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

Scandium Triflate Catalyzed Cycloadditions of Vinyl Diazo Compounds and In Situ Formed Naphthoquinone Methides

Vinyl diazo carbonyl compounds have received great attention and are widely employed in the cycloadditions of in situ formed reactive intermediates. Metal carbenes are predominantly involved in cycloadditions, and transformations of vinyl diazo compounds that do not proceed via the metal carbene pathway have been seldom reported. Herein, scandium-catalyzed cycloadditions of vinyl diazo compounds and in situ formed 2-naphthoquinone-8-methides are achieved, and naphthalene-fused polycyclic products were obtained in up to 88% yield. In the transformation, the nucleophilic conjugate addition of vinyl diazo compounds to in situ formed 2-naphthoquinone-8-methides generates vinyl diazonium intermediates, which undergo an intramolecular Friedel-Crafts reaction and intramolecular transesterification to yield the final product.

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.

Regio- and Diastereoselective Cascade Reactions of Bicyclo[1.1.0]butanes: Access to gem-Difluorinated Carbocyclic Rings

gem-Difluorinated carbocyclic rings are attractive motifs in drug development. Herein, we report the transition-metal free cascade reaction of bicyclo[1.1.0]butanes (BCBs) with gem-difluorocyclopropenes for the synthesis of gem-difluorinated carbocyclic rings with excellent regio- and diastereoselectivity. This method was successfully applied to provide a broad range of gem-difluorinated cyclopentenes and cyclopropanes, which could undergo a variety of difluoromethylene (CF2) retaining transformations.

Aryne Alder-Ene Reaction Enables Arylation of Conformationally Locked Styrenes

We report a transition metal-free aryne Alder-ene (AAE) reaction of conformationally locked styrenes like methylene indane (5,6-fusion) and methylene tetralin (6,6-fusion) to provide aryl-decorated indenes and dihydronaphthalenes. DFT studies compared the transition states between 5,6- and 6,6-fused systems. The synthetic utility of the 5,6- and 6,6-fused products was demonstrated.

Lewis Acid Catalyzed Divergent Reaction of Bicyclo[1.1.0]Butanes With Quinones for the Synthesis of Diverse Polycyclic Molecules

Bicyclo[1.1.0]butanes (BCBs) are highly strained hydrocarbons with unique structural properties and intrinsic reactivity, making them valuable building blocks for constructing complex molecular architectures. Herein, we report the Lewis acid-catalyzed divergent reactions of BCBs with quinones, yielding a diverse array of polycyclic molecules. Using Sc(OTf)₃ as a catalyst, pyrazole-substituted BCBs efficiently undergo formal (3 + 2) cycloaddition reactions with quinones, producing highly substituted bicyclo[2.1.1]hexanes featuring a caged framework. Monosubstituted BCB ketones undergo a sequential cascade involving Alder-ene reaction, 4π electrocyclic ring-opening, and [4 + 2] cycloaddition reaction, yielding fused benzoxepines efficiently. Disubstituted BCB esters, ketones, and amides undergo a tandem isomerization and (3 + 2) cycloaddition process, stereoselectively yielding tetrahydrocyclobuta[b]benzofuran products. Notably, strong Lewis acids such as SnCl₄ and BiBr₃ directly participate in the ring-opening reactions of monosubstituted BCB ketones, generating halogenated cyclobutane derivatives. Additionally, the synthetic potential of these approaches has been further highlighted through scale-up experiments and a range of transformations. This study demonstrates the tunability of reaction pathways based on the diverse substitution patterns of BCBs, providing efficient methods for the synthesis of a range of polycyclic compounds.

Photoinduced Formal Cross-[3+3] Cycloaddition of Vinyldiazo Reagents with Acceptor-Only Diazoalkanes

In this study, we devised an innovative approach for the synthesis of pyrazine derivatives through a photoinduced formal cross-[3+3] cycloaddition between vinyldiazo reagents and acceptor-only diazoalkanes. This method leverages the differential reactivity of two distinct diazo compounds: vinyldiazo reagents, which upon visible light irradiation form key cyclopropane intermediates, and acceptor-only diazoalkanes, which function as 1,3-dipoles to capture the photogenerated reactive species. The reactions proceed exclusively under visible light, yielding 1,4-dihydropyridazines with a broad substrate scope and compatibility with various functional groups. Importantly, the synthesized 1,4-dihydropyridazines can be readily converted to other valuable products. The mechanism, elucidated through UV-vis absorption studies, deuterium labeling experiments, control experiments, and in situ NMR spectroscopy, provides a clear understanding of the observed reactivity.

Generation and Use of Bicyclo[1.1.0]butyllithium under Continuous Flow Conditions

The bicyclo[1.1.0]butyl scaffold has emerged as a valuable bioisostere in drug discovery programs. Here, we present a streamlined approach for the generation of bicyclo[1.1.0]butyllithium and its functionalization with various classes of electrophiles in a one-flow process, eliminating the need for intermediate isolation. In comparison to traditional batch processes, the flow method allows the use of a single organolithium reagent instead of two and operates at significantly higher temperatures (0 °C versus -78 °C), enhancing both practicality and scalability.

σ-Bond insertion reactions of two strained diradicaloids

The development of new synthetic methodologies is instrumental for enabling the discovery of new medicines. The methods that provide efficient access to structural alternatives for aromatic compounds (that is, saturated arene bioisosteres) have become highly coveted1-4. The incorporation of these bioisosteres typically leads to favourable drug-like properties and represents an emerging field of research. Here we report a new synthetic method that furnishes a coveted motif, the bicyclo[2.1.1]hexane scaffold5,6, using mild reaction conditions and an operationally simple protocol. The methodology proceeds through the uncommon coupling of two strained fragments: transiently generated cyclic allenes and bicyclo[1.1.0]butanes, which possess considerable strain energies of about 30 kcal mol-1 (ref. 7) and about 60 kcal mol-1 (ref. 6), respectively. The reaction is thought to proceed by a σ-bond insertion through a diradical pathway. However, rather than requiring an external stimulus to generate radical species, reactivity is thought to arise as a result of innate diradical character present in each reactant. This diradicaloid character8, an underused parameter in reaction design, arises from the severe geometric distortions of each reactant. Our studies provide a means to access functionalized bicyclo[2.1.1]hexanes of value for drug discovery, underscore how geometric distortion of reactants can be used to enable uncommon modes of reactivity and should encourage the further exploration and strategic use of diradicaloids in chemical synthesis.

Blue light-induced diazo cross-coupling: synthesis of allyldiazo compounds through reshuffling of functionalities

In this paper, we describe a new type of cross-coupling between simple diazo and vinyldiazo compounds that gives access to unusual allyldiazo products. Blue light discriminates two diazo compounds towards free carbene formation, triggering sequential cyclopropenation, (3+2) cycloaddition and ring opening rearrangement processes. This strategy involves an overall reshuffle of diazo functionality and olefinic carbons of vinyldiazo compounds with an extrusion of nitrogen. Mechanistic studies including a 15N-labelling experiment demonstrate that the diazo functionality of allyldiazo products derives from simple diazo compounds, while vinyldiazo reagents are selectively decomposed via energy transfer with thioxanthone photocatalyst. The obtained allyldiazo compounds can be efficiently converted into synthetically useful structures such as 1,3-dienes, gem-difluoro-1,4-diene, hydrazine, dihydropyrazole, pyridazine, and bicyclobutane.

Shapeshifting Gabriel Amine Synthesis with Iodo-BCPs

The Gabriel amine synthesis is a textbook method for the preparation of primary amines from alkyl halides. In this work, we demonstrate a Gabriel amine synthesis with iodo-bicyclopentanes to make aminomethyl bicyclobutanes. DFT studies support the concerted rearrangement of a bicyclo[1.1.1]pentyl to a bicyclo[1.1.0]butyl carbocation, initiated by a carbon-halide dissociation. A carboxamide substituent stabilizes the carbocation intermediate with anchimeric assistance.

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.

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.

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.

A Photochemical Strategy towards Michael Addition Reactions of Cyclopropenes

The development of Michael addition reactions to conjugated cyclopropenes is a challenge in organic synthesis due to the fleeting and reactive nature of such strained Michael acceptor systems. Herein, the development of a photochemical approach towards such conjugated cyclopropenes is reported that serves as a strategic entry point to densely functionalized cyclopropanes in a diastereoselective fashion. The process involves the light-mediated generation of transient cyclopropenyl α,β-unsaturated esters from vinyl diazo esters, followed by an organic base catalyzed nucleophilic addition of N-heterocycles to directly access β-N-heterocyclic cyclopropanoic esters. With this synergistic approach, various trisubstituted cyclopropanes bearing N-heteroaryl and N-heterocyclic rings such as indole, pyrrole, benzimidazole, isatin, pyridinone, and quinolinone were accessed efficiently in good yield and decent to good diastereoselectivities. Further, β-indolyl cyclopropanoic acids have been synthesized and were successfully evaluated as FABP-4 inhibitors. Theoretical calculations have been performed to elucidate the mechanism, which was further supported by experimental findings.

Structure-Dependent, Switchable Alder-Ene/[2π + 2σ] Cycloadditions of Vinyl Bicyclo[1.1.0]butanes with α-Ketoesters Enabled by Palladium Catalysis

A structure-dependent, palladium-catalyzed switchable alder-ene/[2π + 2σ] cycloaddition of VBCBs with α-ketoesters has been reported. A variety of cyclobutenes and 2-oxabicyclo[2.1.1]hexanes have been efficiently achieved in good to excellent yields through strain-release-driven alder-ene reactions and [2π + 2σ] cycloadditions, respectively. The potential of this method is illustrated by the scale-up reaction and diverse postsynthetic transformations of the obtained cyclic scaffolds. Additionally, the reaction mechanism and origins of the chemoselectivity have been probed by computational studies.

Ligand-controlled regiodivergent aminocarbonylation of cyclobutanols toward 1,1- and 1,2-substituted cyclobutanecarboxamides

Four-membered carbocycles are among the most sought-after backbones which are commonly found in biologically active molecules. However, difficulties on their producing are existing due to its highly strained ring system. On the other hand, cyclobutanols can be straightforwardly prepared and can serves as precursors for synthesizing cyclobutane derivatives. Here we report an example of regioselective aminocarbonylation of cyclobutanols in which the cyclobutane core remained intact. The method exhibits good functional group compatibility, as well as high regio- and stereoselectivity, offering new pathways for synthesizing several pharmaceuticals. Furthermore, this strategy enables the rapid installation of cyclobutane as a conformational restricted skeleton, greatly facilitating direct access to valuable drug molecules that require conformational restriction.

Bifunctional Lewis Base-Catalyzed (3 + 2) Cycloadditions of Pyrazolone-Derived MBH Carbonates with Arynes

The (3 + 2) cycloaddition of arynes with allylic ylides remains a formidable challenge because both intermediates are highly reactive and prone to spontaneous quenching. Here, we report a (3 + 2) cycloaddition of pyrazolone MBH carbonates with arynes, enabling the efficient synthesis of diverse indene-fused spiropyrazolones. The key is employing a new bifunctional Lewis base catalyst to facilitate the cycloaddition of in situ generated allylic pyridinium ylides with arynes.

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

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

Synthesis of Stereodefined Polysubstituted Bicyclo[1.1.0]butanes

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

DFT Insight into a Strain-Release Mechanism in Bicyclo[1.1.0]butanes via Concerted Activation of Central and Lateral C-C Bonds with Rh(III) Catalysis

Transition-metal-catalyzed, strain-release-driven transformations of "spring-loaded" bicyclo[1.1.0]butanes (BCBs) are considered potent tools in synthetic organic chemistry. Previously proposed strain-release mechanisms involve either the insertion of the central C-C bond of BCBs into a metal-carbon bond, followed by β-C elimination, or the oxidative addition of the central or lateral C-C bond on the transition metal center, followed by reductive elimination. This study, employing DFT calculations on a Rh(III)-catalyzed model system in a three-component protocol involving oxime ether, BCB ester, and ethyl glyoxylate for constructing diastereoselective quaternary carbon centers, introduces an unusual strain-release mechanism for BCBs. In this mechanism, the catalytic reaction is initiated by the simultaneous cleavage of two C-C bonds (the central and lateral C-C bonds), resulting in the formation of a Rh-carbene intermediate. The new mechanism exhibits a barrier of 21.0 kcal/mol, making it energetically more favorable by 11.1 kcal/mol compared to the previously suggested most favorable pathway. This unusual reaction mode rationalizes experimental observation of the construction of quaternary carbon centers, including the excellent E-selectivity and diastereoselectivity. The newly proposed strain-release mechanism holds promise in advancing our understanding of transition-metal-catalyzed C-C bond activation mechanisms and facilitating the synthesis of transition metal carbene complexes.

Umpolung reactivity of strained C-C σ-bonds without transition-metal catalysis

Umpolung is an old and important concept in organic chemistry, which significantly expands the chemical space and provides unique structures. While, previous research focused on carbonyls or imine derivatives, the umpolung reactivity of polarized C-C σ-bonds still needs to explore. Herein, we report an umpolung reaction of bicyclo[1.1.0]butanes (BCBs) with electron-deficient alkenes to construct the C(sp3)-C(sp3) bond at the electrophilic position of C-C σ-bonds in BCBs without any transition-metal catalysis. Specifically, this transformation relies on the strain-release driven bridging σ-bonds in bicyclo[1.1.0]butanes (BCBs), which are emerged as ene components, providing an efficient and straightforward synthesis route of various functionalized cyclobutenes and conjugated dienes, respectively. The synthetic utilities of this protocol are performed by several transformations. Preliminary mechanistic studies including density functional theory (DFT) calculation support the concerted Alder-ene type process of C-C σ-bond cleavage with hydrogen transfer. This work extends the umpolung reaction to C-C σ-bonds and provides high-value structural motifs.