Investigation of a Bicyclo[1.1.1]pentane as a Phenyl Replacement within an LpPLA2 Inhibitor

Photoredox/Copper Cooperatively Catalyzed Arylalkynylation of [1.1.1]Propellane

We present a sp2-sp3/sp3-sp bond formation reaction through a three-component coupling strategy involving terminal alkynes, [1.1.1]propellane, and aryl thianthrenium salts that are prepared from arenes. The reaction employs a dual photo/copper catalysis system and provides a streamlined approach for assembling 1-alkynyl-3-aryl bicyclo[1.1.1]pentane derivatives with a broad spectrum of functional group compatibility. Mechanistic studies suggest that the generation of aryl radicals and copper alkynide intermediates was involved.

Conformationally Stable and Sterically Hindered Bicyclo[1.1.1]pentane-1,3-diammonium Modification of FAPbI3 Enhances the Performance of Perovskite Solar Cells

Solution-processed α-FAPbI3 perovskite films frequently exhibit structural defects and impurities that impede the durable operation of solar cells. In this study, we introduced a conformationally stable, sterically bulky molecular passivator, bicyclo[1.1.1]pentane-1,3-diammonium iodide (BCPDAI), by spin-coating from an isopropanol solution onto the perovskite film surface, followed by thermal annealing. This treatment effectively converted PbI2 and δ-FAPbI3 into a two-dimensional perovskite structure and significantly enhanced the crystal quality of α-FAPbI3. The BCPDAI-treated perovskite films exhibited smoother surface morphology and reduced trap densities for charge carriers, leading to improved power conversion efficiency in the solar cells. Notably, the BCPDAI-modified perovskite films provided the solar cells with enhanced operational stability. Theoretical calculations demonstrated that the high positive charge density of BCPDA2+ confers greater binding energy at the perovskite surface and elevates the diffusion activation energy of the iodide anion.

Highly Functional Allyl-Bicyclo[1.1.1]pentane Synthesis by Radical-Initiated Three-Component Stereoselective Allylation

Rapid access to highly functional allylated BCP synthons can be achieved with good selectivity and yield through a radical, three-component reaction (3CR) regime using various combinations of radical precursors and vinyl-appended heterocycles acting as versatile and modular precursors. This practical process combines mild operating conditions, a wide scope of reaction partners, and the ability to diversify the functionalized allylic scaffolds further using the allyl and other functional groups as synthetic branching points. The developed protocol allows structural alteration and increases the molecular complexity through late-stage drug modifications and drug conjugation approaches. Mechanistic probes demonstrate that the 3CR process is initiated by a selective, light-promoted radical addition to [1.1.1]-propellane, followed by coupling with the vinyl-substituted heterocycle, which represents a formal decarboxylative radical addition/double bond relay/protonation sequence.

Modular access to saturated bioisosteres of anilines via photoelectrochemical decarboxylative C(sp3)-N coupling

In drug development, the substitution of benzene rings in aniline-based drug candidates with saturated bridged bicyclic ring systems often enhances pharmacokinetic properties while preserving biological activity. However, current efforts predominantly focuses on bicyclo[1.1.1]pentylamines, accessing analogs capable of mimicking ortho- and meta-substituted anilines remains challenging due to the lack of a versatile and modular synthetic methods. Herein, we present a modular approach to access a diverse array of saturated bioisosteres of anilines via photoelectrochemical-induced decarboxylative C(sp3)-N Coupling. The success of this reaction hinges on the merging the cooperative ligand-to-metal charge transfer (LMCT) with copper-catalyzed amination. Notably, this net-oxidative C(sp3)-N forming reaction operates under mild electrode potentials and proceeds through hydrogen evolution, eliminating the need for external chemical oxidants. Our research enables the facile decarboxylative amination of a set of sp3-rich small-ring cage carboxylic acids, thus offering a versatile bioisosteric replacement for ortho-, meta-, and para-substituted anilines and di(hetero)aryl amines.

Isosteric 3D Bicyclo[1.1.1]Pentane (BCP) Core-Based Lipids for mRNA Delivery and CRISPR/Cas Gene Editing

Lipid nanoparticles (LNPs) are an essential component of messenger RNA (mRNA) vaccines and genome editing therapeutics. Ionizable amino lipids, which play the most crucial role in enabling mRNA to overcome delivery barriers, have, to date, been restricted to two-dimensional (2D) architectures. Inspired by improved physicochemical properties resulting from the incorporation of three-dimensionality (3D) into small-molecule drugs, we report the creation of 3D ionizable lipid designs through the introduction of bicyclo[1.1.1]pentane (BCP) core motifs. BCP-based lipids enabled efficient in vivo mRNA delivery to the liver and spleen with significantly greater performance over 2D benzene- and cyclohexane-based analogues. Notably, lead BCP-NC2-C12 LNPs mediated ∼90% reduction in the PCSK9 serum protein level via CRISPR/Cas9 gene knockout, outperforming 2D controls and clinically used DLin-MC3-DMA LNPs at the same dose. Here, we introduce BCP-based designs with superior in vivo activity, thereby expanding the chemical scope of ionizable amino lipids from 2D to 3D and offering a promising avenue to improve mRNA and gene editing efficiency for the continued development of genetic medicines.

Halogen Bonding Initiated Difunctionalization of [1.1.1]Propellane via Photoinduced Polarity Match Additions

Bicyclo[1.1.1]pentane (BCP), recognized as a bioisostere for para-disubstituted benzene, has gained widespread interest in drug development due to its ability to enhance the physicochemical properties of pharmaceuticals. In this work, we introduce a photoinduced, halogen bonding-initiated, metal-free strategy for synthesizing various BCP derivatives. This method involves the generation of nucleophilic α-aminoalkyl radicals via halogen-bonding adducts. These undergo selective radical addition to [1.1.1]propellane, yielding electrophilic BCP radicals that subsequently participate in polarity-matched additions, culminating in the difunctionalization of bicyclopentane. The versatility and practicality of this metal-free approach are underscored by its broad substrate scope, which includes late-stage functionalization and a series of valuable transformations, all conducted under mild reaction conditions.

Electron Donor-Acceptor Complex-Driven Divergent Routes to α-Amino Bicyclopentyl Iodides and Methylene Cyclobutanols

Amines are essential due to their structural diversity and biological significance. Introducing the valuable bicyclopentane (BCP) moiety at the α-position of amines offers a promising strategy for developing novel bioactive compounds. This study outlines a divergent synthesis approach to generate α-amino-functionalized bicyclopentyl iodides and methylene cyclobutanols under mild and environmentally sustainable conditions. By employing an electron donor-acceptor (EDA) complex between amine and the inexpensive and readily available inorganic base Cs2CO3, this method circumvents the need for strongly reducing, metal-based photocatalysts typically required for aryl radical generation. This approach enables the α-functionalization of various aliphatic and alicyclic amines via a 1,5-hydrogen atom transfer mechanism.

Photocatalytic Diheteroarylation of [1.1.1]Propellane for the Construction of 1,3-Diheteroaryl Bicyclo[1.1.1]pentanes

Herein, we report a visible light-induced diheteroarylation reaction of [1.1.1]propellane to synthesize 1,3-diheteroaryl bicyclo[1.1.1]pentanes (BCPs). In this approach, heteroaryl radicals are generated from heteroaryl halides via photocatalysis and subsequently added to [1.1.1]propellane. The in situ generated BCP radicals are then trapped by various heterocycles to furnish 1,3-diheteroaryl BCPs. Notably, this strategy features metal-free, mild conditions and utilizes inexpensive catalyst. For the first time, the diheteroarylation of [1.1.1]propellane could be achieved via a radical strategy, allowing for the efficient synthesis of 1,3-diheteroaryl BCPs with various applications in organic and medicinal chemistry.

Benzyl Alcohol Functionalization of [1.1.1]Propellane with Alkanes and Aldehydes

Bicyclo[1.1.1]pentanes (BCPs) play a crucial role in drug discovery research as C(sp3)-rich bioisosteres of benzene rings. However, the preparation of BCPs with strong alkane C(sp3)-H bonds has not been reported to date. In this study, we reported a method for light-induced benzyl alcohol functionalization of [1.1.1]propellane with aliphatic hydrocarbons (which have not previously been explored for this purpose) and aldehydes under metal- and photocatalyst-free conditions. The BCP products could be transformed into various useful derivatives, demonstrating the utility of the method. Notably, we achieved the synthesis of functionalized BCPs with simple alkanes.

Synthesis of 1-Azabicyclo[2.1.1]hexanes via Formal Single Electron Reduction of Azabicyclo[1.1.0]butanes under Photochemical Conditions

C(sp3)-rich heterocycles are privileged building blocks for pharmaceuticals and agrochemicals. Therefore, synthetic methods that provide access to novel saturated nitrogen-containing heterocycles are in high demand. Herein, we report a general synthesis of 1-azabicyclo[2.1.1]hexanes (1-aza-BCH) via a formal cycloaddition of azabicyclo[1.1.0]butanes (ABB) with styrenes under photochemical conditions. To overcome the challenging direct single electron reduction of ABBs, we designed a polar-radical-polar relay strategy that leverages a fast acid-mediated ring-opening of ABBs to form bromoazetidines, which undergo efficient debrominative radical formation to initiate the cycloaddition reaction. The reaction is applicable to a broad range of ABB-ketones and we demonstrate the 1-aza-BCH products can be further functionalized to access larger saturated, conformationally rigid heterocycles.

Direct Diazoarylation of [1.1.1]Propellane with Arenediazonium Salts: A Modular Assembly of Arylated Diazo Bicyclo[1.1.1]pentanes

A mild and concise diazoarylation of [1.1.1]propellane is described, which provides a modular approach to arylated diazo bicyclopentanes (BCPs). This reaction proceeds smoothly under basic conditions without requiring other additives or catalysts. The substrate scope shows that various electron-withdrawing and electron-donating groups are tolerated, and the subsequent modifications provide a novel avenue for assembling arylamino-BCP analogs.

Discovery and Optimization of N-Heteroaryl Indazole LRRK2 Inhibitors

Inhibition of leucine-rich repeat kinase 2 is a genetically supported mechanism for the treatment of Parkinson's disease. We previously disclosed the discovery of an indazole series lead that demonstrated both safety and translational risks. The safety risks were hypothesized to be of unknown origin, so structural diversity in subsequent chemical matter was prioritized. The translational risks were identified due to a low brain Kpu,u in nonhuman primate studies, which raised concern over the use of an established peripheral biomarker as a surrogate for central target engagement. Given these challenges, the team sought to leverage structure- and property-based drug design and expanded efflux transporter profiling to identify structurally distinct leads with enhanced CNS drug-likeness. Herein, we describe the discovery of a "reinvented" indazole series with improved physicochemical properties and efflux transporter profiles while maintaining excellent potency and off-target kinase selectivity, which resulted in advanced lead, compound 23.

Photocatalytic Decarboxylative Alkylation of Cyclic Imine-BF3 Complexes: A Modular Route to Functionalized Azacycles

Alkyl radicals generated via an acridine photocatalyzed decarboxylation reaction of feedstock carboxylic acids engage with a range of cyclic imine-BF3 complexes to provide α-functionalized azacycles in an operationally simple process. A three-component variant of this transformation incorporating [1.1.1]propellane as an additional reaction partner enables the synthesis of valuable bicyclopentane (BCP)-containing azacycles. Reactions exhibit good functional group compatibility, enabling late-stage modification of complex bioactive molecules.

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.

Photochemical tandem reaction of nitrogen containing heterocycles, bicyclo[1.1.1]pentane, and difluoroiodane(III) reagents

A visible light-induced difluoroalkylation/heteroarylation of [1.1.1]propellane with nitrogen containing heterocycles and difluoroiodane(III) reagents was achieved. Various heteroarenes and difluoroiodane(III) reagents exhibited good compatibility, yielding the desired products in moderate to good yields. The accessibility of the reagents and the mild reaction conditions establish this method as an alternative and practical strategy for accessing diverse 1-difluoroalkyl-3-heteroaryl bicyclo[1.1.1]pentanes (BCPs).

Photocatalyzed 1,3-Bromodifluoroallylation of [1.1.1]Propellane with α-Trifluoromethylalkenes and KBr Salts

Herein we unveil a visible-light-driven transition-metal-free 1,3-bromodifluoroallylation of [1.1.1]propellane. This reactivity is harnessed through organophotocatalysis, providing practical synthetic pathways to 1-brominated-3-gem-difluoroallylic bicyclo[1.1.1]pentane derivatives, particularly derived from readily available α-trifluoromethylalkenes and inexpensive KBr salts utilized as precursors for bromine radicals. Mechanistic investigations reveal that bromide anions quench the excited state of the photocatalyst, leading to the formation of bromine radicals, which react in a strain-release radical addition process rather than hydrogen atom abstraction with [1.1.1]propellane.

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.

α-Amino bicycloalkylation through organophotoredox catalysis

Bridged bicycloalkanes such as bicyclo[1.1.1]pentanes (BCPs) and bicyclo[3.1.1]heptanes (BCHeps) are important motifs in contemporary drug design due to their potential to act as bioisosteres of disubstituted benzene rings, often resulting in compounds with improved physicochemical and pharmacokinetic properties. Access to such motifs with proximal nitrogen atoms (i.e. α-amino/amido bicycloalkanes) is highly desirable for drug discovery applications, but their synthesis is challenging. Here we report an approach to α-amino BCPs and BCHeps through the visible-light enabled addition of α-amino radicals to the interbridgehead C-C bonds of [1.1.1] and [3.1.1]propellane respectively. The reaction proceeds under exceptionally mild conditions and displays broad substrate scope, providing access to an array of medicinally-relevant BCP and BCHep products. Experimental and computational mechanistic studies provide evidence for a radical chain pathway which depends critically on the stability of the α-amino radical, as well as effective catalyst turnover.

Synthesis of Azo-Substituted Bicyclo[1.1.1]pentanes (BCPs) via Base-Promoted Halogen Atom Transfer

Because of the three-dimensional bioisosteric feature, bicyclo[1.1.1]pentylamines (BCPAs) are valuable scaffolds in synthetic chemistry and medicinal chemistry. Here, we report a Halogen Atom Transfer (XAT) mediated radical C-N coupling between C3-iodo-BCPs and diazonium salts in the presence of base. Similarly, a multicomponent reaction (MCR) enables the simultaneous construction of the C-C bond and C-N bond simultaneously. Versatile roles of diazonium salts were also explored.

Synthesis of Stereodefined Polysubstituted Bicyclo[1.1.0]butanes

We report a highly diastereoselective synthesis of polysubstituted bicyclobutanes possessing up to three stereodefined quaternary centers and five substituents. Our strategy involves a diastereoselective carbometalation of cyclopropenes followed by a cyclization to furnish the bicyclobutane ring system. This straightforward approach allows for the incorporation of a diverse range of substituents and functional groups, notably without the need for electron-withdrawing functionalities.

Enantioselective Synthesis of α-Chiral Bicyclo[1.1.1]pentanes via Multicomponent Asymmetric Allylic Alkylation

Bicyclo[1.1.1]pentanes (BCPs) have emerged as important structural motifs in drug design. However, asymmetric transformations that provide chiral BCPs bearing an adjacent stereocenter are still scarce. Here, we report a catalytic methodology for the enantioselective synthesis of α-chiral 1,3-difunctionalized BCPs from a three-component coupling of [1.1.1]propellane, a Grignard reagent, and an allylic phosphate. The reaction proceeds via the addition of the Grignard reagent to [1.1.1]propellane followed by an asymmetric N-heterocyclic carbene (NHC)-catalyzed allylic substitution of the resulting BCP-Grignard, providing a broad range of α-chiral BCPs with excellent levels of regioselectivity and enantioselectivity.

(Bio)isosteres of ortho- and meta-substituted benzenes

Saturated bioisosteres of substituted benzenes offer opportunities to fine-tune the properties of drug candidates in development. Bioisosteres of para-benzenes, such as those based on bicyclo[1.1.1]pentane, are now very common and can be used to increase aqueous solubility and improve metabolic stability, among other benefits. Bioisosteres of ortho- and meta-benzenes were for a long time severely underdeveloped by comparison. This has begun to change in recent years, with a number of potential systems being reported that can act as bioisosteres for these important fragments. In this review, we will discuss these recent developments, summarizing the synthetic approaches to the different bioisosteres as well as the impact they have on the physiochemical and biological properties of pharmaceuticals and agrochemicals.

Light-Driven Synthesis and Functionalization of Bicycloalkanes, Cubanes and Related Bioisosteres

Bicycloalkanes, cubanes and their structural analogues have emerged as bioisosteres of (hetero)arenes. To meet increasing demand, the chemical community has developed a plethora of novel synthetic methods. In this review, we assess the progress made in the field of light-driven construction and functionalization of such relevant molecules. We have focused on diverse structural targets, as well as on reaction processes giving access to: (i) [1.1.1]-bicyclopentanes (BCPs); (ii) [2.2.1]-bicyclohexanes (BCHs); (iii) [3.1.1]-bicycloheptanes (BCHeps); and (iv) cubanes; as well as other structurally related scaffolds. Finally, future perspectives dealing with the identification of novel reaction manifolds to access new functionalized bioisosteric units are discussed.

Decarboxylative C-N Coupling of 2,2-Difluorobicyclo[1.1.1]pentane (BCP-F2) Building Blocks

Described herein is our effort toward achieving the decarboxylative functionalization of 2,2-difluorobicyclo[1.1.1]pentane (BCP-F2) building blocks. When compared with the nonfluorinated bicyclo[1.1.1]pentane (BCP) analogues, we discovered divergent reactivities. This is the first successful decarboxylative coupling of BCP-F2 building blocks reported via the photoredox mechanism.

Expanding the Frontier of Linear Drug Design: Cu-Catalyzed Csp -Csp 3 -Coupling of Electron-Deficient SF4 -Alkynes with Alkyl Iodides

Despite the attractive properties of tetrafluorosulfanyl (SF4 ) compounds in drug discovery, medicinal research on SF4 molecules is hindered by the scarcity of suitable synthetic methodologies. Drawing inspiration from the well-established Sonogashira cross-coupling of terminal alkynes under Pd-catalysis, it is envisioned that SF4 -alkynes can serve as effective coupling partners. To overcome the challenges associated with the electron-deficient nature of SF4 -alkynes and the lability of the SF4 unit under transition-metal catalysis, an aryl radical mediated Csp -Csp 3 cross-coupling reaction is successfully developed under Cu catalysis. This methodology facilitates the coupling of SF4 -alkynes with alkyl iodides, leading to the immediate synthesis of SF4 -attached drug-like molecules. These findings highlight the potential impact of SF4 -containing molecules in the drug industry, paving the way for further research in this emerging field.

Spiro[3.3]heptane as a Saturated Benzene Bioisostere

The spiro[3.3]heptane core, with the non-coplanar exit vectors, was shown to be a saturated benzene bioisostere. This scaffold was incorporated into the anticancer drug sonidegib (instead of the meta-benzene), the anticancer drug vorinostat (instead of the phenyl ring), and the anesthetic drug benzocaine (instead of the para-benzene). The patent-free saturated analogs obtained showed a high potency in the corresponding biological assays.

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.

Cross Dehydrogenative Coupling of SF4-Alkyne with Tetrahydroisoquinolines

This study introduces a dual-catalytic method for cross-dehydrogenative coupling (CDC) between tetrahydroisoquinolines and Py-SF4-alkyne using visible-light photoredox catalysis. This protocol enables selective C(sp3)-H alkynylation, expanding the synthetic toolkit for SF4-based molecules. Demonstrating efficiency and substrate versatility, this approach opens new avenues in hexacoordinated tetrafluorinated sulfur chemistry and CDC strategies and holds significant promise for drug discovery and materials science.

Programmable late-stage functionalization of bridge-substituted bicyclo[1.1.1]pentane bis-boronates

Modular functionalization enables versatile exploration of chemical space and has been broadly applied in structure-activity relationship (SAR) studies of aromatic scaffolds during drug discovery. Recently, the bicyclo[1.1.1]pentane (BCP) motif has increasingly received attention as a bioisosteric replacement of benzene rings due to its ability to improve the physicochemical properties of prospective drug candidates, but studying the SARs of C2-substituted BCPs has been heavily restricted by the need for multistep de novo synthesis of each analogue of interest. Here we report a programmable bis-functionalization strategy to enable late-stage sequential derivatization of BCP bis-boronates, opening up opportunities to explore the SARs of drug candidates possessing multisubstituted BCP motifs. Our approach capitalizes on the inherent chemoselectivity exhibited by BCP bis-boronates, enabling highly selective activation and functionalization of bridgehead (C3)-boronic pinacol esters (Bpin), leaving the C2-Bpin intact and primed for subsequent derivatization. These selective transformations of both BCP bridgehead (C3) and bridge (C2) positions enable access to C1,C2-disubstituted and C1,C2,C3-trisubstituted BCPs that encompass previously unexplored chemical space.

Enantioselective synthesis of chiral BCPs

Bicyclo[1.1.1]pentanes (BCPs) have emerged as an interesting scaffold in drug design. These strained molecules can act as bioisosteres of para-substituted phenyl rings, tert-butyl groups or internal alkynes, leading to drug analogues with enhanced pharmacokinetic and physicochemical properties. Thus, catalytic methodologies for the synthesis of BCPs represent a major goal in modern organic synthesis. In particular, asymmetric transformations that provide chiral BCPs bearing an adjacent stereocenter are particularly valuable to expand the chemical space of this important scaffold. In this article, we discuss the available methodologies for the asymmetric synthesis of α-chiral BCPs, their key mechanistic features and their application in bioisosteric replacements in drug design.

Bridge Cross-Coupling of Bicyclo[1.1.0]butanes

Bicyclo[1.1.0]butanes (BCBs) have gained growing popularity in "strain release" chemistry for the synthesis of four-membered-ring systems and para- and meta-disubstituted arene bioisosteres as well as applications in chemoselective bioconjugation. However, functionalization of the bridge position of BCBs can be challenging due to the inherent strain of the ring system and reactivity of the central C-C bond. Here we report the first late-stage bridge cross-coupling of BCBs, mediated by directed metalation/palladium catalysis.

C-F bond activation enables synthesis of aryl difluoromethyl bicyclopentanes as benzophenone-type bioisosteres

Bioisosteric design has become an essential approach in the development of drug molecules. Recent advancements in synthetic methodologies have enabled the rapid adoption of this strategy into drug discovery programs. Consequently, conceptionally innovative practices would be appreciated by the medicinal chemistry community. Here we report an expeditous synthetic method for synthesizing aryl difluoromethyl bicyclopentane (ADB) as a bioisostere of the benzophenone core. This approach involves the merger of light-driven C-F bond activation and strain-release chemistry under the catalysis of a newly designed N-anionic-based organic photocatalyst. This defluorinative coupling methodology enables the direct conversion of a wide variety of commercially available trifluoromethylaromatic C-F bonds (more than 70 examples) into the corresponding difluoromethyl bicyclo[1.1.1]pentanes (BCP) arenes/difluoromethyl BCP boronates in a single step. The strategy can also be applied to [3.1.1]and [4.1.1]propellane systems, providing access to analogues with different geometries. Moreover, we have successfully used this protocol to rapidly prepare ADB-substituted analogues of the bioactive molecule Adiporon. Biological testing has shown that the ADB scaffold has the potential to enhance the pharmacological properties of benzophenone-type drug candidates.

Stereoselective Alder-Ene Reactions of Bicyclo[1.1.0]butanes: Facile Synthesis of Cyclopropyl- and Aryl-Substituted Cyclobutenes

Bicyclo[1.1.0]butanes (BCBs), strained carbocycles comprising two fused cyclopropane rings, have become well-established building blocks in organic synthesis, medicinal chemistry, and chemical biology due to their diverse reactivity profile with radicals, nucleophiles, cations, and carbenes. The constraints of the bicyclic ring system confer high p-character on the interbridgehead C-C bond, leading to this broad reaction profile; however, the use of BCBs in pericyclic processes has to date been largely overlooked in favor of such stepwise, non-concerted additions. Here, we describe the use of BCBs as substrates for ene-like reactions with strained alkenes and alkynes, which give rise to cyclobutenes decorated with highly substituted cyclopropanes and arenes. The former products are obtained from highly stereoselective reactions with cyclopropenes, generated in situ from vinyl diazoacetates under blue light irradiation (440 nm). Cyclobutenes featuring a quaternary aryl-bearing carbon atom are prepared from equivalent reactions with arynes, which proceed in high yields under mild conditions. Mechanistic studies highlight the importance of electronic effects in this chemistry, while computational investigations support a concerted pathway and rationalize the excellent stereoselectivity of reactions with cyclopropenes.

Bicyclopentylation of Alcohols with Thianthrenium Reagents

Herein we present the first method for the synthesis of bicyclo[1.1.1]pentyl (BCP) alkyl ethers from alcohols. The reaction uses BCP-thianthrenium reagents and is catalyzed by a dual copper/photoredox catalyst system. Unlike known alkylations of tertiary alcohols via carbocation intermediates, our Cu-mediated radical process circumvents the labile BCP carbocations. The approach demonstrates a broad tolerance for functional groups when applied to primary, secondary, and even tertiary alcohols. In addition, we highlight the utility of this method in late-stage functionalizations of both natural products and pharmaceuticals as well as in the rapid construction of BCP analogs of known pharmaceuticals that would otherwise be difficult to access.

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 Bicyclo[1.1.0]butanes from Iodo-Bicyclo[1.1.1]pentanes

We describe a two-step process for the synthesis of substituted bicyclo[1.1.0]butanes. A photo-Hunsdiecker reaction generates iodo-bicyclo[1.1.1]pentanes under metal-free conditions at room temperature. These intermediates react with nitrogen and sulfur nucleophiles to afford substituted bicyclo[1.1.0]butane products.

Metal and catalyst-free strategy to access 1,3-thio-heteroaryl BCP derivatives

The widespread presence of bicyclo[1.1.1]pentane (BCP) and sulfur motifs in pharmaceutical compounds underscores the significance of synthesizing suitably functionalized BCP thioethers. In response, we have developed a metal-free and photocatalyst-free strategy that harnesses visible light-induced radical cascades. This approach culminates in the synthesis of essential thio-BCP derivatives, which serve as crucial precursors for the formation of the corresponding sulfoxides, sulfones, and sulfoximines. Importantly, this methodology exhibits potential for large-scale applications, displaying commendable tolerance towards various functional groups while operating under mild reaction conditions.

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.

Synthesis of polysubstituted bicyclo[2.1.1]hexanes enabling access to new chemical space

Saturated bridged-bicyclic compounds are currently under intense investigation as building blocks for pharmaceutical drug design. However, the most common methods for their preparation only provide access to bridgehead-substituted structures. The synthesis of bridge-functionalised species is highly challenging but would open up many new opportunities for molecular design. We describe a photocatalytic cycloaddition reaction that provides unified access to bicyclo[2.1.1]hexanes with 11 distinct substitution patterns. Bridge-substituted structures that represent ortho-, meta-, and polysubstituted benzene bioisosteres, as well as those that enable the investigation of chemical space inaccessible to aromatic motifs can all be prepared using this operationally simple protocol. Proof-of-concept examples of the application of the method to the synthesis of saturated analogues of biorelevant trisubstituted benzenes are also presented.

Preparation of 3,5-Methanobenzo[b]azepines: An sp3-Rich Quinolone Isostere

The replacement of the aromatic ring in bioactive compounds with saturated bioisosteres has become a popular tactic to obtain novel structures with improved physicochemical profiles. In this paper, we describe an efficient synthesis of 3,5-methanobenzo[b]azepine analogues and suggest them as isosteres of quinolones. Quinolones are heteroaromatic, flat rings and considered as privileged scaffolds. An isosteric version of this scaffold with more 3D character would offer new options to expand their use.

Exceptional reactivity of the bridgehead amine on bicyclo[1.1.1]pentane

Bicyclo[1.1.1]pentane (BCP) has received substantial interest in the field of synthetic chemistry recently for its potential use as a benzene isostere in medicinal chemistry. Whereas bicyclo[2.2.2]octane (BCO) has also been used as a bioisostere of benzene, the condensation of BCP-amine with nadic anhydride is significantly easier than that of BCO-amine. Analyses of the geometries and the frontier molecular orbitals of these amines suggest that the low steric hindrance and high intrinsic nucleophilicity of BCP-amine together contribute to its exceptional reactivity.

Metal-Free Photoredox Four-Component Strategy to 1,3-Functionalized BCP Derivatives

Trifluoromethyl bicyclo[1.1.1]pentanes (BCPs) have attracted significant attention from the scientific community and pharmaceutical industries due to their advantageous physicochemical properties as arene bioisosteres. Initial photoredox perfluoroalkylation of [1.1.1]propellane triggers the tandem reaction to the perfluoroalkyl BCP radical followed by Giese addition to an in situ generated electron-deficient alkene by Knoevenagel condensation in a four-component fashion to form 1,3-functionalized BCPs. This strategy provides easy access to various 1,3-functionalized perfluoroalkyl BCP derivatives with the added advantage of nitrile group as a functional handle to diversified transformations. This methodology offers scalability and late-stage derivatization of drug molecules with high chemoselectivity.

Silver(I)-Catalyzed Synthesis of Cuneanes from Cubanes and their Investigation as Isosteres

Bridged or caged polycyclic hydrocarbons have rigid structures that project substituents into precise regions of 3D space, making them attractive as linking groups in materials science and as building blocks for medicinal chemistry. The efficient synthesis of new or underexplored classes of such compounds is, therefore, an important objective. Herein, we describe the silver(I)-catalyzed rearrangement of 1,4-disubstituted cubanes to cuneanes, which are strained hydrocarbons that have not received much attention since they were first described in 1970. The synthesis of 2,6-disubstituted or 1,3-disubstituted cuneanes can be achieved with high regioselectivities, with the regioselectivity being dependent on the electronic character of the cubane substituents. A preliminary assessment of cuneanes as scaffolds for medicinal chemistry suggests cuneanes could serve as isosteric replacements of trans-1,4-disubstituted cyclohexanes and 1,3-disubstituted benzenes. An analogue of the anticancer drug sonidegib was synthesized, in which the 1,2,3-trisubstituted benzene was replaced with a 1,3-disubstituted cuneane.

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.

A General and Practical Route to Functionalized Bicyclo[1.1.1]Pentane-Heteroaryls Enabled by Photocatalytic Multicomponent Heteroarylation of [1.1.1]Propellane

1-Aryl-substituted bicyclo[1.1.1]pentanes (BCPs) are an important class of BCP derivatives with widespread application in drug development. Most syntheses of these materials require multiple chemical steps via BCP electrophiles or nucleophiles derived from [1.1.1]propellane. Although one-step, multicomponent radical cross-coupling reactions could provide a more sustainable and rapid route to access diverse heteroarylated BCPs, current approaches are limited to tertiary alkyl radicals, leading to a decrease in their practical value. In this study, a conceptually different approach enabled by a radical multicomponent heteroarylation of [1.1.1]propellane to access functionalized heteroarylated BCPs is described. Importantly, this protocol is compatible with primary-, secondary-, and tertiary aliphatic radicals, as well as various fluoroalkyl radical sources, thus enabling rapid library generation of sought-after BCP derivatives for drug development.

Photocatalytic Three-Component Synthesis of 3-Heteroarylbicyclo[1.1.1]pentane-1-acetates

Herein, we report a visible-light-induced three-component reaction involving [1.1.1]propellane, diazoates, and various heterocycles for the synthesis of 3-heteroarylbicyclo[1.1.1]pentane-1-acetates. Throughout this reaction, the radicals generated from diazoate species react with [1.1.1]propellane in an addition reaction to form bicyclo[1.1.1]pentane (BCP) radicals that subsequently react with heterocycles, leading to the formation of 1,3-disubstituted BCP acetates. Notably, this methodology exhibits excellent functional group compatibility, high atom economy, and mild reaction conditions, thus facilitating suitable synthetic access to 1,3-disubstituted BCP acetates.

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.

Nickel-Mediated Alkyl-, Acyl-, and Sulfonylcyanation of [1.1.1]Propellane

The replacement of traditional functional groups with polycyclic scaffolds has been increasingly rewarding in medicinal chemistry programs. Over the decades, 1,3-disubstituted bicyclo[1.1.1]pentanes (BCPs) have demonstrated the potential for being competent bioisosteres for aryl-, alkyl- and alkynyl substructures. Although highly desired, mild and versatile synthetic methods to access synthetically valuable BCP-containing building blocks remain limited. Herein, a versatile way to access bridgehead substituted BCP nitriles, a useful BCP building block, is described, enabled by the unexpected selectivity of nickel in the multi-component radical cyanation. Commodity materials including carboxylic acids, amines, sulfonyl chlorides, and alkyl chlorides are engaged to provide a broad spectrum of substituted BCP nitriles in a single-step, multi-component fashion.