Saturated Bioisosteres of ortho-Substituted Benzenes

Bicyclo[1.1.0]butyl Radical Cations: Synthesis and Application to [2π + 2σ] Cycloaddition Reactions

As the chemistry that surrounds the field of strained hydrocarbons, such as bicyclo[1.1.0]butane, continues to expand, it becomes increasingly advantageous to develop alternative reactivity modes that harness their unique properties to access new regions of chemical space. Herein, we report the use of photoredox catalysis to promote the single-electron oxidation of bicyclo[1.1.0]butanes. The synthetic utility of the resulting radical cations is highlighted by their ability to undergo highly regio- and diastereoselective [2π + 2σ] cycloaddition reactions. The most notable feature of this transformation is the breadth of alkene classes that can be employed, including nonactivated alkenes, which have so far been elusive for previous strategies. A rigorous mechanistic investigation, in conjunction with DFT computation, was undertaken in order to better understand the physical nature of bicyclo[1.1.0]butyl radical cations and thus provides a platform from which further studies into the synthetic applications of these intermediates can be built upon.

Stellane at the Forefront: Derivatization and Reactivity Studies of a Promising Saturated Bioisostere of ortho-Substituted Benzenes

This work highlights stellane's cage stability and derivatization opportunities. A diverse range of building blocks were synthesized using modern synthesis protocols to demonstrate stellane's reactivity and chemical tolerance across different reaction systems, proving its promise as a bioisosteric scaffold. It can be utilized in scaffold-based molecular design and is superior in terms of topological precision compared to existing ortho isosteres, as well as monosubstituted benzene mimetics, holding the potential to become a robust platform for future medicinal chemistry studies.

Titanium catalyzed [2σ + 2π] cycloaddition of bicyclo[1.1.0]-butanes with 1,3-dienes for efficient synthesis of stilbene bioisosteres

Natural stilbenes have shown significant potential in the prevention and treatment of diseases due to their diverse pharmacological activities. Here we present a mild and effective Ti-catalyzed intermolecular radical-relay [2σ + 2π] cycloaddition of bicyclo[1.1.0]-butanes and 1,3-dienes. This transformation enables the synthesis of bicyclo[2.1.1]hexane (BCH) scaffolds containing aryl vinyl groups with excellent regio- and trans-selectivity and broad functional group tolerance, thus offering rapid access to structurally diverse stilbene bioisosteres.

Silver-Enabled Cycloaddition of Bicyclobutanes with Isocyanides for the Synthesis of Polysubstituted 3-Azabicyclo[3.1.1]heptanes

Synthesis of bicyclic scaffolds has emerged as an important research topic in modern drug development because they can serve as saturated bioisosters to enhance the physicochemical properties and metabolic profiles of drug candidates. Here we report a remarkably simple silver-enabled strategy to access polysubstituted 3-azabicyclo[3.1.1]heptanes in a single operation from readily accessible bicyclobutanes (BCBs) and isocyanides. The process is proposed to involve a formal (3+3)/(3+2)/retro-(3+2) cycloaddition sequence. This novel protocol allows for rapid generation of molecular complexity from simple starting materials, and the products can be easily derivatized, further enriching the BCB cycloaddition chemistry and the growing set of valuable sp3-rich bicyclic building blocks.

B(C6F5)3-Catalyzed Formal (n + 3) (n = 5 and 6) Cycloaddition of Bicyclo[1.1.0]butanes to Medium Bicyclo[n.1.1]alkanes

Herein, a B(C6F5)3-catalyzed formal (n + 3) (n = 5 and 6) cycloaddition of bicyclo[1.1.0]butanes (BCBs) with imidazolidines/hexahydropyrimidines is described. The reaction provides a modular, atom-economical, and efficient strategy to two libraries of synthetically challenging medium-bridged rings, 2,5-diazabicyclo[5.1.1]nonanes and 2,6-diazabicyclo[6.1.1]decanes, in moderate to excellent yields. This reaction also features simple operation, mild reaction conditions, and broad substrate scope. A scale-up experiment and various synthetic transformations of products further highlight the synthetic utility.

Palladium-Catalyzed Ligand-Controlled Switchable Hetero-(5 + 3)/Enantioselective [2σ+2σ] Cycloadditions of Bicyclobutanes with Vinyl Oxiranes

For nearly 60 years, significant research efforts have been focused on developing strategies for the cycloaddition of bicyclobutanes (BCBs). However, higher-order cycloaddition and catalytic asymmetric cycloaddition of BCBs have been long-standing formidable challenges. Here, we report Pd-catalyzed ligand-controlled, tunable cycloadditions for the divergent synthesis of bridged bicyclic frameworks. The dppb ligand facilitates the formal (5+3) cycloaddition of BCBs and vinyl oxiranes, yielding valuable eight-membered ethers with bridged bicyclic scaffolds in 100% regioselectivity. The Cy-DPEphos ligand promotes selective hetero-[2σ+2σ] cycloadditions to access pharmacologically important 2-oxabicyclo[3.1.1]heptane (O-BCHeps). Furthermore, the corresponding catalytic asymmetric synthesis of O-BCHeps with 94-99% ee has been achieved using chiral (S)-DTBM-Segphos, representing the first catalytic asymmetric cross-dimerization of two strained rings. The obtained O-BCHeps are promising bioisosteres for ortho-substituted benzenes.

2-Oxabicyclo[2.1.1]hexanes: Synthesis, Properties, and Validation as Bioisosteres of ortho- and meta-Benzenes

We have developed a general and practical approach towards 2-oxabicyclo[2.1.1]hexanes with two and three exit vectors via an iodocyclization reaction. The obtained compounds have been easily converted into the corresponding building blocks for use in medicinal chemistry. 2-Oxabicyclo[2.1.1]hexanes have been incorporated into the structure of five drugs and three agrochemicals, and validated biologically as bioisosteres of ortho- and meta-benzenes.

(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.

Synthesis of Borylated Carbocycles by [2 + 2]-Cycloadditions and Photo-Ene Reactions

Saturated bicyclic compounds make up a valuable class of building blocks in the development of agrochemicals and pharmaceuticals. Here, we present the synthesis of borylated bicyclo[2.1.1]hexanes via crossed [2 + 2]-cycloaddition. Due to the presence of the C-B bond, a variety of structures can be easily prepared that are not accessible by other methods. Moreover, a rare photo-ene reaction is also disclosed, allowing for the diastereoselective synthesis of trisubstituted borylated cyclopentanes.

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.

Eu(OTf)3 -Catalyzed Formal Dipolar [4π+2σ] Cycloaddition of Bicyclo-[1.1.0]butanes with Nitrones: Access to Polysubstituted 2-Oxa-3-azabicyclo[3.1.1]heptanes

Herein, we have synthesized multifunctionalized 2-oxa-3-azabicyclo[3.1.1]heptanes, which are considered potential bioisosteres for meta-substituted arenes, through Eu(OTf)3 -catalyzed formal dipolar [4π+2σ] cycloaddition of bicyclo[1.1.0]butanes with nitrones. This methodology represents the initial instance of fabricating bicyclo[3.1.1]heptanes adorned with multiple heteroatoms. The protocol exhibits both mild reaction conditions and a good tolerance for various functional groups. Computational density functional theory calculations support that the reaction mechanism likely involves a nucleophilic addition of nitrones to bicyclo[1.1.0]butanes, succeeded by an intramolecular cyclization. The synthetic utility of this novel protocol has been demonstrated in the concise synthesis of the analogue of Rupatadine.

Assessing the rigidity of cubanes and bicyclo(1.1.1)pentanes as benzene bioisosteres

Aromatic rings are critical core substructures in the majority of pharmaceutical compounds. There is much recent interest in replacing aromatic structures with saturated bioisosteres of benzene, which are generally fused or bridged ring systems. These bioisosteres often show improved solubility properties compared to benzene, and may also undergo fewer unwanted metabolic processes. One key reason why aromatic rings have proven so successful in drug design is their rigidity. This paper uses molecular dynamics simulations supported by crystallographic data to assess the rigidity of bicyclopentane and cubane ring systems as two of the most common benzene bioisosteres and compares this to benzene. Whilst a benzene ring is shown to be more flexible than these two bioisosteres in terms of its dihedral ring flexibility, substituents around the ring tend to behave in a much more similar way in both benzene and the bioisosteric systems.

Photochemistry in Medicinal Chemistry and Chemical Biology

Photochemistry has emerged as a transformative force in organic chemistry, significantly expanding the chemical space accessible for medicinal chemistry. Light-induced reactions enable the efficient synthesis of intricate organic structures and have found applications throughout the different stages of the drug discovery and development processes. Moreover, photochemical techniques provide innovative solutions in chemical biology, allowing precise spatiotemporal drug activation and targeted delivery. In this Perspective, we highlight the already numerous remarkable applications and the even more promising future of photochemistry in medicinal chemistry and chemical biology.

Double Strain-Release [2π+2σ]-Photocycloaddition

In pursuit of potent pharmaceutical candidates and to further improve their chemical traits, small ring systems can serve as a potential starting point. Small ring units have the additional merit of loaded strain at their core, making them suitable reactants as they can capitalize on this intrinsic driving force. With the introduction of cyclobutenone as a strained precursor to ketene, the photocycloaddition with another strained unit, bicyclo[1.1.0]butane (BCB), enables the reactivity of both π-units in the transient ketene. This double strain-release driven [2π+2σ]-photocycloaddition promotes the synthesis of diverse heterobicyclo[2.1.1]hexane units, a pharmaceutically relevant bioisostere. The effective reactivity under catalyst-free conditions with a high functional group tolerance defines its synthetic utility. Experimental mechanistic studies and density functional theory (DFT) calculations suggest that the [2π+2σ]-photocycloaddition takes place via a triplet mechanism.

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.

Energy transfer photocatalysis: exciting modes of reactivity

Excited (triplet) states offer a myriad of attractive synthetic pathways, including cycloadditions, selective homolytic bond cleavages and strain-release chemistry, isomerizations, deracemizations, or the fusion with metal catalysis. Recent years have seen enormous advantages in enabling these reactivity modes through visible-light-mediated triplet-triplet energy transfer catalysis (TTEnT). This tutorial review provides an overview of this emerging strategy for synthesizing sought-after organic motifs in a mild, selective, and sustainable manner. Building on the photophysical foundations of energy transfer, this review also discusses catalyst design, as well as the challenges and opportunities of energy transfer catalysis.

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.

Mechanistic Insight into Lewis Acid-Catalyzed Cycloaddition of Bicyclo[1.1.0]butanes with Ketene: Bicyclo[1.1.0]butanes Serving as an Electrophile

Lewis acid-catalyzed cycloaddition between bicyclo[1.1.0]butanes (BCBs) and unsaturated substrates has recently been demonstrated to be a powerful strategy for synthesizing bicyclo[2.1.1]hexanes. However, their reaction mechanisms remain elusive. This computational work explored the recently developed TMSOTf-catalyzed cycloaddition of BCB ketone to ketene and determined the rate-determining step as the activation of BCB ketone. Contrary to the previous proposal of BCB enolate as the active species, this work instead identified the catalytically active species to be a partially Lewis acid-activated BCB cation, which shows a greater electrophilicity and larger orbital interactions with ketene compared to those of the pristine BCB. The most favorable reaction pathway uniquely utilizes this activated BCB species as an electrophile to react with ketene as a nucleophile, while the previously proposed enolate is relatively inactive. Moreover, the in situ-generated TfO anion is revealed to be non-innocent, and its coordination mode and orientation could affect the reaction kinetics.

A Photochemical Strategy for the Conversion of Nitroarenes into Rigidified Pyrrolidine Analogues

Bicyclic amines are important motifs for the preparation of bioactive materials. These species have well-defined exit vectors that enable accurate disposition of substituents toward specific areas of chemical space. Of all possible skeletons, the 2-azabicyclo[3.2.0]heptane framework is virtually absent from MedChem libraries due to a paucity of synthetic methods for its preparation. Here, we report a modular synthetic strategy that utilizes nitroarenes as flat and easy-to-functionalize feedstocks for the assembly of these sp3-rich materials. Mechanistically, this approach exploits two concomitant photochemical processes that sequentially ring-expand the nitroarene into an azepine and then fold it into a rigid bicycle pyrroline by means of singlet nitrene-mediated nitrogen insertion and excited-state-4π electrocyclization. A following hydrogenolysis provides, with full diastereocontrol, the desired bicyclic amine derivatives whereby the aromatic substitution pattern has been translated into the one of the three-dimensional heterocycle. These molecules can be considered rigid pyrrolidine analogues with a well-defined orientation of their substituents. Furthermore, unsupervised clustering of an expansive virtual database of saturated N-heterocycles revealed these derivatives as effective isosteres of rigidified piperidines. Overall, this platform enables the conversion of nitroarene feedstocks into complex sp3-rich heterocycles of potential interest to drug development.

Synthesis of 2-Azanorbornanes via Strain-Release Formal Cycloadditions Initiated by Energy Transfer

Rigid bicycles are becoming more popular in the pharmaceutical industry because they allow for expansion to new and unique chemical spaces. This work describes a new strategy to construct 2-azanorbornanes, which can act as rigid piperidine/pyrrolidine scaffolds with well-defined exit vectors. To achieve the synthesis of 2-azanorbornanes, new strain-release reagent, azahousane, is introduced along with its photosensitized strain-release formal cycloaddition with alkenes. Furthermore, new reactivity between a housane and an imine is disclosed. Both strategies lead to various substituted 2-azanorbornanes with good selectivities.

1,2-Disubstituted bicyclo[2.1.1]hexanes as saturated bioisosteres of ortho-substituted benzene

Bicyclo[2.1.1]hexanes have been synthesized, characterized, and biologically validated as saturated bioisosteres of the ortho-substituted benzene ring. The incorporation of the 1,2-disubstituted bicyclo[2.1.1]hexane core into the structure of fungicides boscalid (BASF), bixafen (Bayer CS), and fluxapyroxad (BASF) gave saturated patent-free analogs with high antifungal activity.

Intermolecular Formal Cycloaddition of Indoles with Bicyclo[1.1.0]butanes by Lewis Acid Catalysis

Herein, we develop a new approach to directly access architecturally complex polycyclic indolines from readily available indoles and bicyclo[1.1.0]butanes (BCBs) through formal cycloaddition promoted by commercially available Lewis acids. The reaction proceeded through a stepwise pathway involving a nucleophilic addition of indoles to BCBs followed by an intramolecular Mannich reaction to form rigid indoline-fused polycyclic structures, which resemble polycyclic indole alkaloids. This new reaction tolerated a wide range of indoles and BCBs, thereby allowing the one-step construction of various rigid indoline polycycles containing up to four contiguous quaternary carbon centers.

Silver-Catalyzed Dearomative [2π+2σ] Cycloadditions of Indoles with Bicyclobutanes: Access to Indoline Fused Bicyclo[2.1.1]hexanes

Bicyclo[2.1.1]hexanes (BCHs) are becoming ever more important in drug design and development as bridged scaffolds that provide underexplored chemical space, but are difficult to access. Here a silver-catalyzed dearomative [2π+2σ] cycloaddition strategy for the synthesis of indoline fused BCHs from N-unprotected indoles and bicyclobutane precursors is described. The strain-release dearomative cycloaddition operates under mild conditions, tolerating a wide range of functional groups. It is capable of forming BCHs with up to four contiguous quaternary carbon centers, achieving yields of up to 99 %. In addition, a scale-up experiment and the synthetic transformations of the cycloadducts further highlighted the synthetic utility.

Enolate addition to bicyclobutanes enables expedient access to 2-oxo-bicyclohexane scaffolds

We report the synthesis of 2-oxo-bicyclo[2.1.1]hexanes (2-oxo-BCHs) from bicyclobutanes (BCBs) and readily available enolate precursors. Glycine-derived enolates directly give protected 2-oxo-3-amino-BCH derivatives that can be further functionalized. Arylacetate derivatives are also suitable enolate precursors, giving 2-oxo-3-aryl-BCH scaffolds from readily available starting materials.

Hantzsch Esters Enabled [2π+2σ] Cycloadditions of Bicyclo [1.1.0] butanes and Alkenes under Photo Conditions

Hantzsch esters (HEs) are widely recognized as sources of hydride ions (H-) and sacrificial electron donors in their ground state. Here, we report the application of HE as a mediator in [2π+2σ] cycloaddition of bicyclo[1.1.0]butanes (BCBs) with alkenes under photo conditions. Through this strategy, various substituted bicyclo[2.1.1]hexanes can be efficiently prepared.

Enantioselective, Intermolecular [π2+σ2] Photocycloaddition Reactions of 2(1H)-Quinolones and Bicyclo[1.1.0]butanes

1-Substituted bicyclo[1.1.0]butanes add enantioselectively to 2(1H)-quinolones upon irradiation (λ = 366 nm) in the presence of a chiral complexing agent. A two-point hydrogen bond between the quinolone and the template is responsible for stereocontrol in the photocycloaddition reaction. The reaction leads to the formation of products with a chiral bicyclo[2.1.1]hexane skeleton in high enantiomeric excess (91-99% ee). The chiral template can be almost quantitatively (97%) recovered and used in another reaction. A triplet reaction pathway is likely, and sensitization is a suitable tool if the reaction is to be performed with visible light (λ = 420 nm).

Photocyclization by a triplet-triplet annihilation upconversion pair in water - avoiding UV-light and oxygen removal

We present a formal [2 + 2]-cycloaddition of unsaturated ketones enabled by a green-to-ultraviolet triplet-triplet annihilation upconversion (TTA-UC) pair, using commercially available Ru(bpy)32+ and pyrene as sensitizer and annihilator, respectively. In the developed protocol, visible light irradiation at λmax = 520 nm allows for the reaction to proceed without the need for UV-light and the aqueous medium eliminates the need for oxygen removing protocols. Through this study, the application of the readily available upconversion pair is broadened to include cyclization reactions. We showcase the utility of the system by generating bicyclo[2.1.1]hexanes that are valuable bioisosteres of ortho-substituted benzenes, a promising motif for pharmaceuticals.

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.

Strain-Release Photocatalysis

The concept of strain in organic compounds is as old as modern organic chemistry and was initially introduced to justify the synthetic setbacks along the synthesis of small ring systems (pars construens of strain). In the last decades, chemists have developed an arsenal of strain-release reactions (pars destruens of strain) which can generate─with significant driving force─rigid aliphatic systems that can act as three-dimensional alternatives to (hetero)arenes. Photocatalysis added an additional dimension to strain-release processes by leveraging the energy of photons to create chemical complexity under mild conditions. This perspective presents the latest advancements in strain-release photocatalysis─with emphases on mechanisms, catalytic cycles, and current limitations─the unique chemical architectures that can be produced, and possible future directions.

General Synthesis of 3-Azabicyclo[3.1.1]heptanes and Evaluation of Their Properties as Saturated Isosteres

A general approach to 3-azabicyclo[3.1.1]heptanes by reduction of spirocyclic oxetanyl nitriles was developed. The mechanism, scope, and scalability of this transformation were studied. The core was incorporated into the structure of the antihistamine drug Rupatidine instead of the pyridine ring, which led to a dramatic improvement in physicochemical properties.

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.

Lewis Acid Catalyzed Formal (3+2)-Cycloaddition of Bicyclo[1.1.0]butanes with Ketenes

Design, synthesis and application of benzene bioisosteres have attracted a lot of attention in the past 20 years. Recently, bicyclo[2.1.1]hexanes have emerged as highly attractive bioisosteres for ortho- and meta-substituted benzenes. Herein we report a mild, scalable and transition-metal-free protocol for the construction of highly substituted bicyclo[2.1.1]hexan-2-ones through Lewis acid catalyzed (3+2)-cycloaddition of bicyclo[1.1.0]-butane ketones with disubstituted ketenes. The reaction shows high functional group tolerance as documented by the successful preparation of various 3-alkyl-3-aryl as well as 3,3-bisalkyl bicyclo[2.1.1]hexan-2-ones (26 examples, up to 89 % yield). Postfunctionalization of the exocyclic ketone moiety is also demonstrated.

Catalytic Formal [2π+2σ] Cycloaddition of Aldehydes with Bicyclobutanes: Expedient Access to Polysubstituted 2-Oxabicyclo[2.1.1]hexanes

Synthesis of bicyclic scaffolds has attracted tremendous attention because they are playing an important role as saturated bioisosteres of benzenoids in modern drug discovery. Here, we report a BF3 -catalyzed [2π+2σ] cycloaddition of aldehydes with bicyclo[1.1.0]butanes (BCBs) to access polysubstituted 2-oxabicyclo[2.1.1]hexanes. A new kind of BCB containing an acyl pyrazole group was invented, which not only significantly facilitates the reactions, but can also serve as a handle for diverse downstream transformations. Furthermore, aryl and vinyl epoxides can also be utilized as substrates which undergo cycloaddition with BCBs after in situ rearrangement to aldehydes. We anticipate that our results will promote access to challenging sp3 -rich bicyclic frameworks and the exploration of BCB-based cycloaddition chemistry.

2,5-disubstituted bicyclo[2.1.1]hexanes as rigidified cyclopentane variants

Identification of rigid counterparts for common flexible scaffolds is crucial to the advancement of medicinal chemistry. Here we showcase a new class of building blocks, 2,5-disubstituted bicyclo[2.1.1]hexanes that can act as rigidified cis-, or trans-1,3-disubstituted cyclopentanes, common motifs in drugs. The scalable synthesis of these structures was enabled through the use of C-H functionalization logic and cycloaddition reactions.

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.

Properties of Aliphatic Ligand-Based Metal-Organic Frameworks

Ligands with a purely aliphatic backbone are receiving rising attention in the chemistry of coordination polymers and metal-organic frameworks. Such unique features inherent to the aliphatic bridges as increased conformational freedom, non-polarizable core, and low light absorption provide rare and valuable properties for their derived MOFs. Applications of such compounds in stimuli-responsive materials, gas, and vapor adsorbents with high and unusual selectivity, light-emitting, and optical materials have extensively emerged in recent years. These properties, as well as other specific features of aliphatic-based metal-organic frameworks are summarized and analyzed in this short critical review. Advanced characterization techniques, which have been applied in the reported works to obtain important data on the crystal and molecular structures, dynamics, and functionalities, are also reviewed within a general discussion. In total, 132 references are included.

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.

General access to cubanes as benzene bioisosteres

The replacement of benzene rings with sp3-hybridized bioisosteres in drug candidates generally improves pharmacokinetic properties while retaining biological activity1-5. Rigid, strained frameworks such as bicyclo[1.1.1]pentane and cubane are particularly well suited as the ring strain imparts high bond strength and thus metabolic stability on their C-H bonds. Cubane is the ideal bioisostere as it provides the closest geometric match to benzene6,7. At present, however, all cubanes in drug design, like almost all benzene bioisosteres, act solely as substitutes for mono- or para-substituted benzene rings1-7. This is owing to the difficulty of accessing 1,3- and 1,2-disubstituted cubane precursors. The adoption of cubane in drug design has been further hindered by the poor compatibility of cross-coupling reactions with the cubane scaffold, owing to a competing metal-catalysed valence isomerization8-11. Here we report expedient routes to 1,3- and 1,2-disubstituted cubane building blocks using a convenient cyclobutadiene precursor and a photolytic C-H carboxylation reaction, respectively. Moreover, we leverage the slow oxidative addition and rapid reductive elimination of copper to develop C-N, C-C(sp3), C-C(sp2) and C-CF3 cross-coupling protocols12,13. Our research enables facile elaboration of all cubane isomers into drug candidates, thus enabling ideal bioisosteric replacement of ortho-, meta- and para-substituted benzenes.

Catalytic undirected borylation of tertiary C-H bonds in bicyclo[1.1.1]pentanes and bicyclo[2.1.1]hexanes

Catalytic borylations of sp3 C-H bonds occur with high selectivities for primary C-H bonds or secondary C-H bonds that are activated by nearby electron-withdrawing substituents. Catalytic borylation at tertiary C-H bonds has not been observed. Here we describe a broadly applicable method for the synthesis of boron-substituted bicyclo[1.1.1]pentanes and (hetero)bicyclo[2.1.1]hexanes by an iridium-catalysed borylation of the bridgehead tertiary C-H bond. This reaction is highly selective for the formation of bridgehead boronic esters and is compatible with a broad range of functional groups (>35 examples). The method is applicable to the late-stage modification of pharmaceuticals containing this substructure and the synthesis of novel bicyclic building blocks. Kinetic and computational studies suggest that C-H bond cleavage occurs with a modest barrier and that the turnover-limiting step of this reaction is an isomerization that occurs prior to reductive elimination that forms the C-B bond.

Construction of oxygenated 2-azabicyclo[2.2.1]heptanes via palladium-catalyzed 1,2-aminoacyloxylation of cyclopentenes

Herein, we describe a palladium-catalyzed 1,2-aminoacyloxylation of cyclopentenes to synthesize oxygenated 2-azabicyclo[2.2.1]heptanes. This reaction proceeds efficiently with a broad array of substrates. The products could be further functionalized to build up a library of bridged aza-bicyclic structures.

A catalytic alkene insertion approach to bicyclo[2.1.1]hexane bioisosteres

C(sp³)-rich bicyclic hydrocarbon scaffolds, as exemplified by bicyclo[1.1.1]pentanes, play an increasingly high-profile role as saturated bioisosteres of benzenoids in medicinal chemistry and crop science. Substituted bicyclo[2.1.1]hexanes (BCHs) are emerging bicyclic hydrocarbon bioisosteres for ortho- and meta-substituted benzenes, but are difficult to access. Therefore, a general synthetic route to BCHs is needed if their potential as bioisosteres is to be realized. Here we describe a broadly applicable catalytic approach that delivers substituted BCHs by intermolecular coupling between olefins and bicyclo[1.1.0]butyl (BCB) ketones. The SmI₂-catalysed process works for a wide range of electron-deficient alkenes and substituted BCB ketones, operates with SmI₂ loadings as low as 5 mol% and is underpinned by a radical relay mechanism that is supported by density functional theory calculations. The product BCH ketones have been shown to be versatile synthetic intermediates through selective downstream manipulation and the expedient synthesis of a saturated hydrocarbon analogue of the broad-spectrum antimicrobial, phthalylsulfathiazole.

Diboron(4)-Catalyzed Remote [3+2] Cycloaddition of Cyclopropanes via Dearomative/Rearomative Radical Transmission through Pyridine

Ring structures such as pyridine, cyclopentane or their combinations are important motifs in bioactive molecules. In contrast to previous cycloaddition reactions that necessitated a directly bonded initiating functional group, this work demonstrated a novel through-(hetero)arene radical transmission concept for selective activation of a remote bond. An efficient, metal-free and atom-economical [3+2] cycloaddition between 4-pyridinyl cyclopropanes and alkenes or alkynes has been developed for modular synthesis of pyridine-substituted cyclopentanes, cyclopentenes and bicyclo[2.1.1]hexanes that are difficult to access using known methods. This complexity-building reaction was catalyzed by a very simple and inexpensive diboron(4) compound and took place via dearomative/rearomative processes. The substrate scope was broad and more than 100 new compounds were prepared in generally high yields. Mechanistic experiments and density function theory (DFT) investigation supported a radical relay catalytic cycle involving alkylidene dihydropyridine radical intermediates and boronyl radical transfer.

Bicyclo[2.1.1]hexanes by Visible Light-Driven Intramolecular Crossed [2 + 2] Photocycloadditions

Bicyclo[2.1.1]hexanes have become increasingly popular building blocks in medicinal chemistry as bridged scaffolds that provide unexplored chemical space. We herein report a visible light-driven approach to these compounds that relies on an intramolecular crossed [2 + 2] photocycloaddition of styrene derivatives enabled by triplet energy transfer. Bicyclo[2.1.1]hexanes were obtained in good to high yields (19 examples, 61%-quantitative yield) and allowed for further functionalizations by consecutive reactions, thereby opening different pathways to decorate the aliphatic core structure.

Enantioselective Syntheses of 2-Azabicyclo[2.2.1]heptanes via Brønsted Acid Catalyzed Ring-Opening of meso-Epoxides

A chiral phosphoric acid-catalyzed ring-opening of meso-epoxides was developed. A range of 2-azabicyclo[2.2.1]heptanes were obtained in high yields with excellent enantioselectivities. In addition, the hydroxyl and amide groups in the products provided handles for further derivatization.

Practical and Facile Access to Bicyclo[3.1.1]heptanes: Potent Bioisosteres of meta-Substituted Benzenes

There is increasing interest in replacement of the planar aromatic rings of drug candidates with three-dimensional caged scaffolds in order to improve the physical properties, but bioisosteres of meta-substituted benzenes have remained elusive. We focused on the bicyclo[3.1.1]heptane (BCH) scaffold as a novel bioisostere of meta-substituted benzenes, anticipating that [3.1.1]propellane (2) would be a versatile precursor of diversely functionalized BCHs. Here, we describe a practical preparative method for [3.1.1]propellane from newly developed 1,5-diiodobicyclo[3.1.1]heptane (1), as well as difunctionalization reactions of 2 leading to functionalized BCHs. We also report postfunctionalization reactions of these products.

Synthesis of meta-substituted arene bioisosteres from [3.1.1]propellane

Small-ring cage hydrocarbons are popular bioisosteres (molecular replacements) for commonly found para-substituted benzene rings in drug design1. The utility of these cage structures derives from their superior pharmacokinetic properties compared with their parent aromatics, including improved solubility and reduced susceptibility to metabolism2,3. A prime example is the bicyclo[1.1.1]pentane motif, which is mainly synthesized by ring-opening of the interbridgehead bond of the strained hydrocarbon [1.1.1]propellane with radicals or anions4. By contrast, scaffolds mimicking meta-substituted arenes are lacking because of the challenge of synthesizing saturated isosteres that accurately reproduce substituent vectors5. Here we show that bicyclo[3.1.1]heptanes (BCHeps), which are hydrocarbons for which the bridgehead substituents map precisely onto the geometry of meta-substituted benzenes, can be conveniently accessed from [3.1.1]propellane. We found that [3.1.1]propellane can be synthesized on a multigram scale, and readily undergoes a range of radical-based transformations to generate medicinally relevant carbon- and heteroatom-substituted BCHeps, including pharmaceutical analogues. Comparison of the absorption, distribution, metabolism and excretion (ADME) properties of these analogues reveals enhanced metabolic stability relative to their parent arene-containing drugs, validating the potential of this meta-arene analogue as an sp3-rich motif in drug design. Collectively, our results show that BCHeps can be prepared on useful scales using a variety of methods, offering a new surrogate for meta-substituted benzene rings for implementation in drug discovery programmes.

Bicyclobutanes: from curiosities to versatile reagents and covalent warheads

The unique chemistry of small, strained carbocyclic systems has long captivated organic chemists from a theoretical and fundamental standpoint. A resurgence of interest in strained carbocyclic species has been prompted by their potential as bioisosteres, high fraction of sp3 carbons, and limited appearance in the patent literature. Among strained ring systems, bicyclo[1.1.0]butane (BCB) stands apart as the smallest bicyclic carbocycle and is amongst the most strained carbocycles known. Despite the fact that BCBs have been synthesized and studied for well over 50 years, they have long been regarded as laboratory curiosities. However, new approaches for preparing, functionalizing, and using BCBs in "strain-release" transformations have positioned BCBs to be powerful synthetic workhorses. Further, the olefinic character of the bridgehead bond enables BCBs to be elaborated into various other ring systems and function as covalent warheads for bioconjugation. This review will discuss the recent developments in the synthesis and functionalization of BCBs as well as the applications of these strained rings in synthesis and drug discovery. An overview of the properties and the historical context of this interesting structure will be provided.