Factors affecting the endo : exo ratio in diels-alder reactions of cyclopentadiene

The ratio of endo-CHO:exo-CHO in the Diels-Alder addition of a trans-αβ-unsaturated aldehyde to cyclopentadiene can be changed from 1:2 to 8:1, depending on temperature and BF₃-catalysis.     

Metal Coordination Properties of a Chromophoric Desferrioxamine (DFO) Derivative: Insight on the Coordination Stoichiometry and Thermodynamic Stability of Zr4+ Complexes

Desferrioxamine (DFO) is the current “gold standard” chelator for ⁸⁹Zr⁴⁺, which is used to label monoclonal antibodies for applications in immunopositron emission tomography. Recently, controversial data have been reported regarding the speciation and the stability of the complexes formed by DFO with Zr⁴⁺ in solution. To shed some light on this point, we studied the coordination properties in solution ofa chromophoric DFO derivative bearing a substituted pyrimidine residue (DFO–Pm) toward several metal ions (Zr⁴⁺, Cu²⁺, Zn²⁺, Mg²⁺, Ca²⁺, Na⁺, K⁺). Potentiometric titrations showed that DFO–Pm and pristine DFO form complexes with very similar stoichiometry and stability. DFO–Pm, which can consequently be taken as a model system for DFO, provides a photochemical response to metal coordination that can be used to further define the complexes formed. In the critical case of Zr⁴⁺, spectrophotometric measurements allowed the verification of the formation of 1:1 and 2:3 complexes that, together with 2:2 complexes form the coordination model that was obtained through the use of our potentiometric measurements. Additionally, mass spectrometry measurements verified the formation of 1:1 and 2:3 complexes and showed that 1:2 species can be easily generated through the fragmentation of the 2:3 species. In conclusion, the results obtained with DFO–Pm validate the complexation model of Zr⁴⁺/DFO composed of 1:1, 2:2, and 2:3 metal-to-ligand complexes. Convergences and conflicts with other works are addressed.     

A new graphical method for determining stability constants of weak and polynuclear complexes

A graphical method is proposed for differentiating mono- and poly-nuclear complexes as well as for determining the stability constants of weak complexes. The method is based on the effect of dilution on the degree of dissociation of the complex. The precision for log K is ±0.02 for a degree of dissociation between 30 and 70%. The greatest values for log K that can be determined are 4.3, 12.4, 7.5 and 10.7 for 1:1, 2:2, 2:1 and 3:1 ligand: metal complexes, respectively. If log K₁ ? 5.7, the method permits log K₂ to be determined if simultaneous complexes are formed.     

Simultaneous determination of histamine and spermidine by second-derivative synchronous fluorescence spectrometry

The method is based on formation of the fluorescent condensation products with o-phthaldialdehyde; 0.5–2000 ng ml^?1 histamine and 3–700 ng ml^?1 spermidine can be quantified, with relative standard deviations of 2–3%. Histamine/spermidine ratios of 2.5:1–1:30 can be handled. A selectivity study is reported.     

Electrocatalytic performance of poly(o-phenylenediamine)-Pt–Ru nanocomposite for methanol oxidation

The electrocatalytic properties of poly(o-phenylenediamine) (PoPD)-Pt–Ru nanocomposite electrode for methanol oxidation have been investigated by linear sweep and cyclic voltammetry. The Pt–Ru ion concentration ratio in the electrodeposition bath is varied in the ratios of 1:0.25, 1:0.5, 1:1, 1:2, and 1:4. The morphology and particle size of the nanocomposites are obtained from the scanning electron microscopy data. The onset potential for the oxidation of methanol is found to be effectively reduced by 220 mV for the (PoPD)-Pt–Ru (1:1) nanocomposite compared to PoPD-Pt electrode. Also, the PoPD-Pt–Ru (1:1) composite shows a value of 4.2 for the ratio of forward to reverse peak current which is relatively a high value that can be observed among the conducting polymer-based catalysts used for methanol oxidation. The results are substantiated by the polarization and stability data.     

Inhibitory Effect of Salvia miltiorrhiza Extract and Its Active Components on Cervical Intraepithelial Neoplastic Cells

The effective treatment of cervical intraepithelial neoplasia (CIN) can prevent cervical cancer. Salvia miltiorrhiza is a medicinal and health-promoting plant. To identify a potential treatment for CIN, the effect of S. miltiorrhiza extract and its active components on immortalized cervical epithelial cells was studied in vitro. The H8 cell was used as a CIN model. We found that S. miltiorrhiza extract effectively inhibited H8 cells through the CCK8 method. An HPLC–MS analysis revealed that S. miltiorrhiza extract contained salvianolic acid H, salvianolic acid A, salvianolic acid B, monomethyl lithospermate, 9‴-methyl lithospermate B, and 9‴-methyl lithospermate B/isomer. Salvianolic acid A had the best inhibitory effect on H8 cells with an IC₅₀ value of 5.74 ± 0.63 μM. We also found that the combination of salvianolic acid A and oxysophoridine had a synergistic inhibitory effect on H8 cells at molar ratios of 4:1, 2:1, 1:1, 1:2, and 1:4, with salvianolic acid A/oxysophoridine = 1:2 having the best synergistic effect. Using Hoechst33342, flow cytometry, and Western blotting analysis, we found that the combination of salvianolic acid A and oxysophoridine can induce programmed apoptosis of H8 cells and block the cell cycle in the G2/M phase, which was correlated with decreased cyclinB1 and CDK1 protein levels. In conclusion, S. miltiorrhiza extract can inhibit the growth of H8 cells, and the combination of salvianolic acid A (its active component) and oxysophoridine has a synergistic inhibitory effect on H8 cells and may be a potential treatment for cervical intraepithelial neoplasia.     

The micro-determination of gold with p-dimethyl-aminobenzylidenerhodanine

Gold can be determined in the range 4–14 μg within 2% by a simple extraction of chlorauric acid in 0.12 N hydrochloric acid with a solution of p-dimethylaminobenzylidenerhodanine in iso-amyl acetate A 1:1 complex is formed with maximum absorption at 515 mμ.     

Extraction and spectrophotometric determination of vanadium(V) with ferron

Various metal complexes of 7-iodo-8-hydroxyquinoline-5-sulphonic acid (ferron) were found to be selectively extracted into immiscible alcohols. Vanadium(V) is almost completely extracted into n-butanol in a single extraction from solutions which are 0.05 M in sulphuric acid. A sensitive and selective spectrophotometric method can be based on this extraction. Beer's law is obeyed up to 15.3 μg of vanadium per ml; the sensitivity of the color reaction is 0.011 μg of vanadium per cm² at 430 mμ. The interference of iron(III) can be eliminated by adding excess pyrophosphate. The extracted species appears to contain vanadium, ferron and n-butanol in the ratio 1:2:4.     

Quantum chemical study of complexation of trimethylaluminum with chlorine-containing solvents

Complexation of trimethylaluminum with chlorine-containing organic solvents has been studied in an MP2/6-31G(d, tp) approximation. Trimethylaluminum can form complexes with dichloromethane and dichloroethane. For 1:1 and 2:1 complexes, the geometrical and thermodynamic parameters have been determined. For 2:1 complexes, two orientations of trimethylaluminum relative to the chloroalkane molecule are most favorable. Thermodynamic parameters of dimerization and complexation reactions have been studied for reactions of trimethylaluminum with dichloromethane and dichloroethane. For trimethylaluminum, dimerization was found to be preferable to complexation.     

Static atomic displacements of Sn in disordered NiAs/Ni2In type HT-Ni1+δSn

The crystal structures of the intermediate solid solution HT (high temperature) Ni_1+δSn with δ=0.28, 0.52 and 0.61 (refined Ni contents) have been analyzed in detail by X-ray diffraction on single crystals. The previously reported basic atomic arrangement, i.e., a NiAs/Ni₂In structure type (P6₃/mmc, Ni(1) on 2a, 0 0 0, Ni(2) with an occupancy δ on 2d, and Sn on 2c, ), is confirmed. However, strong anisotropic atomic displacements occur for Sn within the a-b plane of the hexagonal unit cell, which require a Gram-Charlier expansion of the probability density function of Sn in order to obtain a good fit to the diffraction data. Direction, magnitude and the concentration dependence of the displacements can be interpreted in terms of the geometrical requirements of the different local atomic configurations in the planes z=±1/4, so that the displacements can be identified as static ones.     

An amide orthoesterification route to N-(1′-alkylthioglucopyranosyl)indoles

The addition of 3-methylindolylmagnesium bromide to tetra-O-benzyl-α-d-gluconothionolactone yields the expected indole N-gluconothioamide as its hemiorthothioamide tautomer. The thiol function is alkylated to yield the corresponding orthothioamide, a 1′-alkylthio-substituted N-glycoside. Alternatively, the 1′-alkylthio-N-glycoside can be accessed from the corresponding indole N-gluconamide via a boron trifluoride-etherate mediated orthoesterification with ethanethiol. Radical reduction of the orthothioamide yields the N-glycosides in 2:1 stereoselectivity in favor of the β-N-glycoside, while reduction via the oxonium ion leads to an improved 6:1 selectivity.     

Polarographic determination of the stability constants of metal complexes based on the shifts in half-wave or peak potentials of the anodic oxidation of mercury: methylthioacetato- and 2,2′-thiobisacetato complexes

The stability constants of metal-ion complexes, other than those of mercury ions, can be determined from the shifts, caused by an excess of that metal ion, in the anodic waves of mercury oxidation in the presence of the ligand.Let a solution contain L, a ligand that can react with mercury ion from the anodic oxidation of mercury to form the complexes HgL_p. The half-wave potential of the anodic wave will be shifted by the presence of an excess of a metal ion M, which can also react with L forming the complexes M_jL (where j = 1, $\text{1}\text{2}$,…1/N). A general expression is derived relating the half-wave potential shift ΔE_{$\text{1}\text{2}$} to the overall stability constants. If the excess of metal ion M is great enough for the 1:1 complex to predominate in solution (and if HgL₂ is the predominating mercury complex), for reversible, diffusion-controlled processes the general equation can be simplified to:

where c_M is the total concentration of metal ion M.This equation provides an easy and fast method for β₁ determination. The simplified equation is best suited for experimental data obtained from techniques such as DP, AC₁ and AC₂ polarography, whose increased precision in ΔE measurement enables poorly-developed dc-polarographic anodic waves to be used for β₁ determination. Since the metal-to-ligand concentration ratio must be large in order to apply the simplified equation, the determination can be carried out at very small ligand concentrations. This fact renders the new method especially useful when the ligand solubility does not allow the high ligand concentrations needed in the DeFord—Hume method to be reached. The adequacy of the deduced equations when applied to polarographic processes which are irreversible or not controlled by diffusion is discussed.The simplified equation is tested using several metal complexes of methylthioacetate and 2,2′-thiobisacetate ligands and comparing some of the values obtained from experimentally measured ΔE's with known β₁ values taken from the literature. Good agreement is found. For the U(VI)—methylthioacetate complex, which has not been previously reported in the literature, log β₁ = 1.75 ± 0.1.     

Sodium: Dibenzo-30-crown-10 aggregates in solution: Na-23 NMR study

(n + 1):n sodium:dibenzo-30-crown-10 complexes are formed in nitromethane solutions. A model based on the coexistence in solution of 1:1, 2:1 and 3:2 complexes can account for the observed Na-23 chemical shifts, transverse and longitudinal relaxation rates. Logarithms of the equilibrium constants of formation at (25±1)°C are (2.1±0.3) and (2.5±0.3) for the 2:1 and 3:2 complexes respectively. The characteristic Na-23 relaxation rates and chemical shifts of the aggregated complexes are compatible with a structure of ion pairs linked by the crown ether.     

New acetylated flavone C-glycosides from Iris lactea

Chemical investigation of the aerial parts of Iris lactea afforded three new flavone C-glycosides including 4?-O-acetyl-embinin (1), 2?,4?-O-diacetyl-embinin (2) and 6″,4?-O-diacetyl-embinin (3) along with the known analogue embinin (4). Their structures were elucidated by 1D and 2D NMR spectroscopic analysis as well as by HRESIMS data. The sugars were characterized following acid hydrolysis of the respective glycosides and TLC analysis compared to known standards. Duplicated signals can be observed in the NMR spectra, indicating the presence of rotamers caused by rotational hindrance around the glycosyl-flavone CC linkage. All isolated compounds were tested for their antimicrobial and cytotoxic activities but found to be inactive.     

Elaboration of the ether cleaving ability and selectivity of the classical Pearlman's catalyst [Pd(OH)2/C]: concise synthesis of a precursor for a myo-inositol pyrophosphate

The cleavage of propargyl, allyl, benzyl, and PMB ethers by Pd(OH)₂/C can be tuned in that order, by varying the reaction conditions. Other moieties such as C-C double bonds, esters, trityl ether, p-bromo and p-nitrobenzyl ethers are stable to these reaction conditions. Cleavage of allyl ethers can be made catalytic by using 1:1 mixture of Pd(OH)₂/C and Pd/C. The synthetic potential of the selective ether cleaving ability of Pd(OH)₂/C, essentially under neutral conditions, has been demonstrated by an efficient synthesis of a precursor for the preparation of an inositol pyrophosphate derivative.     

Regge pole analysis of differential cross sections for elastic molecular scattering

The angle and energy dependence of differential cross sections σ(E, θ) that exhibit diffraction oscillations is analyzed by a Regge representation. It is shown that: (1) the Regge poles can be estimated directly from the measured σ(E, θ); (2) a fixed-energy Regge pole calculation can reproduce the E-dependence of σ(E, θ).     

A potentiometric study of the water-soluble silver(I) complexes of n,n'-bis(2-hydroxyethyl)dithiooxamide : Part III. Determination of the different species

Silver(I) and N,N'-bis(2-hydroxyethyl)dithiooxamide (H₂X) in acidic solutions, form a series of water-soluble polynuclear complexes. These complexes can be represented by two general formulae: H_2qAg_pX_q and H_2q-1Ag_pX_q with q=1,2,3 and p=q, q-1.     

Bis(benzoato)-bis(π-cyclopentadienyl)-titan(IV): Darstellung in wässriger lösung und struktur einer neuen modifikation

The reaction of titanocene dichloride with sodium benzoate in aqueous solution gives titanocene dibenzoate in high yield. Crystals of a modification different from the one described recently by Hoffman et al. can be obtained by recrystallization from chloroform: orthorhombic, space group P2₁2₁2₁, Z = 4 and lattice constants at − 120°C a 21.154(4), b 12.529(4), c 7.574(2) Å. Whereas the TiO bond for one of the benzoate groups is a normal σ-bond, the other one seems to have a considerable additional π-bond character.     

Toward supramolecular polymers incorporating double cavity cucurbituril hosts

We describe the synthesis of a series of guests (1-6) containing two adamantylammonium ions separated by xylylene spacing groups and their complexation properties toward double cavity cucurbituril host bis-ns-CB[10]. We observed the preferential formation of 1:1, 2:2, and oligomeric complexes rather than the desired n:n supramolecular polymers. Guest 7, which contains a longer biphenyl spacer successfully precludes the formation of the 1:1 complex but results in the formation of the 2:2 complex (bis-ns-CB[10]₂·7₂) rather than supramolecular polymer. Guest 8, which contains adamantylammonium, p-xylylene diammonium, and hexanediammonium ion binding regions is shown to reversibly form 2:2 and 1:2 complexes (bis-ns-CB[10]₂·8₂ and bis-ns-CB[10]·8₂) in response to changes in host:guest stoichiometry. Lastly, this equilibrium can be manipulated by the addition of exogenous CB[6], which selectively targets the hexanediammonium ion binding region of 8 and delivers the penta-molecular complex bis-ns-CB[10]·8₂·CB[6]₂.