Novel polymer-supported coupling/dehydrating reagents for use in organic synthesis

Two novel dehydrating reagents and, based on a phosphonium anhydride and an oxyphosphonium triflate respectively, were prepared by reaction of the corresponding polymer-supported phosphine oxides with triflic anhydride. Reagent, based on the novel phosphorus heterocycle 1,1,3,3-tetraphenyl-2-oxa-1,3-diphospholanium bis(trifluoromethanesulfonate), was found to be a useful reagent for ester and amide formation. A wide range of coupling/dehydration-type reactions, such as ester, amide, anhydride, peptide, ether and nitrile formation, were performed in high yield using the more readily prepared polymer-supported triphenylphosphine ditriflate, which was easily recovered and re-used several times without loss of efficiency. With primary alcohols, both reagents and provide an alternative to the Mitsunobu reaction, where the use of azodicarboxylates and chromatography to remove the phosphine oxide by-product can be avoided. The use of 4-dimethylaminopyridine allowed the esterification of secondary alcohols with to proceed in high yield but with retention of configuration.     

Reaction of 1-(2′,3′-epoxy-β-D-lyxofuranosyl)uracil with hydrogen fluoride. The unexpected formation of 1-(3′-fluoro-3′-deoxy-β-D-ribofuranosyl)uracil

Reaction of 1-(2′,3′-epoxy-β- D -lyxofuranosyl)uracil ( 1 ) with hydrogen fluoride afforded 3′-fluoro-3′-deoxyarabinouridine ( 2 , 13%) and 3′-fluoro-3′-deoxyuridine ( 3 , 11%). The structure of 3 was assigned from spectrometric data and confirmed by an unambiguous synthesis from 2′,5′-di-O-trityl-2,3′-anhydrouridine ( 5 ) and hydrogen fluoride.     

Polymerizable group 4 ansa-cyclopentadienyl-amido catalysts for the copolymerization of ethylene with 1-octene

A styrene unit has been successfully incorporated into the half metallocene constrained-geometry framework as [(η⁵-C₅Me₄)SiMe₂(η¹-NC₆H₄CHCH₂)]MX₂ (M = Ti, X = NMe₂, 6a; M = Zr, X = NMe₂, 6b; M = Ti, X = Cl, 7a; M = Zr, X = Cl, 7b). These complexes have been characterized with ¹H NMR, ¹³C NMR spectroscopy together with single crystal X-ray diffraction studies of 6a and 6b. A polystyrene-immobilized constrained-geometry catalyst 8 was formed by the radical copolymerization of 7a with styrene using AIBN as the initiator. The complexes 6a, 6b, 7a and 8 gave active homogeneous catalysts for the copolymerization of ethylene with 1-octene when treated with excess methylalumoxane (MAO). The polymerization results showed that 7a was highly active and effective for the incorporation of comonomer 1-octene whereas the zirconium complex 6b and the immobilized catalyst 8 yield low activities and low incorporations of 1-octene in the products with broad molecular weight distributions.     

ULTRAVIOLET RADIATION-INDUCED PHOSPHOLIPASE A2 ACTIVATION OCCURS IN MAMMALIAN CELL MEMBRANE PREPARATIONS

Ultraviolet erythema in human skin is mediated in part by membrane derivatives of arachidonic acid (AA). UVA (320–400nm) and UVB (290–320nm) have been shown to induce release of AA from intact mammalian cells in culture. In order to investigate the mechanism of this release we examined the effect of UVA and UVB on release of [³H] AA from membrane preparations of murine fibroblasts. C3H 10T1/2 cells were prelabelled for 24 h with [³H] AA. The membrane fractions of the cells were separated after lysis by differential centrifugation. The membranes were irradiated in suspension and the [³H] AA released from the membranes was determined by scintillation spectroscopy of supernatants3–4 h after irradiation. Both UVA and UVB induced release of AA from the membrane preparations. The response to UVB was small but significant, reaching levels approximately 150% of control release at doses of 1,200-4,000 J/m². The response to UVA was larger; doses of 2.5-5.0 J/cm² induced release equal to twice control (200%) levels, while doses of10–20 J/cm² induced maximal release at levels approximately 400% of control. Time course studies with UVB and UVA showed maximal release at 4 h after irradiation. When the membrane preparations were incubated with a polyclonal anti-phospholipase A₂ antibody the UV induced release of [³H] AA was completely inhibited in both UVB (1200 J/m²) and UVA (10 J/cm²) treated cells. These data suggest that activation of phospholipase A₂ is responsible for the UV induced release of AA in mammalian cells and that the mechanism of this activation is due, in part at least, to direct photon-membrane interaction.     

High Compression-Induced Conductivity in a Layered Cu–Br Perovskite

We show that the onset pressure for appreciable conductivity in layered copper-halide perovskites can decrease by ca. 50 GPa upon replacement of Cl with Br. Layered Cu–Cl perovskites require pressures >50 GPa to show a conductivity of 10⁻⁴ S cm⁻¹, whereas here a Cu–Br congener, (EA)₂CuBr₄ (EA=ethylammonium), exhibits conductivity as high as 2×10⁻³ S cm⁻¹ at only 2.6 GPa, and 0.17 S cm⁻¹ at 59 GPa. Substitution of higher-energy Br 4p for Cl 3p orbitals lowers the charge-transfer band gap of the perovskite by 0.9 eV. This 1.7 eV band gap decreases to 0.3 eV at 65 GPa. High-pressure X-ray diffraction, optical absorption, and transport measurements, and density functional theory calculations allow us to track compression-induced structural and electronic changes. The notable enhancement of the Br perovskite's electronic response to pressure may be attributed to more diffuse Br valence orbitals relative to Cl orbitals. This work brings the compression-induced conductivity of Cu-halide perovskites to more technologically accessible pressures.     

Three-Dimensional Collagen Gel Networks for Neural Stem Cell-Based Neural Tissue Engineering

Stem and progenitor cells isolated from the embryonic rat cerebral cortex were immobilized by matrix entrapment in three-dimensional (3D) Type I collagen gels, and cultured in serum-free medium containing basic fibroblast growth factor. The cells trapped within the collagen networks actively proliferated and formed clone-like aggregates. Neurons were the first differentiated cells to appear within the aggregates, followed by generation of astrocytes and oligodendrocytes. In addition, necrotic cores were developed as the aggregate diameter increased and cell viability declined significantly after 3 weeks in culture. To overcome these problems, the cell-collagen constructs were transferred to Rotary Wall Vessel bioreactors for up to 10 weeks. In the rotary culture, the collagen gels compacted 3-4 folds and a long-term growth and differentiation of neural stem and progenitor cells was dynamically maintained. Remarkably, the cell-collagen constructs formed a complex two-layered structure that superficially emulated to a certain extent the cerebral cortex of the embryonic brain in architecture and functionality. The engineered 3D tissue-like constructs displaying characteristic properties of neuronal circuits may have potential use in tissue replacement therapy for injured brain and spinal cord.     

ChemInform Abstract: Tetrakis(tetramethylammonium) Tricarbonatodioxidouranate Octahydrate.

Single crystals of (Me₄N)₄ [UO₂(CO₃)₃]·8H₂O are grown by slow evaporation (25 °C, 6 weeks) of an aqueous solution of uranyl 1‐hydroxyethane‐1,1‐diphosphonate and Me₄NOH.