Synthesen der Nonactinsäure

Two stereoselective syntheses of nonactic acid I, the building block of the macrotetrolide antibiotic nonactin are described. The characteristic cis-configuration of the 2,5-substituents on the tetrahydrofuran ring of I is obtained in the first synthesis by catalytic hydrogenation of the furan derivative X. This key intermediate possesses the carbon skeleton and correct distribution of oxygen functions for conversin into nonactic acid. It is synthesized by an electrophilic substitution of 2-acetonylfuran (VI) with the N-cyclohexyl-N-propenyl nitrosonium ion (V) generated from the corresponding α-chloronitrone (VII) and silver fluoroborate, followed by hydrolysis and oxidation of the aldehyde group. The second synthesis starts with a diol already having the correct configuration of the side chain that contains the hydroxyl group. For this purpose threo-1-octen-5,7-diol (XV) is synthesized from acetylacetone in two steps. Oxidative cleavage of the terminal double bond of this threo-diol yields an aldehyde which is converted by a Wittig reaction, with the carbanion, obtained from diethyl α-methoxycarbonylethyl phosphonate, into the open chain intermediate, 2-methyl-6,8-dihydroxy-2-nonenoic acid methylester (XVIII). Base-catalyzed cyclisation of this α,β-unsaturated dihydroxy ester yields the methyl ester of nonactic acid (I) as the main product.     

Structure of KNCS-complexes of nonactin

The macrotetrolide antibiotic nonactin forms a crystalline complex with KNCS. The crystals are orthorhombic, a = 20.17, b = 15.55, c = 15.33 Å, space group Pnna (D,h) with four molecules per unit cell. The crystal structure has been determined with the help of direct methods and refined by full-matrix least-squares analysis of three-dimensional diffractometer data. The K⁺ ion is surrounded by four O-atoms from tetrahydrofurane rings (K⁺ … O, 2.81–2.88 Å) and by four carbonyl O-atoms (K⁺ … O, 2.73–2.81 Å) in approximately cubic 8-coordination. The observed conformation of the 32-membered ring of the macrotetrolide has approximate S₄(4 )-symmetry and can be described as resembling the seam of a tennis ball with K⁺ at the centre of the ball and with methyl substituents and methylene groups of the tetrahydrofurane rings on the outside. The constitution and configuration of nonactin, as reported on the basis of chemical studies [10], is confirmed by the present analysis. Some implications of the structure are discussed.     

The crystal structure of nonactin

An X-ray crystal structure investigation of the macrotetrolide antibiotic nonactin is reported. The shape of the molecule is discussed with special reference to the conformational changes involved in passing from free nonactin to the K⁺-complex.     

Nonactin biosynthesis: the initial committed step is the condensation of acetate (malonate) and succinate

Nonactin is a macrotetrolide antibiotic produced by Streptomyces griseus subsp. griseus ETH A7796 that has shown activity against the P170-glycoprotein efflux pump associated with multiple drug resistant cancer cells. Nonactin is a polyketide, albeit a highly atypical one. The structure is composed of two units of each of the enantiomers of nonactic acid, arranged in a macrocycle, so that the molecule has S4 symmetry and is achiral. The monomer units, (+)- and (-)-nonactic acid, are derived from acetate, succinate, and propionate, although the exact details of the assembly process are quite unclear. We have used feeding experiments with a series of multiple stable isotope labeled precursors to elucidate the details of the first committed step of nonactic acid biosynthesis. We have found that the (13)C label from 3-ketoadipate is incorporated specifically into both nonactic acid and its homologue, homononactic acid. The data conclusively show that the first committed step of nonactin biosynthesis is the coupling of a succinate derivative with either acetate or malonate. The differentiation into either nonactate or homononactate occurs after the initial condensation; the homologues are not derived from use of a different "starter unit" by the nonactate polyketide synthase. The first step of nonactin biosynthesis involves achiral intermediates; differentiation between the known enantiocomplementary biosynthesis pathways to form each enantiomer of the precursor monomer units likely occurs after the initial condensation reaction.     

Total synthesis of a novel macrotetrolide

The total synthesis of a novel macrotetrolide, an isobutyl nonactin analog, has been achieved in 15% yield by coupling both enantiomers of the corresponding nonactic acid analogs followed by macrolactonization. These building blocks were prepared starting from β-ketoester in nine steps and 34% overall yield, in an efficient and highly stereoselective sequence. The key steps of the strategy are asymmetric hydrogenation, chelation-controlled allylation, intramolecular haloetherification of bishomoallylic ether presenting a trisubstituted double bond deactivated by an ester, and finally a stereoselective reduction of α-bromoester.     

The crystal structure of the NaNCS complex of nonactin

The NaNCS complex of the macrotetrolide antibiotic nonactin, which acts as an ionophore with biological and artificial membranes crystallizes in the space group C2/c, a = 15.55, b = 19.59, c = 15.31 Å, β = 90°. The X-ray crystal structure investigation shows that Na⁺ is coordinated by four carbonyl oxygen atoms (Na⁺ … O, 2.42 Å) and four ether oxygen atoms (Na⁺ … O, 2.77 Å). The cubic coordination by eight equidistant oxygen atoms observed in the K⁺ complex (K⁺ … O, 2.81 Å) is thus deformed. This, however, requires only very small changes in the ligand conformation.     

Der austausch des cyclopentadienylliganden im methylcyclopentadienyl-mangantricarbonyl : Darstellung und zusammensetzung der komplexe mit methyl-substituierten benzolen

The exchange of the cyclopentadienyl ring in methylcyclopentadienylmanganese(I) tricarbonyl by methy-substituted benzenes in the presence of aluminium halide leads to [arene-Mn(CO)₃]⁺-ions. The reaction conditions have been optimised; in addition to the yield the purity of the complexes has been determined in some detail. After mild decomposition the arenes were analysed by GLC. Pure [arene-Mn(CO)₃]⁺-ions can be isolated with benzene, toluene, m- and o-xylene.     

Investigation of the interaction between cobalt sulfide coatings and Ag(I) ions using cyclic voltammetry in KClO4 and NaOH aqueous solutions and X-ray photoelectron spectroscopy

Cobalt sulfide coatings have been investigated by means of cyclic voltammetry in 0.1 M KClO₄ and 0.1 M NaOH solutions and analyzed using X-ray photoelectron spectroscopy. They have been shown to contain CoS(OH), CoS and Co(OH)₂. After treating such Co sulfide coatings with AgNO₃ solution, their composition changes: both the cobalt and oxygen content decreases and Ag (up to 85 at%) appears in the coating as Ag₂S, Ag₂O and metallic Ag. Co(II) compounds react with Ag⁺ ions according to an exchange reaction [CoS+2Ag⁺+2H₂O→Ag₂S+Co(OH)₂+2H⁺]. In the course of the reaction of Co(OH)₂ with silver ions, a redox process occurs, giving metallic silver [Co(OH)₂+Ag⁺+H₂O→Ag°+Co(OH)₃+H⁺ or Co(OH)₂+Ag⁺→Ag°+CoO(OH)+H⁺]. Ag₂S reduction takes place at more positive potentials than Cu reduction; therefore sulfide layers of cobalt modified with silver ions, unlike unmodified ones, may be plated with Cu from both acid and alkaline electrolytes. Electronic Publication     

G-quadruplex-hemin DNAzyme-amplified colorimetric detection of Ag ion

A G-quadruplex-hemin DNAzyme-amplified Ag⁺-sensing method was developed based on the ability of Ag⁺ to stabilize C-C mismatches by forming C-Ag⁺-C base pairs. In this method, only one unlabelled oligonucleotide strand was used. In the absence of Ag⁺, the oligonucleotide strand formed an intramolecular duplex. The G-rich sequence in the oligonucleotide was partially caged in this duplex structure and cannot fold into the G-quadruplex structure. The addition of Ag⁺ promoted the formation of another intramolecular duplex in which C-C mismatches were stabilized by C-Ag⁺-C base pairs, leading to the release of the G-rich sequence which can fold into a G-quadruplex capable to bind hemin to form a catalytically active G-quadruplex-hemin DNAzyme. As a result, a UV-vis absorbance increasing was observed in the H₂O₂-ABTS (2,2′-azinobis(3-ethylbenzothiozoline)-6-sulfonic acid) reaction system. This “turn-on” process allowed the detection of aqueous Ag⁺ at concentrations as low as 6.3 nM using a simple colorimetric technique, showing a high selectivity over a range of other metal ions.     

In situ photoreduction of Ag+-ions by TiO2 nanoparticles deposited on cotton and cotton/PET fabrics

The possibility of in situ photoreduction of Ag⁺-ions using TiO₂ nanoparticles deposited on cotton and cotton/PET fabrics in the presence of amino acid alanine and methyl alcohol has been discussed. The possible interaction between TiO₂, alanine and Ag⁺-ions was evaluated by FTIR analysis. The fabrication of TiO₂/Ag nanoparticles on both fabrics was confirmed by SEM, EDX, XRD, XPS and AAS analyses. Cotton and cotton/PET fabrics impregnated with TiO₂/Ag nanoparticles provided maximum reduction of Gram-negative bacteria Escherichia coli and Gram-positive bacteria Staphylococcus aureus. Although excellent antibacterial activity was preserved after ten washing cycles, a significant amount of silver leached out from the fabrics into the washing bath. The perspiration fastness assessment revealed that smaller amounts of silver were also released from the fabrics into artificial sweat at pH 5.50 and 8.00. In addition, deposited TiO₂/Ag nanoparticles imparted maximum UV protection to fabrics.     

Self-Assembled Multivalent Ag-SR Coordination Polymers with Phosphatase-Like Activity

We show herein the phosphatase-like catalytic activity of coordination polymers obtained after adding Ag⁺-ions to thiols bearing hydrophobic alkyl chains terminated with a 1,4,7-triazacyclononane (TACN) group. The subsequent addition of Zn²⁺ -ions to the self-assembled polymers resulted in the formation of multivalent metal coordination polymers capable of catalysing the transphosphorylation of an RNA-model compound (2-hydroxypropyl-4-nitrophenyl phosphate, HPNPP) with high reactivity. Analysis of a series of metal ions showed that the highest catalytic activity was obtained when Ag⁺-ions were used as the first metal ions to construct the backbone of the coordination polymer through interaction with the -SH group followed by Zn²⁺-ions as the second metal ions complexed by the TACN-macrocycle. Furthermore, it was demonstrated that the catalytic activity could be modulated by changing the length of the hydrophobic alkyl chain.     

Oscillations in the belousov-zhabotinskii system (BZR) catalysed by bis-bipyridine-silver complexes

A new catalytic ion couple [Ag(bipy)₂]⁺/[Ag(bipy)₂]²⁺ was found to catalyze the Belousov-Zhabotinskii reaction between bromate and malonic acid in sulfuric acid solution. Because of the insolubility of [Ag(bipy)₂]⁺ salts the reaction is not homogeneous. Oscillations could be observed to redox potential, the potentials of Ag⁺ and Br^? ion sensitive electrodes, and in light scattering caused by the periodic precipitation and dissolution of Ag^I complex.     

Characterization of [peptide+(Ag)n]+ complexes using matrix-assisted laser desorption/ionization time-of-flight mass spectrometry

Silver ion complexes of peptides [M + (Ag)_n]⁺, M = angiotensin I or substance P where n = 1–8 and 17–23 for angiotensin I and n = 1–5 for substance P, are identified and characterized using matrix‐assisted laser desorption/ionization time‐of‐flight mass spectrometry (MALDI‐TOFMS). The Ag⁺ coordination number exceeds the number of available amino acid residues in angiotensin I whereas the number of observed complexes in substance P is less than the number of amino acid residues in it. The larger coordination number of angiotensin I with Ag⁺ indicates the simultaneous binding of several Ag⁺ ions to the amino acid residue present in it. The lower number of observed complexes in substance P suggests the binding of two or more residues to one Ag⁺ ion. The presence of trifluoroacetic acid in the peptide samples reduces the Ag⁺ coordination ability in both the peptides which indicates that the basic residues in it are already protonated and do not participate in the Ag⁺‐binding process. The Ag⁺ ion also forms a complex with the α‐cyano‐4‐hydroxycinnamic acid (CHCA) matrix and is observed in the MALDI mass spectra and the formation of [CHCA + Ag]⁺, [CHCA + AgNO₃]⁺ and [(CHCA)₂ + Ag]⁺ ions is due to the high binding affinity of Ag⁺ to the CN group of CHCA. Copyright © 2010 John Wiley & Sons, Ltd.     

Visible Light-induced Oxygen Generation and Cyclic Water Cleavage Sensitized by Porphyrins

Zinctetramethylpyridylporphyrin (ZnTMPyP⁴⁺) in acidic aqueous solution sensitizes efficiently oxygen generation by visible light in the presence of acceptors such as Fe³⁺ - and Ag⁺-ions and colloidal RuO₂/TiO₂ redox catalyst. Hydrogen and oxygen are cogenerated under visible light illumination of ZnTMPyP⁴⁺ solutions when a bifunctional catalyst (Pt and RuO₂ codeposited onto TiO₂) is employed.     

Sodium mycophenolate

The title compound, sodium 6‐(1,3‐di­hydro‐4‐hydroxy‐6‐methoxy‐7‐methyl‐3‐oxoisobenzo­furan‐5‐yl)‐4‐methyl­hex‐4‐enoate, Na⁺·C₁₇H₁₉O₆^?, is the sodium salt of the natural immunosuppressant compound mycophenolic acid. It consists of a phthalide moiety carrying four different substituents on the aromatic ring. The anion has no intramolecular hydrogen bonds, but a very strong intermolecular hydrogen bond links the phenolic hydroxy group to the carboxyl group of a neighbouring anion found in the same layer. Within a distance of 2.71 Å, the Na⁺ ion is surrounded by five O atoms from four different anions, forming a distorted square pyramid. This Na—O network forms an infinite two‐dimensional system running parallel to the bc plane.     

Promotion of photocatalytic steam reforming of methane over Ag~0/Ag~+-SrTiO_3

Methane(CH_4) is not only used as a fuel but also as a promising clean energy source for hydrogen generation.The steam reforming of CH4(SRM) using photocatalysts can realize the production of syngas(CO+H_2) with low energy consumption.In this work,Ag~0/Ag~+-loaded SrTi03 nanocomposites were successfully prepared through a photodeposition method.When the loading amount of Ag is 0.5 mol%,the atom ratio of Ag~+ to Ag~0 was found to be 51:49.In this case,a synergistic effect of Ago and Ag~+ was observed,in which Ago was proposed to improve the adsorption of H_2 O to produce hydroxyl radicals and enhance the utilization of light energy as well as the separation of charge carriers.Meanwhile,Ag~0 was regarded as the reduction reaction site with the function of an electron trapping agent.In addition,Ag~+adsorbed the CH4 molecules and acted as the oxidation reaction sites in the process of photocatalytic SRM to further promote electron-hole separation.As a result,0.5 mol% Ag-SrTi03 exhibited enhancement of photocatalytic activity for SRM with the highest CO production rate of 4.3 μmol g~(-1) h~(-1),which is ca.5 times higher than that of pure SrTi03.This work provides a facile route to fabricate nanocomposite with cocatalyst featuring different functions in promoting photocatalytic activity for SRM.     

High-Fidelity,Narcissistic Self-Sorting in the Synthesis of Organometallic Assemblies from Poly-NHC Ligands

Highly selective, narcissistic self-sorting has been observed in the one-pot synthesis of three organometallic molecular cylinders of type [M₃{L-(NHC)₃}₂](PF₆)₃ (M=Ag⁺, Au⁺; L=1,3,5-benzene, triphenylamine, or 1,3,5-triphenylbenzene) from L-(NHC)₃ and silver(I) or gold(I) ions. The molecular cylinders contain only one type of tris-NHC ligand with no crossover products detectable. Transmetalation of the tris-NHC ligands from Ag⁺ to Au⁺ in a one-pot reaction with retention of the supramolecular structures is also demonstrated. High-fidelity self-sorting was also observed in the one-pot reaction of benzene-bridged tris-NHC and tetrakis-NHC ligands with Ag₂O. This study for the first time extends narcissistic self-sorting in metal–ligand interactions from Werner-type complexes to organometallic derivatives.     

Syntheses and Crystal Structures of Copper and Silver Complexes with the Ylide Ph3PCHP(O)PPh2 as Ligand

The reactivity of the hydrolysis product of hexaphenylcarbodiphosphorane, PPh₃CHP(O)Ph₂, towards different soft Lewis acids, such as CuI and Ag[BF₄] are reported. While CuI exclusively binds at the ylidic carbon atom, reaction of the silver cation in CH₂Cl₂ leads to proton abstraction from the solvent to give the cation [PPh₃CH₂P(O)Ph₂]⁺. Surprisingly, Ag⁺ replaces the methyl group of [PPh₃CHMeP(O)Ph₂]⁺ to produce a dimeric complex, in which Ag⁺ is coordinated to C and O forming an eight membered ring. The compounds were characterized by spectroscopic methods and X‐ray diffraction.     

Ag Clusters as Active Species for HC-SCR Over Ag-Zeolites

The addition of hydrogen in the reaction atmosphere is effective in promoting the activity of Ag/alumina and Ag-zeolites on the selective reduction of NO by hydrocarbons (HC-SCR) at low temperatures. The increment of NO conversion over Ag-MFI corresponds to the periodic addition of hydrogen into C₃H₈-SCR conditions. The UV–VIS spectra of Ag-MFI have revealed that the addition of hydrogen results in the formation of Ag_n^δ+ clusters due to partial reduction and agglomeration of Ag species. The coincidence of the formation of the Ag_n^δ+ clusters and the increment of NO conversion suggests that Ag_n^δ+ clusters are the highly active species for HC-SCR. From analysis by H₂-TPR, UV–VIS, and EXAFS, the structure of Ag_n^δ+ clusters on Ag-MFI is identified as being Ag₄²⁺ on average. The formation of Ag clusters was strongly affected by the type of zeolites: The major Ag species are Ag⁺ ions for MOR, Ag_n^δ+ clusters for MFI and BEA, and relatively large metallic Ag_mparticles for Y. The sequence of Ag agglomeration (MOR < MFI < BEA < Y) is in accordance with the strength of the acid sites of zeolites. It can be expected that the interaction between the positive charge of Ag_n^δ+ clusters and acid sites, i.e., the ion-exchange site of zeolites, stabilizes Ag_n^δ+ clusters. The type of Ag species under HC-SCR conditions depends on the concentration of gas-phase oxidants (NO, O₂) and reductants (H₂, HC), and also on the number and strength of the zeolite acid sites.     

LiZr4(PO4)3的Na/Li及Ag/Li离子交换

研究了LiZr2(PO4)3在水溶液中的Na/Li和Ag/Li离子交换行为.结果表明,LiZr2(PO4)3对Na+和Ag+离子均具有很高的选择性,且对Ag+的选择性高于Na+.LiZr2(PO4)3与Ag+的离子交换反应是通过形成固溶体的形式进行的,而与Na+的离子交换反应则是通过置换进行的.温度升高有利于提高LiZr2(PO4)3上Na/Li和Ag/Li的离子交换反应速度.