Catechol pendant polystyrene for solid-phase synthesis

Novel pH-sensitive catechol dyes synthesised by a three component one-pot reaction

The synthesis and characterisation of new dyes based on indolizines bearing catechol groups in their structure is presented. The preparation was carried out through a simple three component one-pot reaction promoted by CuNPs/C, between pyridine-2-carbaldehyde, an aromatic alkyne and a tetrahydroisoquinoline (THIQ) functionalized with catechol groups. The products were isolated in 30%-34% yield, which was considered more than acceptable considering that the catechol hydroxyl groups were not protected prior to reaction. In view of the colour developed by the products and their response to the acidic and basic conditions of the medium, product 3aa was studied by UV-Vis and NMR spectroscopies at different pH values. We concluded that product 3aa suffered two deprotonations at pK_a of 4.4 and 9.5, giving three species in a pH range between 2-12, with colours varying from light red to deep orange. The reversibility of the process observed for 3aa at different pH values, together with its changes in colour, make this new family of products attractive candidates to use them as pH indicators.

Solid-phase synthesis and evaluation of tumour-targeting phenylboronate-based MRI contrast agents

Paramagnetic macrocycles functionalized with phenylboronic moieties have proven to be interesting for MRI applications based on their ability to recognize cancer cells and generate local contrast. However, full use of the potential of this class of compounds is hampered by laborious and inefficient synthetic and, especially, purification procedures. The amphiphilic character of water-soluble phenylboronates renders them difficult compounds to be prepared through conventional solution synthesis due to the tendency to aggregate and form adducts with other nucleophiles. The new strategy described herein exploits the advantage of solid-phase synthesis with the application of DEAM-PS resin for anchorage and the subsequent simplified derivatization of boronates. GdDOTA-EN-PBA and its fluorinated analogue GdDOTA-EN-F2PBA were synthesized in a much easier, faster and economically convenient way to achieve good yields and purity. Furthermore, the effect of electron-withdrawing fluorine atoms on the aromatic ring of the latter compound was investigated by comparing the physico-chemical properties of both compounds as well as their binding affinity towards melanoma cancer cells.

Accelerated hydrolysis of boronic acid in a modified poly(amidoamine) dendrimer: identification of a factor leading to the production of an impurity in boronic acid containing poly(amidoamine) dendrimers

The preparation of boronic acid containing dendrimers is still a difficult task in dendrimer chemistry. In this investigation, an unanticipated hydrolysis product (4) was identified during the preparation of (G:2)-PAMAM-dendri-(4-phenyl boronic acid)14 (3) by acquiring its 1H NMR, 13C NMR, and UV–vis absorption spectra and IR spectra and comparing them with those of 4-hydroxylbenzoic acid (5). Furthermore, an Alizarin Red S staining analysis, ICP-MS, and 11B NMR spectrum indicated the lack of boronic acid in 4. Finally, treatment of 3 with hydrogen peroxide gave a product that was determined to be identical to dendrimer 4. By analyzing the formation of 4 under various conditions, the presence of numerous primary amines in a PAMAM dendrimer did accelerate the hydrolysis of peripheral boronic acid. This investigation revealed an apparent intrinsic problem that needs to be overcome during the preparation of boronic acid containing dendrimers and related boronic acid containing macromolecules.

Boronic acid shell-crosslinked dextran-b-PLA micelles for acid-responsive drug delivery

Herein, 3-carboxy-5-nitrophenylboronic acid (CNPBA) shell-crosslinked micelles based on amphiphilic dextran-block-polylactide (Dex-b-PLA) are prepared and used for efficient intracellular drug deliveries. Due to the reversible pH-dependent binding with diols to form boronate esters, CNPBA modified Dex-b-PLA shows excellent pH-sensitivity. In neutral aqueous conditions, CNPBA-Dex-b-PLA forms shell-crosslinked micelles to enable DOX loading, while in acid conditions, the boronate esters hydrolyze and the micelles de-crosslink to release loaded DOX. In vitro release studies indicate that the release of the DOX cargo is minimized at physiological conditions, while there is a burst release in response to low pHs. The cell viability of CNPBA-Dex-b-PLA investigated by MTT assay was more than 90%, indicating that, as a drug delivery system, CNPBA-Dex-b-PLA has good cytocompatibility. These features suggest that the pH-responsive biodegradable CNPBA-Dex-b-PLA can efficiently load and deliver DOX into tumor cells and enhance the inhibition of cellular proliferation in vitro, providing a favorable platform as a drug delivery system for cancer therapy.

Synthesis and reactivity of solid-supported organotrifluoroborates in Suzuki cross-coupling

Solid-supported organotrifluoroborates were prepared in high yields by ion exchange with Amberlyst resins. The reactivity of solid supported aryltrifluoroborates was evaluated in Suzuki-Miyaura couplings with numerous aryl bromide partners. Electron-rich and -poor substituents were tolerated on both substrates, providing yields up to 90%. Examples of alkyl-, alkenyl-, alkynyl-, and heteroaryltrifluoborates were also successfully cross-coupled to aryl halides.

Click reactions and boronic acids: applications, issues, and potential solutions

Boronic acids have been widely used in a wide range of organic reactions, in the preparation of sensors for carbohydrates, and as potential pharmaceutical agents. With the growing importance of click reactions, inevitably they are also applied to the synthesis of compounds containing the boronic acid moiety. However, such applications have unique problems. Chief among them is the issue of copper-mediated boronic acid degradation in copper-assisted [2,3]-cycloadditions involving an alkyne and an azido compound as the starting materials. This review summarizes recent developments, analyzes potential issues, and discusses known as well as possible solutions.

The thermal and boron-catalysed direct amide formation reactions: mechanistically understudied yet important processes

This review provides a summary and discussion of work reported to date regarding the mechanistic aspects of the catalysed and uncatalysed direct amide formation reaction.

Computational investigation of the oxidative deboronation of boroglycine, H2N-CH2-B(OH)2, Using H2O and H2O2

We report results from a computational investigation of the oxidative deboronation of boroglycine, H2N-CH2-B(OH)2, using H2O and H2O2 as the reactive oxygen species (ROS) to yield aminomethanol, H2N-CH2-OH; these results complement our study on the protodeboronation of boroglycine to produce methylamine, H2N-CH3 (Larkin et al. J. Phys. Chem. A 2007, 111, 6489-6500). Second-order Møller-Plesset (MP2) perturbation theory with Dunning-Woon correlation-consistent (cc) basis sets were used for the calculations with comparisons made to results from density functional theory (DFT) at the PBE1PBE/6-311++G(d,p)(cc-pVDZ) levels. The effects of a bulk aqueous environment were also incorporated into the calculations employing PCM and CPCM methodology. Using H2O as the ROS, the reaction H2O + H2N-CH2-B(OH)2 --> H2N-CH2-OH + H-B(OH)2 was calculated to be endothermic; the value of DeltaH(298)(0) was +12.0 kcal/mol at the MP2(FC)/cc-pVTZ computational level in vacuo and +13.7 kcal/mol in PCM aqueous media; the corresponding value for the activation barrier, DeltaH(double dagger), was +94.3 kcal/mol relative to the separated reactants in vacuo and +89.9 kcal/mol in PCM aqueous media. In contrast, the reaction H2O2 + H2N-CH2-B(OH)2 --> H2N-CH2-OH + B(OH)3 was calculated to be highly exothermic with an DeltaH(298)(0) value of -100.9 kcal/mol at the MP2(FC)/cc-pVTZ computational level in vacuo and -99.6 kcal/mol in CPCM aqueous media; the highest-energy transition state for the multistep process associated with this reaction involved the rearrangement of H2N-CH2-B(OH)(OOH) to H2N-CH2-O-B(OH)2 with a DeltaH(double dagger) value of +23.2 kcal/mol in vacuo relative to the separated reactants. These computational results for boroglycine are in accord with the experimental observations for the deboronation of the FDA approved anticancer drug bortezomib (Velcade, PS-341), where it was found to be the principle deactivation pathway (Labutti et al. Chem. Res. Toxicol. 2006, 19, 539-546).

Design and synthesis of boronic-acid-labeled thymidine triphosphate for incorporation into DNA

The boronic acid moiety is a versatile functional group useful in carbohydrate recognition, glycoprotein pull-down, inhibition of hydrolytic enzymes and boron neutron capture therapy. The incorporation of the boronic-acid group into DNA could lead to molecules of various biological functions. We have successfully synthesized a boronic acid-labeled thymidine triphosphate (B-TTP) linked through a 14-atom tether and effectively incorporated it into DNA by enzymatic polymerization. The synthesis was achieved using the Huisgen cycloaddition as the key reaction. We have demonstrated that DNA polymerase can effectively recognize the boronic acid-labeled DNA as the template for DNA polymerization, that allows PCR amplification of boronic acid-labeled DNA. DNA polymerase recognitions of the B-TTP as a substrate and the boronic acid-labeled DNA as a template are critical issues for the development of DNA-based lectin mimics via in vitro selection.

Water-soluble fluorescent boronic acid compounds for saccharide sensing: substituent effects on their fluorescence properties

Four new naphthalene-based boronic acid compounds (1-4) were synthesized. The effect of various carbohydrates on their fluorescence properties has been studied in aqueous phosphate buffer at pH 7.4. Different substitutions on the aniline group of the naphthalene ring resulted in significant differences in fluorescence properties for these four compounds. Compound 1 shows ratiometric fluorescence changes upon addition of a sugar. Compounds 2 and 3 do not show ratiometric fluorescence changes but show very large fluorescence intensity changes (about 70-fold fluorescence intensity increase). In addition to the quantifiable fluorescence property changes upon sugar addition, the fluorescence color changes of 1-3 are also visible to the naked eye. However, amidation of the aniline nitrogen atom significantly diminishes the fluorescence intensity of compound 4. The crystal structure of one boronic acid provided some insight into the structural features that are important for the fluorescence properties of these compounds.

Boric Acid Catalyzed Chemoselective Esterification of α-Hydroxycarboxylic Acids

Boric acid catalyzes the selective esterification of alpha-hydroxycarboxylic acids without causing significant esterification to occur with other carboxylic acids. The procedure is simple, high-yielding, and applicable to the esterification of alpha-hydroxy carboxylates in the presence of other carboxylic acids including beta-hydroxyacids within the same molecule. [reaction: see text]

Boronic acid compounds as potential pharmaceutical agents

Boronic acid compounds have been used, because of their unique structural features, for the development of potent enzyme inhibitors, boron neutron capture agents for cancer therapy, and as antibody mimics that recognize biologically important saccharides. Consequently, there has been a surge of interests in boronic acid compounds. This study reviews the recent development in this area during the last six years.