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

Enantioselective Synthesis of Tetrahydro-1H-1,3-methanocarbazoles by Formal (3 + 3)-Cycloaddition Using Bicyclo[1.1.0]butanes

Asymmetric synthesis presents many challenges, with the selective formation of chiral bridged polyheterocycles being a notable example. Cycloadditions using bicyclo[1.1.0]butanes (BCB) offer a promising solution along those lines, yet, despite significant advances in that emerging area, asymmetric control has remained limited thus far. Here, we describe an organocatalytic, enantioselective formal (3 + 3)-cycloaddition of BCBs with 1H-indol-3-yl((hetero)aryl)methanol derivatives. This approach enables the rapid and efficient synthesis of chiral tetrahydro-1H-1,3-methanocarbazole derivatives (34 examples) from readily available starting materials, with very good stereochemical control (up to 98:2 er). Successful scale-up experiments and product modification demonstrated the potential of this methodology. Control experiments and DFT calculations provide insights into the mechanistic pathway.

Photoinduced Cobaloxime-Catalyzed Regio- and Diastereoselective Hydrogen-Evolution C(sp3)-H Phosphorylation of Bicyclo[1.1.0]butanes

Radical-initiated functionalization of bicyclo[1.1.0]butanes (BCBs) is a straightforward approach to accessing diverse cyclobutane derivatives. However, selective C(sp3)-H functionalization at the C2 position of BCBs remains scarce. Herein, a mild protocol for the hydrogen-evolution of C2 C(sp3)-H phosphorylation with BCBs enabled by photoinduced cobaloxime catalysis was realized in a regio- and diastereoselective manner. This oxidant- and additional photocatalyst-free method enabled C(sp3)-H phosphorylation with a wide range of BCBs and diarylphosphine oxides. The mechanism was studied via control experiments and DFT calculation. Moreover, the efficiency of this approach was highlighted in the synthesis of high-value, structurally complex molecules.

Palladium-Catalyzed Strain-Enabled [2π + 2σ] Cycloadditions of Vinyl Bicyclo[1.1.0]butanes with Methyleneindolinones

A palladium-catalyzed [2π + 2σ] cycloaddition of vinyl bicyclo[1.1.0]butanes with methyleneindolinones has been developed. The reaction enables the construction of spirobicyclo[2.1.1]hexanes bearing an all-carbon quaternary center in moderate to good yields with excellent diastereoselectivities. This method features a broad substrate scope with good functional group compatibility. The practical utility of this protocol was further demonstrated by gram-scale synthesis and postsynthetic transformations of desired product.

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.

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.

Enantioselective [2π + 2σ] Photocycloaddition Enabled by Brønsted Acid Catalyzed Chromophore Activation

Bicyclo[2.1.1]hexanes have emerged as valuable scaffolds for the design of new pharmaceutical and agrochemical active ingredients. These structures can be efficiently synthesized via [2π + 2σ] photocycloadditions; however, control over the absolute stereochemistry of these strain-releasing reactions has remained challenging. Herein, we demonstrate that Brønsted acid catalyzed chromophore activation of C-acyl imidazoles enables highly enantioselective [2π + 2σ] photocycloadditions. Because this approach is agnostic to the identity of the coupling partner, the same strategy can be used to synthesize several other medicinally relevant strained small-ring structures.

Modular Synthesis of Azidobicyclo[2.1.1]hexanes via (3 + 2) Annulation of α-Substituted Vinyl Azides and Bicyclo[1.1.0]butanes

Here, we present a mild and rapid method to access azidobicyclo[2.1.1]hexanes via formal (3 + 2) cycloaddition of α-substituted vinyl azides and bicyclo[1.1.0]butanes under Lewis acid catalysis. A wide range of α-substituted vinyl azides were tolerated under mild conditions. Notably, the resulting cycloadducts could be transformed into structurally attractive 3-azabicyclo[3.1.1]heptenes through microwave-promoted rearrangement. The utilities were highlighted by copper(I)-catalyzed Huisgen 1,3-dipolar cycloaddition of tertiary alkyl azide and further transformation of the azide and ketone groups.

Divergent Enantioselective Access to Diverse Chiral Compounds from Bicyclo[1.1.0]butanes and α,β-Unsaturated Ketones under Catalyst Control

Achieving structural and stereogenic diversity from the same starting materials remains a fundamental challenge in organic synthesis, requiring precise control over the selectivity. Here, we report divergent catalytic methods that selectively yield either cycloaddition or addition/elimination products from bicyclo[1.1.0]butanes and α,β-unsaturated ketones. By employing chiral Lewis acid or Brønsted acid catalysts, we achieved excellent regio-, diastereo-, and enantioselectivity across all three distinct transformations, affording a diverse array of synthetically valuable chiral bicyclo[2.1.1]hexanes and cyclobutenes. The divergent outcomes are controlled by the differential activation of the substrates by the specific chiral catalyst with the reaction conditions dictating the pathway selectivity. This strategy demonstrates the power of divergent catalysis in creating molecular complexity and diversity, offering a valuable tool for the synthesis of enantioenriched chiral building blocks.

Formal [2σ + 2σ]-Cycloaddition of Aziridines with Bicyclo[1.1.0]butanes: Access to Enantiopure 2-Azabicyclo[3.1.1]heptane Derivatives

Saturated nitrogen heterocycles are among the most significant structural components in small-molecule pharmaceuticals. Herein, a protocol for the construction of enantiopure 2-azabicyclo[3.1.1]heptane derivatives by a stereospecific intermolecular formal cycloaddition of aziridines with bicyclo[1.1.0]butanes is described. The reaction is run by using B(C6F5)3 as a catalytic additive to give access to a library of enantiopure 2-azabicyclo[3.1.1]heptane derivatives (37 examples) under mild and operationally simple conditions. Successful scale-up reactions, mechanistic experiments, density functional theory (DFT) calculations and synthetic applications are presented.

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

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

Lewis Acid-Catalyzed Formal [2π+2σ] Cycloaddition of Bicyclobutanes with Quinoxalin-2(1H)-ones: Access to Quinoxaline-Fused Aza-Bicyclo[2.1.1]hexanes

An efficient synthesis of quinoxaline-fused aza-bicyclo[2.1.1]hexanes bearing multiple quaternary carbon centers via the intermolecular [2π+2σ] cycloaddition of bicyclo[1.1.0]butanes and quinoxalin-2(1H)-ones, facilitated by Lewis acid catalysis, is presented. This reaction is carried out under mild conditions and exhibits a broad substrate scope and excellent functional group tolerance.

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

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

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

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

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

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

Enantioselective [2π + 2σ] Cycloadditions of Bicyclo[1.1.0]butanes with Vinylazaarenes through Asymmetric Photoredox Catalysis

Here we present a highly enantioselective [2π + 2σ] photocycloaddition of bicyclo[1.1.0]butanes (BCBs). The reaction uses a variety of vinylazaarenes as partners and is catalyzed by a polycyclic aromatic hydrocarbon (PAH)-containing chiral phosphoric acid as a bifunctional chiral photosensitizer. A wide array of pharmaceutically important bicyclo[2.1.1]hexane (BCH) derivatives have been synthesized with high yields, enantioselectivity, and diastereoselectivity. In addition to the diverse 1-ketocarbonyl-3-substituted BCBs, α/β-substituted vinylazaarenes are compatible with such an unprecedented photoredox catalytic pathway, resulting in the successful assembly of an all-carbon quaternary stereocenter or two adjacent tertiary stereocenters on the product.