging polysubstituted acrylamides from readily available aromatic amines, cyclopropenones (CpOs), and aryl halides via the synergistic merging of nucleophilic phosphine-mediated amidation and palladium-catalyzed C-H arylation. The reaction is scalable, and some obtained acrylamides proved to be solid state luminogens with obvious aggregation-induced emission (AIE) properties, demonstrating the synthetic potential in drug discovery and material development.