Bicyclic Pyrrolidine Inhibitors of Toxoplasma gondii Phenylalanine t-RNA Synthetase with Antiparasitic Potency In Vitro and Brain Exposure.

Previous studies have shown that bicyclic azetidines are potent and selective inhibitors of apicomplexan phenylalanine tRNA synthetase (PheRS), leading to parasite growth inhibition in vitro and in vivo, including in models of Toxoplasma infection. Despite these useful properties, additional optimization is required for the development of efficacious treatments of toxoplasmosis from this inhibitor series, in particular, to achieve optimal exposure in the brain. Here, we describe a series of PheRS inhibitors built on a new bicyclic pyrrolidine core scaffold designed to retain the exit-vector geometry of the isomeric bicyclic azetidine core scaffold while offering avenues to sample diverse chemical space. Relative to the parent series, bicyclic pyrrolidines retain reasonable potency and target selectivity for parasite PheRS vs host. Further structure-activity relationship studies revealed that the introduction of aliphatic groups improved potency and ADME and PK properties, including brain exposure. The identification of this new scaffold provides potential opportunities to extend the analogue series to further improve selectivity and potency and ultimately deliver a novel, efficacious treatment of toxoplasmosis.

SOMOphilic alkyne vs radical-polar crossover approaches: The full story of the azido-alkynylation of alkenes.

We report the detailed background for the discovery and development of the synthesis of homopropargylic azides by the azido-alkynylation of alkenes. Initially, a strategy involving SOMOphilic alkynes was adopted, but only resulted in a 29% yield of the desired product. By switching to a radical-polar crossover approach and after optimization, a high yield (72%) of the homopropargylic azide was reached. Full insights are given about the factors that were essential for the success of the optimization process.

Bicyclic pyrrolidine inhibitors of Toxoplasma gondii phenylalanine t-RNA synthetase with antiparasitic potency in vitro and brain exposure.

Previous studies have shown that bicyclic azetidines are potent and selective inhibitors of apicomplexan phenylalanine tRNA synthetase (PheRS), leading to parasite growth inhibition in vitro and in vivo, including in models of Toxoplasma infection. Despite these useful properties, additional optimization is required for the development of efficacious treatments of toxoplasmosis from this inhibitor series, in particular to achieve sufficient exposure in the brain. Here, we describe a series of PheRS inhibitors built on a new bicyclic pyrrolidine core scaffold designed to retain the exit-vector geometry of the isomeric bicyclic azetidine core scaffold while offering avenues to sample diverse chemical space. Relative to the parent series, bicyclic pyrrolidines retain reasonable potency and target selectivity for parasite PheRS vs. host. Further structure-activity relationship studies revealed that the introduction of aliphatic groups improved potency, ADME and PK properties, including brain exposure. The identification of this new scaffold provides potential opportunities to extend the analog series to further improve selectivity and potency and ultimately deliver a novel, efficacious treatment of toxoplasmosis.

Acyl-Ethynylbenziodoxolone (acyl-EBX): Access to Ketene Dithioarylacetals.

We report the synthesis of ketene dithioarylacetals in 40-97% yield using thiophenols and acyl-EBXs (ethynylbenziodoxolones) generated in situ from a common hypervalent iodine precursor and alkynyl trifluoroborate salts. The products could be further modified to afford functionalized ketene dithioacetals and various S-substituted heterocycles.

Azido-alkynylation of alkenes through radical-polar crossover.

We report an azido-alkynylation of alkenes allowing a straightforward access to homopropargylic azides by combining hypervalent iodine reagents and alkynyl-trifluoroborate salts. The design of a photocatalytic redox-neutral radical polar crossover process was key to develop this transformation. A variety of homopropargylic azides possessing electron-rich and -poor aryls, heterocycles or ether substituents could be accessed in 34-84% yield. The products are synthetically useful building blocks that could be easily transformed into pyrroles or bioactive amines.

Copper-Catalyzed Alkynylation of Hydrazides: An Easy Access to Functionalized Azadipeptides.

We report a copper-catalyzed alkynylation of azadipeptides using ethynylbenziodoxolone (EBX) reagents. Nonsymmetrical ynehydrazides could be obtained in 25-97% yield using azaglycine derivatives as nucleophiles. The transformation is compatible with most functional groups naturally occurring on amino acid side chains and allows the transfer of silyl-, alkyl-, and aryl-substituted alkynes. The obtained α-alkynyl azaglycine products could be further functionalized by nucleophilic attack or cycloaddition on the triple bond.

Tosyloxybenziodoxolone: A Platform for Performing the Umpolung of Alkynes in One-Pot Transformations.

Ethynylbenziodoxolones (EBXs) are commonly encountered reagents for the electrophilic alkynylation of nucleophiles. Herein, we report a one-pot, two-step process for EBX generation and their direct application in substrate functionalization. Our approach enables us to bypass the originally mandatory isolation and purification of the reagents, resulting in a more efficient synthesis. We could apply this process to seven different transformations involving both two- and one-electron nucleophiles to obtain a large variety of alkynylated products.

Composition of Carbon Clusters in Implanted Silicon Using Atom Probe Tomography.

Atom probe tomography was employed to observe and derive the composition of carbon clusters in implanted silicon. This value, which is of interest to the microelectronic industry when considering ion implantation defects, was estimated not to exceed 2 at%. This measurement has been done by fitting the distribution of first nearest neighbor distances between monoatomic carbon ions (C+ and C2+). Carbon quantification has been considerably improved through the detection of molecular ions, using lower electric field conditions as well as equal proportions of 12C and 13C. In these conditions and using another quantification method, we have shown that the carbon content in clusters approaches 50 at%. This result very likely indicates that clusters are nuclei of the SiC phase.

Bicyclic azetidines kill the diarrheal pathogen Cryptosporidium in mice by inhibiting parasite phenylalanyl-tRNA synthetase.

Cryptosporidium is a protozoan parasite and a leading cause of diarrheal disease and mortality in young children. Currently, there are no fully effective treatments available to cure infection with this diarrheal pathogen. In this study, we report a broad drug repositioning effort that led to the identification of bicyclic azetidines as a new anticryptosporidial series. Members of this series blocked growth in in vitro culture of three Cryptosporidium parvum isolates with EC50 's in 1% serum of <0.4 to 96 nM, had comparable potencies against Cryptosporidium hominis and C. parvum, and was effective in three of four highly susceptible immunosuppressed mice with once-daily dosing administered for 4 days beginning 2 weeks after infection. Comprehensive genetic, biochemical, and chemical studies demonstrated inhibition of C. parvum phenylalanyl-tRNA synthetase (CpPheRS) as the mode of action of this new lead series. Introduction of mutations directly into the C. parvum pheRS gene by CRISPR-Cas9 genome editing resulted in parasites showing high degrees of compound resistance. In vitro, bicyclic azetidines potently inhibited the aminoacylation activity of recombinant ChPheRS. Medicinal chemistry optimization led to the identification of an optimal pharmacokinetic/pharmacodynamic profile for this series. Collectively, these data demonstrate that bicyclic azetidines are a promising series for anticryptosporidial drug development and establish a broad framework to enable target-based drug discovery for this infectious disease.

Copper-Catalyzed Oxyalkynylation of C-S Bonds in Thiiranes and Thiethanes with Hypervalent Iodine Reagents.

We report the oxyalkynylation of thiiranes and thietanes using ethynylbenziodoxolone reagents (EBXs) to readily access functionalized building blocks bearing an alkynyl, a benzoate, and an iodide group. The reaction proceeds with high atom efficiency most likely through an alkynyl-episulfonium intermediate. The transformation is copper-catalyzed and compatible with a large array of thiiranes and thietanes.