Discovery and Characterization of a Chemical Probe Targeting the Zinc-Finger Ubiquitin-Binding Domain of HDAC6.

Histone deacetylase 6 (HDAC6) inhibition is an attractive strategy for treating numerous cancers, and HDAC6 catalytic inhibitors are currently in clinical trials. The HDAC6 zinc-finger ubiquitin-binding domain (UBD) binds free C-terminal diglycine motifs of unanchored ubiquitin polymer chains and protein aggregates, playing an important role in autophagy and aggresome assembly. However, targeting this domain with small molecule antagonists remains an underdeveloped avenue of HDAC6-focused drug discovery. We report SGC-UBD253 (25), a chemical probe potently targeting HDAC6-UBD in vitro with selectivity over nine other UBDs, except for weak USP16 binding. In cells, 25 is an effective antagonist of HDAC6-UBD at 1 µM, with marked proteome-wide selectivity. We identified SGC-UBD253N (32), a methylated derivative of 25 that is 300-fold less active, serving as a negative control. Together, 25 and 32 could enable further exploration of the biological function of the HDAC6-UBD and investigation of the therapeutic potential of targeting this domain.

Photo-Uncaging by C(sp3)-C(sp3) Bond Cleavage Restores ß-Lapachone Activity.

ß-Lapachone is an ortho-naphthoquinone natural product with significant antiproliferative activity but suffers from adverse systemic toxicity. The use of photoremovable protecting groups to covalently inactivate a substrate and then enable controllable release with light in a spatiotemporal manner is an attractive prodrug strategy to limit toxicity. However, visible light-activatable photocages are nearly exclusively enabled by linkages to nucleophilic functional sites such as alcohols, amines, thiols, phosphates, and sulfonates. Herein, we report covalent inactivation of the electrophilic quinone moiety of ß-lapachone via a C(sp3)-C(sp3) bond to a coumarin photocage. In contrast to ß-lapachone, the designed prodrug remained intact in human whole blood and did not induce methemoglobinemia in the dark. Under light activation, the C-C bond cleaves to release the active quinone, recovering its biological activity when evaluated against the enzyme NQO1 and human cancer cells. Investigations into this report of a C(sp3)-C(sp3) photoinduced bond cleavage suggest a nontraditional, radical-based mechanism of release beginning with an initial charge-transfer excited state. Additionally, caging and release of the isomeric para-quinone, α-lapachone, are demonstrated. As such, we describe a photocaging strategy for the pair of quinones and report a unique light-induced cleavage of a C-C bond. We envision that this photocage strategy can be extended to quinones beyond ß- and α-lapachone, thus expanding the chemical toolbox of photocaged compounds.

Synthetic Ionizable Colloidal Drug Aggregates Enable Endosomal Disruption.

Colloidal drug aggregates enable the design of drug-rich nanoparticles; however, the efficacy of stabilized colloidal drug aggregates is limited by entrapment in the endo-lysosomal pathway. Although ionizable drugs are used to elicit lysosomal escape, this approach is hindered by toxicity associated with phospholipidosis. It is hypothesized that tuning the pKa of the drug would enable endosomal disruption while avoiding phospholipidosis and minimizing toxicity. To test this idea, 12 analogs of the nonionizable colloidal drug fulvestrant are synthesized with ionizable groups to enable pH-dependent endosomal disruption while maintaining bioactivity. Lipid-stabilized fulvestrant analog colloids are endocytosed by cancer cells, and the pKa of these ionizable colloids influenced the mechanism of endosomal and lysosomal disruption. Four fulvestrant analogs-those with pKa values between 5.1 and 5.7-disrupted endo-lysosomes without measurable phospholipidosis. Thus, by manipulating the pKa of colloid-forming drugs, a tunable and generalizable strategy for endosomal disruption is established.

Iron-Catalyzed Reductive Cyclization by Hydromagnesiation: A Modular Strategy Towards N-Heterocycles.

A reductive cyclization to prepare a variety of N-heterocycles, through the use of ortho-vinylanilides, is reported. The reaction is catalyzed by an inexpensive and bench-stable iron complex and generally occurs at ambient temperature. The transformation likely proceeds through hydromagnesiation of the vinyl group, and trapping of the in situ generated benzylic anion by an intramolecular electrophile to form the heterocycle. This iron-catalyzed strategy was shown to be broadly applicable and was utilized in the synthesis of substituted indoles, oxindoles and tetrahydrobenzoazepinoindolone derivatives. Mechanistic studies indicated that the reversibility of the hydride transfer step depends on the reactivity of the tethered electrophile. The synthetic utility of our approach was further demonstrated by the formal synthesis of a reported bioactive compound and a family of natural products.

Recent Strategies for Carbon-Halogen Bond Formation Using Nickel.

Nickel catalysis has demonstrated the capability of performing a broad range of synthetically challenging transformations over the last decade. Though recent literature has focused on the formation of C-C and C-N bonds, a variety of breakthroughs in the field of C-X bond generation have also been reported. A diverse range of strategies using nickel have been developed, in an effort to expand the scope and synthetic utility of these halogenation methods. This Minireview will cover six emerging strategies in this field including: oxidatively induced C-X reductive elimination, triflate-to-halogen exchange reactions, directed C-H halogenation, non-directed electrophilic C-H halogenation of arenes, enantioselective α-fluorination of carbonyl containing compounds, and 1,2-difunctionalization-halogenation reactions. The final section has been split into two parts: nickel-catalyzed hydrohalogenation and nickel-catalyzed carbohalogenation reactions.

Aminomethylation of Oxabenzonorbornadienes via the Merger of Photoredox and Nickel Catalysis.

The first aminomethylation of oxabenzonorbornadienes using dual photoredox/nickel catalysis has been disclosed. This cascade reaction allowed the preparation of the cis-aminomethyl dihydronaphthalenols without any prefunctionalization or any use of nucleophilic organometallic species. The control of the regio- and stereoselectivity might be explained by a sequence involving insertion of nickel(0) into the C-O bond followed by the formation of a π-allyl intermediate.

Diastereoselective Nickel-Catalyzed Carboiodination Generating Six-Membered Nitrogen-Based Heterocycles.

A scalable, diastereoselective nickel-catalyzed carboiodination reaction is reported that avoids metal-based reducing agents. Novel anti-dihydroquinolones and previously unreported tetrahydroquinolines are now readily prepared. The generation of anti-dihydroquinolones is noteworthy, as this selectivity is opposite to that of the Pd variant. Mechanistic insight into the nature of the nickel-catalyzed carboiodination reaction was derived experimentally, suggesting a catalyst-controlled cyclization and stereoretentive reductive elimination.

Forming Benzylic Iodides via a Nickel Catalyzed Diastereoselective Dearomative Carboiodination Reaction of Indoles.

A diastereoselective dearomative carboiodination reaction is reported. We report a novel metal-catalyzed approach to install reactive secondary benzylic iodides. Utilizing the unique reactivity of nickel, we have expanded the carboiodination reaction to non-activated aromatic double bonds forming a previously unattainable class of iodides. We also report a broadly applicable method to avoid the use of a metallic reducing agent by utilizing an alkyl phosphite as the ligand. The reaction is thought to proceed through a syn intramolecular carbonickelation across a 2-substituted indole followed by a diastereoretentive reductive elimination of the carbon-iodine bond. The complex iodinated indolines generated in the reaction were obtained in moderate to good yields and good to excellent diastereoselectivity. The products were easily functionalized by a variety of synthetic methods.

Carboiodination Catalyzed by Nickel.

A novel nickel-catalyzed cycloisomerization reaction forming a new carbon-carbon bond while preserving the carbon-halogen bond has been developed. A cheap and readily available Ni-catalyst is employed to generate nitrogen containing heterocycles in good to excellent yields and the procedure is readily scalable. The more readily available aryl bromides were also cyclized with the addition of potassium iodide to generate the respective alkyl iodides. A rare dual ligand system employing a bisphosphine and bisphosphine monoxide was used to achieve enantioenriched products.

Nickel-Catalyzed Intramolecular Arylcyanation for the Synthesis of 3,3-Disubstituted Oxindoles.

A nickel-catalyzed arylcyanation reaction for the synthesis of 3,3-disubstituted oxindoles has been developed. This method features a bench-stable precatalyst system and serves as an economical alternative to the existing palladium-catalyzed arylcyanations described to date. A wide scope of oxindole products were accessible in moderate to good yields, and the rich chemistry of the newly installed nitrile functional group was demonstrated in the synthesis of various oxindole derivatives.

Identification and Structure-Activity Relationship of HDAC6 Zinc-Finger Ubiquitin Binding Domain Inhibitors.

HDAC6 plays a central role in the recruitment of protein aggregates for lysosomal degradation and is a promising target for combination therapy with proteasome inhibitors in multiple myeloma. Pharmacologically displacing ubiquitin from the zinc-finger ubiquitin-binding domain (ZnF-UBD) of HDAC6 is an underexplored alternative to catalytic inhibition. Here, we present the discovery of an HDAC6 ZnF-UBD-focused chemical series and its progression from virtual screening hits to low micromolar inhibitors. A carboxylate mimicking the C-terminal extremity of ubiquitin, and an extended aromatic system stacking with W1182 and R1155, are necessary for activity. One of the compounds induced a conformational remodeling of the binding site where the primary binding pocket opens up onto a ligand-able secondary pocket that may be exploited to increase potency. The preliminary structure-activity relationship accompanied by nine crystal structures should enable further optimization into a chemical probe to investigate the merit of targeting the ZnF-UBD of HDAC6 in multiple myeloma and other diseases.

Small Molecule Antagonists of the Interaction between the Histone Deacetylase 6 Zinc-Finger Domain and Ubiquitin.

Inhibitors of HDAC6 have attractive potential in numerous cancers. HDAC6 inhibitors to date target the catalytic domains, but targeting the unique zinc-finger ubiquitin-binding domain (Zf-UBD) of HDAC6 may be an attractive alternative strategy. We developed X-ray crystallography and biophysical assays to identify and characterize small molecules capable of binding to the Zf-UBD and competing with ubiquitin binding. Our results revealed two adjacent ligand-able pockets of HDAC6 Zf-UBD and the first functional ligands for this domain.

Diastereoselective palladium-catalyzed formate reduction of allylic carbonates en route to polypropionate systems.

Diastereoselective palladium-catalyzed formate reduction of allylic carbonates presents unique opportunities for applications in target-oriented organic synthesis provided that selectivity, in particular stereoselectivity, in the course of this metal-catalyzed reaction can be controlled. This article describes our recent developments on new and efficient metal-catalyzed processes exploiting resident stereocenters on the substrates as a means to control stereoselectivity en route to preparing propionate units containing an array of stereochemical patterns. In particular, the effect of the protecting group, the stereochemistry of the aldol adduct, neighboring substituents, and the olefin geometry were examined. Strategic choice of the above parameters provides entry into three of the four possible diastereomeric triads, namely syn-syn, anti-syn, and anti-anti. Preliminary results indicate that construction of the syn-anti triad is possible, albeit in moderate diastereoselectivity.

Diastereoselective palladium-catalyzed formate reduction of allylic carbonates as a new entry into propionate units.

[reaction: see text] The diastereoselective palladium-catalyzed formate reduction of allylic carbonates is described. Reduction of allylic carbonates under mild conditions (Pd(OAc)(2) (2.5-5 mol %), [n-Bu(3)PH]BF(4) (2.5-5 mol %), HCO(2)H/Et(3)N (1:2) (3 equiv), CH(3)CN (0.05M), 40 degrees C) affords the terminal olefin as the syn isomer in good yields and modest to excellent diastereoselectivity. These compounds, which are useful building blocks for the synthesis of polypropionate units, are the synthetic equivalent of the products obtained from an aldol reaction of an alpha-methyl-beta,gamma-unsaturated aldehyde.

Total synthesis of ionomycin using ring-opening strategies.

[structure: see text] The total synthesis of the polyether antibiotic ionomycin, a calcium ionophore, is described. The synthesis demonstrates the utility of ring-opening methodologies as applied to the synthesis of polypropionate and deoxypolypropionate subunits, which are found in two of the four fragments in the synthesis.

Asymmetric Synthesis of Bryostatin 2.

The potent bryostatin antitumor agents are currently in phase II clinical trials for the treatment of a variety of forms of cancer. Aldol reactions and directed reductions are among the essential steps for the formation of fragments A-C in the total synthesis of the title compound. Coupling of these fragments by sulfone-based olefination and alkylation reactions was followed by macrocyclization and introduction of the enoate moieties on rings B and C.