Organic chemistry. Strain-release amination

Direct Access to Functional 2-Azabicyclo[2.1.1]Hexanes via Hydrodearomative [2π + 2σ] Cycloaddition of Aza-Arenes

Converting planar six-membered aza-arenes into C(sp3)-rich three-dimensional (3D) scaffolds is a promising way to obtain isosteric mimetics of numerous functional products, but it remains to date a formidable challenge due to the high thermodynamic stability and dynamic inertness as well as the selectivity control. Here, by applying a novel non-noble bimetallic Mn/Fe catalyst system, we report, for the first time, an approach for direct construction of functional 3D 2-azabicyclo[2.1.1]hexanes via a hydrodearomatization (HDA) of the aza-arenes and [2π + 2σ] cycloaddition cascade. Mechanistic investigations reveal that the triplet state of Fe(II) facilitates the activation of both aza-arenes and bicyclo[1.1.0]butanes (BCBs). The mild reduction nature of manganese catalysis and the steric effects of Fe(II) coordination result in an 1,4-hydrodearomatization, and the imine species derived from the isomerization of 1,4-hydrogenated aza-arenes are then effectively trapped by the polarized BCBs, thus suppressing the thermodynamically favorable over-hydrogenation of aza-arenes into cyclic amine by-products. Given the features of good substrate and functionality compatibility, high step and atom efficiency, and diversified product post-transformations, the developed chemistry offers a practical platform to access various functional molecules.

Synthesis of 2-(Trifluoromethyl)Azetidines by Strain-Release Reactions of 2-(Trifluoromethyl)-1-Azabicyclo[1.1.0]Butanes

Substituted azetidines are privileged heterocyclic scaffolds in medicinal chemistry and have become synthetic targets of high interest in recent years. With the goal of developing a new access to azetidines incorporating the pharmaceutically relevant trifluoromethyl group, the reactivity of 2-(trifluoromethyl)-1-azabicyclo[1.1.0]butanes was investigated in polar strain-release reactions. By using benzyl chloroformate or trifluoroacetic anhydride as reacting partners, diversely substituted 3-chloroazetidines, 3-substituted azetidines and azetidin-3-ols bearing a trifluoromethyl group at C2 could be readily synthesized. In addition, palladium-catalyzed hydrogenolysis reactions provided an entry to cis-3-aryl-2-trifluoromethyl azetidines.

Pd-Catalyzed Photoinduced Interceptive Decarboxylative Allylation

Herein, we describe a photoinduced Pd-catalyzed interceptive decarboxylative allylation of allyl esters. Our protocol provides a new gateway to enable atom pair swaps or a series of contractions and elongations, thus offering unconventional disconnections and a modular yet broadly applicable tool for rapidly and reliably accessing sp3 architectures in drug discovery.

Strain-Release-Driven Alder-Ene Reaction of Bicyclo[1.1.0]butanes with β-Fluoroalkyl-α,β-Unsaturated Ketones

The Alder-ene reaction of the C-C bond in bicyclo[1.1.0]butanes would provide a unique and efficient synthesis route for cyclobutene frameworks. Herein, we report a regio- and diastereoselective Alder-ene reaction of bicyclo[1.1.0]butanes with β-fluoroalkyl-α,β-unsaturated ketones, giving a wide variety of cyclobutenes with two contiguous centers and diene products. The reaction features atoms and step economies and exhibits broad substrate scope. Several downstream transformations of these cyclobutenes were performed.

Recent advances in the synthesis of bicyclo[4.1.1]octanes

The exploration of bicyclo[n.1.1]alkanes, known for their intricate chemical diversity and potential as benzene bioisosteres, has garnered significant attention over the past two decades. In particular, the past year has seen the emergence of bicyclo[4.1.1]octanes and their structural analogues as promising bioisosteres for meta-substituted arenes and cis-1,3-disubstituted cyclohexanes. To meet the growing demand for bicyclo[4.1.1]octanes, chemists have recently developed innovative (4 + 3) cycloaddition strategies, leveraging bicyclobutanes (BCBs) and 1,4-dipoles for their synthesis. This review provides a comprehensive evaluation of recent advancements in the synthesis and functionalization of these compounds, emphasizing their scope and underlying mechanisms. Additionally, we highlight the challenges and future prospects of identifying novel reaction pathways to access new functionalized bicyclo[4.1.1]octanes.

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

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

Synthesis of polysubstituted cyclobutanes through a photoredox strain-release/[3,3]-rearrangement cascade

Small saturated carbocycles, such as cyclobutanes, with elevated three-dimensionality and rich Csp3 centers are privileged scaffolds in naturally occurring molecules and drug discovery. It remains highly desirable and challenging to develop modular and straightforward strategies to craft densely substituted cyclobutanes. Herein, a photoredox-catalyzed radical strain-release/[3,3]-rearrangement cascade (SRRC) strategy for efficient synthesis of polysubstituted cyclobutanes is disclosed. This protocol operates with readily available α-silylamines as radical precursors, and strained bicyclo[1.1.0]butanes (BCBs) and cyclobutenes as radical acceptors, to access an array of structurally diverse 1,1,3- and 1,1,2-trisubstituted cyclobutanes containing a unique non-natural amino acid scaffold. Mechanistic studies reveal the pivotal reactivity of the silylketene acetal intermediate and the origin of diastereoselectivity. The power and utility of this method are illustrated with diverse transformations and preliminary anticancer assessment.

Rapid Access to 3-Substituted Bicyclo[1.1.1]pentanes

The prevalence of benzene rings in pharmaceutical scaffolds has prompted efforts to identify structural bioisosteres with improved in vivo properties. Notably, bicyclo[1.1.1]pentanes (BCPs)-C(sp3)-enriched, 1,4-disubstituted phenyl bioisosteres-have been leveraged to tune the pharmacokinetic profiles of lead compounds. While 3-arylated BCPs have been widely implemented to confer resistance against oxidative degradation and hydrogen atom transfer (HAT) processes, the analogous 3-alkylated BCPs remain underexplored as bioisosteric "benzylic" C-H motifs. Current methods to install 3-alkylated BCP motifs are heavily reliant on lengthy de novo synthesis and the preparation of reactive [1.1.1]propellane feedstocks, limiting their adoption in drug discovery programs. In this report, we disclose a mild, unified method for the preparation of both alkyl and aryl-substituted BCPs from bench-stable precursors. This method, which proceeds via dual copper-photoredox catalysis, is capable of installing BCP functionalities onto a range of saturated motifs, aryl-containing residues, and medicinally relevant heterocycles.

Ability of strained C atoms to act as an electron donor

There is a great deal of strain within the propellane and pyramidane hydrocarbon molecules. Quantum chemical calculations evaluate how this strain affects the ability of the bridgehead C atom to act as an electron donor in hydrogen, halogen, chalcogen, pnicogen, and tetrel bonds, despite the absence of a formal C lone pair or C[double bond, length as m-dash]C multiple bond. The strain induces the formation of a substantial region of negative electrostatic potential on this C atom which can attract the σ-hole of an electrophile. Each such molecule also contains an occupied molecular orbital that can be described as either a C lone pair or C-C bond, which is spatially disposed to align with, and transfer charge to, a σ* antibonding orbital of an approaching Lewis acid. The degree of strain within the hydrocarbon is closely correlated with the magnitude of the negative electrostatic potential, which is in turn connected with the strength of the ensuing bond. Tetrel bonds are strongest, followed by halogen, both of which contain a significant degree of covalency.

Simultaneous Activation of Bicyclobutanes and Indolyl Alcohols with HFIP: Access to Indole-Fused Bicyclo[3.1.1]Heptanes

The concept of strain release to unleash unique reactivity that drives a wide range of synthetically valuable transformations has long intrigued chemists. Among the various strained systems, highly reactive bicyclo[1.1.0]butanes (BCBs) have recently emerged as versatile building blocks for constructing bicyclic scaffolds. Despite the existence of various activation pathways for BCBs, the use of 1,1,1,3,3,3-hexafluoroisopropan-2-ol (HFIP) to activate BCBs has not been realized so far. Herein, we report the first HFIP-promoted (3 + 3) annulation of BCBs with indolyl alcohols through the simultaneous activation of both partners, facilitating the metal- and photocatalyst-free synthesis of indole-fused bicyclo[3.1.1]heptanes. Mechanistic studies reveal the role of HFIP in activating both components, and the reaction proceeds by an initial (3 + 2) annulation followed by a ring expansion/aromatization cascade.

Diastereoselective 1,3-nitrooxygenation of bicyclo[1.1.0]butanes

Cyclobutanes are strained carbocycles found in many drugs and natural products. Herein, we report a diastereoselective 1,3-nitrooxygenation of bicyclo[1.1.0]butanes with tert-butylnitrite and TEMPO to access 1,1,3-trisubstituted cyclobutanes. Various bicyclo[1.1.0]butanes effectively participated in the radical reaction yielding the substituted cyclobutane scaffolds with excellent yields and diastereoselectivity. The reaction is catalyst-free, easy to perform, and scalable and can be conducted in open air. Products obtained serve as substrates for the synthesis of 1,1,3,3-tetrasubstituted cyclobutanes with good yields and diastereoselectivity.

Recent advances in bridged structures as 3D bioisosteres of ortho-phenyl rings in medicinal chemistry applications

In recent years, a lot of scientists have tried to replace the phenyl ring in bioactive compounds with saturated 3D bioisosteres to obtain novel patentable structures in medicinal chemistry. Among these structures, bridged polycyclic compounds, as 3D bioisosteres of the ortho-substituted phenyl ring - a basic structural element in drugs and agrochemicals - have attracted much attention. The demand for rigid three-dimensional aliphatic skeletons to "escape from flatland" in drug discovery has prompted numerous efforts to broaden their applications and chemical spaces. This review summarises and highlights recent advances (year > 2015) in the medicinal chemistry applications of bridged polycyclic 3D bioisosteres of ortho-phenyl rings, which is expected to inspire further achievements in this area.

Catalytic Asymmetric Synthesis of Chiral Caged Hydrocarbons as Arenes Bioisosteres

The utilization of caged hydrocarbons as bioisosteres for arenes, especially the phenyl ring, in bioactive compounds has resulted in significant enhancements in potency, solubility, and metabolic stability. These improvements highlight the potential of C(sp3)-rich polycyclic scaffolds as a promising motif for the development of drug candidates. However, this strategy has also increased the structural complexity of these molecules, posing synthetic challenges in controlling the chirality of caged and highly decorated bioactive scaffolds. Over the past two years, remarkable progress has been achieved in catalytic asymmetric methodologies for the synthesis of caged hydrocarbons, significantly advancing their utility in chiral drug discovery and development. This minireview provides a comprehensive summary of recent breakthroughs in the catalytic asymmetric synthesis of chiral caged hydrocarbons, encompassing bicyclo[n.1.1]alkanes, cubanes, and related three-dimensional scaffolds. Additionally, we highlight the intriguing applications of enantiomerically pure caged hydrocarbons in biological studies. It is anticipated that this minireview will inspire further advancements in the enantioselective synthesis of these pharmaceutically valuable caged hydrocarbons.

Copper Catalysis with Arynes: Unlocking Site-Selective Arylation of Pyrazoles

Arynes are among the most reactive species in organic chemistry-six-membered rings so strained that their energy rivals that of a hand grenade. 1 Since their discovery in 1902, chemists have used arynes to achieve innovative transformations and access diverse natural products, however, their application for catalytic cross-coupling remains unrealized. 2 A major challenge in late-stage functionalization is the selective N -arylation of unsymmetric pyrazoles to create a core found in blockbuster medicines worth over nineteen billion dollars annually. 3 Traditional cross-coupling methods usually favor one type of regioisomer and thus, limit late-stage access to alternatives that could speed up drug discovery. 4,5 Here, we show that copper catalysis harnesses arynes to achieve switchable arylation of pyrazoles. By tuning metallotautomers via ligand choice, we direct N -arylation to either nitrogen site in a pyrazole, unlocking site-selective control. 6,7 Mechanistic studies reveal how steric and electronic forces guide regioselectivity and turn an unpredictable process into a precise synthetic tool.

Modular Synthesis of 3,3-Disubstituted Azetidines via Azetidinylation Reagents

Azetidines represent an attractive and emerging design option in medicinal chemistry owing to their small size and polar nature, as well as their potential to significantly impact the physicochemical properties of drug molecules. However, traditional methods for the synthesis of 3,3-disubstituted azetidines usually require higher step counts or exhibit poor functional group compatibility. Herein, we report a modular synthesis strategy for 3,3-disubstituted azetidines based on azetidinylation reagents. The practicality of this method is further exemplified by the use of readily available starting materials, mild reaction conditions, and a very broad substrate scope.

Dynamically Generated Carbenium Species via Photoisomerization of Cyclic Alkenes: Mild Friedel-Crafts Alkylation

The torsional strain of trans-configured medium-sized (6-8) cycloalkenes imparts substantial potential energy efficiently toward ionic additions through generated reactive carbenium species. These reactions have been underexplored due to a historical necessity for harsh ultraviolet irradiation. We report here the Friedel-Crafts (FC) type reactivity of arylcycloalkenes (ACs) and π-nucleophiles for the first time with weak Brønsted acid and visible light energy transfer catalysis. Following optimizations using p-fluorophenyl cyclohexene as the AC and 2-methylfuran as the nucleophile, model conditions were obtained to probe the respective influence of the acid catalyst, aryl component of AC, nucleophile, and alicyclic component of AC on the desired FC reactivity. Each parameter was found to critically influence the course of the reaction. Ultimately, a mild, visible light-driven method for the preparation of a variety of 1,1-diarylcyclohexane and 4,4-diarylpiperidine derivatives that is mechanistically distinct from and complementary to other methods of preparation is outlined.

Silver-mediated formal [4π + 2σ] cycloaddition reactions of bicyclobutanes with nitrile imines: access to 2,3-diazobicyclo[3.1.1]heptenes

Despite recent advances in the synthesis of aza-bicyclo[3.1.1]heptanes (aza-BCHeps, which have an sp3-hybridized nitrogen atom) and azabicyclo[3.1.1]heptenes (aza-BCHepes, which have an sp2-hybridized nitrogen atom), which are bioisosteres of pyridine, construction of 2,3-diazobicyclo[3.1.1]heptenes (2,3-diazo-BCHepes), which have both sp2- and sp3-hybridized nitrogen atoms, has yet to be achieved. Herein, we disclose a method for silver-enabled formal [4π + 2σ] cycloaddition reactions between bicyclobutanes and nitrile imines (generated from hydrazonyl chlorides) to furnish a diverse array of 2,3-diazo-BCHepes, which feature both sp2- and sp3-hybridized nitrogen atoms embedded in a BCHepe framework. These compounds have the potential to serve as bioisosteres of both pyridines and pyridazines. Owing to the presence of the sp3-hybridized nitrogen, 2,3-diazo-BCHepes can be expected to exhibit geometries similar to those of aza-BCHepes and much better solubility. We demonstrated the synthetic utility of our method by carrying out a scaled-up reaction and diverse postcatalytic transformations.

Access to Quaternary-Substituted Cyclobutylsilanes by Ring Opening of Arylbicyclobutanes with Silyllithium Reagents

Disclosed herein is the reaction of silyllithium reagents with quaternary-substituted arylbicyclobutanes to diastereoselectively form polysubstituted cyclobutylsilanes by C-C bond cleavage. The bicyclobutanes are generated in situ, by lithium-halogen exchange, from readily accessible (bromomethyl)iodocyclopropane precursors, rendering this a one-pot transformation. The trapping of a generated cyclobutyllithium intermediate with an electrophile was also demonstrated, providing a cyclobutane product with vicinal quaternary stereocenters. The utility of the cyclobutylsilane products was showcased by Tamao-Fleming oxidation to prepare a quaternary-substituted cyclobutanol.

Enantioselective synthesis of 2-substituted bicyclo[1.1.1]pentanes via sequential asymmetric imine addition of bicyclo[1.1.0]butanes and skeletal editing

The substitution of an aromatic ring with a C(sp3)-rich bicyclic hydrocarbon, known as bioisosteric replacement, plays a crucial role in modern drug discovery. Substituted bicyclo[1.1.1]pentanes (BCPs) are particularly noteworthy owing to their uniquely three-dimensional stereochemical complexity. 1,3-Difunctionalized BCPs have been widely used as bioisosteres for para-substituted phenyl rings, and they have been incorporated into numerous lead pharmaceutical candidates. 2-Substituted BCPs (substituted at the bridge position) can function as alternatives to ortho- or meta-substituted arene rings; however, the general and efficient construction of these scaffolds remains challenging, particularly if performed in an enantioselective manner. Here we present an approach for synthesizing enantioenriched 2-substituted BCPs by a nitrogen-atom insertion-and-deletion strategy, involving a chiral Brønsted acid-catalytic enantioselective cycloaddition of bicyclo[1.1.0]butanes with imines and nitrogen deletion of resulting aza-bicyclo[2.1.1]hexanes (aza-BCHs) with generally good enantiopurity retention. Mechanistic experiments verify the radical pathway. Chiral BCPs have been readily incorporated into medicinally relevant molecules, and a drug analogue has been successfully prepared enantioselectively.

Diverse Synthesis of Arene-Fused [n.1.1]-Bridged Molecules via Catalytic Cycloaddition and Rearrangement Reactions

Although great advancement has been made in synthesis of 3D bridged bicyclic[n.1.1]-bioisosteres, facile construction of 2D/3D merged molecules incorporating bridged rings, as novel chemical space in drug discovery, remains a significant challenge. Herein a collective, selective, and diversity-oriented approach for up to 6 types of 2D/3D polycyclic scaffolds featuring bicyclo[n.1.1] substructure is reported. A boronyl radical-catalyzed [2σ+2π] cycloaddition between bicyclo[1.1.0]butanes and ortho-quinone methides afforded spirocyclic compounds containing a bicyclo[2.1.1]hexanes unit, which were used as intermediates for synthesis of three types of 2D/3D scaffolds via judiciously controlled Lewis acid-catalyzed rearrangements. The reaction and rearrangement of para-quinone methides worked analogously and provided another two polycyclic scaffolds.

Spiro-C(sp3)-atom transfer: Creating rigid three-dimensional structures with Ph2SCN2

The introduction of a single C-atom into organic substrates typically results in the formation of flat molecules containing unsaturated C(sp)-centers. Adding a single C(sp3)-atom surrounded by four σ-C-C bonds, which opens up the three-dimensional space, is an unresolved problem in synthetic chemistry. We report the synthesis and application of the diazosulfur ylide Ph2S=C=N2 reagent that combines the reactivity of both sulfur ylides and diazo compounds to create carbon spiro-centers in a general fashion by the sequential or single-step installation of a C(sp3)-atom. New C-C and C-X (where X is O or N) bonds can be created around the C(sp3)-atom, which can ultimately be extended to four C-C σ-bonds in one step without resorting to transition metal catalysis. Ph2SCN2 can also be used to access highly strained frameworks containing (oxa)spiro[2.2]pentanes as well as tricyclic spiro-compounds.

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

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

Strain-release trifluoromethoxylation and pentafluorosulfanoxylation of [1.1.0]bicyclobutanes: expanded access to fluorinated cyclobutane hybrid bioisosteres

Methods for formal bromo-trifluoromethoxylation and bromo-pentafluorosulfanoxylation of [1.1.0]bicyclobutanes using AgOCF3 or AgOSF5 and 1,3-dibromo-5,5-dimethylhydantoin are disclosed. These represent complementary strategies to the syntheses of SF5- and CF3SF4-containing cyclobutanes previously reported from our laboratory, ultimately enabling comparative structural studies and in vitro ADME profiling for various fluorinated cyclobutanes.

"Angular" Spirocyclic Azetidines: Synthesis, Characterization, and Evaluation in Drug Discovery

The previously neglected "angular" spirocyclic azetidines have been synthesized, characterized, and validated in drug discovery. We have shown that these compounds could act as bioisosteres for common saturated six-membered heterocycles. Their incorporation into the structure of the anticancer drug Sonidegib (instead of morpholine), and Danofloxacine (instead of piperazine) provided novel patent-free analogs with similar physicochemical properties and high activity.

Solvent-Dependent Divergent Cyclization of Bicyclo[1.1.0]butanes

Bicyclo[1.1.0]butanes (BCBs) have recently garnered significant research interest as versatile precursors for synthesizing potential [n.1.1] bioisosteres and multi-functionalized cyclobutanes in a straightforward and atom-economical manner. Here, we report a solvent-dependent divergent cyclization of BCBs that provides highly diastereospecific decorated cyclobutanes and oxygen-containing bicyclo[3.1.1]heptanes (BCHeps), which serve as bioisosteres of meta-substituted arenes. Additionally, an unprecedented 1,2-difunctionalization reaction mode for BCBs was explored, thus expanding the chemical space of arene bioisosteres and highly functionalized cyclobutanes.

Synthesis of tertiary alkyl amines via photoinduced copper-catalysed nucleophilic substitution

In view of the high propensity of tertiary alkyl amines to be bioactive, the development of new methods for their synthesis is an important challenge. Transition-metal catalysis has the potential to greatly expand the scope of nucleophilic substitution reactions of alkyl electrophiles; unfortunately, in the case of alkyl amines as nucleophiles, only one success has been described so far: the selective mono-alkylation of primary amines to form secondary amines. Here, using photoinduced copper catalysis, we report the synthesis of tertiary alkyl amines from secondary amines and unactivated alkyl electrophiles, two readily available coupling partners. Utilizing an array of tools, we have analysed the mechanism of this process; specifically, we have structurally characterized the three principal copper-based intermediates that are detected during catalysis and provided support for the key steps of the proposed catalytic cycle, including the coupling of a copper(II)-amine intermediate with an alkyl radical to form a C-N bond.

An efficient and flexible approach for local distortion: distortion distribution analysis enabled by fragmentation

Distortion can play crucial roles in influencing structures and properties, as well as enhancing reactivity or selectivity in many chemical and biological systems. The distortion/interaction or activation-strain model is a popular and powerful method for deciphering the origins of activation energies, in which distortion and interaction energies dictate an activation energy. However, decomposition of local distortion energy at the atomic scale remains less clear and straightforward. Knowing such information should deepen our understanding of reaction processes and improve reaction design. Herein, an efficient, general and flexible fragmentation-based approach was proposed to evaluate local distortion energies for various chemical and biological molecules, which can be obtained computationally and/or experimentally. Moreover, our distortion analysis is readily applicable to multiple structures from molecular dynamics (or the minimum energy path) as well as can be evaluated by different computational chemistry methods. Our systematic analysis shows that our approach not only aids computational and experimental chemists in visualizing (relative) distortion distributions within molecules (distortion map) and identifies the key distorted pieces, but also offers deeper understanding and insights into structures, reaction mechanisms and dynamics in various chemical and biological systems. Furthermore, our analysis offers indices of local distortion energy, which can potentially serve as a new descriptor for multi-linear regression (MLR) or machine learning (ML) modelling.

Synthesis and Functionalization of Sulfoximine-Bicyclo[1.1.0]butanes: Functionalizable, Tuneable and Cysteine-Selective Chiral Warheads

Electrophilic covalent warheads with appropriate reactivity and selectivity are crucial to the investigation of protein function and the discovery of therapeutics. Here we report the synthesis of sulfoximine bicyclo[1.1.0] butanes (BCBs) as novel thiol reactive chiral warheads, achieved in one-pot from methylsulfoximines. Unusually the warhead can then be derivatized, keeping the BCB intact, over 3 vectors: i) sulfoximine N-modification instills a broad range of strain-release reactivity; ii) sp2-cross-coupling reactions on aryl-BCB-sulfoximines allows direct diversification, and iii) functionalization of the BCB motif itself is achieved by metalation and trapping with electrophiles. The BCB sulfoximines are shown to react selectively with cysteine including in a protein model (CDK2) under biocompatible conditions. Preliminary data indicate suitability for chemoproteomic applications, and enantioselective cysteine-labelling. The reactivity of sulfoximine BCBs with electron withdrawing groups on nitrogen is comparable to acrylamides with low to moderate reactivity.

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.

Stereoselective Radical Acylfluoroalkylation of Bicyclobutanes via N-Heterocyclic Carbene Catalysis

Cyclobutanes are prominent structural components in natural products and drug molecules. With the advent of strain-release-driven synthesis, ring-opening reactions of bicyclo[1.1.0]butanes (BCBs) provide an attractive pathway to construct these three-dimensional structures. However, the stereoselective difunctionalization of the central C-C σ-bonds remains challenging. Reported herein is a covalent-based organocatalytic strategy that exploits radical NHC catalysis to achieve diastereoselective acylfluoroalkylation of BCBs under mild conditions. The Breslow enolate acts as a single electron donor and provides an NHC-bound ketyl radical with appropriate steric hindrance, which effectively distinguishes between the two faces of transient cyclobutyl radicals. This operationally simple method tolerates various fluoroalkyl reagents and common functional groups, providing a straightforward access to polysubstituted cyclobutanes (75 examples, up to >19 : 1 d.r.). The combined experimental and theoretical investigations of this organocatalytic system confirm the formation of the NHC-derived radical and provide an understanding of how stereoselective radical-radical coupling occurs.

Single-Molecule Conductance of Staffanes

We report the first conductance measurements of [n]staffane (bicyclopentane) oligomers in single-molecule junctions. Our studies reveal two quantum transport characteristics unique to staffanes that emerge from their strained bicyclic structure. First, though staffanes are composed of weakly conjugated C-C σ-bonds, staffanes carry a shallower conductance decay value (β=0.84±0.02 n-1) than alkane chain analogs (β=0.96±0.03 n-1) when measured with the scanning tunneling microscopy break junction (STM-BJ) technique. Staffanes are thus more conductive than other σ-bonded organic backbones reported in the literature on a per atom basis. Density functional theory (DFT) calculations suggest staffane backbones are more effective conduits for charge transport because their significant bicyclic ring strain destabilizes the HOMO-2 energy, aligning it more closely with the Fermi energy of gold electrodes as oligomer order increases. Second, the monostaffane is significantly lower conducting than expected. DFT calculations suggest that short monostaffanes sterically enforce insulating gauche interelectrode orientations over syn orientations; these steric effects are alleviated in longer staffanes. Moreover, we find that [2-5]staffane wires may accommodate axial mechanical strain by "rod-bending". These findings show for the first time how bicyclic ring strain can enhance charge transmission in saturated molecular wires. These studies showcase the STM-BJ technique as a valuable tool for uncovering the stereoelectronic proclivities of molecules at material interfaces.

Asymmetric Construction of Chiral 2-Azetines and Axially Chiral Tetrasubstituted Allenes Via Phosphine Catalysis

Chiral 2-azetines and allenes are highly valuable structural units in natural products and useful chemicals. However, enantioselective synthesis of both 2-azetines and allenes has been extremely challenging. Herein, we present asymmetric construction of chiral 2-azetines (70-98 % yields and up to 96 % ee) through chiral phosphine-catalyzed [2+2] annulation of yne-enones with sulfamate-derived cyclic imines. These 2-azetines were easily transformed into chiral allenes upon treatment with Et3SiH, BF3 ⋅ Et2O and water at rt for 2 minutes. Based on the above transformations, a concise one-pot synthetic procedure combining [2+2] annulation of yne-enones and sulfamate-derived cyclic imines under phosphine catalysis and sequential reduction/isomerization/ring-opening reaction through Et3SiH, BF3 ⋅ Et2O and water was thus set up, providing axially chiral tetrasubstituted allenes in satisfactory yields and enantioselectivities (56-90 % yields and up to 91 % ee).

Synthesis of fluorine-containing bicyclo[4.1.1]octenes via photocatalyzed defluorinative (4 + 3) annulation of bicyclo[1.1.0]butanes with gem-difluoroalkenes

Although bicyclo[4.1.1] systems are privileged scaffolds in many natural products and drug molecules, efficient synthetic approaches to these systems remain underdeveloped. In this work, we disclose a photoredox-catalyzed defluorinative (4 + 3) annulation of bicyclo[1.1.0]butanes with gem-difluoroalkenes, which provides practical and straightforward access to the fluorine-containing bicyclo[4.1.1]octenes. Our protocol is characterized by mild conditions, broad substrate scope, excellent functional group tolerance and good to excellent yields. Notably, the ease and variety of product derivatizations further enrich the diversity and complexity of the fluorine-containing bicyclo[4.1.1] systems.

Strain-Release-Driven Electrochemical Skeletal Rearrangement of Non-Biased Alkyl Cyclopropanes/Butanes

Capitalizing the inherent strain energy within molecules, strain-release-driven reactions have been widely employed in organic synthesis. Small cycloalkanes like cyclopropanes and cyclobutanes, with their moderate ring strain, typically require dense functionalization to induce bias or distal activation of (hetero) aromatic rings via single-electron oxidation for relieving the tension. In this study, we present a pioneering direct activation of alkyl cyclopropanes/butanes through electrochemical oxidation. This approach not only showcases the potential for ring-opening of cyclopropane/butane under electrochemical conditions but also streamlines the synthesis of diverse oxazolines and oxazines. The applicability of our method is exemplified by its broad substrate scopes. Notably, the products derived from cyclobutanes undergo a formal ring contraction to cyclopropanes, introducing an intriguing aspect to our discoveries. These discoveries mark a significant advancement in strain-release-driven skeletal rearrangement reactions of moderately strained rings, offering sustainable and efficient synthetic pathways for future endeavours.

Asymmetric synthesis of atropisomers featuring cyclobutane boronic esters facilitated by ring-strained B-ate complexes

The strain-release-driven reactions of bicyclo[1.1.0]butanes (BCBs) have received significant attention from chemists. Notably, 1,2-migratory reactions enabled by BCB-derived B-ate complexes effectively complement the reactions initiated by common BCBs. The desired products are particularly valuable for late-stage transformations due to the presence of the C-B bond. However, asymmetric reactions mediated by BCB-derived boronate complexes have progressed slowly. In this study, we develop an asymmetric synthesis of atropisomers featuring cis-cyclobutane boronic esters facilitated by 1,2-carbon or boron migration of ring-strained B-ate complexes, achieving high enantioselectivity. The reaction is compatible with various aryl, alkenyl, alkyl boronic esters and B2pin2, and shows good compatibility with natural product derivatives. Mechanistic studies are conducted to understand stereoselective control in the dynamic kinetic asymmetric transformations (DYKATs). The target products can undergo a series of transformations, further demonstrating the practicality of this methodology.

Harnessing Oxetane and Azetidine Sulfonyl Fluorides for Opportunities in Drug Discovery

Four-membered heterocycles such as oxetanes and azetidines represent attractive and emergent design options in medicinal chemistry due to their small and polar nature and potential to significantly impact the physiochemical properties of drug molecules. The challenging preparation of these derivatives, especially in a divergent manner, has severely limited their combination with other medicinally and biologically important groups. Consequently, there is a substantial demand for mild and effective synthetic strategies to access new oxetane and azetidine derivatives and molecular scaffolds. Here, we report the development and use of oxetane sulfonyl fluorides (OSFs) and azetidine sulfonyl fluorides (ASFs), which behave as precursors to carbocations in an unusual defluorosulfonylation reaction pathway (deFS). The small-ring sulfonyl fluorides are activated under mild thermal conditions (60 °C), and the generated reactive intermediates couple with a broad range of nucleophiles. Oxetane and azetidine heterocyclic, -sulfoximine, and -phosphonate derivatives are prepared, several of which do not have comparable carbonyl analogs, providing new chemical motifs and design elements for drug discovery. Alternatively, a SuFEx pathway under anionic conditions accesses oxetane-sulfur(VI) derivatives. We demonstrate the synthetic utility of novel OSF and ASF reagents through the synthesis of 11 drug analogs, showcasing their potential for subsequent diversification and facile inclusion into medicinal chemistry programs. Moreover, we propose the application of the OSF and ASF reagents as linker motifs and demonstrate the incorporation of pendant groups suitable for common conjugation reactions. Productive deFS reactions with E3 ligase recruiters such as pomalidomide and related derivatives provide new degrader motifs and potential PROTAC linkers.

Catalytic Intermolecular Asymmetric [2π + 2σ] Cycloadditions of Bicyclo[1.1.0]butanes: Practical Synthesis of Enantioenriched Highly Substituted Bicyclo[2.1.1]hexanes

The high percentage of sp3-hybridized carbons and the presence of chiral carbon centers could contribute to increased molecular complexity, enhancing the likelihood of clinical success of drug candidates. Three-dimensional (3D) bridged motifs have recently garnered significant interest in medicinal chemistry. Bicyclo[2.1.1]hexanes (BCHs) are emerging 3D benzene bioisosteres, but the synthesis of chiral, highly substituted BCHs has been underexplored. Herein, we disclose the Lewis acid-catalyzed asymmetric intermolecular [2π + 2σ] cycloaddition of bicyclo[1.1.0]butanes with coumarins, 2-pyrone, or chromenes to access diverse enantioenriched 1,2,3,4-tetrasubstituted BCHs bearing vicinal tertiary-quaternary stereocenters. The key to success is the introduction of chiral bisoxazoline ligands to effectively suppress the side reactions, inhibit significant racemic background reactions, and fine-tune the reactivity and regio-, enantio-, and diastereoselectivities of the reactions. The resulting BCHs hold significant potential as benzene bioisosteres in the synthesis of chiral BCHex-Sonidegib and BCHex-BMS-202, mimicking the anticancer drug Sonidegib and the PD-1/PD-L1 inhibitor BMS-202, respectively. The outcome highlights the positive impact of bioisosteric replacement on physicochemical properties, while maintaining comparable antitumor activity to their aryl-containing counterparts.

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.

Recent advances in photochemical strain-release reactions of bicyclo[1.1.0]butanes

Bicyclo[1.1.0]butanes (BCBs) are attractive compounds for their beautiful "butterfly" conformations, distinctive properties, and novel reactivities. As soon as the first example had been synthesized, a wide range of strain-release reactions were explored for the preparation of cyclobutanes and bicyclic systems in the ground state or excited state. In particular, with the demand for the construction of rigid three-dimensional aliphatic skeletons to "escape from flatland" in drug discovery programs, numerous efforts have been devoted in this area to expanding the boundaries of their reactivities and broadening the chemical space of their attractive bioisosteric products. In recent years, with the great resurgence and dramatic evolution of photochemistry, photochemical strain-release reactions generally relying on single electron transfer (SET) or energy transfer (EnT) strategies can provide much more opportunities and capability for innovative transformations of BCBs. In this review, we summarize and highlight the recent advances (year > 2016) of this topic and hope that it will inspire much more wonderful chemistry of BCBs.

Stereocontrolled Synthesis of Cyclobutylamines Enabled by Lewis Acid-Catalyzed (3 + 2 + 2) Cycloaddition/Ring-Opening Strategy of Bicyclo[1.1.0]butanes with Triazinanes

Herein, we develop a modular and efficient "cycloaddition/ring-opening" strategy of bicyclo[1.1.0]butanes (BCBs) with triazinanes to provide a series of syn-diastereoselective cyclobutylamines via 2,4-diazabicyclo[4.1.1]octanes (aza-BCOs). The reaction features simple operation, mild reaction conditions, and a broad substrate scope. Mechanistic studies suggest that the cycloaddition follows a stepwise (3 + 2 + 2) rather than (4 + 3) cycloaddition, involving an SN2 nucleophilic addition of formaldimine to Lewis acid-activated BCB species. A scale-up experiment and various synthetic transformations of the product further highlight the synthetic utility. We expect that our findings will encourage the exploration of BCB chemistry to access more synthetically challenging cyclobutane frameworks.

Lewis Acid-Catalyzed Unusual (4+3) Annulation of para-Quinone Methides with Bicyclobutanes: Access to Oxabicyclo[4.1.1]Octanes

Over the past few years, there has been a surge of interest in the chemistry of bicyclobutanes (BCBs). Although BCBs have been used to synthesize bicyclo[2.1.1]hexanes and bicyclo[3.1.1]heptanes, the synthesis of bicyclo[4.1.1]octanes has remained elusive. Herein, we report the first Lewis acid-catalyzed unexpected (4+3) annulation of para-quinonemethides (p-QMs) with BCBs allowing the synthesis of oxabicyclo[4.1.1]octanes proceeding under mild conditions. With 5 mol % of Bi(OTf)3, the reaction afforded the (4+3) annulated product in high regioselectivity and good functional group compatibility via a simultaneous Lewis acid activation of BCBs and p-QMs. The reaction is likely initiated by the 1,6-addition of Lewis acid activated BCBs to p-QMs followed by the C2-selective intramolecular addition of the phenol moiety to the generated cyclobutyl cation intermediate. Moreover, detailed mechanistic studies provided insight into the mechanism of the reaction.

Catalytic Asymmetric Reactions of Ketimines and Alkenes via [2 + 2] Cycloaddition: Chemical Reactivity Controlled by Switching a Heteroatom

Azetidine units are commonly found in natural products and biologically active drugs. The [2 + 2] cycloaddition of imines and alkenes has been extensively used in the synthesis of such structures, while enantioselective approaches remain elusive. Herein, an efficient B(C6F5)3/chiral phosphoric acid-catalyzed asymmetric [2 + 2] cycloaddition of ketimines and aryl vinyl selenides was presented, delivering valuable chiral azetidines with excellent stereoselectivities (>20:1 dr and up to 96:4 er). What's even more interesting was that when a "Se" atom was switched to an "S" atom, the reaction proceeded through a [2 + 2] cycloaddition/ring-opening cascade process, affording a range of chiral thioacetals with high enantioselectivities (up to 98:2 er), which were also important organic sulfur compounds. Mechanistic experiments, coupled with density functional theory (DFT) calculations, shed light on a mechanism involving stepwise [2 + 2] cycloaddition and ring-opening processes, with the initial alkenylation step identified as crucial for achieving stereoselective control.

gem-Difluorobicyclo[2.1.1]hexanes via Photochemical [2π + 2σ] Cycloaddition Initiated by Oxidative Activation of gem-Difluorodienes

The incorporation of fluorine atoms into three-dimensional sp3-rich scaffolds represents an attractive tactic during bioisosteric evolution campaigns by endowing bioisosteric candidates with improved pharmacokinetic properties. Photo- or Lewis acid-mediated bicyclo[1.1.0]butane cycloaddition has offered an efficient approach for the construction of numerous regular bicyclo[n.1.1] scaffolds (n = 1-5) but remains a significant challenge to the synthesis of related 3D fluorinated scaffolds. Herein, we unveiled a photochemical single-electron oxidative strategy for gem-difluorodiene activation and subsequent [2π + 2σ] cycloaddition with bicyclo[1.1.0]butanes to provide a broad range of gem-difluorobicyclo[2.1.1]hexane scaffolds containing several post-transformable handles. A combination of experimental and computational mechanistic studies suggested that the conjugated π system of gem-difluorodiene plays important dual roles in promoting its preferential single-electron oxidation and stabilizing various radical-involved intermediates during the cyclization.

Aza-ortho-Quinone Methide Promoted Strain-Release-Driven Conversion of Azabicyclo[1.1.0]butanes into Functionalized Azetidines

A strategy for activating azabicyclo[1.1.0]butane (ABB) with in situ generated aza-ortho-quinone methide, promoted by HFIP, is reported. This unified activation, vis-à-vis strain-release-driven N/C3-functionalization, features a new means to prepare functionalized azetidines from ABB. Additionally, the newly installed motif on azetidine nitrogen could be forged into an indoline via Pd-catalyzed hydroamination, leveraging access to medicinally relevant scaffolds.

Diastereoselective dearomative cycloaddition of bicyclobutanes with pyridinium ylides: a modular approach to multisubstituted azabicyclo[3.1.1]heptanes

Diastereoselective (3+3) cycloaddition between bicyclobutanes and pyridinium ylides forms azabicyclo[3.1.1]heptanes via pyridine dearomatization. These reactions proceed under ambient conditions with no need for photochemistry or catalysis, and tolerate a wide range of functional gorups. The resulting multicyclic ring systems have diverse synthetic handles for further transformations, making them potentially valuable for the design of Csp3-rich drug candidates. These include semi-reduction of the dihydropyridine, and diastereoselective photochemical skeletal rearrangement to give a tetrasubstituted cyclobutane.

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.

Electrochemical Selenized Reaction of N-Arylbicyclo[1.1.0]butane-1-carboxamides: Access to 3-(Arylselanyl)spiro[cyclobutane-1,3'-indolin]-2'-one Derivatives

A novel selenized reaction of N-arylbicyclo [1.1.0]butane-1-carboxamides with diselenide for the synthesis of polycyclic indoline derivatives is developed under electrochemical conditions. The synthesis is achieved by the bicyclo[1.1.0]butane strain-release reaction and intramolecular cyclization process. In addition, this approach features a wide range of substrates, good group tolerance, shorter reaction time, and mild conditions.

Strain-Releasing Ring-Opening Diphosphinations for the Synthesis of Diphosphine Ligands with Cyclic Backbones

Diphosphine ligands based on cyclobutane, bicyclo[3.1.1]heptane, and bicyclo[4.1.1]octane were synthesized from the corresponding highly strained, small, cyclic organic molecules, i.e., bicyclo[1.1.0]butane, [3.1.1]propellane, and [4.1.1]propellane, employing a ring-opening diphosphination. Under photocatalytic conditions, the three-component reaction of a diarylphosphine oxide, one of the aforementioned strained molecules, and a diarylchlorophosphine results in the smooth formation of the corresponding diphosphines in high yield. The obtained diphosphines can be expected to find applications in functional molecules due to their unique structural characteristics, which likely impart specific properties on their associated metal complexes and coordination polymers (e.g., a zigzag-type structure). The feasibility of the initial radical addition can be estimated using density-functional-theory calculations using the artificial force induced reaction (AFIR) method. This study focuses on the importance of integrating experimental and computational methods for the design and synthesis of new diphosphination reactions that involve strained, small, cyclic organic molecules.

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.