The Structural Basis for Hydrolysis Resistance in the Esters of (2-Formylphenyl)boronic Acid 2,4-Dinitrophenylhydrazones

Design, development, synthesis, and crystal structure of the prototype of a new class of deep blue-fluorescing boron heterocycle estrogen mimics

To further the development of boron heterocyclic compounds that are useful to medicinal chemistry, we demonstrate how the class of compounds known as the diazaborines can be elaborated to produce an exceptionally close structural mimic of a natural estrogen. After building progressively closer models, a benzyloxy-substituted formylphenylboronic acid was ultimately condensed with a hydroxymethylated β-hydrazinocyclopentenone to give, after debenzylation, an isosteric mimic (diazaborine 1) of the naturally-occurring estrogen equilenin and the prototype of a new class of boron heterocycle estrogen mimics. X-ray crystallography revealed the prototype to be planar, with a transmolecular interoxygen distance virtually identical to that found in equilenin and with a strong hydrogen-bond-donating hydroxyl group. From this it can be anticipated that members of this unique class of boron heterocycle estrogen mimics will be found to possess useful biological properties. Furthermore, the prototype was found to fluoresce in the deep blue region of the visible spectrum, and so the development of members serving as light-emitting probes in biochemical and biological studies can also be anticipated.

Preliminary investigations into the synthesis and antimicrobial activities of boron-containing capsaicinoids

This preliminary study reports on the synthesis of two new boron-capsaicin derivatives containing either a short or long chain aliphatic tail group using an iridium catalyzed hydroboration reaction with pinacolborane. The boronate ester groups reside on the terminal position of the tail group and are necessary for the bioactivity of these compounds. Indeed, both compounds showed considerable activity against two Gram-positive bacteria, including Vancomycin-resistant Enterococcus. Vancomycin is considered the last resort medication for the treatment of septicemia, and new antibacterial agents that can treat sepsis are of paramount importance. The more lipophilic boron compound with the longer aliphatic chain also showed antifungal activity against Saccharomyces cerevisiae.

Synthesis, reactivity, and antimicrobial properties of boron-containing 4-ethyl-3-thiosemicarbazide derivatives

The addition of 4-ethyl-3-thiosemicarbazide to benzaldehyde and boronic acid containing derivatives afforded the corresponding thiosemicarbazones (1–3) or benzodiazaborines (4–6) depending on the position of the boronic acid within the ring. All compounds have been characterized fully including an X-ray diffraction study of the methoxy-containing benzodiazaborine 6. Attempts to coordinate thiosemicarbazones 2 and 3 to palladium(II) acetate were unsuccessful; however, addition of the non-boron-containing derivative 1 to palladium afforded complex 7 whose molecular structure was determined by an X-ray diffraction study. The initial bioactivities of compounds 1–7 were examined against two fungi, Aspergillus niger and Saccharomyces cerevisiae, and two bacteria, Bacillus cereus and Pseudomonas aeruginosa.

Formation of hydrazones and stabilized boron-nitrogen heterocycles in aqueous solution from carbohydrazides and ortho-formylphenylboronic acids

A recent addition to the suite of fast bioorthogonal reactions combines hydrazines and hydroxylamines with ortho-carbonyl substituted phenylboronic acids. Carbohydrazides are easily incorporated into biomolecules, making them appealing substrates in these reactions. Here we show that simple alkyl carbohydrazides form a single product with ortho-formylphenylboronic acid in an organic solvent and in the solid state. The solution structures of the products formed from the carbohydrazides in buffered aqueous solution, however, are markedly different from those identified in the organic solvent and solid state. The reactants form a mixture of hydrazone and heterocyclic products, the relative composition of which varies with pH. The observed reversibility of bioconjugates using carbohydrazide can thus be explained by the reversibility of the boron-nitrogen bond in the heterocycle. In contrast, the inclusion of an α-amine into the carbohydrazide substrate yields a single product in which both nitrogens are bonded to boron. These tricyclic structures are the same in organic solvent, solid state and aqueous solution from pH 4 to pH 9. Bioconjugates formed with α-amino carbohydrazides are stable to SDS-PAGE, while those formed with simple alkyl carbohydrazides are not. We propose that the inclusion of an intramolecular stabilizing ligand into a carbohydrazide substrate is a generally applicable principle that may be exploited to form boronic acid-based bioconjugates with a defined structure and resistance to hydrolysis.

A colour-tunable chiral AIEgen: reversible coordination, enantiomer discrimination and morphology visualization

We present a conceptually new approach to synthesise a boron-containing Aggregation-Induced Emissive Luminogen (AIEgen) with a chiral chromophore. An intramolecular N-B coordinating bond results in a low-energy transition that renders the material red-emissive in a solid state. By competitive binding of nucleophiles to the boron atom, this bond is replaced in favour of an intermolecular coordinating bond, which results in a tremendous blue-shift in both the absorption and emission. A supportive DFT computation elucidates that a breakage of the intramolecular N-B coordinating bond causes a tremendous loss of conjugation in the LUMO, resulting in a larger energy gap. Owing to the fact that our scaffold is intrinsically chiral and Lewis-acidic, we demonstrate how our AIEgen discriminates between two pairs of enantiomers in a simple UV-vis measurement. Furthermore, the binding capabilities are exploited to stain polymer blends that comprised a non-coordinating and a Lewis-basic polymer. The red fluorescence that originates only from domains of the non-coordinating polymer is conveniently detected by a fluorescence microscope. Thus, compared to current analytical methods, we present a cheaper and faster methodology to study the micro-morphologies of certain polymer blends.

A zwitterion produced by a strong intramolecular N→B interaction in 1-hydroxy-2-(pyridin-2-ylcarbonyl)benzo[d][1,2,3]diazaborinine

2-Acylated 2,3,1-benzodiazaborines can display unusual structures and reactivities. The crystal structure analysis of the boron heterocycle obtained by condensing 2-formylphenylboronic acid and picolinohydrazide reveals it to be an N→B-chelated zwitterionic tetracycle (systematic name: 1-hydroxy-11-oxo-9,10,17λ5-triaza-1λ4-boratetracyclo[8.7.0.02,7.012,17]heptadeca-3,5,7,12,14,16-hexaen-17-ylium-1-uide), C13H10BN3O2, produced by the intramolecular addition of the Lewis basic picolinoyl N atom of 1-hydroxy-2-(pyridin-2-ylcarbonyl)benzo[d][1,2,3]diazaborinine to the boron heterocycle B atom acting as a Lewis acid. Neither of the other two pyridinylcarbonyl isomers (viz. nicotinoyl and isonicotinoyl) are able to adopt such a structure for geometric reasons. A favored yet reversible chelation equilibrium provides an explanation for the slow D2O exchange observed for the OH resonance in the1H NMR spectrum, as well as for its unusual upfield chemical shift. Deuterium exchange may take place solely in the minor open (unchelated) species present in solution.

Synthesis, characterization, and antibacterial activity of structurally complex 2-acylated 2,3,1-benzodiazaborines and related compounds

A set of 2-acylated 2,3,1-benzodiazaborines and some related boron heterocycles were synthesized, characterized, and tested for antibacterial activity against Escherichia coli and Mycobacterium smegmatis. By high-field solution NMR, the heretofore unknown class of 2-acyl-1-hydroxy-2,3,1-diazaborines has been found to be able to exist in several interconvertable structural forms along a continuum comprised of an open hydrazone a, a monomeric B-hydroxy diazaborine b, and an anhydro dimer c. X-Ray crystallography of one of the anhydro dimers, 17c, revealed it to have an unprecedented structure featuring a double intramolecular O→B chelation. The crystal structure of another compound, 37, showed it to be based on a new pentacyclic B heterocycle framework. Nine compounds were found to possess activities against E. coli, and two others were active against M. smegmatis. The finding that these two contain isoniazid covalently embedded in their structures suggests that they might possibly be acting as prodrugs of this well-known antituberculosis agent in vivo.

1-Hydroxy-1H-benzo[d][1,2,6]oxazaborinin-4(3H)-one

The structure of the title compound, C(7)H(6)BNO(3), a new boron heterocycle, prepared by the condensation of (2-ethoxycarbonylphenyl)boronic acid and hydroxylamine, reveals the specific mode of intramolecular condensation between a phenylboronic acid and an ortho hydroxamic acid substituent. The crystal structure shows that dehydration occurs to form a planar oxazaborinine ring possessing both phenol-like B-O-H and lactam functional groups. In the extended structure, intermolecular hydrogen bonding generates a 14-membered ring. To our knowledge, this is the first crystal structure determination involving a six-membered ring that exhibits consecutive B-OH, O, NH, and C=O functional groups.