Exploring Cuneanes as Potential Benzene Isosteres and Energetic Materials: Scope and Mechanistic Investigations into Regioselective Rearrangements from Cubanes

Stellane at the Forefront: Derivatization and Reactivity Studies of a Promising Saturated Bioisostere of ortho-Substituted Benzenes

This work highlights stellane's cage stability and derivatization opportunities. A diverse range of building blocks were synthesized using modern synthesis protocols to demonstrate stellane's reactivity and chemical tolerance across different reaction systems, proving its promise as a bioisosteric scaffold. It can be utilized in scaffold-based molecular design and is superior in terms of topological precision compared to existing ortho isosteres, as well as monosubstituted benzene mimetics, holding the potential to become a robust platform for future medicinal chemistry studies.

(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.

Assessing the rigidity of cubanes and bicyclo(1.1.1)pentanes as benzene bioisosteres

Aromatic rings are critical core substructures in the majority of pharmaceutical compounds. There is much recent interest in replacing aromatic structures with saturated bioisosteres of benzene, which are generally fused or bridged ring systems. These bioisosteres often show improved solubility properties compared to benzene, and may also undergo fewer unwanted metabolic processes. One key reason why aromatic rings have proven so successful in drug design is their rigidity. This paper uses molecular dynamics simulations supported by crystallographic data to assess the rigidity of bicyclopentane and cubane ring systems as two of the most common benzene bioisosteres and compares this to benzene. Whilst a benzene ring is shown to be more flexible than these two bioisosteres in terms of its dihedral ring flexibility, substituents around the ring tend to behave in a much more similar way in both benzene and the bioisosteric systems.

Computational Research on Ag(I)-Catalyzed Cubane Rearrangement: Mechanism, Metal and Counteranion Effect, Ligand Engineering, and Post-Transition-State Desymmetrization

Ag(I) salts have demonstrated superior catalytic activity in the cubane-cuneane rearrangement. This research presents a comprehensive mechanistic investigation using high-level computations. The reaction proceeds via oxidative addition (OA) of Ag(I) to the C-C bond, followed by C-Ag bond cleavage and subsequent dynamically concerted carbocation rearrangement. The OA of Ag(I) exhibits significant more electrophilic nature than classical transition metal-induced OA, and the superior catalytic activity of Ag(I) is attributed to the accessibility of a highly electrophilic "bare" Ag+ center and a relatively weak Ag-C bond. However, the highly Lewis acidic nature of the Ag(I) center limits the substrate scope. To address this problem, ligand and counteranion screening was conducted, revealing that chiral biarylether ligands in combination with BF4- as the counteranion offer both enhanced reactivity and improved chemoselectivity while suppressing the Lewis acidity. Additionally, quasi-classical molecular dynamics simulations indicate the possibility of a novel desymmetrization pathway through post-transition-state dynamics in the biarylether-Ag(I)-BF4- system, thereby providing a potential avenue for enantioselective cuneane synthesis.

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.

Scaffold Editing of Cubanes into Homocubanes, Homocuneanes via Cuneanes

The selective synthesis of cage-type hydrocarbons through the editing of the highly symmetric molecule cubane can be anticipated as one of the efficient approaches. In this paper, we identify a catalyst that facilitates the efficient scaffold isomerization of cubanes into homocubanes. This approach, which involves the direct synthesis of homocubanol esters, is promising as a novel method for the synthesis of phenoxy bioisosteres. Additionally, we observed that the isomerization of 1,4-bis(acyloxymethl)cubane results in the generation of both D2 - and C2 -symmetrical bishomocubanes. The same catalyst was also applied to the isomerization of acyloxymethylcuneanes, producing homocuneanol esters.

Photoinduced Formation of Cubyl Aryl Thioethers and Synthesis of Monocubyl Analogue of Dapsone

1,4-Disubstituted cubyl aryl thioethers were generated from the corresponding iodocubanes and aryl thiolates upon UV irradiation in dimethyl sulfoxide at room temperature. This simple procedure was found to be compatible with a variety of substituted aryl thiolates. This finding paved the way to a synthesis of the monocubyl analogue of dapsone, a key molecule in the treatment of leprosy, also known as Hansen's disease, and of acne.

C(sp2)-H cyclobutylation of hydroxyarenes enabled by silver-π-acid catalysis: diastereocontrolled synthesis of 1,3-difunctionalized cyclobutanes

Ring-opening of bicyclo[1.1.0]butanes (BCBs) is emerging as a powerful strategy for 1,3-difunctionalized cyclobutane synthesis. However, reported radical strain-release reactions are typically plagued with diastereoselectivity issues. Herein, an atom-economic protocol for the highly chemo- and diastereoselective polar strain-release ring-opening of BCBs with hydroxyarenes catalyzed by a π-acid catalyst AgBF4 has been developed. The use of readily available starting materials, low catalyst loading, high selectivity (up to >98 : 2 d.r.), a broad substrate scope, ease of scale-up, and versatile functionalizations of the cyclobutane products make this approach very attractive for the synthesis of 1,1,3-trisubstituted cyclobutanes. Moreover, control experiments and theoretical calculations were performed to illustrate the reaction mechanism and selectivity.

Silver(I)-Catalyzed Synthesis of Cuneanes from Cubanes and their Investigation as Isosteres

Bridged or caged polycyclic hydrocarbons have rigid structures that project substituents into precise regions of 3D space, making them attractive as linking groups in materials science and as building blocks for medicinal chemistry. The efficient synthesis of new or underexplored classes of such compounds is, therefore, an important objective. Herein, we describe the silver(I)-catalyzed rearrangement of 1,4-disubstituted cubanes to cuneanes, which are strained hydrocarbons that have not received much attention since they were first described in 1970. The synthesis of 2,6-disubstituted or 1,3-disubstituted cuneanes can be achieved with high regioselectivities, with the regioselectivity being dependent on the electronic character of the cubane substituents. A preliminary assessment of cuneanes as scaffolds for medicinal chemistry suggests cuneanes could serve as isosteric replacements of trans-1,4-disubstituted cyclohexanes and 1,3-disubstituted benzenes. An analogue of the anticancer drug sonidegib was synthesized, in which the 1,2,3-trisubstituted benzene was replaced with a 1,3-disubstituted cuneane.