Acid Pinacol Ester Research Articles

Lysosome‐Targeting Amplifiers of Reactive Oxygen Species as Anticancer Prodrugs

AbstractCancer cells produce elevated levels of reactive oxygen species, which has been used to design cancer specific prodrugs. Their activation relies on at least a bimolecular process, in which a prodrug reacts with ROS. However, at low micromolar concentrations of the prodrugs and ROS, the activation is usually inefficient. Herein, we propose and validate a potentially general approach for solving this intrinsic problem of ROS‐dependent prodrugs. In particular, known prodrug 4‐(N‐ferrocenyl‐N‐benzylaminocarbonyloxymethyl)phenylboronic acid pinacol ester was converted into its lysosome‐specific analogue. Since lysosomes contain a higher concentration of active ROS than the cytoplasm, activation of the prodrug was facilitated with respect to the parent compound. Moreover, it was found to exhibit high anticancer activity in a variety of cancer cell lines (IC50=3.5–7.2 μm) and in vivo (40 mg kg−1, NK/Ly murine model) but remained weakly toxic towards non‐malignant cells (IC50=15–30 μm).

Lysosome-Targeting Amplifiers of Reactive Oxygen Species as Anticancer Prodrugs.

Cancer cells produce elevated levels of reactive oxygen species, which has been used to design cancer specific prodrugs. Their activation relies on at least a bimolecular process, in which a prodrug reacts with ROS. However, at low micromolar concentrations of the prodrugs and ROS, the activation is usually inefficient. Herein, we propose and validate a potentially general approach for solving this intrinsic problem of ROS-dependent prodrugs. In particular, known prodrug 4-(N-ferrocenyl-N-benzylaminocarbonyloxymethyl)phenylboronic acid pinacol ester was converted into its lysosome-specific analogue. Since lysosomes contain a higher concentration of active ROS than the cytoplasm, activation of the prodrug was facilitated with respect to the parent compound. Moreover, it was found to exhibit high anticancer activity in a variety of cancer cell lines (IC50 =3.5-7.2 μm) and in vivo (40 mg kg-1 , NK/Ly murine model) but remained weakly toxic towards non-malignant cells (IC50 =15-30 μm).

Benzoyl Peroxide Detection in Real Samples and Zebrafish Imaging by a Designed Near-Infrared Fluorescent Probe

A novel near-infrared fluorescence off-on probe, (E)-3,3-dimethyl-1-propyl-2-(2-(6-(2-(4,4,5,5-tetramethyl-1,3,2-dioxaborolan-2-yl)benzyloxy)-2,3-dihydro-1H-xanthen-4-yl)vinyl)-3H-indolium (1), is developed and applied to benzoyl peroxide (BPO) detection in real samples and fluorescence imaging in living cells and zebrafish. When arylboronate as the recognition unit is connected to a stable hemicyanine skeleton, the probe is readily prepared, which exhibits superior analytical performance, such as near-infrared fluorescence emission over 700 nm and high sensitivity with a low detection limit of 47 nM. Upon reaction with BPO, phenylboronic acid pinacol ester is oxidized, followed by hydrolysis and 1,4-elimination of o-quinone methide to release fluorophore. In addtion, the probe displays high selectivity toward BPO over other common substances, which makes it of great potential use in quantitative and simple detection of BPO in wheat flour and antimicrobial agent. More importantly, the probe has been successfully demonstrated for monitoring BPO in living HeLa cells and zebrafish. The probe with superior properties could be of great potential use in other biosystems and in vivo studies.

A Zinc Catalyzed C(sp3 )-C(sp2 ) Suzuki-Miyaura Cross-Coupling Reaction Mediated by Aryl-Zincates.

The Suzuki–Miyaura (SM) reaction is one of the most important methods for C−C bond formation in chemical synthesis. In this communication, we show for the first time that the low toxicity, inexpensive element zinc is able to catalyze SM reactions. The cross‐coupling of benzyl bromides with aryl borates is catalyzed by ZnBr2, in a process that is free from added ligand, and is compatible with a range of functionalized benzyl bromides and arylboronic acid pinacol esters. Initial mechanistic investigations indicate that the selective in situ formation of triaryl zincates is crucial to promote selective cross‐coupling reactivity, which is facilitated by employing an arylborate of optimal nucleophilicity.

Structure–Property Correlations of Reactive Oxygen Species-Responsive and Hydrogen Peroxide-Eliminating Materials with Anti-Oxidant and Anti-Inflammatory Activities

To develop reactive oxygen species (ROS)-responsive anti-inflammatory materials and establish their structure–property correlations, a series of H2O2-eliminating materials (OxbCDs) were designed and synthesized by conjugating different phenylboronic acid pinacol ester (PBAP) groups onto a biocompatible scaffold compound β-cyclodextrin via varied linker groups. Both the H2O2-triggered hydrolysis profiles and H2O2-eliminating capacities of these materials were dependent on the chemical structure of the PBAP moieties. Together with the elucidation of hydrolysis mechanisms, we established structure–property correlations of these OxbCD materials. Extensive in vitro experiments revealed nanoparticles (NPs) based on OxbCDs showed no adverse biological effects on normal cells. OxbCD NPs could effectively inhibit inflammatory responses and oxidative stress in stimulated macrophages. Consistently, OxbCD NPs efficaciously alleviated the symptoms of peritonitis in mice, with respect to reducing the counts of neutroph...

Effect of Pyridine Addition on the Efficiency of Copper‐Mediated Radiofluorination of Aryl Pinacol Boronates

AbstractThe rise of the transition‐metal‐mediated and/or catalyzed radiofluorination reactions has led us to investigate possibilities for the improvement of the radiolabeling yields of pinacol esters of arylboronic acids (arylBPin) in the presence of Cu(OTf)2(py)4. Addition of small amounts pyridine to the reaction medium with DMF as a solvent and [18F]F−/K222Cs+complex as [18F]fluoride source was shown to have a significant positive effect on the radiofluorination yields. The effect was investigated in depth using 4‐biphenylboronic acid pinacol ester as a model compound, with up to 20 % increase in radiochemical conversion observed at optimized conditions. Maximum effect was observed at pyridine‐to‐catalyst ratio of 30:1. The findings were applied to a representative electron‐rich ring systems that are traditionally difficult or impossible to radiolabel with nucleophilic fluoride, leading to products such as 2‐[18F]fluoroanisole and protected 4‐L‐[18F]fluorophenylalanine. Given the ever‐present pressure to develop new methods for introduction of fluorine‐18 into various substrates to facilitate easy and robust production of clinically‐relevant radiotracers for PET imaging we believe our findings can be of significant practical interest for radiofluorination of ortho‐substituted substrates in particular.

N-heterocyclic Carbene–Cu-Catalyzed Enantioselective Conjugate Additions with Alkenylboronic Esters as Nucleophiles

Catalytic enantioselective conjugate additions with easily accessible alkenylboronic acid pinacol esters as nucleophiles promoted by chiral copper complexes of N-heterocyclic carbenes are presented. These processes constitute an unprecedented instance of conjugate additions of a variety of functionalized alkenyl groups and afford desired products that are otherwise difficult to access in up to 98% yield and 99.5:0.5 enantiomeric ratio. The origins of ligand-controlled enantioselectivity are elucidated with density functional theory (DFT) calculations.

Ni-Catalyzed Alkene Carboacylation via Amide C-N Bond Activation.

We report Ni-catalyzed formal carboacylation of o-allylbenzamides with arylboronic acid pinacol esters. The reaction is triggered by oxidative addition of an activated amide C-N bond to a Ni(0) catalyst and proceeds via alkene insertion into a Ni(II)-acyl bond. The exo-selective carboacylation reaction generates 2-benzyl-2,3-dihydro-1H-inden-1-ones in moderate to high yields (46-99%) from a variety of arylboronic acid pinacol esters and substituted o-allylbenzamides. These results show that amides are practical substrates for alkene carboacylation via amide C-N bond activation, and this approach bypasses challenges associated with alkene carboacylation triggered by C-C bond activation.

One-Pot, Three-Step Synthesis of Cyclopropylboronic Acid Pinacol Esters from Synthetically Tractable Propargylic Silyl Ethers.

Simple propargylic silyl ethers can be converted to complex cyclopropylboronic acid pinacol esters in an efficient one-pot procedure. Terminal acetylenes undergo a Schwartz's reagent catalyzed hydroboration; subsequent addition of further Schwartz's reagent and Lewis acid-mediated activation of neighboring silyl ether allows cyclization to access a range of cyclopropylboronic acid pinacol esters. The scope includes aromatic, aliphatic, quaternary, and spiro substituted cyclopropyl rings, which can be transformed via Suzuki coupling into a range of lead-like substituted cyclopropyl aryl products.

Practical, Broadly Applicable, α-Selective, Z-Selective, Diastereoselective, and Enantioselective Addition of Allylboron Compounds to Mono-, Di-, Tri-, and Polyfluoroalkyl Ketones.

A practical method for enantioselective synthesis of fluoroalkyl-substituted Z-homoallylic tertiary alcohols has been developed. Reactions may be performed with ketones containing a polylfluoro-, trifluoro-, difluoro-, and monofluoroalkyl group along with an aryl, a heteroaryl, an alkenyl, an alkynyl, or an alkyl substituent. Readily accessible unsaturated organoboron compounds serve as reagents. Transformations were performed with 0.5-2.5 mol % of a boron-based catalyst, generated in situ from a readily accessible valine-derived aminophenol and a Z- or an E-γ-substituted boronic acid pinacol ester. With a Z organoboron reagent, additions to trifluoromethyl and polyfluoroalkyl ketones proceeded in 80-98% yield, 97:3 to >98:2 α:γ selectivity, >95:5 Z:E selectivity, and 81:19 to >99:1 enantiomeric ratio. In notable contrast to reactions with unsubstituted allylboronic acid pinacol ester, additions to ketones with a mono- or a difluoromethyl group were highly enantioselective as well. Transformations were similarly efficient and α- and Z-selective when an E-allylboronate compound was used, but enantioselectivities were lower. In certain cases, the opposite enantiomer was favored (up to 4:96 er). With a racemic allylboronate reagent that contains an allylic stereogenic center, additions were exceptionally α-selective, affording products expected from γ-addition of a crotylboron compound, in up to 97% yield, 88:12 diastereomeric ratio, and 94:6 enantiomeric ratio. Utility is highlighted by gram-scale preparation of representative products through transformations that were performed without exclusion of air or moisture and through applications in stereoselective olefin metathesis where Z-alkene substrates are required. Mechanistic investigations aided by computational (DFT) studies and offer insight into different selectivity profiles.

Cu-Catalyzed Stereoselective Borylation of gem-Difluoroalkenes with B2pin2.

A novel and efficient method for the synthesis of (Z)-fluorinated alkenylboronic acid pinacol esters via Cu-catalyzed stereoselective borylation of gem-difluoroalkenes using bis(pinacolato)diboron (B2pin2) as the boron source with the assistance of NaOtBu and Xantphos at room temperature was developed.

Hydrogen Peroxide-Responsive Nanoprobe Assists Circulating Tumor Cell Identification and Colorectal Cancer Diagnosis.

In the clinic, numeration of circulating tumor cells (CTCs) plays a critical role in cancer diagnosis and treatment, but conventional CTC identification and counting that rely on specific antibodies to characterize a cell's surface antigens are costive and with limitations. Importantly, false positive or negative results may occur due to the high heterogeneity and epithelial-mesenchymal transition (EMT) of CTCs. Herein we demonstrate a novel and effective CTC detecting nanoprobe that could rapidly respond to the high level of endogenous H2O2 of CTCs and report the signal through fluorescence emission. Briefly, a hydrophobic coumarin-benzene boronic acid pinacol ester (Cou-Bpin) was grafted onto hydrophilic glycol chitosan (GC) to form an amphiphilic molecule, which further assembled into micellar nanoparticles in aqueous solution. This new nanoprobe was highly sensitive to H2O2 with a detection limit of 0.1 μM and could rapidly enter the cells within 30 min. Upon exposure to intracellular H2O2, the nanoprobe exhibited remarkable one-photon and two-photon luminescent characteristics, which were suitable for imaging of endogenous H2O2 of various human colorectal cancer cells and assist the identification of CTCs. Compared to a conventional CTC counting assay, the nanoprobe-based CTC numeration could overcome the false-negative findings due to the low expression of cytokeratin 19 (CK19). In a clinic test, CTC counting results based on the new nanoprobe match better to the postoperative pathological results of four clinic patients who had colorectal cancer at different stages.

Synthesis of ( E )-prop-1-ene-1,3-diyldibenzene derivatives via direct decarboxylative coupling of α,β-unsaturated carboxylic acids with benzyl boronic acid pinacol ester

Abstract The first copper-catalyzed cross-coupling reaction between benzyl boronic acid pinacol ester and α,β-unsaturated carboxylic acids was described. The ready availability of the starting materials and excellent E selectivity make this protocol a safe and operationally convenient strategy for the efficient synthesis of (E)-prop-1-ene-1,3-diyldibenzene derivatives.

Synthesis and anti-obesity effects in vivo of Crotadihydrofuran C as a novel PPAR\u03b3 antagonist from Crotalaria albida

Crotadihydrofuran C (CC) from the herbs of Crotalaria albida is able to inhibit adipocyte differentiation and lipid accumulation. However, the effects of CC on obesity and metabolic disorders have not yet been elucidated. In our study, the first enantioselective synthesis of the 2-isopropenyl dihydrofuran isoflavone skeleton (CC) is described. The convenient and efficient synthetic protocols developed skilfully solve the problems of the ortho-para directing group and Suzuki coupling reaction using a boronic acid pinacol ester that was more stable and easy to obtain. Furthermore, CC treatment of high-fat diet (HFD)-fed obese mice remarkably reduced their body weight, fat mass, and lipid level as well as improved insulin resistance and non-alcoholic fatty liver disease (NAFLD). A TR-FRET assay showed that CC was specifically bound to PPARγ LBD, which was further confirmed by the molecular docking study. These results suggest that CC could be a useful and potential natural product for treating metabolic diseases, including obesity, hyperlipidemia insulin resistance and NAFLD, without toxic side-effects.

Synthesis and evaluation of 18F-labeled CJ-042794 for imaging prostanoid EP4 receptor expression in cancer with positron emission tomography

The potent and selective prostanoid EP4 receptor antagonist CJ-042794 was radiolabeled with 18F, and evaluated for imaging EP4 receptor expression in cancer with positron emission tomography (PET). The fluorination precursor, arylboronic acid pinacol ester 4, was prepared in 4 steps with 42% overall yield. 18F-CJ-042794 was synthesized via a copper-mediated 18F-fluorination reaction followed by base hydrolysis, and was obtained in 1.5±1.1% (n=2) decay-corrected radiochemical yield. PET/CT imaging and biodistribution studies in mice showed that 18F-CJ-042794 was excreted through both renal and hepatobiliary pathways with significant retention in blood. The EP4-receptor-expressing LNCaP prostate cancer xenografts were clearly visualized in PET images with 1.12±0.08%ID/g (n=5) uptake value and moderate tumour-to-muscle contrast ratio (2.73±0.22) at 1h post-injection. However, the tumour uptake was nonspecific as it could not be blocked by co-injection of cold standard, precluding the application of 18F-CJ-042794 for PET imaging of EP4 receptor expression in cancer.

Unveiling a New Aspect of Simple Arylboronic Esters: Long-Lived Room-Temperature Phosphorescence from Heavy-Atom-Free Molecules.

Arylboronic esters can be used as versatile reagents in organic synthesis, as represented by Suzuki-Miyaura cross-coupling. Here we report a serendipitous finding that simple arylboronic esters are phosphorescent in the solid state at room temperature with a lifetime on the order of several seconds. The phosphorescence properties of arylboronic esters are remarkable in light of the general notion that phosphorescent organic molecules require heavy atoms and/or carbonyl groups for the efficient generation of a triplet excited state. Theoretical calculations on phenylboronic acid pinacol ester indicated that this molecule undergoes an out-of-plane distortion at the (pinacol)B-Cipso moiety in the T1 excited state, which is responsible for its phosphorescence. A compound survey with 19 arylboron compounds suggested that the phosphorescence properties might be determined by solid-state molecular packing rather than by the patterns and numbers of boron substituents on the aryl units. The present finding may update the general notion of phosphorescent organic molecules.

Enantioselective Allyl-, and Allenylboration of Aldehydes Catalyzed by Chiral Hydroxyl Carboxylic Acid

Asymmetric allylboration of aldehydes with allylboronic acid pinacol ester catalyzed by chiral hydroxyl carboxylic acid is described. This reaction provides synthetically useful homoallyl alcohols in high yield with good to high enantioselectivity. The present catalytic protocol was also examined in asymmetric allenylboration of aldehydes at elevated temperature to afford chiral homopropargyl alcohols with reasonable asymmetric induction.

Bismuth-catalyzed allylation of para-quinone methides.

An efficient 1,6-allylation addition of para-quinone methides to allylboronic acid pinacol ester catalyzed by bismuth(iii) triflate was developed. The reaction proceeded smoothly in high yields under mild conditions with only 0.5-5 mol% catalyst.

Synthesis and spectral properties of fluorene substituted cyclic and polymeric phosphazenes

Two new fluorene chromophores were prepared by the Suzuki cross-coupling reactions of 9,9-dimethylfluorene-2-boronic acid pinacol ester with bromophenol or 3,5-dibromophenol, respectively. The cyclic compounds were obtained by the reactions of chromophores with hexachlorocyclotriphosphazene (N3P3Cl6). For the comparison of spectral properties of cyclic compounds, the poly(bis-fluorenyl)phosphazene was also prepared from nucleophilic displacement reactions of chromophore with polydichlorophosphazene (Cl2PN)n. The synthesized compounds were characterized by standard spectroscopic methods (MALDI-TOF mass spectrometry, gel permeation chromatography for poly(bis-fluorenyl)phosphazene, FT-IR, 1H, 13C and 31P NMR spectroscopy) and their structure was confirmed by elemental analysis. The spectral properties were analysised by UV–Vis and fluorescence spectroscopy, thermogravimetric analysis, differential scanning calorimetry and cyclic voltammetry.

Evaluation of Allylboronic Acid Pinacol Ester as Effective Shutdown Overcharge Additive for Lithium Ion Cells

Allylboronic acid pinacol ester (ABPE) was investigated as shutdown overcharge additive to increase the intrinsic safety of lithium ion cells during operation at elevated charge cutoff potentials up to 4.5 V vs. Li/Li+. It was demonstrated that the additive had no negative influence on the cycling performance of LiNi1/3Co1/3Mn1/3O2 (NMC-111)/graphite full cells operated in a standard operation voltage range between 2.5 V and 4.2 V. Electrochemical impedance spectroscopy was used to study the influence of the electrolyte additive on the impedance before and after overcharge and concomitant cell shutdown. Thereby, an immense increase in the charge transfer resistance after electrode shutdown was observed, thus, hinting to the formation of a lithium ion-insulating layer on the positive electrode surface. To further elucidate the working principle of the shutdown additive, surface investigations by means of scanning electron microscopy and X-ray photoelectron spectroscopy were carried out. The existence of a surface layer consisting mainly of polymeric species was proven. The presented results open up a new family of compounds for overcharge protection.