Mechanism-Based Design of Quinoline Potassium Acyltrifluoroborates for Rapid Amide-Forming Ligations at Physiological pH.

Potassium acyltrifluoroborates (KATs) undergo chemoselective amide-forming ligations with hydroxylamines. Under aqueous, acidic conditions these ligations can proceed rapidly, with rate constants of ∼20 M-1 s-1. The requirement for lower pH to obtain the fastest rates, however, limits their use with certain biomolecules and precludes in vivo applications. By mechanistic investigations into the KAT ligation, including kinetic studies, X-ray crystallography, and DFT calculations, we have identified a key role for a proton in accelerating the ligation. We applied this knowledge to the design and synthesis of 8-quinolyl acyltrifluoroborates, a new class of KATs that ligates with hydroxylamines at pH 7.4 with rate constants >4 M-1 s-1. We trace the enhanced rate at physiological pH to unexpectedly high basicity of the 8-quinoline-KATs, which leads to their protonation even under neutral conditions. This proton assists the formation of the key tetrahedral intermediate and activates the leaving groups on the hydroxylamine toward a concerted 1,2-BF3 shift that leads to the amide product. We demonstrate that the fast ligations at pH 7.4 can be carried out with a protein substrate at micromolar concentrations.

Photocontrolled Cobalt Catalysis for Selective Hydroboration of α,ß-Unsaturated Ketones.

Selectivity between 1,2 and 1,4 addition of a nucleophile to an α,ß-unsaturated carbonyl compound has classically been modified by the addition of stoichiometric additives to the substrate or reagent to increase their "hard" or "soft" character. Here, we demonstrate a conceptually distinct approach that instead relies on controlling the coordination sphere of a catalyst with visible light. In this way, we bias the reaction down two divergent pathways, giving contrasting products in the catalytic hydroboration of α,ß-unsaturated ketones. This includes direct access to previously elusive cyclic enolborates, via 1,4-selective hydroboration, providing a straightforward and stereoselective route to rare syn-aldol products in one-pot. DFT calculations and mechanistic experiments confirm two different mechanisms are operative, underpinning this unusual photocontrolled selectivity switch.

Palladium-mediated Suzuki-Miyaura Cross-Coupling Reaction of Potassium Boc-protected aminomethyltrifluoroborate with DNA-Conjugated aryl bromides for DNA-Encoded chemical library synthesis.

A mild reaction for DNA-compatible, palladium promoted Suzuki-Miyaura cross-coupling reaction of potassium Boc-protected aminomethyltrifluoroborate with DNA-conjugated aryl bromides has been developed efficiently. This novel DNA encoded chemistry reaction proceeded well with a wide range of functional group tolerance, including aryl bromides and heteroaryl bromides. Further, the utility our DNA conjugated aminomethylated arene products is demonstrated by reaction with various types of reagents (including amide formation with carboxylic acids, alkylation with aldehydes, and carbamoylation with amines) as would be desired for the production of a DNA encoded library.

Uptake of [¹8F]tetrafluoroborate in MCF-7 Breast Cancer Cells is Induced after Stimulation of the Sodium Iodide Symporter.

BACKGROUND: The human sodium iodide symporter (hNIS) has been the most important target in nuclear medicine regarding thyroid-related diseases. Although hNIS-expression can also be determined in extra-thyroidal tumors, imaging hNIS with positron emission tomography has not been exploited clinically. OBJECTIVE: Here, we evaluated the accumulation of the novel hNIS-substrate [18F]tetrafluoroborate ([18F]TFB) in the endogenously hNIS-expressing breast cancer cell line MCF-7 after an improved radiosynthesis and pharmacological stimulation. METHODS: [18F]TFB was prepared under mild reaction conditions (40°C, 25 min) and its uptake properties were investigated in MCF-7 cells pretreated with a combination of all-trans retinoic acid plus methasone-derivatives and compared to the clinically established tracers [131I]iodide and [99mTc]pertechnetate. Specificity of the tracer accumulation was assessed by inhibition experiments using NaBF4, KSO3F, KI and KIO3. RESULTS: [18F]TFB was obtained with a radiochemical yield of 24.0 ± 6.6 % (n = 17) within 40 min after high pressure liquid chromatography-separation and with 26.8 ± 6.2 % (n = 13) within 45 min after adapting the procedure on a synthesis module using higher starting activities (> 10 GBq). After pharmacological treatment, a 4-fold increase in hNIS-expression on the MCF-7 cell surface was achieved, resulting in a significantly higher [18F]TFB uptake into the cells (up to 58-fold) as compared to control experiments. Inhibition studies using various NIS-substrates confirmed the specificity of [18F]TFB for hNIS. CONCLUSION: [18F]TFB was shown to be a promising hNIS-substrate in our model using the human MCF-7 breast cancer cell line mandating in vivo evaluations in xenografted studies and in patients.

Research and Development of Zincoborates: Crystal Growth, Structural Chemistry and Physicochemical Properties.

Borates have been regarded as a rich source of functional materials due to their diverse structures and wide applications. Therein, zincobrates have aroused intensive interest owing to the effective structural and functional regulation effects of the strong-bonded zinc cations. In recent decades, numerous zincoborates with special crystal structures were obtained, such as Cs3Zn6B9O21 and AZn2BO3X2 (A = Na, K, Rb, NH4; X = Cl, Br) series with KBe2BO3F2-type layered structures were designed via substituting Be with Zn atoms, providing a feasible strategy to design promising non-linear optical materials; KZnB3O6 and Ba4Na2Zn4(B3O6)2(B12O24) with novel edge-sharing [BO4]5- tetrahedra were obtained under atmospheric pressure conditions, indicating that extreme conditions such as high pressure are not essential to obtain edge-sharing [BO4]5--containing borates; Ba4K2Zn5(B3O6)3(B9O19) and Ba2KZn3(B3O6)(B6O13) comprise two kinds of isolated polyborate anionic groups in one borate structure, which is rarely found in borates. Besides, many zincoborates emerged with particular physicochemical properties; for instance, Bi2ZnOB2O6 and BaZnBO3F are promising non-linear optical (NLO) materials; Zn4B6O13 and KZnB3O6 possess anomalous thermal expansion properties, etc. In this review, the synthesis, crystal structure features and properties of representative zincoborates are summarized, which could provide significant guidance for the exploration and design of new zincoborates with special structures and excellent performance.

Hydrated Zinc Borates and Their Industrial Use.

Zinc borates are important chemical products having industrial applications as functional additives in polymers, bio-composites, paints and ceramics. Of the thirteen well documented hydrated binary zinc borates, Zn[B3O4(OH)3] (2ZnO∙3B2O3∙3H2O) is manufactured in the largest quantity and is known as an article of commerce as 2ZnO∙3B2O3∙3.5H2O. Other hydrated zinc borates in commercial use include 4ZnO∙B2O3∙H2O, 3ZnO∙3B2O3∙5H2O and 2ZnO∙3B2O3∙7H2O. The history, chemistry, and applications of these and other hydrated zinc borate phases are briefly reviewed, and outstanding problems in the field are highlighted.

A near-infrared fluorescent probe for evaluating endogenous hydrogen peroxide during ischemia/reperfusion injury.

Hydrogen peroxide (H2O2), as a major component of reactive oxygen species (ROS), plays an important role in normal physiological processes. A H2O2 burst also occurs in the ischemia/reperfusion (I/R) process and causes a series of physiological and pathological injuries. Therefore, it is important to determine concentration fluctuations of H2O2. Here we develop a ratiometric fluorescent probe, Cy-ArB, which shows high selectivity and sensitivity toward H2O2. The fluorescence response of the probe is triggered by the reaction of borate esters with H2O2, and this process releases a near-infrared heptamethine cyanine fluorophore which has the ability of mitochondrial tracing. Hence, the probe can be used for real-time monitoring of H2O2 fluctuations in the mitochondrial respiration chain. Finally, we explore the fluctuations of H2O2 in cells and in vivo during the I/R process using the probe Cy-ArB. The results of our experiments prove that our probe is a potential candidate for clinical surgery pre-evaluation.

H2 O2 -Responsive Organosilica-Doxorubicin Nanoparticles for Targeted Imaging and Killing of Cancer Cells Based on a Synthesized Silane-Borate Precursor.

Doxorubicin (Dox) is a widely used fluorescent chemotherapy drug. Its primary delivery systems, based on physical adsorption to silica nanoparticles, can lead to low drug loading. Direct loading of Dox via covalent bonds during the formation of silica nanoparticles has never been reported. In this work, we designed and synthesized a silane-borate precursor, which contains not only an alkoxysilane moiety to form organosilica nanoparticles but also a phenylboronic acid moiety to react with diol-containing compounds. Using this compound, the covalent loading of Dox during the preparation of organosilica nanoparticles was effectively realized with a high drug loading content up to 22.4 %. Further modification by hyaluronic acid (HA) bestowed the Si-Dox@HA nanoparticles with the ability to target CD44-overexpressing cancer cells. The Si-Dox@HA nanoparticles exhibited H2 O2 -responsive release of about 80 % Dox and displayed seven-fold selectivity for killing cancer cells over normal cells, relative to Dox and Si-Dox nanoparticles. Moreover, these Si-Dox@HA nanoparticles are also suitable for targeted fluorescence imaging of CD44-overexpressing cancer cells.

Adhesive Gold Nanoparticles for Easy and Controlled Surface Coating.

Gold nanoparticles (Au NPs) are one of the most important nanomaterials due to their unique properties and broad applications. Among these applications, decorating Au NPs on universal surfaces is highly desired. Herein, we report adhesive Au NPs functionalized by borated dopamine dithiocarbamate. Such Au NPs are nonreactive in colloidal solution but can be activated at an acidic pH to produce adhesive Au NPs and initiate spontaneous surface coating through deprotected catechol-mediated reactions. Easy and controllable surface coating was achieved on materials with distinguished chemical and physical properties because of the high reactivity of catechol. Adhesive Au NPs represent new surface coating method with wide application potentials.

Convergent Synthesis of the NS5B Inhibitor GSK8175 Enabled by Transition Metal Catalysis.

A convergent eight-stage synthesis of the boron-containing NS5B inhibitor GSK8175 is described. The previous route involves 13 steps in a completely linear sequence, with an overall 10% yield. Key issues include a multiday SNAr arylation of a secondary sulfonamide using HMPA as solvent, multiple functional group interconversions after all of the carbon atoms are installed (including a Sandmeyer halogenation), use of carcinogenic chloromethyl methyl ether to install a protecting group late in the synthesis, and an unreliable Pd-catalyzed Miyaura borylation as the penultimate step. We have devised an orthogonal approach using a Chan-Lam coupling between a halogenated aryl pinacol boronate ester and an aryl methanesulfonamide. This reaction is performed using a cationic Cu(I) precatalyst, which can be easily generated in situ using KPF6 as a halide abstractor. High-throughput screening revealed a new Pd catalyst system to effect the penultimate borylation chemistry using simple monodentate phosphine ligands, with PCyPh2 identified as optimal. Reaction progress analysis of this borylation indicated likely mass-transfer rate limitations under standard conditions using KOAc as the base. We have devised a K2CO3/pivalic acid system as an alternative, which dramatically outperforms the standard conditions. This new synthesis proceeds in eight stages with a 20% overall yield.

Effect of antioxidant supplements on lipid peroxidation levels in primary cortical neuron cultures.

Oxidative stress, specifically lipid peroxidation, is a major driving force in neurodegenerative processes. However, the exact role of lipid peroxidation remains elusive as reliable real-time detection and quantification of lipid peroxyl radicals proves to be challenging in vitro and in vivo. Motivated by this methodological limitation, we have optimized conditions for real-time imaging and quantification of lipid peroxyl radical generation in primary neuron cultures using the lipophilic fluorogenic antioxidant H4BPMHC (8-((6-hydroxy-2,5,7,8-tetramethylchroman-2-yl)-methyl)-1,5-di(3-chloropropyl)-pyrromethene fluoroborate), an α-tocopherol analog probe. By subjecting neurons to different antioxidant conditions in the presence and absence of lipid peroxidation inducing stressors (Haber-Weiss reagents), we maximized H4BPMHC sensitivity and confirmed its potential to temporally resolve subtle and marked differences in lipid peroxidation levels in real-time. Herein we report imaging and quantification of homeostatic and induced lipid peroxidation in primary neuron cultures, supporting the use of this probe for investigating healthy and diseased states. Overall these results provide the necessary foundation and impetus towards using H4BPMHC for elucidating and mapping lipid peroxyl radical contributions to ROS-associated pathological processes in neurons.

Catalytic Carbo- and Aminoboration of Alkenyl Carbonyl Compounds via Five- and Six-Membered Palladacycles.

A palladium(II)-catalyzed alkene difunctionalization reaction has been developed, wherein B2pin2 is used to trap chelation-stabilized alkylpalladium(II) intermediates that are formed upon nucleopalladation. A range of carbon and nitrogen nucleophiles were found to be suitable coupling partners in this transformation, providing moderate to high yields. Both 3-butenoic and 4-pentenoic acid derivatives were reactive substrate classes, affording ß,γ- and γ,δ-difunctionalized carboxylic acid derivatives. This work represents a new strategy to synthesize highly functionalized secondary boronates that complements existing methods.

Mechanistic Studies of Formal Thioboration Reactions of Alkynes.

Several formal heteroborylative cyclization reactions have been recently reported, but little physical-organic and mechanistic data are known. We now investigate the catalyst-free formal thioboration reaction of alkynes to gain mechanistic insight into B-chlorocatecholborane (ClBcat) in its new role as an alkynophilic Lewis acid in electrophilic cyclization/dealkylation reactions. In kinetic studies, the reaction is second-order globally and first-order with respect to both the 2-alkynylthioanisole substrate and the ClBcat electrophile, with activation parameters of ΔG‡ = 27.1 ± 0.1 kcal mol-1 at 90 °C, ΔH‡ = 13.8 ± 1.0 kcal mol-1, and ΔS‡ = -37 ± 3 cal mol-1 K-1, measured over the range 70-90 °C. Carbon kinetic isotope effects supported a rate-determining AdE3 mechanism wherein alkyne activation by neutral ClBcat is concerted with cyclative attack by nucleophilic sulfur. A Hammett study found a ρ+ of -1.7, suggesting cationic charge buildup during the cyclization and supporting rate-determining concerted cyclization. Studies of the reaction with tris(pentafluorophenyl)borane (B(C6F5)3), an activating agent capable of cyclization but not dealkylation, resulted in the isolation of a postcyclization zwitterionic intermediate. Kinetic studies via UV-vis spectroscopy with this boron reagent found second-order kinetics, supporting the likely relevancy of intermediates in this system to the ClBcat system. Computational studies comparing ClBcat with BCl3 as an activating agent showed why BCl3, in contrast to ClBcat, failed to mediate the complete the cyclization/demethylation reaction sequence by itself. Overall, the results support a mechanism in which the ClBcat reagent serves a bifunctional role by sequentially activating the alkyne, despite being less electrophilic than other known alkyne-activating reagents and then providing chloride for post-rate-determining demethylation/neutralization of the resulting zwitterionic intermediate.

Single-Electron Transmetalation: Photoredox/Nickel Dual Catalytic Cross-Coupling of Secondary Alkyl ß-Trifluoroboratoketones and -esters with Aryl Bromides.

The first cross-coupling of secondary alkyl ß-trifluoroboratoketones and -esters has been achieved through application of photoredox/nickel dual catalysis. Although the related ß-trifluoroboratoamides have been effectively cross-coupled via Pd-catalysis, the corresponding ketones and esters had proven recalcitrant prior to this report. Reactions occur under mild conditions, and a variety of functional groups and sterically and electronically diverse reaction partners are tolerated.

α-Arylation/Heteroarylation of Chiral α-Aminomethyltrifluoroborates by Synergistic Iridium Photoredox/Nickel Cross-Coupling Catalysis.

Direct access to complex, enantiopure benzylamine architectures using a synergistic iridium photoredox/nickel cross-coupling dual catalysis strategy has been developed. New C(sp(3))-C(sp(2)) bonds are forged starting from abundant and inexpensive natural amino acids.

New 3-(heteroaryl)-2-iminocoumarin-based borate complexes: synthesis, photophysical properties, and rational functionalization for biosensing/biolabeling applications.

Members of a series of boron difluoride complexes with 3-(heteroaryl)-2-iminocoumarin ligands bearing both a phenolic hydroxyl group (acting as a fluorogenic center) and an N-aryl substituent (acting as a stabilizing moiety) have been synthesized in good yields by applying a straightforward two-step method. These novel fluorogenic dyes belong to the family of "Boricos" (D. Frath et al., Chem. Commun.- 2013, 49, 4908-4910) and are the first examples of phenol-based fluorophores of which the photophysical properties in the green-yellow spectral range are dramatically improved by N,N-chelation of a boron atom. Modulation of their fluorescence properties through reversible chemical modification of their phenol moieties has been demonstrated by the preparation of the corresponding 2,4-dinitrophenyl (DNP) ethers, which led to a dramatic "OFF-ON" fluorescence response upon reaction with thiols. Additionally, to expand the scope of these "7-hydroxy-Borico" derivatives, particularly in biolabeling, amine or carboxylic acid functionalities amenable to (bio)conjugation have been introduced within their scaffold. Their utility has been demonstrated in the preparation of fluorescent bovine serum albumin (BSA) conjugates and "Borico"-DOTA-like scaffolds in an effort to design novel monomolecular multimodal fluorescence- radioisotope imaging agents.

o-Boronato- and o-Trifluoroborato-Phosphonium Salts Supported by L-α-Amino Acid Side Chain.

The synthesis of o-boronato- and o-trifluoroborato-phosphonium salts supported by the L-amino acid side chain is described. The synthesis of these new class of amino acid derivatives was achieved by stereoselective quaternization of o-(pinacolato)boronatophenylphosphine with ß- or γ-iodo amino acid derivatives which are prepared from L-serine or L-aspartic acid, respectively. The quaternization of the phosphine was performed using either iodo amino ester or carboxylic acid derivatives. In addition, free carboxylic acid and amine derivatives were obtained by saponification or HCl acidolysis of o-boronato-phosphonium amino esters, respectively. The usefulness of these compounds in peptide coupling was demonstrated by coupling an o-boronato-phosphonium amino ester with an aspartic acid moiety. When the o-boronato-phosphonium amino acid or dipeptide derivatives were mixed with fluoride, the corresponding o-trifluoroborated products were cleanly and rapidly obtained in high isolated yields. The hydrolysis of these compounds at room temperature using a phosphate buffer pH 7/CD3CN mixture has shown only traces of free fluoride F(-) after several days. Finally, a preliminary radiolabeling essay has proven the facile [(18)F]-fluoride incorporation and high stability of the radiolabeled product in aqueous conditions. Indeed, this new class of boron-phosphonium amino acid derivatives shows promising properties for their applications in synthesis and labeling of peptides.

(Z)- or (E)-Selective hydrogenation of potassium (3,3,3-trifluoroprop-1-yn-1-yl)trifluoroborate: route to either isomer of ß-trifluoromethylstyrenes.

A Pd-catalyzed hydrogenation of potassium (3,3,3-trifluoroprop-1-yn-1-yl)trifluoroborate providing either the (Z)- or (E)-isomer of the vinylborate in >98% purity is described. The initially formed (Z)-isomer of the alkene is transformed to the (E)-isomer with time, irrespective of the catalyst used; coupling with bromo- and iodoarenes provides a variety of (Z)- or (E)-ß-trifluoromethylstyrenes. Also, a safe synthesis of the alkynyltrifluoroborate from HFC-245fa and BF3·OEt2 has been described.

One-pot synthesis of an indole-substituted 7,8-dicarba-nido-dodecahydroundecaborate(-1).

Carbaboranes are increasingly used as pharmacophores to replace phenyl substituents in established drug molecules. In contrast to traditional organic chemistry, elaborate procedures to introduce functionality frequently fail in the case of carbaboranes and their chemistry is often hampered by degradation of the cluster. Herein, the development of a one-pot synthesis of a water-soluble N-nido-dicarbaborato indole is presented, including a proposed mechanism for the reaction sequence. These studies provide useful synthetic tools for the conjugation of two important pharmacophores, indoles and carbaboranes.

Dual mode fluorescent (18)F-PET tracers: efficient modular synthesis of rhodamine-[cRGD]2-[(18)F]-organotrifluoroborate, rapid, and high yielding one-step (18)F-labeling at high specific activity, and correlated in vivo PET imaging and ex vivo fluorescence.

The design of dual mode fluorescent-PET peptidic tracers that can be labeled with [(18)F]fluoride at high specific activity and high yield has been challenged by the short half-life of (18)F and its aqueous indolence toward nucleophilic displacement, that often necessitates multistep reactions that start with punctiliously dry conditions. Here we present a modular approach to constructing a fluorescent dimeric peptide with a pendant radioprosthesis that is labeled in water with [(18)F]fluoride ion in a single, user-friendly step. The modular approach starts with grafting a new zwitterionic organotrifluoroborate radioprosthesis onto a pentaerythritol core with three pendent alkynes that enable successive grafting of a bright fluorophore (rhodamine) followed by two peptides (cylcoRGD). The construct is labeled with [(18)F]fluoride via isotope exchange within 20 min in a single step at high specific activity (>3 Ci/µmol) and in good yield to provide 275 mCi and high radiochemical purity. Neither drying of the [(18)F]fluoride ion solution nor HPLC purification of the labeled tracer is required. Facile chemical synthesis of this dual mode tracer along with a user-friendly one-step radiolabeling method affords very high specific activity. In vivo PET images of the dual mode tracer are acquired at both high and low specific activities. At very high specific activity, i.e., 3.5 Ci/µmol, tumor uptake is relatively high (5.5%ID/g), yet the associated mass is below the limits of fluorescent detection. At low specific activity, i.e., 0.01 Ci/µmol, tumor uptake in the PET image is reduced by approximately 50% (2.9%ID/g), but the greater associated mass enables fluorescence detection in the tumor. These data highlight a facile production of a dual mode fluorescent-PET tracer which is validated with in vivo and ex vivo images. These data also define critical limitations for the use of dual mode tracers in small animals.