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PubMed2005

Synthesis of beta,beta-disubstituted vinyl boronates via the ruthenium-catalyzed Alder ene reaction of borylated alkynes and alkenes.

Ruthenium-catalyzed Alder ene reactions between borylated alkynes and terminal alkenes give the corresponding beta,beta-disubstituted vinyl boronates with high selectivity for the branched isomer. The stereochemistry of the vinyl boronate moiety was the result of a formal trans addition of the ene subunit across the alkyne, which is the opposite stereochemical outcome observed for other internal alkynes.

Alkenes↗
PubMed2005

Boronate-based fluorescent probes for imaging cellular hydrogen peroxide.

The syntheses, properties, and biological applications of the Peroxysensor family, a new class of fluorescent probes for hydrogen peroxide, are presented. These reagents utilize a boronate deprotection mechanism to provide high selectivity and optical dynamic range for detecting H2O2 in aqueous solution over similar reactive oxygen species (ROS) including superoxide, nitric oxide, tert-butyl hydroperoxide, hypochlorite, singlet oxygen, ozone, and hydroxyl radical. Peroxyresorufin-1 (PR1), Peroxyfluor-1 (PF1), and Peroxyxanthone-1 (PX1) are first-generation probes that respond to H2O2 by an increase in red, green, and blue fluorescence, respectively. The boronate dyes are cell-permeable and can detect micromolar changes in H2O2 concentrations in living cells, including hippocampal neurons, using confocal microscopy and two-photon microscopy. The unique combination of ROS selectivity, membrane permeability, and a range of available excitation/emission colors establishes the potential value of PR1, PF1, PX1, and related probes for interrogating the physiology and pathology of cellular H2O2.

Boron Compounds↗
PubMed2006

Synthesis and photophysical properties of borondipyrromethene dyes bearing aryl substituents at the boron center.

Several borondipyrromethene (Bodipy) dyes bearing an aryl nucleus linked directly to the boron center have been prepared under mild conditions. The choice of Grignard or lithio organo-metallic reagents allows the isolation of B(F)(aryl) or B(aryl)2 derivatives; where aryl refers to phenyl, anisyl, naphthyl, or pyrenyl fragments. A single crystal, X-ray structure determination for the bis-anisyl compound shows that the sp3 hybridized boron center remains pseudo-tetrahedral and that the B-C bond distances are 1.615 and 1.636 A. All compounds are electrode active but replacement of the fluorine atoms by aryl fragments renders the Bodipy unit more easily oxidized by 100 mV in the B(F)(aryl) and 180 mV in the B(aryl)2 compounds whereas reduction is made more difficult by a comparable amount. Strong fluorescence is observed from the Bodipy fluorophore present in each of the new dyes, with the radiative rate constant being independent of the nature of the aryl substituent. The fluorescence quantum yields are solvent dependent and, at least in some cases (aryl = anisyl or pyrenyl), nonradiative decay from the first-excited singlet state is strongly activated. There is no indication, however, for population of a charge-transfer state, in which the aryl substituent acts as donor and the Bodipy fragment functions as acceptor, that is strongly coupled to the ground state. Instead, it is conjectured that nonradiative decay involves a conformational change driven by the solvophobic effect. Thus, the rate of nonradiative decay in any given solvent increases with increasing surface accessibility (or molar volume) of the aryl substituent. Intramolecular energy transfer from pyrene or naphthalene residues to Bodipy is quantitative.

Boron Compounds↗
PubMed1990

Analogues of carbamyl aspartate as inhibitors of dihydroorotase: preparation of boronic acid transition-state analogues and a zinc chelator carbamylhomocysteine.

Dihydroorotase (DHO) catalyzes the conversion of carbamyl aspartate (CA) to dihydroorotate (DO) in the de novo pyrimidine biosynthetic pathway. Few effective inhibitors of DHO have been reported, and thus blockade of this reaction has not been widely pursued as a strategy for development of antitumor agents. Utilizing two mechanism-based strategies, we have designed and prepared potential DHO inhibitor analogues of CA. One strategy replaced the gamma-carboxyl moiety of CA with a boronic acid. This substitution yields compounds which form stable charged tetrahedral intermediates and mimic the enzyme-substrate transition state. Preparation of the boronic acid analogues of CA and its carboxylic acid esters focused on a Curtius rearrangement as a key step following a malonic ester synthesis. This was followed by carbamoylation of the free amine under nonaqueous neutral conditions with Si(NCO)4. The ethyl ester was a competitive inhibitor of DHO with an apparent Ki of 5.07 microM, while the nonesterified analogue and the methyl ester were not effective inhibitors. None of the compounds were cytotoxic against L1210 cells in culture. An active-site-directed sulfhydryl-containing zinc chelator was also prepared. This analogue irreversibly inhibited the enzyme, but it also was ineffective in L1210 growth inhibition.

Amidohydrolases↗
PubMed2002

Synthesis of (aminoalkylamine)-N-aminoalkyl)azanonaborane(11) derivatives for boron neutron capture therapy.

New boron-containing polyamine have been synthesized: (aminoalkylamine)-N-(aminoalkyl)azanonaborane(11) derivatives [H(2)N(CH(2))(n)H(2)NB(8)H(11)NH(CH(2))(n)NH(2)], where n = 4-6 and 12, and [H(2)N(CH(2))(3)H(2)NB(8)H(11)NH(CH(2))(4)NH(2)]. (4-Aminobutylamine)-N-(4-aminobutyl)azanonaborane and (3-aminopropylamine)-N-(4-aminobutyl)azanonaborane were less toxic in vitro (LD(50) of approximately 700 and approximately 1100 microM, respectively) than spermine, while (4-aminobutylamine)-N-isopropylazanonaborane with its hydrophobic isopropyl group and those with n = 5, 6, and 12 were already toxic under similar conditions (LD(50) << 500 microM). These compounds may be useful as delivery agents for boron neutron capture therapy.

Animals↗
PubMed2004

Total synthesis of the boron-containing ion carrier antibiotic macrodiolide tartrolon B.

The first total synthesis of the boron-containing macrodiolide antibiotic tartrolon B is reported in full detail. Two convergent approaches to the target compound are described, the first of which eventually failed, due to sensitive functionality. In the second, successful route the key step was a stereoselective boron-mediated aldol addition of a bicyclic acetonide protected ketone to a diene-aldehyde. In this case the synthesis could be completed without major problems, using a Yamaguchi dimerization macrolactonization endgame.

Aldehydes↗
PubMed2006

SnO2 nanoparticle-functionalized boron nitride nanotubes.

Boron nitride nanotubes (BNNTs) were synthesized by a carbon-free chemical vapor deposition method using boron and metal oxide as reactants. Then SnO(2) nanoparticles were functionalized on them via a simple wet chemistry method. Detailed transmission electron microscopy (TEM) observations reveal that SnO(2) nanoparticles may cover the tube surface or be encapsulated in tube channels. The lattice distances of both BNNT and SnO(2) have been changed due to the strong interactions between them. The band gap energy of SnO(2) particles is found enlarged due to the size effect and interaction with BNNTs.

Boron Compounds↗
PubMed2006

Effects of surfactant and boron doping on the BWF feature in the Raman spectrum of single-wall carbon nanotube aqueous dispersions.

We examine the Breit-Wigner-Fano (BWF) line shape in the Raman spectra of carbon single-wall nanotubes (SWNTs) dispersed in aqueous suspensions. Bundling and electronic effects are studied by comparing undoped SWNTs (C-SWNTs) to boron-doped nanotubes (B-SWNTs) in a variety of different surfactant solutions. For SWNTs dispersed with nonionic surfactants that are less effective in debundling than ionic surfactants, the Raman spectra retain a large BWF feature. However, we demonstrate that even for SWNTs dispersed as isolated nanotubes by ionic surfactants the BWF feature may be present and that the intensity of the BWF is highly sensitive to the specific surfactant. In particular, surfactants with electron-donating groups tend to enhance the BWF feature. Also, modification of the SWNT electronic properties by boron doping leads to enhanced surfactant dispersion relative to undoped C-SWNTs and also to modification of the BWF feature. These observations are in agreement with reports demonstrating an enhancement of the BWF by bundling but also agree with reports that suggest electron donation can enhance the BWF feature even for isolated SWNTs. Importantly, these results serve to caution against using the lack or presence of a BWF feature as an independent measure of SWNT aggregation in surfactant dispersions.

Benzenesulfonates↗
PubMed2006

Electrocrystallization of Phase I, CuTCNQ (TCNQ = 7,7,8,8-Tetracyanoquinodimethane), on indium tin oxide and boron-doped diamond electrodes.

The electrochemical reduction of TCNQ to TCNQ*- in acetonitrile in the presence of [Cu(MeCN)4]+ has been undertaken at boron-doped diamond (BDD) and indium tin oxide (ITO) electrodes. The nucleation and growth process at BDD is similar to that reported previously at metal electrodes. At an ITO electrode, the electrocrystallization of more strongly adhered, larger, branched, needle-shaped phase I CuTCNQ crystals is detected under potential step conditions and also when the potential is cycled over the potential range of 0.7 to -0.1 V versus Ag/AgCl (3 M KCl). Video imaging can be used at optically transparent ITO electrodes to monitor the growth stage of the very large branched crystals formed during the course of electrochemical experiments. Both in situ video imaging and ex situ X-ray diffraction and scanning electron microscopy (SEM) data are consistent with the nucleation of CuTCNQ taking place at a discrete number of preferred sites on the ITO surface. At BDD electrodes, ex situ optical images show that the preferential growth of CuTCNQ occurs at the more highly conducting boron-rich areas of the electrode, within which there are preferred sites for CuTCNQ formation.

Boron↗
PubMed2005

A molecular dynamics study of the structural dependence of boron oxide nanoparticles on shape.

Molecular dynamics simulation is employed to study the effect of varying nanoparticle shape on the structure of boron oxide nanoparticles. Two nanoshapes are investigated and compared: a sphere of diameter 16 A and a cube of dimension 16 x 16 x 16 A. A many-body polarization model is employed within the simulation, accounting for dipole moments induced by local electric fields. The resulting network is described by a short-range structure consisting of planar BO(3) units, while the intermediate-range structure is described by six-membered planar boroxol rings. Both the fraction of boroxol rings and their locations differ between the two nanoshapes. All planar boroxol rings within the spherical simulation are located on the interior, while planar rings within the cubic simulation aggregate to the cube walls. In addition, structural differences appear between the two shapes at longer ranges, including the formation of "layers" aligned parallel to the walls of the cube, reminiscent of both the low-density crystalline phase and the high-density amorphous form of boron oxide.

Boron Compounds↗
PubMed2000

Thiol ester-boronic acid cross-coupling. Catalysis using alkylative activation of the palladium thiolate intermediate.

Thiol esters and boronic acids do not participate in cross-coupling in the presence of palladium catalysts. However, efficient palladium-catalyzed thiol ester-boronic acid cross-coupling is observed when simple alkylating agents are present. Alkylative conversion of the very stable palladium-thiolate bond to a labile palladium-thioether bond is presumed to be crucial to the catalysis. Of the systems studied, 4-halo-n-butyl thiol esters were most effective in this cross-coupling.

Alkylation↗
PubMed2000

Copper-mediated cross-coupling of aryl boronic acids and alkyl thiols.

[reaction: see text] The cross-coupling of aryl boronic acids and alkanethiols mediated by copper(II) acetate and pyridine in anhydrous dimethylformamide affords aryl alkyl sulfides in good yield with a wide variety of substituted aryl boronic acids. The method is applicable to the synthesis of aryl sulfides of cysteine.

Alkylation↗
PubMed2001

Substituted alkyne synthesis under nonbasic conditions: copper carboxylate-mediated, palladium-catalyzed thioalkyne-boronic acid cross-coupling.

[figure: see text] A new methodology for the synthesis of substituted alkynes is described. In the presence of copper(I) thiophene-2-carboxylate (CuTC) or copper (I) 3-methylsalicylate (CuMeSal), the palladium-catalyzed cross-coupling of thioalkyne derivatives with boronic acids affords functionalized alkynes in yields ranging from 39 to 91%. This coupling occurs efficiently under mild, nonbasic conditions with a wide variety of thioalkynes and boronic acids, providing a reaction complementary to the Sonogashira protocol.

Alkynes↗
PubMed2001

Highly fructose selective transport promoted by boronic acids based on a pentaerythritol core.

We have designed and synthesized a highly lipophilic boronic acid (11) with a molecular shape that makes it much more effective at carrying sugars through organic membranes than a previously used steroidal boronic acid. The corresponding diboronic acid (12) was also found to transport fructose ahead of glucose with a very high selectivity (7.6:1.0). Modeling suggests that 12 is able to carry two fructose molecules at once in a complex stabilized through hydrogen bonding and ion pairing.

Boronic Acids↗
PubMed2006

Understanding the origins of remote asymmetric induction in the boron aldol reactions of beta-alkoxy methyl ketones.

We report theoretical studies into the remote 1,5-stereoinduction shown by certain types of beta-alkoxy methyl ketones in boron-mediated aldol reactions with achiral aldehydes. For a range of common alkoxy groups, our calculations are in excellent agreement with experimentally observed diastereoselectivities. In the aldol transition structures, a stabilizing hydrogen bond between the alkoxy oxygen and formyl proton leads to preferential formation of the 1,5-adduct, by minimizing steric interactions between the beta-alkyl group and one of the ligands on boron.

Boron↗
PubMed2006

Oxidative deboronation of the peptide boronic acid proteasome inhibitor bortezomib: contributions from reactive oxygen species in this novel cytochrome P450 reaction.

Bortezomib (1) is a potent first-in-class dipeptidyl boronic acid proteasome inhibitor employed in the treatment of patients with relapsed multiple myeloma where the disease is refractory to conventional lines of therapy. The potency of 1 is owed primarily to the presence of the boronic acid moiety, one which is suited to establish a tetrahedral intermediate with the active site N-terminal threonine residue of the proteasome. Hence, deboronation of 1 represents a deactivation pathway for this chemotherapeutic agent. Deboronation of 1 affords a near equal mixture of diastereomeric carbinolamide metabolites (M1/M2) and represents the principal metabolic pathway observed in humans. In vitro results from human liver microsomes and human cDNA-expressed cytochrome P450 enzymes (P450) indicate a role for P450 in the deboronation of 1. Use of 18O-labeled oxygen under controlled atmospheres confirmed an oxidative mechanism in the P450-mediated deboronation of 1, as 18O was found incorporated in both M1 and M2. Chemically generated reactive oxygen species (ROS), such as those generated as byproducts during P450 catalysis, were also found to deboronate 1 resulting in the formation of M1 and M2. Known to undergo efficient redox cycling, P450 2E1 was found to catalyze the deboronation of 1 predominantly to the carbinolamide metabolites M1 and M2, as well as to a pair of peroxycarbinolamides, 2 and 3. The presence of superoxide dismutase (SOD) and catalase prevented the deboronation of 1, thus, supporting the involvement of ROS in the P450 2E1-catalyzed deboronation reaction. The presence of SOD and catalase also protected 1 against P450 3A4-catalyzed deboronation, albeit to a lesser extent. The remaining deboronation activity observed in the P450 3A4 reaction may suggest the involvement of the more conventional activated enzyme-oxidants previously described for P450. Our present findings indicate that the oxidase activity of P450 (i.e., formation of ROS) represents a mechanism of deboronation.

Boranes↗