Zinc Ammonio-dodecaborates: Synthesis, Lewis Acid Strength, and Reactivity.

Two series of zinc salts, [EtZn][A] and Zn[A]2, with weakly coordinating anions [A]- as counterions have been prepared, and their activities as catalysts for hydrosilylation reactions of 1-hexene, benzophenone, and acetophenone have been investigated. The counterions and per- and partially chlorinated 1-ammonio-closo-dodecaborate anions [Me3NB12Cl11]- [1]-, [Pr3NB12H5Cl6]- [2]-, [Bu3NB12H4Cl7]- [3]-, and [Hex3NB12H5Cl6]- [4]- were chosen as potential and more readily available alternatives to carborate anions such as [CHB11Cl11]- and [HexCB11Cl11]-. The basicity of anion [4]- was determined as being close to that of the triflimide anion [N(SO2CF3)2]-, and the fluoride ion affinities (FIAs) of compounds [EtZn][2] and Zn[2]2 are lower than those of the Lewis acids B(C6F5)3 and Zn[HexCB11Cl11]2. The higher anion basicity and the resulting lower Lewis acidity of the zinc centers result in low activity in 1-hexene hydrosilylation catalysis and only moderate activity in the hydrosilylation catalysis of benzophenone and acetophenone.

Rapid Enantioselective and Diastereoconvergent Hybrid Organic/Biocatalytic Entry into the Oseltamivir Core.

A formal synthesis of the antiviral drug (-)-oseltamivir (Tamiflu) has been accomplished starting from m-anisic acid via a dissolving metal or electrochemical Birch reduction. The correct absolute stereochemistry is efficiently set through enzyme-catalyzed carbonyl reduction on the resultant racemic α,ß-unsaturated ketone. A screen of a broad ketoreductase (KRED) library identified several that deliver the desired allylic alcohol with nearly perfect facial selectivity at the new center for each antipodal substrate, indicating that the enzyme also is able to completely override inherent diastereomeric bias in the substrate. Conversion is complete, with d-glucose serving as the terminal hydride donor (glucose dehydrogenase). For each resulting diastereomeric secondary alcohol, O/N-interconversion is then efficiently effected either by synfacial [3,3]-sigmatropic allylic imidate rearrangement or by direct, stereoinverting N-Mitsunobu chemistry. Both stereochemical outcomes have been confirmed crystallographically. The α,ß-unsaturation is then introduced via an α-phenylselenylation/oxidation/pyrolysis sequence to yield the targeted (S)-N-acyl-protected 5-amino-1,3-cyclohexadiene carboxylates, key advanced intermediates for oseltamivir pioneered by Corey (N-Boc) and Trost (N-phthalamido), respectively.

Synthesis and Characterization of Cu(II) and Mixed-Valence Cu(I)Cu(II) Clusters Supported by Pyridylamide Ligands.

A group of copper complexes supported by polydentate pyridylamide ligands H2bpda and H2ppda were synthesized and characterized. The two Cu(II) dimers [CuII2(Hbpda)2(ClO4)2] (1) and [CuII2(ppda)2(DMF)2] (2) were constructed by using neutral ligands to react with Cu(II) salts. Although the dimers showed similar structural features, the second-sphere interactions affect the structures differently. With the application of Et3N, the tetranuclear cluster (HNEt3)[CuII4(bpda)2(µ3-OH)2(ClO4)(DMF)3](ClO4)2 (3) and hexanuclear cluster (HNEt3)2[CuII6(ppda)6(H2O)2(CH3OH)2](ClO4)2 (4) were prepared under similar reaction conditions. The symmetrical and unsymmetrical arrangement of the ligand donors in ligands H2bpda and H2ppda led to the dramatic conformation difference of the two Cu(II) complexes. As part of our effort to explore mixed-valence copper chemistry, the triple-decker pentanuclear cluster [CuII3CuI2(bpda)3(µ3-O)] (5) was prepared. XPS examination demonstrated the localized mixed-valence properties of complex 5. Magnetic studies of the clusters with EPR evidence showed either weak ferromagnetic or antiferromagnetic interactions among copper centers. Due to the trigonal-planar conformation of the trinuclear Cu(II) motif with the µ3-O center, complex 5 exhibits geometric spin frustration and engages in antisymmetric exchange interactions. DFT calculations were also performed to better interpret spectroscopic evidence and understand the electronic structures, especially the mixed-valence nature of complex 5.

Triple-Negative Breast Cancer Cells Exhibit Differential Sensitivity to Cardenolides from Calotropis gigantea.

Triple-negative breast cancers (TNBC) are aggressive and heterogeneous cancers that lack targeted therapies. We implemented a screening program to identify new leads for subgroups of TNBC using diverse cell lines with different molecular drivers. Through this program, we identified an extract from Calotropis gigantea that caused selective cytotoxicity in BT-549 cells as compared to four other TNBC cell lines. Bioassay-guided fractionation of the BT-549 selective extract yielded nine cardenolides responsible for the selective activity. These included eight known cardenolides and a new cardenolide glycoside. Structure-activity relationships among the cardenolides demonstrated a correlation between their relative potencies toward BT-549 cells and Na+/K+ ATPase inhibition. Calotropin, the compound with the highest degree of selectivity for BT-549 cells, increased intracellular Ca2+ in sensitive cells to a greater extent than in the resistant MDA-MB-231 cells. Further studies identified a second TNBC cell line, Hs578T, that is also highly sensitive to the cardenolides, and mechanistic studies were conducted to identify commonalities among the sensitive cell lines. Experiments showed that both cardenolide-sensitive cell lines expressed higher mRNA levels of the Na+/Ca2+ exchanger NCX1 than resistant TNBC cells. This suggests that NCX1 could be a biomarker to identify TNBC patients that might benefit from the clinical administration of a cardiac glycoside for anticancer indications.

Convenient Access to Gallium(I) Cations through Hydrogen Elimination from Cationic Gallium(III) Hydrides.

Although gallium hydrides XnGaH3-n (X = monoanionic substituent) are usually stable compounds, cationic arene-solvated species [H2Ga(arene)2]+ spontaneously eliminate dihydrogen at room temperature to afford the arene-solvated gallium(I) compounds [Ga(PhF)2][CHB11Cl11] (1) and [Ga(Ph3CH)][B(C6F5)4] (3). A key requirement appears to be the presence of a weakly coordinating anion. Use of the more basic triflimide anion, [NTf2]-, reverses the stability, i.e., the gallium(III) hydride H2GaNTf2 (4) is more stable than the gallium(I) compound GaNTf2 (5). The experimental results are supported by DFT calculations. Compounds 1 and 3 can be used as catalysts for the oligomerization of 2,4,4-trimethyl-1-pentene and the hydrosilylation of benzophenone and 1-hexene.

Not Limited to Iron: A Cobalt Heme-NO Model Facilitates N-N Coupling with External NO in the Presence of a Lewis Acid to Generate N2 O.

Some bacterial heme proteins catalyze the coupling of two NO molecules to generate N2 O. We previously reported that a heme Fe-NO model engages in this N-N bond-forming reaction with NO. We now demonstrate that (OEP)CoII (NO) similarly reacts with 1 equiv of NO in the presence of the Lewis acids BX3 (X=F, C6 F5 ) to generate N2 O. DFT calculations support retention of the CoII oxidation state for the experimentally observed adduct (OEP)CoII (NO⋅BF3 ), the presumed hyponitrite intermediate (P.+ )CoII (ONNO⋅BF3 ), and the porphyrin π-radical cation by-product of this reaction, and that the π-radical cation formation likely occurs at the hyponitrite stage. In contrast, the Fe analogue undergoes a ferrous-to-ferric oxidation state conversion during this reaction. Our work shows that cobalt hemes are chemically competent to engage in the NO-to-N2 O conversion reaction.

Lewis Acid Activation of the Ferrous Heme-NO Fragment toward the N-N Coupling Reaction with NO To Generate N2O.

Bacterial NO reductase (bacNOR) enzymes utilize a heme/non-heme active site to couple two NO molecules to N2O. We show that BF3 coordination to the nitrosyl O-atom in (OEP)Fe(NO) activates it toward N-N bond formation with NO to generate N2O. 15N-isotopic labeling reveals a reversible nitrosyl exchange reaction and follow-up N-O bond cleavage in the N2O formation step. Other Lewis acids (B(C6F5)3 and K+) also promote the NO coupling reaction with (OEP)Fe(NO). These results, complemented by DFT calculations, provide experimental support for the cis: b3 pathway in bacNOR.

Carbon-Nitrogen and Nitrogen-Nitrogen Bond Formation from Nucleophilic Attack at Coordinated Nitrosyls in Fe and Ru Heme Models.

The conversion of inorganic NOx species to organo-N compounds is an important component of the global N-cycle. Reaction of a C-based nucleophile, namely the phenyl anion, with the ferric heme nitrosyl [(OEP)Fe(NO)(5-MeIm)]+ generates a mixture of the C-nitroso derivative (OEP)Fe(PhNO)(5-MeIm) and (OEP)Fe(Ph). The related reaction with [(OEP)Ru(NO)(5-MeIm)]+ generates the (OEP)Ru(PhNO)(5-MeIm) product. Reactions with the N-based nucleophile diethylamide results in the formation of free diethylnitrosamine, whereas the reaction with azide results in N2O formation; these products derive from attack of the nucleophiles on the bound NO groups. These results provide the first demonstrations of C-N and N-N bond formation from attack of C-based and N-based nucleophiles on synthetic ferric-NO hemes.

Opportunistic Sampling of Roadkill as an Entry Point to Accessing Natural Products Assembled by Bacteria Associated with Non-anthropoidal Mammalian Microbiomes.

Few secondary metabolites have been reported from mammalian microbiome bacteria despite the large numbers of diverse taxa that inhabit warm-blooded higher vertebrates. As a means to investigate natural products from these microorganisms, an opportunistic sampling protocol was developed, which focused on exploring bacteria isolated from roadkill mammals. This initiative was made possible through the establishment of a newly created discovery pipeline, which couples laser ablation electrospray ionization mass spectrometry (LAESIMS) with bioassay testing, to target biologically active metabolites from microbiome-associated bacteria. To illustrate this process, this report focuses on samples obtained from the ear of a roadkill opossum (Dideiphis virginiana) as the source of two bacterial isolates (Pseudomonas sp. and Serratia sp.) that produced several new and known cyclic lipodepsipeptides (viscosin and serrawettins, respectively). These natural products inhibited biofilm formation by the human pathogenic yeast Candida albicans at concentrations well below those required to inhibit yeast viability. Phylogenetic analysis of 16S rRNA gene sequence libraries revealed the presence of diverse microbial communities associated with different sites throughout the opossum carcass. A putative biosynthetic pathway responsible for the production of the new serrawettin analogues was identified by sequencing the genome of the Serratia sp. isolate. This study provides a functional roadmap to carrying out the systematic investigation of the genomic, microbiological, and chemical parameters related to the production of natural products made by bacteria associated with non-anthropoidal mammalian microbiomes. Discoveries emerging from these studies are anticipated to provide a working framework for efforts aimed at augmenting microbiomes to deliver beneficial natural products to a host.

Hydride Attack on a Coordinated Ferric Nitrosyl: Experimental and DFT Evidence for the Formation of a Heme Model-HNO Derivative.

Heme-HNO species are crucial intermediates in several biological processes. To date, no well-defined Fe heme-HNO model compounds have been reported. Hydride attack on the cationic ferric [(OEP)Fe(NO)(5-MeIm)]OTf (OEP = octaethylporphyrinato dianion) generates an Fe-HNO product that has been characterized by IR and (1)H NMR spectroscopy. Results of DFT calculations reveal a direct attack of the hydride on the N atom of the coordinated ferric nitrosyl.

A bridged di-iron porphyrin hyponitrite complex as a model for biological N2O production from hyponitrite.

Heme-hyponitrites are intermediates that form at the bimetallic active sites of bacterial nitric oxide reductases. To probe a possible effect of the Fe-Fe distance on hyponitrite stability, we prepared a bridged bis-porphyrin Fe-hyponitrite compound, namely [(OEP-CH2)Fe]2(µ2,η(1),η(1)-ONNO). Its υNO of 992 cm(-1) (υ15NO of 976 cm(-1)) is close to the υNO of 983 cm(-1) reported previously by us for the crystallographically characterized [(OEP)Fe]2(µ2,η(1),η(1)-ONNO) compound. The bridged bis-porphyrin Fe-hyponitrite complex is unstable with respect to N2O production, supporting the role of the bis-Fe porphyrin system in hyponitrite conversion to N2O. The preparation and crystallographic determination of the bridging sulfato derivative is also reported.

Chlorination of 1-Carba-closo-dodecaborate and 1-Ammonio-closo-dodecaborate Anions.

Fully chlorinated carborate and dodecaborate cages such as [CHB11Cl11]- and [Me3NB12Cl11]- are prominent examples of valuable and chemically rather inert weakly coordinating anions. While both anions can be obtained by chlorination of the precursors [CH12B11]- and [H3NB12H11]- with SO2Cl2 followed by methylation for the synthesis of [Me3NB12Cl11]-, best results were found using photochemical chlorination with SO2Cl2 for [CH12B11]- and thermal chlorination with SO2Cl2 for [H3NB12H11]-. The hexachlorinated anion [n-Pr3NB12H5Cl6]- was formed readily within 30 min by chlorination of [n-Pr3NB12H11]-, but attempts to synthesize isopropyl-substituted ammonio-dodecaborates with a higher chlorination number resulted in the formation of mixtures and partial decomposition. The silver and trityl salts of the anions [CHB11Cl11]-, [Me3NB12Cl11]-, and [n-Pr3NB12H5Cl6]- as well as the contact ion-pair [Et2Al][Me3NB12Cl11] were also prepared, and the compounds [Ag(NCMe)][Me3NB12Cl11], [Et2Al][Me3NB12Cl11], and [Et4N][i-Pr3NB12H5Cl6] were also characterized by X-ray crystallography.

Synthesis and Characterization of Copper Complexes with CuICuI, Cu1.5Cu1.5m and CuIICuII Core Structures Supported by a Flexible Dipyridylamide Ligand.

A series of copper complexes supported by a simple dipyridylamide ligand (H2pcp) were isolated and characterized. Treatment of H2pcp with NaH and copper(I) salts led to the formation of [Cu2(2pcp)2] (1a) and {Na[(Cu2(2pcp)2)2]PF6}n (1b). The X-ray crystal structures of both complexes feature CuICuI cores with close Cu···Cu interactions. Electrochemical studies of 1a showed a reversible one-electron oxidation wave in CH2Cl2. On the basis of the work on 1a, we began studying the mixed-valence copper species supported by this ligand. The reaction of H2pcp with Cu(OAc)2 and CuCl in different stoichiometries yielded [Cu2(2pcp)2Cl] (2) and [Cu3(2pcp)2Cl2] (3). X-ray crystallography and spectroscopic characterization suggested delocalized Cu1.5Cu1.5 core structures of both compounds. These results further inspired us to explore the coordination properties of H2pcp toward CuII ions. The complexes [HNEt3][Cu2(2pcp)3(ClO4)](ClO4) (4a), [Cu2(2pcp)3(NO3)] (4b), and [Cu2(2pcp)3(H2O)]BF4 (4c) featuring dinuclear CuIICuII cores were prepared and characterized by X-ray crystallography and spectroscopic methods. Structural analysis of these complexes implied that the accommodation of CuICuI, Cu1.5Cu1.5, and CuIICuII is attributed to the structural flexibility of the ligand H2pcp. Complexes 1a, 2, 3, and 4a were examined by X-ray photoelectron spectroscopy, which confirmed the oxidation state assignments. Computational studies were also performed to provide insight into the electronic structures of these complexes.

Identification of Compounds with Efficacy against Malaria Parasites from Common North American Plants.

Some of the most valuable antimalarial compounds, including quinine and artemisinin, originated from plants. While these drugs have served important roles over many years for the treatment of malaria, drug resistance has become a widespread problem. Therefore, a critical need exists to identify new compounds that have efficacy against drug-resistant malaria strains. In the current study, extracts prepared from plants readily obtained from local sources were screened for activity against Plasmodium falciparum. Bioassay-guided fractionation was used to identify 18 compounds from five plant species. These compounds included eight lupane triterpenes (1-8), four kaempferol 3-O-rhamnosides (10-13), four kaempferol 3-O-glucosides (14-17), and the known compounds amentoflavone and knipholone. These compounds were tested for their efficacy against multi-drug-resistant malaria parasites and counterscreened against HeLa cells to measure their antimalarial selectivity. Most notably, one of the new lupane triterpenes (3) isolated from the supercritical extract of Buxus sempervirens, the common boxwood, showed activity against both drug-sensitive and -resistant malaria strains at a concentration that was 75-fold more selective for the drug-resistant malaria parasites as compared to HeLa cells. This study demonstrates that new antimalarial compounds with efficacy against drug-resistant strains can be identified from native and introduced plant species in the United States, which traditionally have received scant investigation compared to more heavily explored tropical and semitropical botanical resources from around the world.

Texas Native Plants Yield Compounds with Cytotoxic Activities against Prostate Cancer Cells.

There remains a critical need for more effective therapies for the treatment of late-stage and metastatic prostate cancers. Three Texas native plants yielded three new and three known compounds with antiproliferative and cytotoxic activities against prostate cancer cells with IC50 values in the range of 1.7-35.0 µM. A new sesquiterpene named espadalide (1), isolated from Gochnatia hypoleuca, had low micromolar potency and was highly effective in clonogenic assays. Two known bioactive germacranolides (2 and 3) were additionally isolated from G. hypoleuca. Dalea frutescens yielded two new isoprenylated chalcones, named sanjuanolide (4) and sanjoseolide (5), and the known sesquiterpenediol verbesindiol (6) was isolated from Verbesina virginica. Mechanistic studies showed that 1-4 caused G2/M accumulation and the formation of abnormal mitotic spindles. Tubulin polymerization assays revealed that 4 increased the initial rate of tubulin polymerization, but did not change total tubulin polymer levels, and 1-3 had no effects on tubulin polymerization. Despite its cytotoxic activity, compound 6 did not initiate changes in cell cycle distribution and has a mechanism of action different from the other compounds. This study demonstrates that new compounds with significant biological activities germane to unmet oncological needs can be isolated from Texas native plants.

Synthesis and reactivity of indium(I) 1-carba-closo-undecachlorododecaborate.

The arene-solvated indium(I) species [In(C7H8)3][CHB11Cl11] (1) and [In(C6H5Br)1.5][CHB11Cl11] (2) were obtained by a redox reaction involving the silver salt Ag[CHB11Cl11] and indium powder at 80 °C in a toluene or bromobenzene solution. These thermally stable compounds react with triphenylphosphine and the N-heterocyclic carbene 1,3-bis(2,6-diisopropylphenyl)imidazol-2-ylidene under reduction of indium(I) to indium metal and oxidation of the ligands to phosphonium and imidazolium cations contrary to the more commonly observed disproportionation reactions. The presence of 2 equiv of carbene led to deprotonation of the anion to give the dianion [CB11Cl11](2-). Interactions of In(+) with soft donor ligands such as phosphines, olefins, alkynes, and aromatics are weak, and a crystalline solid was only obtained with the nonvolatile phosphinoacetylene Mes2PC≡CPh (Mes = 2,4,6-Me3C6H2). The structure of this compound displays In···C interactions involving the triple bond and the π system of one mesityl group but no In···P contact. Solutions of 2 in fluorobenzene also showed moderate activity as the catalyst for intramolecular hydroamination of primary and secondary aminopentenes. The new compounds were characterized by multinuclear NMR spectroscopy and X-ray diffraction for compounds 1, 2, and 4-6.

Experimental and theoretical studies on halide binding with a p-xylyl-based azamacrocycle.

A p-xylyl-based macrocycle L has been synthesized and its binding properties with halides have been investigated by (1)H NMR titrations, single crystal X-ray diffraction analysis, and density functional theory (DFT) calculations. As investigated by (1)H NMR titrations, the ligand preferentially binds a halide in a 1:2 binding mode, with the association constants (in log K2) of 2.82, 2.70, 2.28, and 2.20 for fluoride, chloride, bromide, and iodide, respectively. The overall binding trend was found to be in the order of fluoride > chloride > bromide > iodide, reflecting that the binding strength correlates with the relative basicity and size of the respective halide. Crystallographic studies indicate that the ligand forms 1:2 complexes with chloride, bromide and iodide. In the chloride complex, the ligand is hexaprotonated and each chloride is held via three NH···Cl(-) bonds. The ligand is tetraprotonated for the other complexes, where each halide is H-bonded to two secondary ammonium NH(+) groups via NH···X(-) bonds. The results of DFT calculations performed on [H6L](6+) at M062x/6-311G (d,p) level in both gas and solvent phases, suggest that the ligand binds halides with the binding energy in the order of F(-) > Cl(-) > Br(-) > I(-), supporting the experimental data obtained from (1)H NMR studies. Results from DFT calculations further indicate that a 1:2 binding is energetically more favorable than a 1:1 binding of the ligand.

Synthesis, molecular structure, and spectroelectrochemistry of a nitrosyl iron porphyrin containing an unsymmetrical xanthene-linked porphyrin core.

Synthetic nitrosyl porphyrins with meso-aryl substituents are potential models for the biologically-important NO-bound P460 heme cofactor. A five-coordinate iron nitrosyl tetraaryl-porphyrin (HTPPX-CO2H)Fe(NO) containing a xanthene-based meso substituent has been prepared. The crystal structure of this formally {FeNO}7 complex reveals an ordered axial and bent NO ligand (∠FeNO=142.5(6)Å) displaying an off-axis tilt of the nitrosyl N atom from the heme normal by 9.2°. Surprisingly, the porphyrin core does not display the expected asymmetry in FeN(por) distances frequently observed in iron nitrosyl porphyrins. The redox behavior as determined by cyclic voltammetry reveals, in contrast to most (por)Fe(NO) compounds, a fast NO dissociation after electrooxidation in CH2Cl2 to result in a net chemically-irreversible oxidation at Epa=+0.77V vs Ag/AgCl. IR spectroelectrochemistry reveals a recombination, on the spectroelectrochemistry time-scale, of the dissociated NO on oxidation with electrogenerated [(HTPPX-CO2H)Fe]+.

Encapsulation and selectivity of sulfate with a furan-based hexaazamacrocyclic receptor in water.

A furan-based hexaazamacrocycle encapsulates a sulfate anion in its cavity showing strong affinity and selectivity for sulfate in water over a wide range of inorganic anions. The DFT calculations demonstrate that the receptor provides binding sites as hydrogen bonding donors and electrostatic positive charges for the strong binding of sulfate.

Polyketide glycosides from Bionectria ochroleuca inhibit Candida albicans biofilm formation.

One of the challenges presented by Candida infections is that many of the isolates encountered in the clinic produce biofilms, which can decrease these pathogens' susceptibilities to standard-of-care antibiotic therapies. Inhibitors of fungal biofilm formation offer a potential solution to counteracting some of the problems associated with Candida infections. A screening campaign utilizing samples from our fungal extract library revealed that a Bionectria ochroleuca isolate cultured on Cheerios breakfast cereal produced metabolites that blocked the in vitro formation of Candida albicans biofilms. A scale-up culture of the fungus was undertaken using mycobags (also known as mushroom bags or spawn bags), which afforded four known [TMC-151s C-F (1-4)] and three new [bionectriols B-D (5-7)] polyketide glycosides. All seven metabolites exhibited potent biofilm inhibition against C. albicans SC5314, as well as exerted synergistic antifungal activities in combination with amphotericin B. In this report, we describe the structure determination of the new metabolites, as well as compare the secondary metabolome profiles of fungi grown in flasks and mycobags. These studies demonstrate that mycobags offer a useful alternative to flask-based cultures for the preparative production of fungal secondary metabolites.