Copper-Catalyzed Cross-Coupling between Alkyl (Pseudo)halides and Bicyclopentyl Grignard Reagents

Rapid Access to 3-Substituted Bicyclo[1.1.1]pentanes

The prevalence of benzene rings in pharmaceutical scaffolds has prompted efforts to identify structural bioisosteres with improved in vivo properties. Notably, bicyclo[1.1.1]pentanes (BCPs)-C(sp3)-enriched, 1,4-disubstituted phenyl bioisosteres-have been leveraged to tune the pharmacokinetic profiles of lead compounds. While 3-arylated BCPs have been widely implemented to confer resistance against oxidative degradation and hydrogen atom transfer (HAT) processes, the analogous 3-alkylated BCPs remain underexplored as bioisosteric "benzylic" C-H motifs. Current methods to install 3-alkylated BCP motifs are heavily reliant on lengthy de novo synthesis and the preparation of reactive [1.1.1]propellane feedstocks, limiting their adoption in drug discovery programs. In this report, we disclose a mild, unified method for the preparation of both alkyl and aryl-substituted BCPs from bench-stable precursors. This method, which proceeds via dual copper-photoredox catalysis, is capable of installing BCP functionalities onto a range of saturated motifs, aryl-containing residues, and medicinally relevant heterocycles.

Synthesis of Amcenestrant (SAR439859): A Copper-Catalyzed Cross-Coupling Reaction as a Sustainable Alternative to Palladium-Catalyzed Suzuki Reaction

A cross-coupling reaction between an enol triflate and an aryl Grignard reagent using a copper catalyst, followed by a deprotection step, a Mitsunobu reaction, and a saponification, allowed for the synthesis of Amcenestrant (SAR439859). This approach, avoiding an expensive and toxic transition metal, is as efficient as the classical route but less expensive for accessing this selective estrogen-receptor degrader (SERD).

Light-enabled scalable synthesis of bicyclo[1.1.1]pentane halides and their functionalizations

In 2012, bicyclo[1.1.1]pentanes were demonstrated to be bioisosteres of the benzene ring. Here, we report a general scalable reaction between alkyl iodides and propellane that provides bicyclo[1.1.1]pentane iodides in milligram, gram and even kilogram quantities. The reaction is performed in flow and requires just light; no catalysts, initiators or additives are needed. The reaction is clean enough that, in many cases, evaporation of the reaction mixture provides products in around 90% purity that can be directly used in further transformations without any purification. Combined with the subsequent functionalization, >300 bicyclo[1.1.1]pentanes for medicinal chemistry have been prepared. So far, this is the most general and scalable approach towards functionalized bicyclo[1.1.1]pentanes.

Synthesis of 1,3-disubstituted bicyclo[1.1.1]pentanes by cross-coupling induced by transition metals - formation of C-C bonds

The synthesis of 1,3-disubstituted bicyclo[1.1.1]pentanes (BCPs), by forming a C-C bond, can be achieved by cross-coupling reactions using transition metal catalysts. Two main strategies are described to access these 1,3-disubstituted BCPs, either from nucleophilic BCPs or electrophilic BCPs. Mechanisms are included where relevant.

Up-Scale Synthesis of p-(CH2=CH)C6H4CH2CH2CH2Cl and p-ClC6H4SiR3 by CuCN-Catalyzed Coupling Reactions of Grignard Reagents with Organic Halides

Grignard reagents featuring carbanion characteristics are mostly unreactive toward alkyl halides and require a catalyst for the coupling reaction. With the need to prepare p-(CH₂=CH)C₆H₄CH₂CH₂CH₂Cl on a large scale, the coupling reaction of p-(CH₂=CH)C₆H₄MgCl with BrCH₂CH₂CH₂Cl was attempted to screen the catalysts, and CuCN was determined to be the best catalyst affording the desired compound in 80% yield with no formation of Wurtz coupling side product CH₂=CHC₆H₄-C₆H₄CH=CH₂. The p-(CH₂=CH)C₆H₄Cu(CN)MgCl species was proposed as an intermediate based on the X-ray structure of PhCu(CN)Mg(THF)₄Cl. p-ClC₆H₄MgCl did not react with sterically encumbered R₃SiCl (R = n-Bu or n-octyl). However, the reaction took place with the addition of 3 mol % CuCN catalyst, affording the desired compound p-ClC₆H₄SiR₃. The structures of p-(CH₂=CH)C₆H₄CH₂CH₂CH₂MgCl and p-ClC₆H₄MgCl were also elucidated, which existed as an aggregate with MgCl₂, suggesting that some portion of the Grignard reagents were possibly lost in the coupling reaction due to coprecipitation with the byproduct MgCl₂. R₃SiCl (R = n-Bu or n-octyl) was also prepared easily and economically with no formation of R₄Si when SiCl₄ was reacted with 4 equiv of RMgCl. Using the developed syntheses, [p-(CH₂=CH)C₆H₄CH₂CH₂CH₂]₂Zn and iPrN[P(C₆H₄-p-SiR₃)₂]₂, which are potentially useful compounds for the production of PS-block-PO-block-PS and 1-octene, respectively, were efficiently synthesized with substantial cost reductions.

Development of tumor-targeting aza-vesamicol derivatives with high affinity for sigma receptors for cancer theranostics

As sigma receptors are highly expressed on various cancer cells, radiolabeled sigma receptor ligands have been developed as imaging and therapeutic probes for cancer. Previously, we synthesized and evaluated a radioiodinated vesamicol derivative, 2-(4-[¹²⁵I](4-iodophenyl)piperidine)cyclohexanol ((+)-[¹²⁵I]pIV), and a radioiodinated aza-vesamicol derivative, trans-2-(4-(3-[¹²⁵I](4-iodophenyl)propyl)piperazin-1-yl)cyclohexan-1-ol ([¹²⁵I]2), as sigma-1 receptor-targeting probes. In order to obtain sigma receptor-targeting probes with superior biodistribution characteristics, we firstly synthesized twelve bromine-containing aza-vesamicol derivatives and evaluated their affinity for sigma receptors. One such derivative exhibited high selectivity for the sigma-1 receptor and another exhibited high affinity for both the sigma-1 and sigma-2 receptors. Thus, their halogen-substituted iodine- and radioiodine-containing compounds were prepared. The ¹²⁵I-labeled compounds exhibited high uptake in tumor and lower uptake in non-target tissues than the two previously developed and evaluated ¹²⁵I-labeled sigma receptor-targeting probes, [¹²⁵I]pIV and [¹²⁵I]2. Therefore, these novel radioiodine-labeled compounds should be promising as sigma receptor-targeting probes.

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.

Highly Regioselective Addition of Allylic Zinc Halides and Various Zinc Enolates to [1.1.1]Propellane

We report a range of highly regioselective openings of [1.1.1]propellane with various allylic zinc halides, as well as zinc enolates of ketones, esters and nitriles. The resulting zincated bicyclopentanes (BCPs) were trapped with a range of electrophiles including acyl chlorides, sulfonothioates, hydroxylamino benzoates, tosyl cyanide as well as aryl and allyl halides, generating highly functionalized BCP-derivatives. The unusually high regioselectivity of these reactions has been rationalized using DFT calculations. A bioisostere of the synthetic opioid pethidine was prepared in 95 % yield in one step using this method.