Reduction of 1-Deoxy -13–oxotaxanes

Numerous reports have described the semisynthesis of paclitaxel and its anologues from different taxoids 1. However, to the best of our knowledge almost all taxoids have a C-1-hydroxyl. Heretofore, the semisynthesis from taxinine, an abundant natural taxoid lacking of C-1-hydroxyl, has only reached the point of C-13 ketone intermediate 2. We have also encountered difficulties in reducing C-13 ketone to corresponding α- hydroxy group using the same conditions as in total synthesis of paclita…     

Hydrogen splitting by pyramidalized 13–15 donor–acceptor cryptands: A computational study

A series of new donor–acceptor cryptands, where pyramidalized donor (azaadamantane) and acceptor (bora/ala/adamantane) molecules are spatially oriented toward each other and linked via aromatic spacer, are constructed and computationally studied at M06-2X and ωB97X-D levels of theory. Kinetic stability of the perfluorinated bora- and ala-adamantane with respect to F migration to group 13 element is demonstrated. The effectiveness of the constructed cryptands, featuring pyramidalized perfluorinated acceptor moieties, in the heterolytic splitting of molecular hydrogen is predicted. Hydrogen splitting is highly exothermic and exergonic and is accompanied by small activation barriers. The most promising candidates for the experimental studies are identified. © 2019 Wiley Periodicals, Inc.     

Lewis acid stabilized group 13–15 element analogs of ethylene

Stabilization of hydrogen-substituted group 13–15 compounds H₂EE′H₂ (E = B, Al, Ga; E′ = P, As, Sb) by Lewis acids is considered at B3LYP/def2-TZVP, B3LYP-D3/def2-TZVP and M06-2X/def2-TZVP levels of theory. It is shown, that for many Lewis acids additional reactivity beyond the DA complex formation with H₂EE′H₂ monomer is expected. In case of complexation with E(C₆F₅)₃, F/H exchange reactions with group 13 bound hydrides are predicted to be exothermic and accompanied by the activation energies which are smaller than dissociation of the complex into components. In case of complex formation with transition metal (TM) carbonyls, additional O → Al, TM–C → Al interactions are observed, which in several cases lead to cyclic structures. The most promising candidates for the experimental studies have been identified. Synthetic approaches to the most promising LA-only stabilized compounds are recommended.     

Low-temperature dehydrogenation reaction–separation membranes using zeolite 13X polyethylacrylate

We prepared blend and pseudo-interpenetrating network (PIPN) membranes containing zeolite 13X with the dehydrogenation catalyst (Ti or Ni) and polyethylacrylate (PEA). Membranes were tested by a single cell reactor using the dehydrogenation reaction of cyclohexane to benzene at low temperatures (323–360 K). Proton nuclear magnetic resonance (NMR) was used to analyze the composition of the throughput and the feed in our experiments. Using 2 wt% of Ti or Ni on 40 wt% of zeolite with 60 wt% PEA the conversion ratio of benzene to cyclohexane was about 4–8%. The feed did not contain measurable amounts of benzene, hence the separation was complete. Scanning electron microscopy (SEM) results showed that the blend membrane developed cracks considerably earlier than the PIPN membrane. We also find the pseudo-first-order rate constant and its activation energy for the dehydrogenation reaction in the membrane by using a phenomenological diffusion–reaction model. The latter values are consistent with surface diffusion control of the kinetics.     

Synthesis of the C1–C13 fragment of eribulin mesylate

Synthesis of the C1–C13 fragment of eribulin mesylate has been accomplished. It features a highly stereoselective construction of a trans-dihydropyran framework using three key reactions: (1) Sharpless epoxidation, (2) regioselective ring opening, and (3) olefin metathesis.     

From Primary Silylphosphanes and Arsanes to Novel Group 13–15 Clusters

Abstract

The reaction of the primary silylphosphanes and -arsanes 1 with [LiAlH₄] leads. under evolution of H2 and depending on the solvent, to the different unusual clusters 2 and 3. Compound 2 has a rhombododecahedral skeleton. Surprisingly, the same reaction of the starting materials in DME instead of Et₂O as solvent hrnishes the triple ion pair 3.     

Sequence determination and resonance assignments of an Azomonas siderophore using 13C natural abundance 13C–1H HNCA experiment

The determination of the amino acid sequence of small peptides as siderophores is commonly performed using long-range HMBC correlation experiments. However, this type of experiment can lead to ambiguities in the assignment resulting in erroneous primary sequence determination. In this paper, we propose to use an HNCA type experiment on a ¹⁵N-labelled sample in order to avoid these ambiguities. This experiment was successfully used to determine the primary sequence of azoverdin, a siderophore produced by Azomonas.     

NEW POLYHETEROATOMIC CYCLIC MOLECULES CONTAINING ELEMENTS of the 13.–16. GROUP

During studies of the reactions of ─N(H)SiMe ₃ and ─N(Me)SiMe ₃ derivatives of Cl ₃ PNSO ₂ Cl with acetonitrile and BCl ₃ we have obtained six-membered polyheteroatomic cycles ?P(Cl ₂ )NSO ₂ (Cl)N(H) C(Me)N? and ?P(Cl ₂ )NS(O)(Cl)OB(Cl ₂ )N(Me)?.^{1, 2} In the system Ph ₂ PCl ₃ , H ₂ NSO ₂ Cl and HN(SiMe ₃ ) ₂ we have identified and isolated several P─N─S cycles, e.g. the reaction of Ph ₂ PCl ₃ with H ₂ NSO ₂ Cl gives Ph ₂ ClPNSO ₂ Cl³ which with HN(SiMe ₃ ) ₂ reacts to ?S(O ₂ )N(H)P(Ph) ₂ N(H)SO ₂ N(H)P(Ph) ₂ N(H)?, ?S(O ₂ )N(H)S(O ₂ )N(H)P(Ph) ₂ N(H)P(Ph) ₂ N(H)? and ?[S(O ₂ )N(H) P(Ph) ₂ NP(Ph) ₂ N(H)]⁺? Cl^?; Ph ₂ PCl ₃ with HN(SiMe ₃ ) ₂ gives N[P(Ph) ₂ N(H)SiMe ₃ ] ₂ ⁺ Cl^?, and H ₂ NSO ₂ Cl with HN(SiMe ₃ ) ₂ leads to SO ₂ (NHSiMe ₃ ) ₂ . The reaction of Ph ₂ PCl ₃ with HN(SiMe ₃ ) ₂ gives N(P(Ph) ₂ NHSiMe ₃ ) ₂ Cl in a very good yield which was further used to syntheses of metal-containing heterocycles. By the reaction of N[P(Ph) ₂ N(H)SiMe ₃ ] ₂ ⁺Cl^? with some covalent halogenides we have obtained six-membered heterocycles containing B, As, In, and Sn. The same cyclic compounds can also obtained by the reaction of N[P(Ph ₂ )NH ₂ ] ₂ ⁺Cl^? or HN(P(R ₂ )N(H)SiMe ₃ ) ₂ with covalent halogenides.^4?6 However, the synthetic route via N[P(Ph) ₂ NHSiMe ₃ ] ₂ ⁺Cl^? is more convenient and gives the compounds in almost quantitative yields. The identity of all compounds was unambiguously establised by their X-ray structure determination.     

Performance of CdTe detector in the 13–1333 keV energy range

The photopeak efficiency, peak to valley ratio and energy resolution of a 3×3×1 mm³ CdTe detector were determined experimentally for 13–1333 keV photon energy by using polyester coated radioisotopes ²⁴¹Am, ²²Na, ⁵⁴Mn, ⁵⁷Co, ⁶⁰Co, ¹⁰⁹Cd, ¹³⁷Cs and ¹³³Ba. The data were analyzed by using PX4 from Amptek. The experimental values were fitted to an analytical function of photon energy, and an agreement was observed for the entire range of the studied energies. The results have shown that the CdTe detector has a high performance due to both the improved charge collection efficiency comparable with that of SI GaAs detectors (Vittone et al., 1999), and the low leakage current. Also, CdTe detector is very attractive for field application as it works at room temperature.     

Heat capacity and thermodynamic functions of YVO4 in the 13–347 K region

The heat capacity of YVO₄ was measured by adiabatic calorimetry in the region of 13.11–347.14 K. The values of thermodynamic functions (the entropy, enthalpy change and reduced Gibbs function) were calculated using smoothed heat capacity values. The value of the Gibbs energy of formation from simple compounds was calculated.     

Triad sequence determination of ethylene–propylene copolymers – application of quantitative 13C NMR

A methodology for acquiring fully quantitative ¹³C{¹H} NMR spectra for high performance ethylene–propylene copolymers has been proposed. To minimize the spectral acquisition time without sacrificing spectral quality, different amounts of chromium(III)acetylacetonate relaxation agent has been added to optimize its concentration. The study demonstrates the critical setting of delay time for determining six triad distributions from eight discrete set of resonances which otherwise leads to inaccurate determination of triad concentrations. It allows precise integral measurements of low intensity resonances depending on copolymer composition, and significant reduction of experimental time.     

Synthesis and Properties of The Solid Solutions Formed in The Fe2V4O13–Cr2V4O13 System

DTA and XRD studies of the Fe₂V₄O₁₃–Cr₂V₄ O₁₃ system have shown that continuous solid solutions of a Fe_2–xCr_xV₄O₁₃ type, bearing a Fe₂ V₄ O₁₃ structure, are formed in the system. With the increasing degree of the Cr3+ ion incorporation into the Fe₂ V₄ O₁₃ structure, a contraction of the solid solution crystal lattice develops. Solid solutions of a Fe_2–x Cr_x V₄ O₁₃ type melt incongruently, their melting temperature increasing from 953 to 1003 K with increase in the degree of the Cr³⁺ ion incorporation. The solid product of melting Fe_2–x Cr_x V₄ O₁₃ solid solutions for 0.2<x >1.2 is the Fe_1–x Cr_x VO₄ solution phase, and for x ≤0.2 and x ≥1.4 – the Fe_1–x Cr_x VO₄ phase as well as FeVO₄ or CrVO₄ , respectively. This revised version was published online in July 2006 with corrections to the Cover Date.     

Full 1H and 13C NMR spectral assignment of conjugated valerolactone metabolites isolated from urine of black tea consumers by means of SPE-prepLC–MS–LC–MS-NMR

The health benefits of black tea have been linked to polyphenol metabolites that target specific modes of action in the human body. A major bottleneck in unravelling the underlying mechanisms is the preparative isolation of these metabolites, which hampers their structural elucidation and assessment of in vitro bioactivity. A solid phase extraction (SPE)-preparative liquid chromatography (prepLC)–MS–LC–MS-NMR workflow was implemented for preparative isolation of conjugated valerolactone metabolites of catechin-based polyphenols from urine of black tea consumers. First, the urine was cleaned and preconcentrated using an SPE method. Subsequently, the clean urine concentrate was injected on a preparative LC column, and conjugated valerolactones were obtained by MS-guided collection. Reconstituted fractions were further separated on an analytical LC column, and valerolactone fractions were collected in an MS-guided manner. These were reconstituted in methanol-d₄ and identified and quantified using 1D and 2D homo- and hetereonuclear NMR experiments (at a field strength of 14.1 T), in combination with mass spectrometry. This resulted in the full spectral ¹H and ¹³C NMR assignments of five conjugated valerolactones. These metabolites were collected in quantities of 8–160 μg and purities of 70–91%. The SPE-prepLC–MS–LC–MS-NMR workflow is suitable for isolating metabolites that occur at sub-μM concentrations in a complex biofluid such as urine. The workflow also provides an alternative for cumbersome and expensive de novo synthesis of tea metabolites for testing in bioactivity assays or for use as authentic analytical standards for quantification by mass spectrometry.     

Structural characterization of drawn (ethylene–vinyl alcohol) copolymer by high-resolution solid-state 13C NMR

High-resolution solid-state ¹³C NMR spectra of drawn (ethylene–vinyl alcohol) copolymer (EVOH) and their ¹³C spin-lattice relaxation times (T₁) were measured in order to elucidate the structure and dynamics of EVOH in the drawn state. It is found that the ¹³C chemical shift of the CH carbon for drawn EVOH moved upfield as compared with that of undrawn EVOH. Further, the mobility for the CH carbons of low molecular motion components are increased by drawing the EVOH sample. From these experimental results, the changes in the structure and dynamics of the drawn EVOH sample can be elucidated.     

Structural Composition of Butadiene–Styrene Copolymers,According to 1Н and 13С NMR Data

Russian Journal of Applied Chemistry - Commercial samples of butadiene–styrene copolymers were studied by NMR spectroscopy in a magnetic field of 16.4 T (protons Larmor frequency 700 MHz). A...     

Electrochemical reactivity of Li–Mn–O and Li–Fe–Mn–O spinels

Electrochemical reductive dissolution of Li–Mn–O and Li–Fe–Mn–O spinels and Li⁺ extraction/insertion in these oxides were performed using voltammetry of microparticles. Both electrochemical reactions are sensitive to the Fe/(Fe+Mn) ratio, specific surface area, Li content in tetrahedral positions, and Mn valence, and can be used for electrochemical analysis of the homogeneity of the elemental and phase composition of synthetic samples. The peak potential (E _P) of the reductive dissolution of the Li–Mn–O spinel is directly proportional to the logarithm of the specific surface area. E _P of Li–Fe–Mn–O spinels is mainly controlled by the Fe/(Fe+Mn) ratio. Li⁺ insertion/extraction can be performed with Mn-rich Li–Fe–Mn–O spinels in aqueous solution under an ambient atmosphere and it is sensitive to the regularity of the spinel structure, in particularly to the amount of Li in tetrahedral positions and the Mn valence. Electronic Publication     

Bubble points of hydrogen chloride–water–isopropanol and hydrogen chloride–water–isopropanol–benzene systems and liquid–liquid equilibria of hydrogen chloride–water–benzene and hydrogen chloride–water–isopropanol–benzene systems

Bubble points of the HCl–water–isopropanol and the HCl–water–isopropanol–benzene systems and liquid–liquid equilibria (LLE) of the HCl–water–benzene and the HCl–water– isopropanol–benzene systems were measured at 25–85°C and 30–70°C, respectively. The electrolyte nonrandom two-liquid model proposed by Chen et al. [C.-C. Chen, H.I. Britt, J.F. Boston, L.B. Evans, AIChE J. 28 (1982) 588–596] can satisfactorily correlate bubble points and liquid–liquid equilibria of the present mixed-solvent electrolyte systems over the entire range of temperature and concentrations using only binary adjustable parameters.     

On hydrolysis of Ba–Bi–O and K–Ba–Bi–O oxide systems

Fundamentally different behavior of Ba–Bi–O (Ba : Bi = 11 : 4, 1 : 1, 2: 3, and 1 : 5 mol/mol) and K_nBa_mBi_m+nO_y (m = n = 1, 2,...; exhibiting superconducting properties with T_c = 28–32 K) oxides and BaO₂ in hydrolysis reactions has been revealed by means of potentiometry and chemical analysis. Products of the oxides treatment with water do not contain H₂O₂, evidencing the absence of peroxide ions in their structure. The perovskite-type barium-bismuth(III) oxides are completely hydrolyzed into Ba(OH)₂ and Bi₂O₃ at room temperature.     

Determination of 13 endocrine disrupting chemicals in sediments by gas chromatography–mass spectrometry using subcritical water extraction coupled with dispersed liquid–liquid microextraction and derivatization

In this study, a sample pretreatment method was developed for the determination of 13 endocrine disrupting chemicals (EDCs) in sediment samples based on the combination of subcritical water extraction (SWE) and dispersed liquid–liquid microextraction (DLLME). The subcritical water that provided by accelerated solvent extractor (ASE) was the sample solution (water) for the following DLLME and the soluble organic modifier that spiked in the subcritical water was also used as the disperser solvent for DLLME in succession. Thus, several important parameters that affected both SWE and DLLME were investigated, such as the extraction solvent for DLLME (chlorobenzene), extraction time for DLLME (30 s), selection of organic modifier for SWE (acetone), volume of organic modifier (10%) and extraction temperature for SWE (150 °C). In addition, good chromatographic behavior was achieved for GC–MS after derivatisation by using N,O-bis(trimethylsilyl) trifluoroacetamide (BSTFA). As a result, proposed method sensitive and reliable with the limits of detection (LODs) ranging from 0.006 ng g⁻¹ (BPA) to 0.639 ng g⁻¹ (19-norethisterone) and the relative standard deviations (RSDs) between 1.5% (E2) and 15.0% (DES). Moreover, the proposed method was compared with direct ASE extraction that reported previously, and the results showed that SWE–DLLME was more promising with recoveries ranging from 42.3% (dienestrol) to 131.3% (4,5α-dihydrotestosterone), except for diethylstilbestrol (15.0%) and nonylphenols (29.8%). The proposed method was then successfully applied to determine 13 EDCs sediment of Humen outlet of the Pearl River, 12 of target compounds could be detected, and 10 could be quantitative analysis with the total concentration being 39.6 ng g⁻¹, and which indicated that the sediment of Humen outlet was heavily contaminated by EDCs.