Synthesis and Biopharmaceutical Evaluation of Imatinib Analogues Featuring Unusual Structural Motifs

Nucleophilic Substitution of 1,3-Diiodobicyclo[1.1.1]pentane: Synthesis of Bicyclo[1.1.1]pentylpyridinium, Quinolinium, Isoquinolinium, and Pyrazolium Salts

In this study, we describe the synthesis of bicyclo[1.1.1]pentane salts by the nucleophilic reaction of 1,3-diodobicyclo[1.1.1]pentane (DIBCP) with several classes of nucleophiles. The bicyclo[1.1.1]pentane fragments are established isosteres for tbutyl, alkynyl, and 1,4-diaryl structural units, whose synthesis is typically achieved by addition to the unstable, cryogenically stored, [1.1.1]propellane precursor. In contrast, DIBCP is a stable crystalline solid, with the potential to be a feedstock in the synthesis of BCP fragments. This work provides a straightforward, practical synthetic route to bicyclo[1.1.1]pentylpyridinium, quinolinium, isoquinolinium and pyrazolium salts. This transformation displays a broad substrate scope, good yield profile, with several of the BCP products being fully characterized by single-crystal X-ray crystallography. The reaction proceeds by nucleophilic substitution on 1,3-diodobicyclo[1.1.1]pentane (DIBCP), and we provide detailed computational analysis, showing the role of two nucleophiles in stabilizing a key carbocation intermediate. The synthesized salts are isosteres of existing arylpyridinium and arylquinolinium salts used within pharmaceuticals and high-value commodity chemicals within the industrial chemical sector. Finally, the synthetic utility of these salts is examined, providing practical synthetic routes to N-pyridin-4-one and N-quinolin-4-one substituted bicyclo[1.1.1]pentanes.

Enantioselective Synthesis of Axially Chiral Alkylidenecyclobutanes via Palladium-Catalyzed N-Tosylhydrazone-Based Carbene Coupling

The synthesis of axially chiral alkylidenecyclobutanes remains challenging due to the requirement of both an efficient asymmetric catalytic system and preservation of its inherent strained ring structure. We herein disclose an enantioselective carbene cross-coupling reaction of cyclobutanecarbaldehyde-derived N-tosylhydrazones with aryl bromides, enabled by palladium catalysis in combination with an elaborately modified sulfinamide phosphine ligand (Sadphos). This method demonstrates the feasibility of constructing axial chirality on a strained metal carbene intermediate precisely through a sequential process of enantiodetermined migratory insertion followed by central-to-axial-chirality-transfer β-H elimination. The reaction provides access to diverse alkylidenecyclobutanes featuring a heteroatom-substituted, tertiary and all-carbon quaternary stereocenter with excellent yields (up to 95%) and high enantioselectivities (up to 95% ee). Moreover, both enantiomers can be selectively obtained by choosing either cis- or trans-cyclobutane substrates in a stereospecific manner.

(3 + 2)-Cycloaddition of bicyclobutanes and thioketones: access to 2-thiabicyclo[2.1.1]hexanes without the use of catalysts or light

A novel approach to the synthesis of a 2-thiabicyclo[2.1.1]hexane scaffold has been described. This method utilizes two highly reactive species: bicyclo[1.1.0]butanes (BCBs) and thioketones. Their high reactivity enabled the formation of the desired product to occur under ambient conditions, without the need for catalysts, additives or light irradiation. To the best of our knowledge, this is the first rational synthesis of this specific skeleton. A variety of carbonyl-substituted BCBs, with or without a substituent at the other bridgehead, and thioketones were examined.

Shifting Lithium Amide Reactivity to the Radical Domain: Regioselective Radical C-H Functionalization of 3-Iodooxetane for the Synthesis of 1,5-Dioxaspiro[2.3]hexanes

Strained spiro-heterocycles (SSHs) have gained significant attention within the medicinal chemistry community as promising sp3-rich bioisosteres for their aromatic and non-spirocyclic counterparts. We herein report access to an unprecedented spiro-heterocycle-1,5-dioxaspiro[2.3]hexane. Our synthetic approach leverages a lithium-amide induced single-electron transfer to benzophenones generating an N-centered radical and a ketyl radical anion-reminiscent of a frustrated radical pair. This pair works synergistically to selectively abstract the β-hydrogen from 3-iodooxetane, initiating an exergonic radical-radical coupling reaction. This process enables the formation of the desired bond between the oxetane core and benzophenone derivatives, ultimately yielding the novel 1,5-dioxaspiro[2.3]hexane core. The stability and synthetic utility of the novel 1,5-dioxaspiro[2.3]hexane motif are showcased. An in-depth mechanistic investigation is presented, including cyclic voltammetry studies, as well as computational calculations and experiments to support the mechanism of this new single electron synthetic tactic.

Difunctionalization of bicyclo[1.1.0]butanes enabled by merging C-C cleavage and ruthenium-catalysed remote C-H activation

The high fraction of sp 3-hybridized carbon atom (Fsp 3) character of cyclobutane derivatives renders them as highly promising bioisosteres for otherwise typically flat arenes. Here, to address the current needs in medicinal chemistry for Fsp 3-rich molecules, we disclose a distinct strategy that exploits the merger of C-C scission in bicyclo[1.1.0]butanes (BCBs) with ruthenium-catalysed remote C-H functionalization of heteroarenes, affording densely substituted cyclobutanes in a chemo-controlled manner. This approach enabled the rapid and efficient synthesis of versatile tri- and tetrasubstituted cyclobutanes by coupling a wide range of mono- or disubstituted BCBs with heteroarenes and alkyl halides under mild reaction conditions, featuring ample substrate scope. The C-C/C-H functionalization was ensured by a multifunctional ruthenium(II) catalyst that enabled ruthenacycle-mediated halogen-atom transfer (Ru-XAT), as well as the selective functionalization of BCBs by strain release. Experimental and computational mechanistic studies unravelled a multi-catalysis manifold, while the C-H/C-C functionalization strategy allowed for telescoping late-stage modification.

Recent Advances in Radical Coupling Reactions Directly Involving Bicyclo[1.1.1]pentane (BCP)

BCP (bicyclo[1.1.1]pentane) is an ideal saturated carbon bioisostere, instead of the traditional benzene group, which has been extensively developed. As a novel building block, BCP could be directly involved in a variety of synthetic methods and widely used in the last-stage modification of drugs, attracting much attention from organic chemists and pharmacists. Radical-type cross-coupling reactions involving BCP enable the simultaneous formation of multiple chemical bonds (e.g., C-C, C-N, C-B, C-S, and C-Si) through metal catalysis, photocatalysis, metal-photo synergistic catalysis, and other catalytic systems. Various radical precursors have been explored, facilitating cross-coupling reactions that directly incorporate BCP. This review highlights these state-of-the-art radical couplings of BCP since 2017, organized by reaction components with emphasis on the scope of substrates, reaction mechanisms, and synthetic applications.

Uncatalyzed Diboron Activation by a Strained Hydrocarbon: Experimental and Theoretical Study of [1.1.1]Propellane Diborylation

The synthesis of strained carbocyclic building blocks is relevant for Medicinal Chemistry, and methylenecyclobutanes are particularly challenging with current synthetic technology. Careful inspection of the reactivity of [1.1.1]propellane and diboron reagents has revealed that bis(catecholato)diboron (B2cat2) can produce a bis(borylated) methylenecyclobutane in a few minutes at room temperature. This reaction constitutes the first example of B-B bond activation by a special apolar hydrocarbon and also the first time that propellane is electrophilically activated by boron. Mechanistic studies including in situ NMR kinetics and DFT calculations demonstrate that the diboron moiety can be directly activated through coordination with the inverted sigma bond of propellane, and reveal that DMF is involved in the stabilization of diboronate ylide intermediates rather than the activation of the B-B bond. These results enable new possibilities for both diboron and propellane chemistry, and for further developments in the synthesis of methylenecyclobutanes based on propellane strain release.

Light-enabled scalable synthesis of bicyclo[1.1.1]pentane halides and their functionalizations

In 2012, bicyclo[1.1.1]pentanes were demonstrated to be bioisosteres of the benzene ring. Here, we report a general scalable reaction between alkyl iodides and propellane that provides bicyclo[1.1.1]pentane iodides in milligram, gram and even kilogram quantities. The reaction is performed in flow and requires just light; no catalysts, initiators or additives are needed. The reaction is clean enough that, in many cases, evaporation of the reaction mixture provides products in around 90% purity that can be directly used in further transformations without any purification. Combined with the subsequent functionalization, >300 bicyclo[1.1.1]pentanes for medicinal chemistry have been prepared. So far, this is the most general and scalable approach towards functionalized bicyclo[1.1.1]pentanes.

Catalytic asymmetric synthesis of meta benzene isosteres

Although aromatic rings are common elements in pharmaceutically active compounds, the presence of these motifs brings several liabilities with respect to the developability of a drug1. Nonoptimal potency, metabolic stability, solubility and lipophilicity in pharmaceutical compounds can be improved by replacing aromatic rings with non-aromatic isosteric motifs2. Moreover, whereas aromatic rings are planar and lack three-dimensionality, the binding pockets of most pharmaceutical targets are chiral. Thus, the stereochemical configuration of the isosteric replacements may offer an added opportunity to improve the affinity of derived ligands for target receptors. A notable impediment to this approach is the lack of simple and scalable catalytic enantioselective syntheses of candidate isosteres from readily available precursors. Here we present a previously unknown palladium-catalysed reaction that converts hydrocarbon-derived precursors to chiral boron-containing nortricyclanes and we show that the shape of these nortricyclanes makes them plausible isosteres for meta disubstituted aromatic rings. With chiral catalysts, the Pd-catalysed reaction can be accomplished in an enantioselective fashion and subsequent transformation of the boron group provides access to a broad array of structures. We also show that the incorporation of nortricyclanes into pharmaceutical motifs can result in improved biophysical properties along with stereochemistry-dependent activity. We anticipate that these features, coupled with the simple, inexpensive synthesis of the functionalized nortricyclane scaffold, will render this platform a useful foundation for the assembly of new biologically active agents.

A General Approach for the Synthesis of Cyanoisopropyl Bicyclo[1.1.1]pentane (BCP) Motifs by Energy Transfer Process

Bicyclo[1.1.1]pentane (BCP) heteroaryls make up an important class of BCP derivatives in drug discovery. Herein, we report the visible-light-mediated synthesis of cyanoisopropyl BCP-heteroaryls motifs from N-containing heterocycles, [1.1.1]propellane, and AIBN (2,2'-azobis(isobutyronitrile)) through three-component cascade reaction. Importantly, this protocol is compatible with pyrazinones, quinoxaline-2(1H)-one, azauracils, quinoline derivatives, and imidazo[1,2-b]pyridazine, as well as various phenyl disulfide derivatives; thus, this operationally simple and general methodology could enable rapid library generation of sought-after BCP derivatives for drug development.

Lewis acid-catalyzed diastereoselective formal ene reaction of thioindolinones/thiolactams with bicyclobutanes

Bicyclo[1.1.0]butanes (BCBs), featuring two fused cyclopropane rings, have found widespread application in organic synthesis. Their versatile reactivity towards radicals, nucleophiles, cations, and carbenes makes them suitable for various reactions, including ring-opening and annulation strategies. Despite this versatility, their potential as enophiles in an ene reaction remains underexplored. Considering this and given the challenges of achieving diastereoselectivity in ring-opening reactions of BCBs, herein, we present a unique method utilizing BCBs as enophiles in a mild and diastereoselective Sc(OTf)3-catalyzed formal ene reaction with thioindolinones/thiolactams, delivering 1,3-disubstituted cyclobutane derivatives in high yields and excellent regio- and diastereoselectivity. Notably, structurally different thiolactam derivatives underwent diastereoselective addition to BCBs, affording the corresponding cyclobutanes. The synthesized thioindole-substituted cyclobutanes could serve as a versatile tool for subsequent functional group manipulations.

Three-dimensional saturated C(sp3)-rich bioisosteres for benzene

Benzenes, the most ubiquitous structural moiety in marketed small-molecule drugs, are frequently associated with poor 'drug-like' properties, including metabolic instability, and poor aqueous solubility. In an effort to overcome these limitations, recent developments in medicinal chemistry have demonstrated the improved physicochemical profiles of C(sp3)-rich bioisosteric scaffolds relative to arenes. In the past two decades, we have witnessed an exponential increase in synthetic methods for accessing saturated bioisosteres of monosubstituted and para-substituted benzenes. However, until recent discoveries, analogous three-dimensional ortho-substituted and meta-substituted biososteres have remained underexplored, owing to their ring strain and increased s-character hybridization. This Review summarizes the emerging synthetic methodologies to access such saturated motifs and their impact on the application of bioisosteres for ortho-substituted, meta-substituted and multi-substituted benzene rings. It concludes with a perspective on the development of next-generation bioisosteres, including those within novel chemical space.

Diversified Synthesis of Chiral Fluorinated Cyclobutane Derivatives Enabled by Regio- and Enantioselective Hydroboration

The diversified synthesis of chiral fluorinated cyclobutane derivatives has remained a difficult task in synthetic chemistry. Herein, we present an approach for asymmetric hydroboration and formal hydrodefluorination of gem-difluorinated cyclobutenes through rhodium catalysis, providing chiral gem-difluorinated α-boryl cyclobutanes and monofluorinated cyclobutenes with excellent regio- and enantioselectivity, respectively. The key to the success of the two transformations relies on an efficient, mild and highly selective rhodium-catalyzed asymmetric hydroboration with HBPin (pinacolborane), in which the subsequent addition of a base, and a catalytic amount of palladium in some cases, results in the formation of formal hydrodefluorination products with the four-membered ring retained. The obtained chiral gem-difluorinated α-boryl cyclobutanes are versatile building blocks that provide a platform for the synthesis of enantioenriched fluorinated cyclobutane derivatives to a great diversity.

Rapid sp3-Enriched Scaffold Generation via a Selective Aziridine Amide Ring-Opening Reaction

Sp3-enriched small molecules play a critical role in developing drug candidates. While designing analogues with greater sp3 character, a methodology utilizing a less explored cyclic-aziridine amide ring-opening reaction to generate sp3-enriched scaffolds has been developed and reported. This methodology enables rapid access to substructures with higher fsp3 values, attracting greater attention within the past few decades. The reaction exhibits a wide reaction scope, featuring a highly sterically hindered phenolic ether, thiophenolic ethers, protected aniline formations, and aliphatic/heteroaromatic ring-containing aziridine amides as substrates. Additionally, this reaction provides access to congested tertiary ether formations through regioselective transformation, applicable to an extensive range of drug discovery targets, construction of complex small molecules, and natural product syntheses. The scaffolds developed show improved physicochemical properties.

Organometallic Bridge Diversification of Bicyclo[1.1.1]pentanes

Bicyclo[1.1.1]pentane (BCP) derivatives have attracted significant recent interest in drug discovery as alkyne, tert-butyl and arene bioisosteres, where their incorporation is frequently associated with increased compound solubility and metabolic stability. While strategies for functionalisation of the bridgehead (1,3) positions are extensively developed, platforms allowing divergent substitution at the bridge (2,4,5) positions remain limited. Recent reports have introduced 1-electron strategies for arylation and incorporation of a small range of other substituents, but are limited in terms of scope, yields or practical complexity. Herein, we show the synthesis of diverse 1,2,3-trifunctionalised BCPs through lithium-halogen exchange of a readily accessible BCP bromide. When coupled with medicinally relevant product derivatisations, our developed 2-electron "late stage" approach provides rapid and straightforward access to unprecedented BCP structural diversity (>20 hitherto-unknown motifs reported). Additionally, we describe a method for the synthesis of enantioenriched "chiral-at-BCP" bicyclo[1.1.1]pentanes through a novel stereoselective bridgehead desymmetrisation.

Structurally diverse rearranged sesquiterpenoids, including a pair of rare tautomers, from the aerial parts of Daphne penicillata

Eight structurally diverse rearranged sesquiterpenoids, including seven undescribed sesquiterpenoids (1a/1b and 3-8) were obtained from the aerial parts of Daphne penicillata. 1a/1b, 3, 5 and 6 possess rare rearranged guaiane skeletons and 4 represents the first example of rearranged carotene sesquiterpenoids. Their structures and absolute configurations were determined by extensive spectroscopic analyses, NMR and ECD calculations. Interestingly, 1a and 1b were a pair of magical interconverting epimers that may interconvert by retro-aldol condensation. The mechanism of interconversion has been demonstrated indirectly by 9-OH derivatization of 1a/1b and a hypothetical biogenetic pathway was proposed. All compounds were evaluated for anti-inflammatory and cytotoxic activities. Among them, 1a/1b and 2 exhibited potential inhibitory activities on the production of NO against LPS-induced BV2 microglial cells.

The latest perspectives of small molecules FMS kinase inhibitors

FMS kinase is a type III tyrosine kinase receptor that plays a central role in the pathophysiology and management of several diseases, including a range of cancer types, inflammatory disorders, neurodegenerative disorders, and bone disorders among others. In this review, the pathophysiological pathways of FMS kinase in different diseases and the recent developments of its monoclonal antibodies and inhibitors during the last five years are discussed. The biological and biochemical features of these inhibitors, including binding interactions, structure-activity relationships (SAR), selectivity, and potencies are discussed. The focus of this article is on the compounds that are promising leads and undergoing advanced clinical investigations, as well as on those that received FDA approval. In this article, we attempt to classify the reviewed FMS inhibitors according to their core chemical structure including pyridine, pyrrolopyridine, pyrazolopyridine, quinoline, and pyrimidine derivatives.

Applications of Bioisosteres in the Design of Biologically Active Compounds

The design of bioisosteres represents a creative and productive approach to improve a molecule, including by enhancing potency, addressing pharmacokinetic challenges, reducing off-target liabilities, and productively modulating physicochemical properties. Bioisosterism is a principle exploited in the design of bioactive compounds of interest to both medicinal and agricultural chemists, and in this review, we provide a synopsis of applications where this kind of molecular editing has proved to be advantageous in molecule optimization. The examples selected for discussion focus on bioisosteres of carboxylic acids, applications of fluorine and fluorinated motifs in compound design, some applications of the sulfoximine functionality, the design of bioisosteres of drug-H2O complexes, and the design of bioisosteres of the phenyl ring.

Synthesis of 1,3-disubstituted bicyclo[1.1.1]pentanes by cross-coupling induced by transition metals - formation of C-C bonds

The synthesis of 1,3-disubstituted bicyclo[1.1.1]pentanes (BCPs), by forming a C-C bond, can be achieved by cross-coupling reactions using transition metal catalysts. Two main strategies are described to access these 1,3-disubstituted BCPs, either from nucleophilic BCPs or electrophilic BCPs. Mechanisms are included where relevant.

2-Oxabicyclo[2.2.2]octane as a new bioisostere of the phenyl ring

The phenyl ring is a basic structural element in chemistry. Here, we show the design, synthesis, and validation of its new saturated bioisostere with improved physicochemical properties - 2-oxabicyclo[2.2.2]octane. The design of the structure is based on the analysis of the advantages and disadvantages of the previously used bioisosteres: bicyclo[1.1.1]pentane, bicyclo[2.2.2]octane, and cubane. The key synthesis step is the iodocyclization of cyclohexane-containing alkenyl alcohols with molecular iodine in acetonitrile. 2-Oxabicyclo[2.2.2]octane core is incorporated into the structure of Imatinib and Vorinostat (SAHA) drugs instead of the phenyl ring. In Imatinib, such replacement leads to improvement of physicochemical properties: increased water solubility, enhanced metabolic stability, and reduced lipophilicity. In Vorinostat, such replacement results in a new bioactive analog of the drug. This study enhances the repertoire of available saturated bioisosteres of (hetero)aromatic rings for the use in drug discovery projects.

Conquering the Synthesis and Functionalization of Bicyclo[1.1.1]pentanes

Bicyclo[1.1.1]pentanes (BCPs) have become established as attractive bioisosteres for para-substituted benzene rings in drug design. Conferring various beneficial properties compared with their aromatic "parents," BCPs featuring a wide array of bridgehead substituents can now be accessed by an equivalent variety of methods. In this perspective, we discuss the evolution of this field and focus on the most enabling and general methods for BCPs synthesis, considering both scope and limitation. Recent breakthroughs on the synthesis of bridge-substituted BCPs are described, as well as methodologies for postsynthesis functionalization. We further explore new challenges and directions for the field, such as the emergence of other rigid small ring hydrocarbons and heterocycles possessing unique substituent exit vectors.

Lewis acid-catalyzed diastereoselective carbofunctionalization of bicyclobutanes employing naphthols

Traditional radical-mediated ring-opening of bicyclo[1.1.0]butanes (BCBs) for cyclobutane synthesis suffers from poor diastereoselectivity. Although few reports on BCB ring-opening via polar mechanisms are available, the Lewis acid-catalyzed diastereoselective ring-opening of BCBs using carbon nucleophiles is still underdeveloped. Herein, we report a mild and diastereoselective Bi(OTf)₃-catalyzed ring-opening of BCBs employing 2-naphthols. The anticipated carbofunctionalized trisubstituted cyclobutanes were obtained via a bicoordinated bismuth complex and the products are formed in good to excellent yields with high regio- and diastereoselectivity. The scope of the reaction was further extended using electron-rich phenols and naphthylamine. The functionalization of the synthesized trisubstituted cyclobutanes shows the synthetic utility of the present method.

2-Oxabicyclo[2.1.1]hexanes as saturated bioisosteres of the ortho-substituted phenyl ring

The ortho-substituted phenyl ring is a basic structural element in chemistry. It is found in more than three hundred drugs and agrochemicals. During the past decade, scientists have tried to replace the phenyl ring in bioactive compounds with saturated bioisosteres to obtain novel patentable structures. However, most of the research in this area has been devoted to the replacement of the para-substituted phenyl ring. Here we have developed saturated bioisosteres of the ortho-substituted phenyl ring with improved physicochemical properties: 2-oxabicyclo[2.1.1]hexanes. Crystallographic analysis revealed that these structures and the ortho-substituted phenyl ring indeed have similar geometric properties. Replacement of the phenyl ring in marketed agrochemicals fluxapyroxad (BASF) and boscalid (BASF) with 2-oxabicyclo[2.1.1]hexanes dramatically improved their water solubility, reduced lipophilicity and most importantly retained bioactivity. This work suggests an opportunity for chemists to replace the ortho-substituted phenyl ring in bioactive compounds with saturated bioisosteres in medicinal chemistry and agrochemistry.

Skeletal Editing Approach to Bridge-Functionalized Bicyclo[1.1.1]pentanes from Azabicyclo[2.1.1]hexanes

Azabicyclo[2.1.1]hexanes (aza-BCHs) and bicyclo[1.1.1]pentanes (BCPs) have emerged as attractive classes of sp3-rich cores for replacing flat, aromatic groups with metabolically resistant, three-dimensional frameworks in drug scaffolds. Strategies to directly convert, or "scaffold hop", between these bioisosteric subclasses through single-atom skeletal editing would enable efficient interpolation within this valuable chemical space. Herein, we describe a strategy to "scaffold hop" between aza-BCH and BCP cores through a nitrogen-deleting skeletal edit. Photochemical [2+2] cycloadditions, used to prepare multifunctionalized aza-BCH frameworks, are coupled with a subsequent deamination step to afford bridge-functionalized BCPs, for which few synthetic solutions currently exist. The modular sequence provides access to various privileged bridged bicycles of pharmaceutical relevance.

Photoinduced Diastereoselective Aminoalkylation of Cubanes

The unique properties of rigid, nonconjugated hydrocarbons provide many opportunities to design molecular building blocks for a variety of applications, but the development of suitable conditions for alkylation of cubanes is quite challenging. Herein, a photoinduced method for aminoalkylation of cubanes is reported. The benign conditions reported allow the incorporation of a wide variety of (hetero)arylimine reaction partners with broad functional group tolerance and high diastereoselectivity.

Strain-Release Driven Epoxidation and Aziridination of Bicyclo[1.1.0]butanes via Palladium Catalyzed σ-Bond Nucleopalladation

The development of preparative methods for the synthesis of four-membered carbocycles is gaining increasing importance due to the widespread utility of cyclic compounds in medicinal chemistry. Herein, we report the development of a new methodology for the production of spirocyclic epoxides and aziridines containing a cyclobutane motif. In a two-step one-pot process, a bicyclo[1.1.0]butyl sulfoxide is lithiated and added to a ketone, aldehyde or imine, and the resulting intermediate is cross-coupled with an aryl triflate through C-C σ-bond alkoxy- or aminopalladation with concomitant epoxide or aziridine formation. After careful optimization, a remarkably efficient reaction was conceived that tolerated a broad variety of both aromatic and aliphatic substrates. Lastly, through several high yielding ring-opening reactions, we demonstrated the excellent applicability of the products as modular building blocks for the introduction of three-dimensional structures into target molecules.

Strain-Release Driven Epoxidation and Aziridination of Bicyclo[1.1.0]butanes via Palladium Catalyzed σ-Bond Nucleopalladation

The development of preparative methods for the synthesis of four-membered carbocycles is gaining increasing importance due to the widespread utility of cyclic compounds in medicinal chemistry. Herein, we report the development of a new methodology for the production of spirocyclic epoxides and aziridines containing a cyclobutane motif. In a two-step one-pot process, a bicyclo[1.1.0]butyl sulfoxide is lithiated and added to a ketone, aldehyde or imine, and the resulting intermediate is cross-coupled with an aryl triflate through C-C σ-bond alkoxy- or aminopalladation with concomitant epoxide or aziridine formation. After careful optimization, a remarkably efficient reaction was conceived that tolerated a broad variety of both aromatic and aliphatic substrates. Lastly, through several high yielding ring-opening reactions, we demonstrated the excellent applicability of the products as modular building blocks for the introduction of three-dimensional structures into target molecules.

Synthesis of 3-borylated cyclobutanols from epihalohydrins or epoxy alcohol derivatives

There is an increasing interest in cyclobutanes within the medicinal chemistry community. Therefore, methods to prepare cyclobutanes that contain synthetic handles for further elaboration are of interest. Herein, we report a new approach for the synthesis of 3-borylated cyclobutanols via a formal [3 + 1]-cycloaddition using readily accessible 1,1-diborylalkanes and epihalohydrins or epoxy alcohol derivatives. 1-Substituted epibromohydrin starting materials provide access to borylated cyclobutanols containing substituents at three of the four positions on the cyclobutane core, and enantioenriched epibromohydrins lead to enantioenriched cyclobutanols with high levels of enantiospecificity (>98%). Finally, derivatization studies demonstrate the synthetic utility of both the OH and Bpin handles.

General and Practical Route to Diverse 1-(Difluoro)alkyl-3-aryl Bicyclo[1.1.1]pentanes Enabled by an Fe-Catalyzed Multicomponent Radical Cross-Coupling Reaction

Bicyclo[1.1.1]pentanes (BCPs) are of great interest to the agrochemical, materials, and pharmaceutical industries. In particular, synthetic methods to access 1,3-dicarbosubsituted BCP-aryls have recently been developed, but most protocols rely on the stepwise C-C bond formation via the initial manipulation of BCP core to make the BCP electrophile or nucleophile followed by a second step (e.g., transition-metal-mediated cross-coupling step) to form the second key BCP-aryl bond. Moreover, despite the prevalence of C-F bonds in bioactive compounds, one-pot, multicomponent cross-coupling methods to directly functionalize [1.1.1]propellane to the corresponding fluoroalkyl BCP-aryl scaffolds are lacking. In this work, we describe a conceptually different approach to access diverse (fluoro)alkyl BCP-aryls at low temperatures and fast reaction times enabled by an iron-catalyzed multicomponent radical cross-coupling reaction from readily available (fluoro)alkyl halides, [1.1.1]propellane, and Grignard reagents. Further, experimental and computational mechanistic studies provide insights into the mechanism and ligand effects on the nature of C-C bond formation. Finally, these studies are used to develop a method to rapidly access synthetic versatile (difluoro)alkyl BCP halides via bisphosphine-iron catalysis.

Beyond Bioisosteres: Divergent Synthesis of Azabicyclohexanes and Cyclobutenyl Amines from Bicyclobutanes

The development of two divergent and complementary Lewis acid catalyzed additions of bicyclobutanes to imines is described. Microscale high-throughput experimentation was integral to the discovery and optimization of both reactions. N-arylimines undergo formal (3+2) cycloaddition with bicyclobutanes to yield azabicyclo[2.1.1]hexanes in a single step; in contrast, N-alkylimines undergo an addition/elimination sequence to generate cyclobutenyl methanamine products with high diastereoselectivity. These new products contain a variety of synthetic handles for further elaboration, including many functional groups relevant to pharmaceutical synthesis. The divergent reactivity observed is attributed to differences in basicity and nucleophilicity of the nitrogen atom in a common carbocation intermediate, leading to either nucleophilic attack (N-aryl) or E1 elimination (N-alkyl).

Synthesis and Exploitation of the Biological Profile of Novel Guanidino Xylofuranose Derivatives

The synthesis and biological evaluation of novel guanidino sugars as isonucleoside analogs is described. 5-Guanidino xylofuranoses containing 3- O -saturated/unsaturated hydrocarbon or aromatic-containing moieties were accessed from 5-azido xylofuranoses via reduction followed by guanidinylation with N , N '-bis( tert -butoxycarbonyl)- N ''-triflylguanidine. Molecules comprising novel types of isonucleosidic structures including 5-guanidino 3- O -methyl-branched N -benzyltriazole isonucleosides and a guanidinomethyltriazole 3'- O -dodecyl xylofuranos-5'-yl isonucleoside were accessed. The guanidinomethyltriazole derivative and a 3- O -dodecyl ( N -Boc)guanidino xylofuranose were revealed as selective inhibitors of acetylcholinesterase ( K i = 22.87 and 7.49 µM, respectively). The latter also showed moderate antiproliferative effects in chronic myeloid leukemia (K562) and breast cancer (MCF-7) cells. An aminomethyltriazole 5'-isonucleoside was the most potent molecule with low micromolar GI 50 values in both cells (GI 50 = 6.33 μM, 8.45 μM), similar to that of the drug 5-fluorouracil in MCF-7 cells. Moreover, the most bioactive compounds showed low toxicity in human fibroblasts, further indicating their interest as promising lead molecules.

Electrophilic Activation of [1.1.1]Propellane for the Synthesis of Nitrogen-Substituted Bicyclo[1.1.1]pentanes

Strategies commonly used for the synthesis of functionalised bicyclo[1.1.1]pentanes (BCP) rely on the reaction of [1.1.1]propellane with anionic or radical intermediates. In contrast, electrophilic activation has remained a considerable challenge due to the facile decomposition of BCP cations, which has severely limited the applications of this strategy. Herein, we report the electrophilic activation of [1.1.1]propellane in a halogen bond complex, which enables its reaction with electron-neutral nucleophiles such as anilines and azoles to give nitrogen-substituted BCPs that are prominent motifs in drug discovery. A detailed computational analysis indicates that the key halogen bonding interaction promotes nucleophilic attack without sacrificing cage stabilisation. Overall, our work rehabilitates electrophilic activation of [1.1.1]propellane as a valuable strategy for accessing functionalised BCPs.

Bridge Functionalisation of Bicyclo[1.1.1]pentane Derivatives

"Escaping from flatland", by increasing the saturation level and three-dimensionality of drug-like compounds, can enhance their potency, selectivity and pharmacokinetic profile. One approach that has attracted considerable recent attention is the bioisosteric replacement of aromatic rings, internal alkynes and tert-butyl groups with bicyclo[1.1.1]pentane (BCP) units. While functionalisation of the tertiary bridgehead positions of BCP derivatives is well-documented, functionalisation of the three concyclic secondary bridge positions remains an emerging field. The unique properties of the BCP core present considerable synthetic challenges to the development of such transformations. However, the bridge positions provide novel vectors for drug discovery and applications in materials science, providing entry to novel chemical and intellectual property space. This Minireview aims to consolidate the major advances in the field, serving as a useful reference to guide further work that is expected in the coming years.

A Modular Access to 1,2- and 1,3-Disubstituted Cyclobutylboronic Esters by Consecutive Radical Additions

A modular approach to substituted cyclobutylboronic esters is described. It proceeds by successive intermolecular radical additions of xanthates to pinacolato 1-cyclobutenylboronate and to pinacolato bicyclo[1.1.0]but-1-ylboronate. Success hinges on tuning the stability of the α-boryl radical by exploiting the stabilizing influence of the trivalent boronic ester and the slightly destabilizing cyclobutane, which increases the σ-character of the radical. Reductive removal of the xanthate group finally provides a range of 1,2- and 1,3-disubstituted cyclobutylboronic esters. The contrast with cyclopropylboronic esters is striking, since the strong destabilization by the highly strained cyclopropane ring allows the first radical addition to take place but not the second. Furthermore, the first adducts are geminal xanthyl boronic esters that can be converted into cyclobutanones. This chemistry furnishes cyclobutylboronic esters that would be quite difficult to obtain otherwise and thus complements existing methods.

Bioisosteres of the Phenyl Ring: Recent Strategic Applications in Lead Optimization and Drug Design

The benzene moiety is the most prevalent ring system in marketed drugs, underscoring its historic popularity in drug design either as a pharmacophore or as a scaffold that projects pharmacophoric elements. However, introspective analyses of medicinal chemistry practices at the beginning of the 21st century highlighted the indiscriminate deployment of phenyl rings as an important contributor to the poor physicochemical properties of advanced molecules, which limited their prospects of being developed into effective drugs. This Perspective deliberates on the design and applications of bioisosteric replacements for a phenyl ring that have provided practical solutions to a range of developability problems frequently encountered in lead optimization campaigns. While the effect of phenyl ring replacements on compound properties is contextual in nature, bioisosteric substitution can lead to enhanced potency, solubility, and metabolic stability while reducing lipophilicity, plasma protein binding, phospholipidosis potential, and inhibition of cytochrome P450 enzymes and the hERG channel.

Put a ring on it: application of small aliphatic rings in medicinal chemistry

Aliphatic three- and four-membered rings including cyclopropanes, cyclobutanes, oxetanes, azetidines and bicyclo[1.1.1]pentanes have been increasingly exploited in medicinal chemistry for their beneficial physicochemical properties and applications as functional group bioisosteres. This review provides a historical perspective and comparative up to date overview of commonly applied small rings, exemplifying key principles with recent literature examples. In addition to describing the merits and advantages of each ring system, potential hazards and liabilities are also illustrated and explained, including any significant chemical or metabolic stability and toxicity risks.

Excipient-Free Pure Drug Nanoparticles Fabricated by Microfluidic Hydrodynamic Focusing

Nanoprecipitation is one of the most versatile methods to produce pure drug nanoparticles (PDNPs) owing to the ability to optimize the properties of the product. Nevertheless, nanoprecipitation may result in broad particle size distribution, low physical stability, and batch-to-batch variability. Microfluidics has emerged as a powerful tool to produce PDNPs in a simple, reproducible, and cost-effective manner with excellent control over the nanoparticle size. In this work, we designed and fabricated T- and Y-shaped Si-made microfluidic devices and used them to produce PDNPs of three kinase inhibitors of different lipophilicity and water-solubility, namely imatinib, dasatinib and tofacitinib, without the use of colloidal stabilizers. PDNPs display hydrodynamic diameter in the 90-350 nm range as measured by dynamic light scattering and a rounded shape as visualized by high-resolution scanning electron microscopy. Powder X-ray diffraction and differential scanning calorimetry confirmed that this method results in highly amorphous nanoparticles. In addition, we show that the flow rate of solvent, the anti-solvent, and the channel geometry of the device play a key role governing the nanoparticle size.

Investigation of the Reactivity of 1-Azido-3-iodobicyclo[1.1.1]pentane under "Click" Reaction Conditions

The bicyclo[1.1.1]pentane (BCP) unit is under scrutiny as a bioisostere in drug molecules. We employed methodologies for the synthesis of different BCP triazole building blocks from one precursor, 1-azido-3-iodobicyclo[1.1.1]pentane, by "click" reactions and integrated cycloaddition-Sonogashira coupling reactions. Thereby, we accessed 1,4-disubstituted triazoles, 5-iodo-1,4,5-trisubstituted triazoles, and 5-alkynylated 1,4,5-trisubstituted triazoles. This gives entry to the synthesis of multiply substituted BCP triazoles on either a modular or a one-pot basis. These methodologies were further utilized for appending porphyrin moieties onto the BCP core.

Decagram Synthesis of Dimethyl 1,4-Cubanedicarboxylate Using Continuous-Flow Photochemistry

The highly strained cubane system is of great interest as a scaffold and rigid linker in both pharmaceutical and materials chemistry. A straightforward approach is reported for the scale-up of a [2+2] photocycloaddition step using convenient home-made flow photoreactors to access dimethyl 1,4-cubanedicarboxylate on decagram-scale in 33–40% yield over 8 steps. The process is demonstrated on 3.4 g·h–1 input with 30 minutes residence time, enabling to reduce the process time and to avoid the use of batch photoreactors. Completion of the characterisation of the photocycloadduct and its hydrates is reported.

Recent Advances and Outlook for the Isosteric Replacement of Anilines

Many promising drug candidates and pharmaceutical compounds fail due to idiosyncratic adverse drug reactions (IADRs), often arising from the formation of reactive metabolites. Among the "structural alerts" responsible, anilines are well-known to undergo deleterious metabolic processing, yet isosteric replacement strategies remain limited. Herein we discuss current art and potential new avenues of saturated isosteres to mitigate aniline-related toxicities.