A New Route to Bicyclo[1.1.1]pentan-1-amine from 1-Azido-3-iodobicyclo[1.1.1]pentane

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.

Radical Reactions in Organic Synthesis: Exploring in-, on-, and with-Water Methods

Radical reactions in water or aqueous media are important for organic synthesis, realizing high-yielding processes under non-toxic and environmentally friendly conditions. This overview includes (i) a general introduction to organic chemistry in water and aqueous media, (ii) synthetic approaches in, on, and with water as well as in heterogeneous phases, (iii) reactions of carbon-centered radicals with water (or deuterium oxide) activated through coordination with various Lewis acids, (iv) photocatalysis in water and aqueous media, and (v) synthetic applications bioinspired by naturally occurring processes. A wide range of chemical processes and synthetic strategies under different experimental conditions have been reviewed that lead to important functional group translocation and transformation reactions, leading to the preparation of complex molecules. These results reveal how water as a solvent/medium/reagent in radical chemistry has matured over the last two decades, with further discoveries anticipated in the near future.

Accelerated Synthesis of Bicyclo[1.1.1]pentylamines: A High-Throughput Approach

Strained bicyclic substructures such as bicyclo[1.1.1]pentylamines (BCPAs) are increasingly targeted in medicinal chemistry as arylamine bioisosteres. Here, we leverage high-throughput automated synthesis to rapidly develop library-amenable reaction conditions and maximize design space to expand access to BCPAs. This new protocol relies on a copper-mediated C-N coupling approach and uses accessible and bench-stable iodo-BCP building blocks. Its applicability has been exemplified by incorporating BCPs in drug-like compounds, providing straightforward access to a library of valuable aniline-like isosteres.

The emerging role of radical chemistry in the amination transformation of highly strained [1.1.1]propellane: Bicyclo[1.1.1]pentylamine as bioisosteres of anilines

Bicyclo[1.1.1]pentylamines (BPCAs), emerging as sp³-rich surrogates for aniline and its derivatives, demonstrate unique structural features and physicochemical profiles in medicinal and synthetic chemistry. In recent years, compared with conventional synthetic approaches, the rapid development of radical chemistry enables the assembly of valuable bicyclo[1.1.1]pentylamines scaffold directly through the amination transformation of highly strained [1.1.1]propellane. In this review, we concisely summarize the emerging role of radical chemistry in the construction of BCPAs motif, highlighting two different and powerful radical-involved strategies including C-centered and N-centered radical pathways under appropriate conditions. The future direction concerning BCPAs is also discussed at the end of this review, which aims to provide some inspiration for the research of this promising project.

Hydroamination of Unactivated Alkenes with Aliphatic Azides

We report here an efficient and highly diastereoselective intermolecular anti-Markovnikov hydroamination of unactivated alkenes with aliphatic azides in the presence of silane. The system tolerates a wide range of azides and alkenes and operates with alkene as limiting reagent. Mechanistic studies suggest a radical chain pathway that involves aminium radical formation, radical addition to alkenes and HAT from silane to β-aminium alkyl radical. The use of sterically bulky silane is proposed to contribute to the excellent diastereoselectivity for HAT. Computational analysis uncovers the reaction pathway of aliphatic azide activation with silyl radical for aminyl radical formation.

Synthetic routes to bicyclo[1.1.1]pentylamines: booming toolkits for drug design

With the flourishing progress of modern medicinal chemistry, the bicyclo[1.1.1]pentylamines (BCPAs) have come to the fore as bioisosteres of arylamine motifs to reduce the growing concern about arylamines’ risks related...

Electrophilic Activation of [1.1.1]Propellane for the Synthesis of Nitrogen-Substituted Bicyclo[1.1.1]pentanes

Strategies commonly used for the synthesis of functionalised bicyclo[1.1.1]pentanes (BCP) rely on the reaction of [1.1.1]propellane with anionic or radical intermediates. In contrast, electrophilic activation has remained a considerable challenge due to the facile decomposition of BCP cations, which has severely limited the applications of this strategy. Herein, we report the electrophilic activation of [1.1.1]propellane in a halogen bond complex, which enables its reaction with electron‐neutral nucleophiles such as anilines and azoles to give nitrogen‐substituted BCPs that are prominent motifs in drug discovery. A detailed computational analysis indicates that the key halogen bonding interaction promotes nucleophilic attack without sacrificing cage stabilisation. Overall, our work rehabilitates electrophilic activation of [1.1.1]propellane as a valuable strategy for accessing functionalised BCPs.

Twofold Radical-Based Synthesis of N,C-Difunctionalized Bicyclo[1.1.1]pentanes

Bicyclo[1.1.1]pentylamines (BCPAs) are of growing importance to the pharmaceutical industry as sp³-rich bioisosteres of anilines and N-tert-butyl groups. Here we report a facile synthesis of 1,3-disubstituted BCPAs using a twofold radical functionalization strategy. Sulfonamidyl radicals, generated through fragmentation of α-iodoaziridines, undergo initial addition to [1.1.1]propellane to afford iodo-BCPAs; the newly formed C-I bond in these products is then functionalized via a silyl-mediated Giese reaction. This chemistry also translates smoothly to 1,3-disubstituted iodo-BCPs. A wide variety of radical acceptors and iodo-BCPAs are accommodated, providing straightforward access to an array of valuable aniline-like isosteres.

A continuous flow synthesis of [1.1.1]propellane and bicyclo[1.1.1]pentane derivatives

A continuous flow process to generate [1.1.1]propellane on demand is presented rendering solutions of [1.1.1]propellane that can directly be derivatised into various bicyclo[1.1.1]pentane (BCP) species. This was realised in throughputs up to 8.5 mmol h-1 providing an attractive and straightforward access to gram quantities of selected BCP building blocks. Lastly, a continuous photochemical transformation of [1.1.1]propellane into valuable BCPs bearing mixed ester/acyl chloride moieties was developed.

Investigation of the Reactivity of 1-Azido-3-iodobicyclo[1.1.1]pentane under "Click" Reaction Conditions

The bicyclo[1.1.1]pentane (BCP) unit is under scrutiny as a bioisostere in drug molecules. We employed methodologies for the synthesis of different BCP triazole building blocks from one precursor, 1-azido-3-iodobicyclo[1.1.1]pentane, by "click" reactions and integrated cycloaddition-Sonogashira coupling reactions. Thereby, we accessed 1,4-disubstituted triazoles, 5-iodo-1,4,5-trisubstituted triazoles, and 5-alkynylated 1,4,5-trisubstituted triazoles. This gives entry to the synthesis of multiply substituted BCP triazoles on either a modular or a one-pot basis. These methodologies were further utilized for appending porphyrin moieties onto the BCP core.

Sodium Bicyclo[1.1.1]pentanesulfinate: A Bench-Stable Precursor for Bicyclo[1.1.1]pentylsulfones and Bicyclo- [1.1.1]pentanesulfonamides

Herein, we present the synthesis of the bench-stable sodium bicyclo[1.1.1]pentanesulfinate (BCP-SO2 Na) and its application in the synthesis of bicyclo[1.1.1]pentyl (BCP) sulfones and sulfonamides. The salt can be obtained in a four-step procedure from commercially available precursors in multigram scale without the need for column chromatography or crystallization. Sulfinates are known to be useful precursors in radical and nucleophilic reactions and are widely used in medicinal chemistry. This building block enables access to BCP sulfones and sulfonamides avoiding the volatile [1.1.1]propellane which is favorable for the extension of SAR studies. Further, BCP-SO2 Na enables the synthesis of products that were not available with previous methods. A chlorination of BCP-SO2 Na and subsequent reaction with a Grignard reagent provides a new route to BCP sulfoxides. Several products were analyzed by single-crystal X-ray diffraction.

Synthesis of Enantioenriched α-Chiral Bicyclo[1.1.1]pentanes

Bicyclo[1.1.1]pentanes (BCPs), useful surrogates for para-substituted arenes, alkynes, and tert-butyl groups in medicinal chemistry, are challenging to prepare when featuring stereogenic centers adjacent to the BCP. We report the development of an efficient route to α-chiral BCPs, via highly diastereoselective asymmetric enolate functionalization. We also describe the application of this chemistry to the synthesis of BCP analogues of phenylglycine and tarenflurbil, the single enantiomer of the NSAID flurbiprofen.

Synthesis and applications of highly functionalized 1-halo-3-substituted bicyclo[1.1.1]pentanes

Bicyclo[1.1.1]pentanes (BCPs) are important bioisosteres of 1,4-disubstituted arenes, tert-butyl and acetylenic groups that can impart physicochemical benefits on drug candidates. Here we describe the synthesis of BCPs bearing carbon and halogen substituents under exceptionally mild reaction conditions, via triethylborane-initiated atom-transfer radical addition ring-opening of tricyclo[1.1.1.01,3]pentane (TCP) with alkyl halides. This chemistry displays broad substrate scope and functional group tolerance, enabling application to BCP analogues of biologically-relevant targets such as peptides, nucleosides, and pharmaceuticals. The BCP halide products can be converted to the parent phenyl/tert-butyl surrogates through triethylborane-promoted dehalogenation, or to other derivatives including carbonyls, alcohols, and heterocycles.

A practical metal-free homolytic aromatic alkylation protocol for the synthesis of 3-(pyrazin-2-yl)bicyclo[1.1.1]pentane-1-carboxylic acid

As a part of our ongoing synthetic quest to expand the frontiers of contemporary medicinal chemistry, we now report an expedient synthesis of a potentially useful bicyclo[1.1.1]pentane building block, 3-(pyrazin-2-yl)bicyclo[1.1.1]pentane-1-carboxylic acid. This report also showcases the application of this motif as a probe in a biological study.

Radical Multicomponent Carboamination of [1.1.1]Propellane

Three-dimensional, small-ring scaffolds are very important in modern drug discovery to expand the available drug-like chemical space and to optimize drug candidates. Among them, bicyclo[1.1.1]pentane (BCP) is regarded as a high-value bioisostere for a phenyl ring or tert-butyl group; it provides an option to generate drug-like molecules with good passive permeability, high aqueous solubility, and improved metabolic stability, though the lack of methodology to functionalize BCP remains a significant challenge. Here we present an efficient method, developed with the aid of density functional theory calculations, for the synthesis of multifunctionalized BCP derivatives by means of a radical multicomponent carboamination of [1.1.1]propellane. This reaction features mild conditions, one-pot operation, and gram-scale synthetic capability, and opens up a unique and highly efficient route for the synthesis of multifunctionalized BCP derivatives, including synthetically useful 3-substituted BCP-amines.

Toward Resolving the Resveratrol Conundrum: Synthesis and in Vivo Pharmacokinetic Evaluation of BCP-Resveratrol

Over the last few decades, resveratrol has gained significance due to its impressive array of biological activities; however, its true potential as a drug has been severely constrained by its poor bioavailability. Indeed, several studies have implicated this bioavailability trait as a major road-block to resveratrol's potential clinical applications. To mitigate this pharmacokinetic issue, we envisioned a tactical bioisosteric modification of resveratrol to bicyclo[1.1.1]pentane (BCP) resveratrol. Relying on the beneficial bioisosteric potential demonstrated by the BCP-scaffold, we hypothesized that BCP-resveratrol would have an inherently better in vivo PK profile as compared to its natural counterpart. To validate such a hypothesis, it was necessary to secure a synthetic access to this novel structure. Herein we describe the first synthesis of BCP-resveratrol and disclose its PK properties.

Improving Nonspecific Binding and Solubility: Bicycloalkyl Groups and Cubanes as para-Phenyl Bioisosteres

Bicycloalkyl groups have been previously described as phenyl group bioisosteres. This article describes the synthesis of new building blocks allowing their introduction into complex molecules, and explores their use as a means to modify the physicochemical properties of drug candidates and improve the quality of imaging agents. In particular, the replacement of an aromatic ring with a bicyclo[1.1.1]pentane-1,3-diyl (BCP) group improves aqueous solubility by at least 50-fold, and markedly decreases nonspecific binding (NSB) as measured by CHI(IAM), the chromatographic hydrophobicity index on immobilized artificial membranes. Structural variations with the bicyclo[2.2.2]octane-1,4-diyl group led to more lipophilic molecules and did not show the same benefits regarding NSB or solubility, whereas substitutions with cubane-1,4-diyl showed improvements for both parameters. These results confirm the potential advantages of both BCP and cubane motifs as bioisosteric replacements for optimizing para-phenyl-substituted molecules.

Strain-Release Heteroatom Functionalization: Development, Scope, and Stereospecificity

Driven by the ever-increasing pace of drug discovery and the need to push the boundaries of unexplored chemical space, medicinal chemists are routinely turning to unusual strained bioisosteres such as bicyclo[1.1.1]pentane, azetidine, and cyclobutane to modify their lead compounds. Too often, however, the difficulty of installing these fragments surpasses the challenges posed even by the construction of the parent drug scaffold. This full account describes the development and application of a general strategy where spring-loaded, strained C–C and C–N bonds react with amines to allow for the “any-stage” installation of small, strained ring systems. In addition to the functionalization of small building blocks and late-stage intermediates, the methodology has been applied to bioconjugation and peptide labeling. For the first time, the stereospecific strain-release “cyclopentylation” of amines, alcohols, thiols, carboxylic acids, and other heteroatoms is introduced. This report describes the development, synthesis, scope of reaction, bioconjugation, and synthetic comparisons of four new chiral “cyclopentylation” reagents.

Organic chemistry. Strain-release amination

To optimize drug candidates, modern medicinal chemists are increasingly turning to an unconventional structural motif: small, strained ring systems. However, the difficulty of introducing substituents such as bicyclo[1.1.1]pentanes, azetidines, or cyclobutanes often outweighs the challenge of synthesizing the parent scaffold itself. Thus, there is an urgent need for general methods to rapidly and directly append such groups onto core scaffolds. Here we report a general strategy to harness the embedded potential energy of effectively spring-loaded C-C and C-N bonds with the most oft-encountered nucleophiles in pharmaceutical chemistry, amines. Strain-release amination can diversify a range of substrates with a multitude of desirable bioisosteres at both the early and late stages of a synthesis. The technique has also been applied to peptide labeling and bioconjugation.

Radical fluorination powered expedient synthesis of 3-fluorobicyclo[1.1.1]pentan-1-amine

Exploration of novel chemical space, a modern trend in medicinal chemistry, is heavily reliant on synthetic access to new and interesting building blocks. In this direction, the following work describes an expedient synthesis of one such moiety, 3-fluorobicyclo[1.1.1]pentan-1-amine, by employing radical fluorination.

Expedient synthesis of 3-phenylbicyclo[1.1.1]pentan-1-amine via metal-free homolytic aromatic alkylation of benzene

The inquisitiveness on the use of the BCP motif as a modern lead optimization tactic entails reliable synthetic access. In that direction, we disclose a new and versatile approach to 3-phenylbicyclo[1.1.1]pentan-1-amine, via metal-free homolytic alkylation of benzene.