Clp-targeting BacPROTACs impair mycobacterial proteostasis and survival.

The ClpC1:ClpP1P2 protease is a core component of the proteostasis system in mycobacteria. To improve the efficacy of antitubercular agents targeting the Clp protease, we characterized the mechanism of the antibiotics cyclomarin A and ecumicin. Quantitative proteomics revealed that the antibiotics cause massive proteome imbalances, including upregulation of two unannotated yet conserved stress response factors, ClpC2 and ClpC3. These proteins likely protect the Clp protease from excessive amounts of misfolded proteins or from cyclomarin A, which we show to mimic damaged proteins. To overcome the Clp security system, we developed a BacPROTAC that induces degradation of ClpC1 together with its ClpC2 caretaker. The dual Clp degrader, built from linked cyclomarin A heads, was highly efficient in killing pathogenic Mycobacterium tuberculosis, with >100-fold increased potency over the parent antibiotic. Together, our data reveal Clp scavenger proteins as important proteostasis safeguards and highlight the potential of BacPROTACs as future antibiotics.

BacPROTACs mediate targeted protein degradation in bacteria.

Hijacking the cellular protein degradation system offers unique opportunities for drug discovery, as exemplified by proteolysis-targeting chimeras. Despite their great promise for medical chemistry, so far, it has not been possible to reprogram the bacterial degradation machinery to interfere with microbial infections. Here, we develop small-molecule degraders, so-called BacPROTACs, that bind to the substrate receptor of the ClpC:ClpP protease, priming neo-substrates for degradation. In addition to their targeting function, BacPROTACs activate ClpC, transforming the resting unfoldase into its functional state. The induced higher-order oligomer was visualized by cryo-EM analysis, providing a structural snapshot of activated ClpC unfolding a protein substrate. Finally, drug susceptibility and degradation assays performed in mycobacteria demonstrate in vivo activity of BacPROTACs, allowing selective targeting of endogenous proteins via fusion to an established degron. In addition to guiding antibiotic discovery, the BacPROTAC technology presents a versatile research tool enabling the inducible degradation of bacterial proteins.

Iodine-Promoted Controlled and Selective Oxidation of (Aryl)(Heteroaryl)Methanes.

The development of direct and controlled oxidation of C(sp3)-H bonds is of great importance. Herein, an iodine-catalyzed controlled oxidation of (aryl)(heteroaryl)methanes to (aryl)(heteroaryl)methanols is disclosed under metal-free reaction conditions. A catalytic system comprised of iodine/silyl chloride with HI as an additive in the presence of dimethyl sulfoxide selectively oxidizes the C(sp3)-H bonds without being overoxidized to corresponding ketones. Therapeutically important aryl heteroaryl methanol derivatives were obtained in good yields. The preliminary mechanistic investigation proves that the primary source of oxygen is DMSO.

Visible-Light-Driven Halogen-Bond-Assisted Direct Synthesis of Heteroaryl Thioethers Using Transition-Metal-Free One-Pot C-I Bond Formation/C-S Cross-Coupling Reaction.

An efficient protocol for the synthesis of thioether directly from heteroarenes has been developed in the presence of visible light in a one-pot manner at room temperature. This method involves two sequential reactions in a single pot where the formation of the iodinated heteroarene is followed by a transition-metal-free C-S coupling reaction. A wide range of heteroarene and thiol partners (including aliphatic thiols) have been used for the synthesis of thioethers. NMR studies and DFT calculations revealed the presence of a halogen bond between the thiolate anion (halogen bond acceptor) and iodoheteroarene (halogen bond donor). This halogen bonded complex on photoexcitation facilitates the electron transfer from the thiolate anion to the iodoheteroarene at room temperature.

Aldosterone Glucuronide, an Important Biomarker: Synthesis and Structure Elucidation of Novel Isomers.

Aldosterone 1 is a mineralocorticoid, it has great influence on the blood pressure and its glucuronide is an important marker for the detection of several diseases. Here, we describe the chemical synthesis of different aldosterone-18- and 20-glucuronides. Reaction of trimethylsilyl 2,3,4-tri- acetyl-1-ß-glucuronic acid methyl ester 5 b and aldosterone diacetate 11 in the presence of TMSOTf gave the 18-α-glucuronide 9 a. The 18-ß-glucuronide 15 b and the 20-ß-glucuronide 16 b could be obtained by reaction of methyl 2,3,4-tri-O-isobutyryl-1α-glucuronate trichloroacetimidate 14 and aldosterone 21-acetate 8 in the presence of TMSOTf or BF3 ⋅OEt2 . Finally, reaction of aldosterone 21-acetate 8 and methyl 2,3,4-triacetyl-1α-glucuronate trichloroacetimidate 19 in the presence of TMSOTf gave the corresponding methyl 18-ß-triacetylglucuronate 9 b, which was transformed into the desired aldosterone-18-ß-glucuronide 3 by two enzyma- tic transformations.

Iodonium Ion-Catalyzed Domino Synthesis of Z-Selective α,ß-Diphenylthio Enones from Easily Accessible Secondary Alcohols.

The application of stabilized iodonium ions in organic synthesis remains largely unexplored. Herein, a metal-free domino synthesis of Z-selective α,ß-diphenylthio enones is developed from easily available benzylic secondary alcohols employing thiophenol-stabilized iodonium ion as a catalyst. This methodology was further extended to five-membered and six-membered cyclic secondary alcohols. The UV-vis experiments suggest the formation of thiol-coordinated iodine(I) intermediates. Several control experiments establish that the reaction proceeds via the oxidation of alcohol to ketone, α-thiolation of ketones followed by α,ß-unsaturation, and finally the ß-thiolation of α,ß-unsaturated ketones to generate bis-vinyl sulfides. The in situ-generated stabilized iodonium ion is highly efficient to catalyze multiple functional group transformations in a domino manner.

Halogen Bond-Assisted Electron-Catalyzed Atom Economic Iodination of Heteroarenes at Room Temperature.

A halogen bond-assisted electron-catalyzed iodination of heteroarenes has been developed for the first time under atom economic condition at room temperature. The iodination is successful with just 0.55 equiv of iodine and 0.50 equiv of peroxide. The kinetic study indicates that the reaction is elusive in the absence of a halogen bond between the substrate and iodine. The formation of a halogen bond, its importance in lowering the activation barrier for this reaction, the presence of radical intermediates in a reaction mixture, and the regioselectivity of the reaction have been demonstrated with several control experiments, spectroscopic analysis, and quantum chemical calculations. Allowing the formation of the halogen bond may offer a new strategy to generate the reactive radical intermediates and to enable the otherwise elusive electron-catalyzed reactions under mild reaction conditions.

Metal-Free Halogen(I) Catalysts for the Oxidation of Aryl(heteroaryl)methanes to Ketones or Esters: Selectivity Control by Halogen Bonding.

Metal-free halogen(I) catalysts were used for the selective oxidation of aryl(heteroaryl)methanes [C(sp3 )-H] to ketones [C(sp2 )=O] or esters [C(sp3 )-O]. The synthesis of ketones was performed with a catalytic amount of NBS in DMSO solvent. Experimental studies and density functional theory (DFT) calculations supported the formation of halogen bonding (XB) between the heteroarene and N-bromosuccinimide, which enabled imine-enamine tautomerism of the substrates. No additional activator was required for this crucial step. Isotope-labeling and other supporting experiments suggested that a Kornblum-type oxidation with DMSO and aerobic oxygenation with molecular oxygen took place simultaneously. A background XB-assisted electron transfer between the heteroarenes and halogen(I) catalysts was responsible for the formation of heterobenzylic radicals and, thus, the aerobic oxygenation. For selective acyloxylation (ester formation), a catalytic amount of iodine was employed with tert-butyl hydroperoxide in aliphatic carboxylic acid solvent. Several control reactions, spectroscopic studies, and Time-Dependent Density Functional Theory (TD-DFT) calculations established the presence of acetyl hypoiodite as an active halogen(I) species in the acetoxylation process. With the help of a selectivity study, for the first time we report that the strength of the XB interaction and the frontier orbital mixing between the substrates and acyl hypoiodites determined the extent of the background electron-transfer process and, thus, the selectivity of the reaction.

Homo-BacPROTAC-induced degradation of ClpC1 as a strategy against drug-resistant mycobacteria.

Antimicrobial resistance is a global health threat that requires the development of new treatment concepts. These should not only overcome existing resistance but be designed to slow down the emergence of new resistance mechanisms. Targeted protein degradation, whereby a drug redirects cellular proteolytic machinery towards degrading a specific target, is an emerging concept in drug discovery. We are extending this concept by developing proteolysis targeting chimeras active in bacteria (BacPROTACs) that bind to ClpC1, a component of the mycobacterial protein degradation machinery. The anti-Mycobacterium tuberculosis (Mtb) BacPROTACs are derived from cyclomarins which, when dimerized, generate compounds that recruit and degrade ClpC1. The resulting Homo-BacPROTACs reduce levels of endogenous ClpC1 in Mycobacterium smegmatis and display minimum inhibitory concentrations in the low micro- to nanomolar range in mycobacterial strains, including multiple drug-resistant Mtb isolates. The compounds also kill Mtb residing in macrophages. Thus, Homo-BacPROTACs that degrade ClpC1 represent a different strategy for targeting Mtb and overcoming drug resistance.

Proton-Coupled Electron Transfer: Transition-Metal-Free Selective Reduction of Chalcones and Alkynes Using Xanthate/Formic Acid.

Highly chemoselective reduction of α,ß-unsaturated ketones to saturated ketones and stereoselective reduction of alkynes to ( E)-alkenes has been developed under a transition-metal-free condition using a xanthate/formic acid mixture through proton-coupled electron transfer (PCET). Mechanistic experiments and DFT calculations support the possibility of a concerted proton electron-transfer (CPET) pathway. This Birch-type reduction demonstrates that a small nucleophilic organic molecule can be used as a single electron-transfer (SET) reducing agent with a proper proton source.

CBr4 as a Halogen Bond Donor Catalyst for the Selective Activation of Benzaldehydes to Synthesize α,ß-Unsaturated Ketones.

CBr4 has been employed as a halogen bond donor catalyst for the selective activation of aldehyde, to achieve an efficient solvent- and metal-free C═C bond forming reaction in the presence of strong acid sensitive groups such as methoxy, cyanide, ester, and ketal for the synthesis of α,ß-unsaturated ketones. This unique capability of CBr4 to act as a halogen bond donor has been explored and established using UV-vis as well as IR spectroscopy. Moreover, this unprecedented methodology enables the synthesis of the pharmaceutically important molecule licochalcone A.

Halogen-bonded iodonium ion catalysis: a route to α-hydroxy ketones via domino oxidations of secondary alcohols and aliphatic C-H bonds with high selectivity and control.

A domino synthesis of α-hydroxy ketones has been developed from benzylic secondary alcohols employing catalytic iodonium ions stabilized by DMSO. The reaction proceeds through an unprecedented sequential oxidation of alcohols to ketone and its α-hydroxylation in a controlled manner. The spectroscopic evidence establishes the possibility of formation of a stable halogen-bonded adduct between DMSO and iodonium ions.

A versatile and one-pot strategy to synthesize α-amino ketones from benzylic secondary alcohols using N-bromosuccinimide.

A metal-free one-pot strategy has been developed for the first time to synthesize pharmaceutically important α-amino ketones from readily available benzylic secondary alcohols and amines using N-bromosuccinimide. This new reaction proceeds via three consecutive steps involving oxidation of alcohols, α-bromination of ketones, and nucleophilic substitution of α-bromo ketones to give α-amino ketones. Importantly, this novel one-pot greener reaction avoids direct usage of toxic and corrosive bromine. This methodology has been employed efficiently to synthesize pharmaceutically important amfepramone and pyrovalerone in a single step.