Rapid and Scalable Halosulfonylation of Strain-Release Reagents

Bis(bicyclo[1.1.1]pentyl)chlorophosphine as a Precursor for the Preparation of Bis(bicyclo[1.1.1]pentyl)phosphines

Dialkylchlorophosphines are among the most versatile building blocks for tertiary phosphine ligands, but their synthesis relies on the nucleophilic substitution of PCl3, leaving substituents that require P-H precursors largely inaccessible. The primary phosphine reagent iPr2NPH2·BH3 can serve as a doubly protected PH2Cl proxy, enabling the synthesis of bis(bicyclo[1.1.1]pentyl)chlorophosphine (Bcp2PCl) for the first time. Bcp2PCl serves as a general reagent for the preparation of a family of bis(bicyclo[1.1.1]pentyl) alkyl- and arylphosphines, including new members of privileged phosphine ligand scaffolds.

(Bio)isosteres of ortho- and meta-substituted benzenes

Saturated bioisosteres of substituted benzenes offer opportunities to fine-tune the properties of drug candidates in development. Bioisosteres of para-benzenes, such as those based on bicyclo[1.1.1]pentane, are now very common and can be used to increase aqueous solubility and improve metabolic stability, among other benefits. Bioisosteres of ortho- and meta-benzenes were for a long time severely underdeveloped by comparison. This has begun to change in recent years, with a number of potential systems being reported that can act as bioisosteres for these important fragments. In this review, we will discuss these recent developments, summarizing the synthetic approaches to the different bioisosteres as well as the impact they have on the physiochemical and biological properties of pharmaceuticals and agrochemicals.

Synthesis of Heterocycle-Substituted Bicyclo[3.1.1]heptanes and Aza-bicyclo[3.1.1]heptanes via Photocatalytic Minisci Reaction

A route toward heterocycle-functionalized bicyclo[3.1.1]heptanes (BCHeps) and aza-bicyclo[3.1.1]heptanes (aza-BCHeps) has been developed, using mild, photocatalytic Minisci-like conditions to introduce various heterocycles at the bridgehead position from readily available N-hydroxyphthalimide esters of the corresponding carboxylic acids. This chemistry enables access to heterocycle-functionalized BCHep-containing structures that are highly relevant in medicinal chemistry research as potential bioisosteres of meta-substituted arenes and pyridines.

Photoredox-Catalyzed Alkylarylation of N-Aryl Bicyclobutyl Amides with α-Carbonyl Alkyl Bromides: Access to 3-Spirocyclobutyl Oxindoles

A visible-light-induced radical alkylarylation of N-aryl bicyclobutyl amides with α-carbonyl alkyl bromides for the synthesis of functionalized 3-spirocyclobutyl oxindoles is described in which β-selective radical addition of the alkyl radical to N-aryl bicyclobutyl amides forms a key radical intermediate followed by interception with intrinsic arene functional group. This approach can be applicable to a wide range of α-carbonyl alkyl bromides, including primary, secondary, and tertiary α-bromoalkyl esters, ketones, nitriles, and nitro compounds.

Transition-Metal-Free Approach for Z-Vinyl Fluorides by Hydrofluorination of Alkynes bearing SF4 and SF5 Groups

The synthesis of vinyl fluorides plays a crucial role in various scientific disciplines, including pharmaceutical and materials sciences. Herein, we present a direct and stereoselective hydrofluorination method for the synthesis of Z isomers of vinyl fluorides from alkynes containing unexplored SF5 and SF4 groups. Our strategy employed tetrabutylammonium fluoride (TBAF) as a fluorine source. It demonstrates high compatibility with aryls, biaryls, heteroaryls, and tert-alkyl groups, allowing facile incorporation of SF5 and SF4 groups across the triple bond without any transition-metal catalysts. This approach avoids the potential decomposition of the SF5 or SF4 units via coordination with transition metals or acidic protic sources. Remarkably, this transformation proceeded at room temperature without any additional additives, providing the Z isomer of vinyl fluorides in excellent yield and high selectivity. The presence of a water molecule as a hydrate in TBAF is essential for efficient conversion. This methodology opens new avenues for the synthesis of enchanting SF5 - and SF4 -containing fluorinated vinylic scaffolds, thereby providing advanced opportunities for novel drug discovery and fluorinated polymers.

Strain-enabled radical spirocyclization cascades: rapid access to spirocyclobutyl lactones and - lactams

Spirocyclobutane derivatives have gained significant attention in drug discovery programs due to their broad spectrum of biological activities and clinical applications. Ring-strain in organic molecules is a powerful tool to promote reactivity by releasing strain energy, allowing the construction of complex molecules selectively and efficiently. Herein, we report the first strain-enabled radical spirocyclization cascades for the synthesis of functionalized spirocyclobutyl lactones and - lactams, which are finding increasing applications in medicinal chemistry. The reaction of interelement compounds with bicyclobutane (BCB) allyl esters and - amides proceeds with high chemoselectivity under simple, catalyst-free conditions using blue light irradiation. The reaction has been successfully extended to synthesize bis-spirocycles. To introduce a more diverse set of functional groups, we have developed a dual photoredox/nickel catalytic system capable of mediating the carbosulfonylation of BCB allyl amides. The reaction shows broad applicability across various (hetero)aryl halides, aryl sulfinates, and BCB allyl amides, operates under mild conditions and demonstrates excellent functional group compatibility. The functional groups introduced during the cascade reactions served as versatile handles for further synthetic elaboration.

Organometallic Bridge Diversification of Bicyclo[1.1.1]pentanes

Bicyclo[1.1.1]pentane (BCP) derivatives have attracted significant recent interest in drug discovery as alkyne, tert-butyl and arene bioisosteres, where their incorporation is frequently associated with increased compound solubility and metabolic stability. While strategies for functionalisation of the bridgehead (1,3) positions are extensively developed, platforms allowing divergent substitution at the bridge (2,4,5) positions remain limited. Recent reports have introduced 1-electron strategies for arylation and incorporation of a small range of other substituents, but are limited in terms of scope, yields or practical complexity. Herein, we show the synthesis of diverse 1,2,3-trifunctionalised BCPs through lithium-halogen exchange of a readily accessible BCP bromide. When coupled with medicinally relevant product derivatisations, our developed 2-electron "late stage" approach provides rapid and straightforward access to unprecedented BCP structural diversity (>20 hitherto-unknown motifs reported). Additionally, we describe a method for the synthesis of enantioenriched "chiral-at-BCP" bicyclo[1.1.1]pentanes through a novel stereoselective bridgehead desymmetrisation.

Chemoselective Reactions of Functionalized Sulfonyl Halides

Chemoselective transformations of functionalized sulfonyl fluorides and chlorides are surveyed comprehensively. It is shown that sulfonyl fluorides provide an excellent selectivity control in their reactions. Thus, numerous conditions are tolerated by the SO2 F group - from amide and ester formation to directed ortho-lithiation and transition-metal-catalyzed cross-couplings. Meanwhile, sulfur (VI) fluoride exchange (SuFEx) is also compatible with numerous functional groups, thus confirming its title of "another click reaction". On the contrary, with a few exceptions, most transformations of functionalized sulfonyl chlorides typically occur at the SO2 Cl moiety.

2-Oxabicyclo[2.2.2]octane as a new bioisostere of the phenyl ring

The phenyl ring is a basic structural element in chemistry. Here, we show the design, synthesis, and validation of its new saturated bioisostere with improved physicochemical properties - 2-oxabicyclo[2.2.2]octane. The design of the structure is based on the analysis of the advantages and disadvantages of the previously used bioisosteres: bicyclo[1.1.1]pentane, bicyclo[2.2.2]octane, and cubane. The key synthesis step is the iodocyclization of cyclohexane-containing alkenyl alcohols with molecular iodine in acetonitrile. 2-Oxabicyclo[2.2.2]octane core is incorporated into the structure of Imatinib and Vorinostat (SAHA) drugs instead of the phenyl ring. In Imatinib, such replacement leads to improvement of physicochemical properties: increased water solubility, enhanced metabolic stability, and reduced lipophilicity. In Vorinostat, such replacement results in a new bioactive analog of the drug. This study enhances the repertoire of available saturated bioisosteres of (hetero)aromatic rings for the use in drug discovery projects.

General Synthesis of 3-Azabicyclo[3.1.1]heptanes and Evaluation of Their Properties as Saturated Isosteres

A general approach to 3-azabicyclo[3.1.1]heptanes by reduction of spirocyclic oxetanyl nitriles was developed. The mechanism, scope, and scalability of this transformation were studied. The core was incorporated into the structure of the antihistamine drug Rupatidine instead of the pyridine ring, which led to a dramatic improvement in physicochemical properties.

Conquering the Synthesis and Functionalization of Bicyclo[1.1.1]pentanes

Bicyclo[1.1.1]pentanes (BCPs) have become established as attractive bioisosteres for para-substituted benzene rings in drug design. Conferring various beneficial properties compared with their aromatic "parents," BCPs featuring a wide array of bridgehead substituents can now be accessed by an equivalent variety of methods. In this perspective, we discuss the evolution of this field and focus on the most enabling and general methods for BCPs synthesis, considering both scope and limitation. Recent breakthroughs on the synthesis of bridge-substituted BCPs are described, as well as methodologies for postsynthesis functionalization. We further explore new challenges and directions for the field, such as the emergence of other rigid small ring hydrocarbons and heterocycles possessing unique substituent exit vectors.

2-Oxabicyclo[2.1.1]hexanes as saturated bioisosteres of the ortho-substituted phenyl ring

The ortho-substituted phenyl ring is a basic structural element in chemistry. It is found in more than three hundred drugs and agrochemicals. During the past decade, scientists have tried to replace the phenyl ring in bioactive compounds with saturated bioisosteres to obtain novel patentable structures. However, most of the research in this area has been devoted to the replacement of the para-substituted phenyl ring. Here we have developed saturated bioisosteres of the ortho-substituted phenyl ring with improved physicochemical properties: 2-oxabicyclo[2.1.1]hexanes. Crystallographic analysis revealed that these structures and the ortho-substituted phenyl ring indeed have similar geometric properties. Replacement of the phenyl ring in marketed agrochemicals fluxapyroxad (BASF) and boscalid (BASF) with 2-oxabicyclo[2.1.1]hexanes dramatically improved their water solubility, reduced lipophilicity and most importantly retained bioactivity. This work suggests an opportunity for chemists to replace the ortho-substituted phenyl ring in bioactive compounds with saturated bioisosteres in medicinal chemistry and agrochemistry.

Nickel-Mediated Alkyl-, Acyl-, and Sulfonylcyanation of [1.1.1]Propellane

The replacement of traditional functional groups with polycyclic scaffolds has been increasingly rewarding in medicinal chemistry programs. Over the decades, 1,3-disubstituted bicyclo[1.1.1]pentanes (BCPs) have demonstrated the potential for being competent bioisosteres for aryl-, alkyl- and alkynyl substructures. Although highly desired, mild and versatile synthetic methods to access synthetically valuable BCP-containing building blocks remain limited. Herein, a versatile way to access bridgehead substituted BCP nitriles, a useful BCP building block, is described, enabled by the unexpected selectivity of nickel in the multi-component radical cyanation. Commodity materials including carboxylic acids, amines, sulfonyl chlorides, and alkyl chlorides are engaged to provide a broad spectrum of substituted BCP nitriles in a single-step, multi-component fashion.

A General and Modular Approach to BCP Alkylamines via Multicomponent Difunctionalization of [1.1.1]Propellane

Over the past decade, bicyclo[1.1.1]pentane (BCP) motifs have come to the fore as valuable pharmaceutical bioisosteres of para-disubstituted benzenes. However, the limited approaches and requisite multistep syntheses of useful BCP building blocks are hampering early discovery research in medicinal chemistry. Herein we report the development of a modular strategy for the divergent preparation of functionalized BCP alkylamines. In this process, a general method to introduce fluoroalkyl groups to BCP scaffolds using readily available and easy-to-handle fluoroalkyl sulfinate salts was also developed. Moreover, this strategy can also be extended to S-centered radicals for incorporation of sulfones and thioethers into the BCP core. Overall, this multicomponent strategy enables rapid construction of BCP-type bioisosteres for applications in drug discovery.