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

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.

Synthesis of 1-Substituted Bicyclo[2.1.1]hexan-2-ones via a Sequential SmI2-Mediated Pinacol Coupling and Acid-Catalyzed Pinacol Rearrangement Reaction

A two-step procedure, combining a SmI2-mediated transannular pinacol coupling reaction with an acid-catalyzed pinacol rearrangement process, was employed to prepare a diverse range of 1-substituted bicyclo[2.1.1]hexan-5-ones from cyclobutanedione derivatives. To underscore the significance of these bicyclic ketones in drug synthesis, an sp3-rich analog of nitazoxanide, a well-known antiparasitic and antiviral agent, was synthesized.

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.

Lewis acid-catalyzed (3 + 2) annulation of bicyclobutanes with ynamides: access to 2-amino-bicyclo[2.1.1]hexenes

Strain-release driven annulations with bicyclo[1.1.0]butanes (BCBs) have become an attractive area of research for the synthesis of bioisosteric bicyclohexane derivatives, which play a vital role in drug discovery. Interestingly, the utilization of the inherent strain in BCBs for the synthesis of functionalized amino-bicyclo[2.1.1]hexenes, which may spatially mimic substituted benzenes and anilines, has received only scant attention. Herein, we report the Sc(OTf)3-catalyzed (3 + 2) annulation of BCBs with ynamides for the facile synthesis of 2-amino-bicyclo[2.1.1]hexenes in one step under mild conditions. The reaction likely proceeds via nucleophilic addition facilitated by the nitrogen lone pair from the alkynyl group of the ynamides to the unsubstituted side of the BCBs, followed by the annulation of the resulting enolate with the keteniminium species. For the first time, the C-C triple bond of ynamides was utilized as the coupling partner for BCBs, resulting in products adorned with a functionalizable amino group and an integrated strained alkene moiety.

Lewis Acid Catalyzed Cycloaddition of Bicyclobutanes with Ynamides for the Synthesis of Polysubstituted 2-Amino-bicyclo[2.1.1]hexenes

Synthesis of bicyclic scaffolds has gained significant attention in drug discovery due to their potential to mimic benzene bioisosteres. Here, we present a mild and scalable Sc(OTf)3-catalyzed [3+2] cycloaddition of bicyclo[1.1.0]butanes (BCBs) with ynamides, yielding a diverse array of polysubstituted 2-amino-bicyclo[2.1.1]hexenes in good to excellent yields. These products offer valuable starting materials for the construction of novel functionalized bicyclo[1.1.0]butanes. Preliminary mechanistic studies indicate that the reaction involves a nucleophilic addition of ynamides to bicyclo[1.1.0]butanes, followed by an intramolecular cyclization of in situ generated enolate and keteniminium ion. We expect that these findings will encourage utilization of complex bioisosteres and foster further investigation into BCB-based cycloaddition chemistry.

Three-dimensional saturated C(sp3)-rich bioisosteres for benzene

Benzenes, the most ubiquitous structural moiety in marketed small-molecule drugs, are frequently associated with poor 'drug-like' properties, including metabolic instability, and poor aqueous solubility. In an effort to overcome these limitations, recent developments in medicinal chemistry have demonstrated the improved physicochemical profiles of C(sp3)-rich bioisosteric scaffolds relative to arenes. In the past two decades, we have witnessed an exponential increase in synthetic methods for accessing saturated bioisosteres of monosubstituted and para-substituted benzenes. However, until recent discoveries, analogous three-dimensional ortho-substituted and meta-substituted biososteres have remained underexplored, owing to their ring strain and increased s-character hybridization. This Review summarizes the emerging synthetic methodologies to access such saturated motifs and their impact on the application of bioisosteres for ortho-substituted, meta-substituted and multi-substituted benzene rings. It concludes with a perspective on the development of next-generation bioisosteres, including those within novel chemical space.

Facile access to bicyclo[2.1.1]hexanes by Lewis acid-catalyzed formal cycloaddition between silyl enol ethers and bicyclo[1.1.0]butanes

Saturated three-dimensional carbocycles have gained increasing prominence in synthetic and medicinal chemistry. In particular, bicyclo[2.1.1]hexanes (BCHs) have been identified as the molecular replacement for benzenes. Here, we present facile access to a variety of BCHs via a stepwise two-electron formal (3 + 2) cycloaddition between silyl enol ethers and bicyclo[1.1.0]butanes (BCBs) under Lewis acid catalysis. The reaction features wide functional group tolerance for silyl enol ethers, allowing the efficient construction of two vicinal quaternary carbon centers and a silyl-protected tertiary alcohol unit in a streamlined fashion. Interestingly, the reaction with conjugated silyl dienol ethers can provide access to bicyclo[4.1.1]octanes (BCOs) equipped with silyl enol ethers that facilitate further transformation. The utilities of this methodology are demonstrated by the late-stage modification of natural products, transformations of tertiary alcohol units on bicyclo[2.1.1]hexane frameworks, and derivatization of silyl enol ethers on bicyclo[4.1.1]octanes, delivering functionalized bicycles that are traditionally inaccessible.

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

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

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.

Bicyclo[m.n.k]alkane Building Blocks as Promising Benzene and Cycloalkane Isosteres: Multigram Synthesis, Physicochemical and Structural Characterization

Electrophilic double bond functionalization - intramolecular enolate alkylation sequence was used to obtain a series of bridged and fused bicyclo[m.n.k]alkane derivatives (i. e., bicyclo[4.1.1]octanes, bicyclo[2.2.1]heptanes, bicyclo[3.2.1]octanes, bicyclo[3.1.0]hexanes, and bicyclo[4.2.0]heptanes). The scope and limitations of the method were established, and applicability to the multigram synthesis of target bicyclic compounds was illustrated. Using the developed protocols, over 50 mono- and bifunctional building blocks relevant to medicinal chemistry were prepared. The synthesized compounds are promising isosteres of benzene and cycloalkane rings, which is confirmed by their physicochemical and structural characterization (pKa , LogP, and exit vector parameters (EVP)). "Rules of thumb" for the upcoming isosteric replacement studies were proposed.

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

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

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

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

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

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

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

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

Synthesis of polysubstituted bicyclo[2.1.1]hexanes enabling access to new chemical space

Saturated bridged-bicyclic compounds are currently under intense investigation as building blocks for pharmaceutical drug design. However, the most common methods for their preparation only provide access to bridgehead-substituted structures. The synthesis of bridge-functionalised species is highly challenging but would open up many new opportunities for molecular design. We describe a photocatalytic cycloaddition reaction that provides unified access to bicyclo[2.1.1]hexanes with 11 distinct substitution patterns. Bridge-substituted structures that represent ortho-, meta-, and polysubstituted benzene bioisosteres, as well as those that enable the investigation of chemical space inaccessible to aromatic motifs can all be prepared using this operationally simple protocol. Proof-of-concept examples of the application of the method to the synthesis of saturated analogues of biorelevant trisubstituted benzenes are also presented.