Practical synthesis of 1,4-dioxane derivative of the closo-dodecaborate anion and its ring opening with acetylenic alkoxides

Closo-dodecaborate-based dianionic surfactants with distorted classical morphology: Synthesis and atypical micellization in water

HYPOTHESIS

To challenge the classical concept of step-like micellization of ionic surfactants with singular critical micelle concentration, novel amphiphilic compounds with bulky dianionic head and the alkoxy tail connected via short linker, which can complex sodium cations, were synthesized in the form of disodium salts.

EXPERIMENT

The surfactants were synthesized by opening of a dioxanate ring attached to closo-dodecaborate by activated alcohol, which allows for attachment of alkyloxy tails of desired length to boron cluster dianion. The synthesis of the compounds with high cationic purity (sodium salt) is described. Self-assembly of the surfactant compound at air/water interface and in bulk water was studied by tensiometry, light and small angle X-ray scattering, electron microscopy, NMR spectroscopy, MD simulations and by isothermal titration calorimetry, ITC. The peculiarities in the micelle structure and formation were revealed by thermodynamic modelling and MD simulations of the micellization process.

FINDINGS

In an atypical process, the surfactants self-assemble in water to form relatively small micelles, where the aggregation number is decreasing with the surfactant concentration. The extensive counterion binding is a key characteristic of the micelles. The analysis strongly indicates complex compensation between the degree of bound sodium ions and the aggregation number. For the first time, a three-step thermodynamic model was used to estimate the thermodynamic parameters associated with micellization process. Diverse micelles differing in size and counterion binding can (co-)exist in the solution over the broad concentration and temperature range. Thus, the concept of step-like micellization was found inappropriate for these types of micelles.

Facile Attachment of Halides and Pseudohalides to Dodecaborate(2-) via Pd-catalyzed Cross-Coupling

Cross-coupling reactions with [B12H11I]2− as one partner have been used successfully for Kumada and Buchwald Hartwig couplings with Pd catalysis. Here, we found that the iodide could be substituted easily, and unexpectedly, with other halides such as Br and Cl, and with pseudohalides such as cyanide, azide, and isocyanate. We found that for Cl, Br, N3, and NCO, tetrabutylammonium salts—or sodium salts—were successful halide sources, whereas for cyanide, CuCN was the only halide source that allowed a successful exchange. The azide could be reacted further in a click reaction with triazoles. While no substitution with fluoride occurred, tetrabutylammonium fluoride in the presence of water led to [B12H11OH]2−. Yields were high to very high, and reaction times were short when using a microwave oven as a heating source.

Design of the New Closo-Dodecarborate-Containing Gemcitabine Analogue for the Albumin-Based Theranostics Composition

Combination therapy is becoming an increasingly important treatment strategy because multi-drugs can maximize therapeutic effect and overcome potential mechanisms of drug resistance. A new albumin-based theranostic containing gemcitabine closo-dodecaborate analogue has been developed for combining boron neutron capture therapy (BNCT) and chemotheraphy. An exo-heterocyclic amino group of gemcitabine was used to introduce closo-dodecaborate, and a 5′-hydroxy group was used to tether maleimide moiety through an acid-labile phosphamide linker. The N-trifluoroacylated homocysteine thiolactone was used to attach the gemcitabine analogue to human serum albumin (HSA) bearing Cy5 or Cy7 fluorescent dyes. The half-maximal inhibitory concentration (IC50) of the designed theranostic relative to T98G cells was 0.47 mM with the correlation coefficient R = 0.82. BNCT experiments resulted in a decrease in the viability of T98G cells, and the survival fraction was ≈ 0.4.

New Azido Coumarins as Potential Agents for Fluorescent Labeling and Their "Click" Chemistry Reactions for the Conjugation with closo-Dodecaborate Anion

Novel fluorescent 7-methoxy- and 7-(diethylamino)-coumarins modified with azido-group on the side chain have been synthesized. Their photophysical properties and single crystals structure characteristics have been studied. In order to demonstrate the possibilities of fluorescent labeling, obtained coumarins have been tested with closo-dodecaborate derivative bearing terminal alkynyl group. Cu^I catalyzed Huisgen 1,3-dipolar cycloaddition reaction has led to fluorescent conjugates formation. The absorption-emission spectra of the formed conjugates have been presented. The antiproliferative activity and uptake of compounds against several human cell lines were evaluated.

Selective Synthesis of 1,4-Dioxane from Oxirane Dimerization over ZrO2/TiO2 Catalyst at Low Temperature

We report a route to produce 1,4-dioxane from oxirane by using ZrO2/TiO2 as catalyst. The composite oxide ZrO2/TiO2 was prepared by a coprecipitation method and the catalytic performance was tested through the synthesis of 1,4-dioxane from oxirane in a pipe reactor. The X-ray diffraction (XRD) shows that ZrO2 and TiO2 are in crystal form. When the mass percentage of ZrO2 is 25%, the composite oxide ZrO2/TiO2 presents as an amorphous form. The sample 25%ZrO2/TiO2 exhibits a specific surface area of 269.5 m2·g−1 and pore volume of 1.34 mL·g−1. The catalyst has 670 µmol·g−1 of NH3-TPD acid, and the characterization of ammonia infrared spectroscopy (NH3-FTIR) shows that both Brønsted and Lewis acids are present on the surface of the catalyst. The reaction mechanism was analyzed according to the distribution of product. The test of catalytic performance showed 100.0% conversion of oxirane and 86.2% selectivity of 1,4-dioxane at the optimal operation conditions: atmospheric pressure, reaction temperature 75 °C and gaseous hourly space velocity of 1200.0 h−1. The catalyst exhibits good catalytic performance stability after continuous use for 720 h.

Iodophenyl-conjugated closo-dodecaborate as a promising small boron molecule that binds to serum albumin and accumulates in tumor

The development of novel boron carriers applicable to various cancers is required for further expansion of boron neutron capture therapy (BNCT). In this study, we took advantage of the fact that serum albumin accumulates in tumors and developed a boron compound that interacts non-covalently with the serum albumin. 4-Iodophenylbutanamide was chosen as an albumin ligand and conjugated with closo-dodecaborate (boron-conjugated 4-iodophenylbutanamide: BC-IP). BC-IP was found to be water soluble with low cytotoxicity. The IC₅₀ values of BC-IP were 475 µM for U87MG cells, 738 µM for HeLa cells, and > 1000 µM for A549 cells. The dissociation constant (K_d) value of BC-IP to HSA was 148 ± 8 μM, while that of disodium closo-dodecaborate (4) was > 1000 μM. Significant tumor accumulation was observed in the U87MG tumor mouse model 3 h after injection. The boron concentration in the tumor reached a maximum of 11 μgB/g at 3 h and gradually decreased to 2.4 and 2.3 μgB/g at 12 and 24 h, respectively.

New approaches to the functionalization of the 1-carba-closo-decaborate anion

Two new approaches to the functionalization of the 1-carba-closo-decaborate anion [1-CB₉H₁₀]^- at boron atoms via the ring-opening of its 1,4-dioxane derivative with various nucleophiles and Pd-catalysed cross-coupling of its iodo derivative with aromatic amines and heteroaromatics were developed.

Sweet Boron: Boron-Containing Sugar Derivatives as Potential Agents for Boron Neutron Capture Therapy

Boron neutron capture therapy (BNCT) is a binary type of radiotherapy for the treatment of cancer. Due to recent developments of neutron accelerators and their installation in some hospitals, BNCT is on the rise worldwide and is expected to have a significant impact on patient treatments. Therefore, there is an increasing need for improved boron delivery agents. Among the many small molecules and delivery systems developed, a significant amount of recent research focused on the synthesis of boron-containing sugar and amino acid derivatives to exploit specific transport proteins, as d-glucose transporter 1 (GLUT1) and large neutral amino acid transporter (LAT1), overexpressed by tumor cells. This review will discuss the last year’s achievements in the synthesis and some biological evaluation of boronated sugars derivatives. The compounds described in this review are intrinsically asymmetric due to the presence of chiral sugar moieties, often joined to boron clusters, which are structural elements with high symmetry.

Solid-State, Thermal Synthesis of Peptide/Protein-Boron Cluster Conjugates

Anionic boron clusters can be used to increase the pharmaceutical properties of the peptides. Here, we describe the method of synthesis of peptide/protein-boron cluster conjugates using solid-state, thermal reaction on two different peptides: thymosin β4 (Tβ4) and lysozyme. 1,4-dioxane oxonium derivatives of anionic boron clusters are used as donors of boron clusters. This procedure allows to conjugate anionic boron clusters to native peptides without loss of the activity of the peptides.

Boron-Containing Lipids and Liposomes: New Conjugates of Cholesterol with Polyhedral Boron Hydrides

A series of boron-containing lipids were prepared by reactions of cyclic oxonium derivatives of polyhedron boranes and metallacarboranes (closo-dodecaborate anion, cobalt and iron bis(dicarbollides)) with amine and carboxylic acids which are derived from cholesterol. Stable liposomal formulations, on the basis of synthesized boron-containing lipids, hydrogenated soybean l-α-phosphatidylcholine and (HSPC) 1,2-distearoyl-sn-glycero-3-phosphoethanolamine-N-[methoxy(polyethylene glycol)-2000] (DSPE-PEG) as excipients, were prepared and then characterized by dynamic light scattering (DLS) that revealed the formation of particles to be smaller than 200 nm in diameter. The resulting liposomal formulations showed moderate to excellent loading and entrapment efficiency, thus justifying the design of the compounds to fit in the lipid bilayer and ensuring ease of in vivo use for future application. The liposomal formulations based on cobalt and iron bis(dicarbollide)-based lipids were found to be nontoxic against both human breast normal epithelial cells MCF-10A and human breast cancer cells MCF-7.

Blue Emitting Star-Shaped and Octasilsesquioxane-Based Polyanions Bearing Boron Clusters. Photophysical and Thermal Properties

High boron content systems were prepared by the peripheral functionalisation of 1,3,5-triphenylbenzene (TPB) and octavinylsilsesquioxane (OVS) with two different anionic boron clusters: closo-dodecaborate (B12) and cobaltabisdicarbollide (COSAN). TPB was successfully decorated with three cluster units by an oxonium ring-opening reaction, while OVS was bonded to eight clusters by catalysed metathesis cross-coupling. The resulting compounds were spectroscopically characterised, and their solution-state photophysical properties analysed. For TPB, the presence of COSAN dramatically quenches the fluorescence emission (λem = 369 nm; ΦF = 0.8%), while B12-substituted TPB shows an appreciable emission efficiency (λem = 394 nm; ΦF = 12.8%). For octasilsesquioxanes, the presence of either COSAN or B12 seems to be responsible for ∼80 nm bathochromic shift with respect to the core emission, but both cases show low emission fluorescence (ΦF = 1.4-1.8%). In addition, a remarkable improvement of the thermal stability of OVS was observed after its functionalisation with these boron clusters.

Focus on Chemistry of the 10-Dioxane-nido-7,8-dicarba-undecahydrido Undecaborate Zwitterion; Exceptionally Easy Abstraction of Hydrogen Bridge and Double-Action Pathways Observed in Ring Cleavage Reactions with OH- as Nucleophile

Ring cleavage of cyclic ether substituents attached to a boron cage via an oxonium oxygen atom are amongst the most versatile methods for conjoining boron closo-cages with organic functional groups. Here we focus on much less tackled chemistry of the 11-vertex zwitterionic compound [10-(O-(CH2-CH2)2O)-nido-7,8-C2B9H11] (1), which is the only known representative of cyclic ether substitution at nido-cages, and explore the scope for the use of this zwitterion 1 in reactions with various types of nucleophiles including bifunctional ones. Most of the nitrogen, oxygen, halogen, and sulphur nucleophiles studied react via nucleophilic substitution at the C1 atom of the dioxane ring, followed by its cleavage that produces six atom chain between the cage and the respective organic moiety. We also report the differences in reactivity of this nido-cage system with the simplest oxygen nucleophile, i.e., OH-. With compound 1, reaction proceeds in two possible directions, either via typical ring cleavage, or by replacement of the whole dioxane ring with -OH at higher temperatures. Furthermore, an easy deprotonation of the hydrogen bridge in 1 was observed that proceeds even in diluted aqueous KOH. We believe this knowledge can be further applied in the design of functional molecules, materials, and drugs.

Design of S–S bond containing maleimide-conjugated closo-dodecaborate (SSMID): identification of unique modification sites on albumin and investigation of intracellular uptake

The S–S bond containing maleimide-conjugated closo-dodecaborate (SSMID) was synthesised to identify the binding sites in bovin serum albumin (BSA).

Organotin Dyes Bearing Anionic Boron Clusters as Cell-Staining Fluorescent Probes

Within the cell nucleus, in the nucleoli, ribosomal RNAs are synthesized and participate in several biological processes. To better understand nucleoli-related processes, their visualization is often required, for which specific markers are needed. Herein, we report the design of novel fluorescent organotin compounds derived from 4-hydroxy-N'-((2-hydroxynaphthalen-1-yl)methylene)benzohydrazide and their cytoplasm and nucleoli staining of B16F10 cells in vitro. Tin compounds bearing an aliphatic carbon chain (-C₁₂ H₂₅ ) and an electron-donating group (-OH) were prepared, and the latter could be derivatized to bear the boron cluster anions [B₁₂ H₁₂ ]^2- and [3,3'-Co(1,2-C₂ B₉ H₁₁ )₂ ]^- (COSAN). All of the conjugates have been fully characterized and their luminescence properties have been assessed. In general, they show good quantum yields in solution (24-49 %), those for the COSAN derivatives being lower. Remarkably, the linking of [B₁₂ H₁₂ ]^2- and COSAN to the complexes made them more soluble, without being detrimental to their luminescence properties. Living B16F10 cells were treated with all of the compounds to determine their fluorescence staining properties; the compounds bearing the aliphatic chain showed a reduced staining capacity due to the formation of aggregates. Notably, the complexes bearing different boron clusters showed different staining effects; those bearing [B₁₂ H₁₂ ]^2- showed extraordinary staining of the nucleoli and cytoplasm, whereas those bearing COSAN were only detected in the cytoplasm. The remarkable fluorescence staining properties shown by these organotin compounds make them excellent candidates for fluorescence bioimaging in vitro.

Maleimide-functionalized closo-dodecaborate albumin conjugates (MID-AC): Unique ligation at cysteine and lysine residues enables efficient boron delivery to tumor for neutron capture therapy

Maleimide-conjugating closo-dodecaborate sodium form 5c (MID) synthesized by the nucleophilic ring-opening reaction of closo-dodecaborate-1,4-dioxane complex 2 with tetrabutylammonium (TBA) azide was found to conjugate to free SH of cysteine and lysine residues in BSA under physiological conditions, forming highly boronated BSA that showed high and selective accumulation in tumor and significant tumor growth inhibition in colon 26 tumor-bearing mice subjected to thermal neutron irradiation.

Synthesis of oligo-closo-dodecaborates by Hüisgen click reaction as encapsulated agents for the preparation of high-boron-content liposomes for neutron capture therapy

High boron-content molecules and their encapsulated liposomes.

Cyanide free contraction of disclosed 1,4-dioxane ring as a route to cobalt bis(dicarbollide) derivatives with short spacer between the boron cage and terminal functional group

The 1,4-dioxane derivative of cobalt bis(dicarbollide) reacts with dialkylsulfides and triphenylphosphine to give the corresponding sulfonium and phosphonium derivatives.

Tetra-butyl-ammonium 2-[2,5-dimethyl-3-(4-nitro-phen-yl)-2,3-dihydro-1,2,4-oxadiazo-lium-4-yl]nona-hydro-closo-deca-borate

The title ionic compound, C16H36N(+)·C10H20B10N3O3(-), consists of a tetra-butyl-ammonium cation and a closo-deca-borate cluster anion, which is bound to the substituted 2,3-dihydro-1,2,4-oxadiazole ring through a B-N bond [1.540 (2) Å]. The distances between connected B atoms in the deca-borate cluster range from 1.689 (2) to 1.844 (2) Å. The 2,3-dihydro-1,2,4-oxadiazole ring adopts an envelope conformation with the N atom as the flap atom.