Alkyl and Aryl Thiol Addition to [1.1.1]Propellane: Scope and Limitations of a Fast Conjugation Reaction

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.

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.

Rapid and Scalable Halosulfonylation of Strain-Release Reagents

Sulfonylated aromatics are commonplace motifs in drugs and agrochemicals. However, methods for the direct synthesis of sulfonylated non-classical arene bioisosteres, which could improve the physicochemical properties of drug and agrochemical candidates, are limited. Here we report a solution to this challenge: a one-pot halosulfonylation of [1.1.1]propellane, [3.1.1]propellane and bicyclo[1.1.0]butanes that proceeds under practical, scalable and mild conditions. The sulfonyl halides used in this chemistry feature aryl, heteroaryl and alkyl substituents, and are conveniently generated in situ from readily available sulfinate salts and halogen atom sources. This methodology enables the synthesis of an array of pharmaceutically and agrochemically relevant halogen/sulfonyl-substituted bioisosteres and cyclobutanes, on up to multidecagram scale.

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.

Synthesis of Sulfur-Substituted Bicyclo[1.1.1]pentanes by Iodo-Sulfenylation of [1.1.1]Propellane

Thiols easily react with [1.1.1]propellane to give sulfur-substituted bicyclo[1.1.1]pentanes in radical reactions, but this reactivity is not replicated in the case of heterocyclic thiols. Herein, we address this issue by electrophilically activating [1.1.1]propellane to promote its iodo-sulfenylation with 10 classes of heterocyclic thiols in two protocols that can be conducted on a multigram scale without exclusion of air or moisture.

Radical Multicomponent Alkyl Alkynylation of Propellane via Synergistic Photoredox and Copper Catalysis

Bicyclo[1.1.1]pentanes (BCPs) are important bioisosteres of aryl, tert-butyl groups, and internal alkynes that can impact key physicochemical properties on drug candidates. Herein, we describe a novel and efficient reaction to synthesize alkyl-alkynyl-substituted BCP derivatives by synergistic photoredox and copper catalysis at room temperature. The mild reaction conditions, simple protocol, broad functional group tolerance, and high efficiency of this procedure make it a valuable strategy for accessing alkynyl-substituted BCPs.

Radical Acylation of [1.1.1]Propellane with Aldehydes: Synthesis of Bicyclo[1.1.1]pentane Ketones

Bicyclo[1.1.1]pentanes (BCPs) are widely utilized in drug design as sp³-rich bioisosteres for tert-butyl, internal alkynes, and aryl groups. A general and mild method for radical acylation of [1.1.1]propellane with aldehydes has been developed. The protocol provides straightforward access to bicyclo[1.1.1]pentane ketones with a broad substrate scope. The synthetic utility of this methodology is demonstrated by the late-stage modification of bioactive molecules and the versatile transformation of bicyclo[1.1.1]pentane ketones, making it useful for drug discovery.

Synthesis of α-Quaternary Bicyclo[1.1.1]pentanes through Synergistic Organophotoredox and Hydrogen Atom Transfer Catalysis

Bicyclo[1.1.1]pentanes (BCPs) are important in drug design as sp³-rich bioisosteres of arenes and tert-butyl groups; however, the preparation of BCPs with adjacent quaternary carbons is barely known. We report a facile synthesis of α-quaternary BCPs using organophotoredox and hydrogen atom transfer catalysis in which α-keto radicals, generated through oxidation of β-ketocarbonyls, undergo efficient addition to [1.1.1]propellane. The BCP products can be transformed into a variety of useful derivatives, including enantioenriched BCPs featuring α-quaternary stereocenters.

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.

Radical-mediated sulfonyl alkynylation, allylation, and cyanation of propellane

Bicyclo[1.1.1]pentane (BCP) is widely applied as the bioisostere for aryl, internal alkynes, and tert-butyl groups in medicinal chemistry. We herein disclose an efficient and practical preparation of sulfonyl alkynyl/allyl/cyano-substituted BCP derivatives through a novel radical-mediated difunctionalization of propellane. The radical alkynylation, allylation, and cyanation processes readily proceed under mild photochemical conditions. The synthetic method features broad functional group tolerance, high product diversity, gram-scale preparation, and excellent atom-economy.

A continuous flow synthesis of [1.1.1]propellane and bicyclo[1.1.1]pentane derivatives

A continuous flow process to generate [1.1.1]propellane on demand is presented rendering solutions of [1.1.1]propellane that can directly be derivatised into various bicyclo[1.1.1]pentane (BCP) species. This was realised in throughputs up to 8.5 mmol h-1 providing an attractive and straightforward access to gram quantities of selected BCP building blocks. Lastly, a continuous photochemical transformation of [1.1.1]propellane into valuable BCPs bearing mixed ester/acyl chloride moieties was developed.

Direct catalytic asymmetric synthesis of α-chiral bicyclo[1.1.1]pentanes

Bicyclo[1.1.1]pentanes (BCPs) are important motifs in contemporary drug design as linear spacer units that improve pharmacokinetic profiles. The synthesis of BCPs featuring adjacent stereocenters is highly challenging, but desirable due to the fundamental importance of 3D chemical space in medicinal chemistry. Current methods to access these high-value chiral molecules typically involve transformations of pre-formed BCPs, and can display limitations in substrate scope. Here we describe an approach to synthesize α-chiral BCPs involving the direct, asymmetric addition of simple aldehydes to [1.1.1]propellane, the predominant BCP precursor. This is achieved by combining a photocatalyst and an organocatalyst to generate a chiral α-iminyl radical cation intermediate, which installs a stereocenter simultaneously with ring-opening of [1.1.1]propellane. The reaction proceeds under mild conditions, displays broad scope, and provides an array of α-chiral BCPs in high yield and enantioselectivity. We also present a theoretical model for stereoinduction in this mode of photoredox organocatalysis.

α-Cyclodextrin Encapsulation of Bicyclo[1.1.1]pentane Derivatives: A Storable Feedstock for Preparation of [1.1.1]Propellane

The bicyclo[1.1.1]pentane (BCP) scaffold is useful in medicinal chemistry, and many protocols are available for synthesizing BCP derivatives from [1.1.1]propellane. Here, we report (1) the α-cyclodextrin (α-CD) encapsulation of BCP derivatives, affording a stable, readily storable material from which BCPs can be easily and quantitatively recovered and (2) new and simple protocols for deiodination reaction of 1,3-diiodo BCP to afford [1.1.1]propellane in protic/aprotic/polar/non-polar solvents. The combination of these methodologies enables simple, on-demand preparation of [1.1.1]propellane in various solvents under mild conditions from α-CD capsules containing 1,3-diiodo BCP.

Highly Regioselective Addition of Allylic Zinc Halides and Various Zinc Enolates to [1.1.1]Propellane

We report a range of highly regioselective openings of [1.1.1]propellane with various allylic zinc halides, as well as zinc enolates of ketones, esters and nitriles. The resulting zincated bicyclopentanes (BCPs) were trapped with a range of electrophiles including acyl chlorides, sulfonothioates, hydroxylamino benzoates, tosyl cyanide as well as aryl and allyl halides, generating highly functionalized BCP-derivatives. The unusually high regioselectivity of these reactions has been rationalized using DFT calculations. A bioisostere of the synthetic opioid pethidine was prepared in 95 % yield in one step using this method.

Copper-Catalyzed Cross-Coupling between Alkyl (Pseudo)halides and Bicyclopentyl Grignard Reagents

The development of a copper-catalyzed cross-coupling between primary and secondary (pseudo)halides and bicyclopentyl Grignard reagents is reported. Highly strained bicyclopentanes can be cross-coupled with a large panel of primary alkyl mesylates and secondary alkyl iodides. The catalytic system is simple and cheap, and the reaction is general and chemoselective.

Iridium-Catalyzed Enantioselective Synthesis of α-Chiral Bicyclo[1.1.1]pentanes by 1,3-Difunctionalization of [1.1.1]Propellane

Bicyclo[1.1.1]pentanes (BCPs) have found application as bioisosteres of aromatic rings in drug development. However, catalytic construction of this motif with adjacent stereocenters with high enantioselectivity from readily available starting materials still constitutes a significant synthetic challenge. Herein we report a direct stereoselective synthesis of α-chiral allylic BCPs by 1,3-difunctionalization of [1.1.1]propellane with Grignard reagents and allyl carbonates using iridium catalysis. This mild protocol proceeds via initial organometallic addition to [1.1.1]propellane followed by asymmetric allylic substitution, providing the products with high enantioselectivities over a broad range of substrates. Further derivatization of the products demonstrates the applicability of this method to the preparation of structurally diverse libraries of chiral BCP derivatives.

Divergent Strain-Release Amino-Functionalization of [1.1.1]Propellane with Electrophilic Nitrogen-Radicals

Herein we report the development of a photocatalytic strategy for the divergent preparation of functionalized bicyclo[1.1.1]pentylamines. This approach exploits, for the first time, the ability of nitrogen-radicals to undergo strain-release reaction with [1.1.1]propellane. This reactivity is facilitated by the electrophilic nature of these open-shell intermediates and the presence of strong polar effects in the transition-state for C-N bond formation/ring-opening. With the aid of a simple reductive quenching photoredox cycle, we have successfully harnessed this novel radical strain-release amination as part of a multicomponent cascade compatible with several external trapping agents. Overall, this radical strategy enables the rapid construction of novel amino-functionalized building blocks with potential application in medicinal chemistry programs as p-substituted aniline bioisosteres.

Rationalizing the diverse reactivity of [1.1.1]propellane through σ-π-delocalization

[1.1.1]Propellane is the ubiquitous precursor to bicyclo[1.1.1]pentanes (BCPs), motifs of high value in pharmaceutical and materials research. The classical Lewis representation of this molecule places an inter-bridgehead C-C bond along its central axis; 'strain relief'-driven cleavage of this bond is commonly thought to enable reactions with nucleophiles, radicals and electrophiles. We propose that this broad reactivity profile instead derives from σ-π-delocalization of electron density in [1.1.1]propellane. Using ab initio and DFT calculations, we show that its reactions with anions and radicals are facilitated by increased delocalization of electron density over the propellane cage during addition, while reactions with cations involve charge transfer that relieves repulsion inside the cage. These results provide a unified framework to rationalize experimental observations of propellane reactivity, opening up opportunities for the exploration of new chemistry of [1.1.1]propellane and related strained systems that are useful building blocks in organic synthesis.

Sodium Bicyclo[1.1.1]pentanesulfinate: A Bench-Stable Precursor for Bicyclo[1.1.1]pentylsulfones and Bicyclo- [1.1.1]pentanesulfonamides

Herein, we present the synthesis of the bench-stable sodium bicyclo[1.1.1]pentanesulfinate (BCP-SO2 Na) and its application in the synthesis of bicyclo[1.1.1]pentyl (BCP) sulfones and sulfonamides. The salt can be obtained in a four-step procedure from commercially available precursors in multigram scale without the need for column chromatography or crystallization. Sulfinates are known to be useful precursors in radical and nucleophilic reactions and are widely used in medicinal chemistry. This building block enables access to BCP sulfones and sulfonamides avoiding the volatile [1.1.1]propellane which is favorable for the extension of SAR studies. Further, BCP-SO2 Na enables the synthesis of products that were not available with previous methods. A chlorination of BCP-SO2 Na and subsequent reaction with a Grignard reagent provides a new route to BCP sulfoxides. Several products were analyzed by single-crystal X-ray diffraction.

Strain release – an old tool for new transformations

This Feature Article provides an overview of research advances in the chemistry of spring-loaded molecules, focusing mainly on strain-release transformations.

Saturated bioisosteres of benzene: where to go next?

The replacement of para-substituted benzenes with saturated bi- and polycyclic bioisosteres - bicyclo[1.1.1]pentane, bicyclo[2.2.2]octane and cubane, - often increases the potency, selectivity and metabolic stability of bioactive compounds. The currently remaining challenge for chemists, however, is to rationally design, synthesize and validate the saturated bioisosteres for ortho- and meta-substituted benzenes.

Reactions of 2-Aryl-1,3-Dithianes and [1.1.1]Propellane

Bicyclo[1.1.1]pentanes (BCPs) have sparked the interest of medicinal chemists due to their recent discovery as bioisosteres of aromatic rings. To study the biological activity of this relatively new class of bioisosteres, reliable methods to incorporate BCPs into target molecules are in high demand, as reflected by a flurry of methods for BCP synthesis in recent years. In this work, we disclose a general method for the synthesis of BCP-containing dithianes which, upon deprotection, provide access to BCP analogues of medicinally abundant diarylketones. A broad scope of 2-aryl-1,3-dithianes, including several heterocyclic derivatives, react with [1.1.1]propellane to afford 26 new derivatives in good to excellent yields. Further transformation of the dithiane portion into a variety of functional groups demonstrates the robustness of the products. A computational study indicates that the reaction of 2-aryl-1,3-dithianes and [1.1.1]propellane proceeds via a two-electron pathway.

Insertion of [1.1.1]propellane into aromatic disulfides

Herein we present the synthesis of symmetrically and unsymmetrically substituted 1,3-bissulfanylbicyclo[1.1.1]pentanes from disulfides and [1.1.1]propellane. Bicyclo[1.1.1]pentanes (BCPs) recently gained interest as rigid linkers and as bioisosters of para-substituted benzene and alkyne moieties. The most promising precursor for BCPs is [1.1.1]propellane (1). The available methods to synthesize BCPs are quite limited and many groups contribute to the development of novel methods. The insertion of 1 into disulfide bonds is known, but has never been thoroughly investigated. In this study, we show that an UV initiated radical reaction can be used to synthesize symmetrically and unsymmetrically substituted BCP sulfides by reaction of [1.1.1]propellane (1) with disulfides. Depending on the ratio of 1 to the disulfide, only the BCP product (with up to 98% yield) or a mixture of BCP and [2]staffane can be obtained. The reaction tolerates functional groups such as halogens, alkyl and methoxy groups. The separation of the corresponding BCP and [2]staffane products is challenging but possible by column chromatography and preparative TLC in most cases. Single crystal X-ray diffraction analysis confirms the rod-like structure of the [2]staffanes that is often required in material applications.

Synthesis of Enantioenriched α-Chiral Bicyclo[1.1.1]pentanes

Bicyclo[1.1.1]pentanes (BCPs), useful surrogates for para-substituted arenes, alkynes, and tert-butyl groups in medicinal chemistry, are challenging to prepare when featuring stereogenic centers adjacent to the BCP. We report the development of an efficient route to α-chiral BCPs, via highly diastereoselective asymmetric enolate functionalization. We also describe the application of this chemistry to the synthesis of BCP analogues of phenylglycine and tarenflurbil, the single enantiomer of the NSAID flurbiprofen.

Nonconjugated Hydrocarbons as Rigid-Linear Motifs: Isosteres for Material Sciences and Bioorganic and Medicinal Chemistry

Nonconjugated hydrocarbons, like bicyclo[1.1.1]pentane, bicyclo[2.2.2]octane, triptycene, and cubane are a unique class of rigid linkers. Due to their similarity in size and shape they are useful mimics of classic benzene moieties in drugs, so-called bioisosteres. Moreover, they also fulfill an important role in material sciences as linear linkers, in order to arrange various functionalities in a defined spatial manner. In this Review article, recent developments and usages of these special, rectilinear systems are discussed. Furthermore, we focus on covalently linked, nonconjugated linear arrangements and discuss the physical and chemical properties and differences of individual linkers, as well as their application in material and medicinal sciences.

A radical exchange process: synthesis of bicyclo[1.1.1]pentane derivatives of xanthates

A new approach for the installation of the bicyclo[1.1.1]pentane unit on the xanthate moiety by means of a radical exchange process.

Synthesis and applications of highly functionalized 1-halo-3-substituted bicyclo[1.1.1]pentanes

Bicyclo[1.1.1]pentanes (BCPs) are important bioisosteres of 1,4-disubstituted arenes, tert-butyl and acetylenic groups that can impart physicochemical benefits on drug candidates. Here we describe the synthesis of BCPs bearing carbon and halogen substituents under exceptionally mild reaction conditions, via triethylborane-initiated atom-transfer radical addition ring-opening of tricyclo[1.1.1.01,3]pentane (TCP) with alkyl halides. This chemistry displays broad substrate scope and functional group tolerance, enabling application to BCP analogues of biologically-relevant targets such as peptides, nucleosides, and pharmaceuticals. The BCP halide products can be converted to the parent phenyl/tert-butyl surrogates through triethylborane-promoted dehalogenation, or to other derivatives including carbonyls, alcohols, and heterocycles.