A novel approach toward the synthesis of kendomycin: selective synthesis of a C-aryl glycoside as a single atropisomer

[reaction: see text] A convergent and concise route to an advanced precursor 2b of kendomycin (1) has been developed by applying a S(N)1 ring cyclization as a key step. The resulting C-aryl glycoside was initially isolated as a rotameric mixture, but after MOM protection of the o-hydroxyl of the phenol, the conformation was frozen to the desired kendomycin-like atropisomer.     

Olefin terpolymerizations. III. Symmetry,sterics, and monomer structure in ansa-zirconocenium catalysis of EPDM synthesis

Ethylenebis (η⁵-fluorenyl) zirconium dichloride ( 1 ) and rac-dimethylsilylene bis (1-η⁵-in-denyl) zirconium dichloride ( 2 ) were activated with methylaluminoxane (MAO) to catalyze ethylene (E) propylene (P) copolymerizations. The former produces high MW copolymer at 20°C rich in ethylene with reactivity ratio values of r_E = 1.7 and r_P <0.01, whereas the latter produces lower MW random copolymers with r_E = 1.32 and r_p = 0.36. Ethylidene norbornene (ENB) complexes with 1/MAO but does not undergo insertion in the presence of E and P. In contrast, 2/MAO catalyzes terpolymerization incorporating 9-15 mol % of ENB with slightly lower MW and activity than the corresponding copolymerizations. In comparison, 1,4–hexadiene was incorporated by 2/MAO with much lower A and MW . Terpolymerizations were also conducted with vinylcyclohexene using both catalyst systems. The steric and electronic effects in these processes were discussed. © 1995 John Wiley & Sons, Inc.     

Toward an Understanding of the Chemical Etiology for DNA Minor‐Groove Recognition by Polyamides

Crescent‐shaped polyamides composed of aromatic amino acids, i.e., 1‐methyl‐1H‐imidazole Im , 1‐methyl‐1H‐pyrrole Py , and 3‐hydroxy‐1H‐pyrrole Hp , bind in the minor groove of DNA as 2 : 1 and 1 : 1 ligand/DNA complexes. DNA‐Sequence specificity can be attributed to shape‐selective recognition and the unique corners or pairs of corners presented by each heterocycle(s) to the edges of the base pairs on the floor of the minor groove. Here we examine the relationship between heterocycle structure and DNA‐sequence specificity for a family of five‐membered aromatic amino acids. By means of quantitative DNase‐I footprinting, the recognition behavior of polyamides containing eight different aromatic amino acids, i.e., 1‐methyl‐1H‐pyrazole Pz , 1H‐pyrrole Nh , 5‐methylthiazole Nt , 4‐methylthiazole Th , 3‐methylthiophene Tn , thiophene Tp , 3‐hydroxythiophene Ht , and furan Fr , were compared with the polyamides containing the parent‐ring amino acids Py, Im , and Hp for their ability to discriminate between the four Watson? Crick base pairs in the DNA minor groove. Analysis of the data and molecular modeling showed that the geometry inherent to each heterocycle plays a significant role in the ability of polyamides to differentiate between DNA sequences. Binding appears sensitive to changes in curvature complementarity between the polyamide and DNA. The Tn / Py pair affords a modest 3‐fold discrimination of T?A vs. A?T and suggests that an S‐atom in the thiophene ring prefers to lie opposite T not A.     

Monitoring of environmental contaminants: 10 years of application of k0-I INAA

Examples of the application of k ₀ standardized instrumental neutron activation analysis (k ₀ -INAA) to aerosols and biological monitors in the last 10 years at Instituto Tecnológico e Nuclear (ITN) are given. As an analytical technique, INAA, in association with the k ₀ method was applied to these materials in four different projects, aiming at monitoring concentrations of heavy metals and others elements in the atmosphere in the Portuguese territory. In these studies we analysed the lichen Parmelia sulcata Taylor and olive tree bark as monitors as well as aerosol samples. For each project some representative results are presented, followed by a discussion of the application of this technique to environmental studies.     

Efficient method to locate double bond positions in conjugated trienes

The double bond positions of 11 conjugated trienes were unambiguously located through a simple derivatization method amenable to nanogram-scale analyses. The trienes were reacted with the powerful dienophile 4-methyl-1,2,4-triazoline-3,5-dione (MTAD), and the mass spectra of the resulting cycloadducts exhibited large diagnostic fragments which allowed the unequivocal location of the double bonds in the parent triene in most cases. Catalytic hydrogenation of the cycloadducts produced saturated compounds with characteristic mass spectral fragments from which the positions of the trienes in the parent compounds could be readily confirmed. Application of the method was demonstrated by the microscale identification of two conjugated triene and one conjugated diene components from extracts of the sex pheromone gland of the saturniid moth Automeris cecrops pamina.     

Synthesis and structural comparison of a series of divalent Ln(Tp(R,R)')2 (Ln = Sm, Eu, Yb) and trivalent Sm(Tp(Me2))2X (X = F, Cl, I, BPh4) complexes

Reaction of LnI2 (Ln = Sm, Yb) with two equivalents of NaTp(Me2) or reduction of Eu(Tp(Me2))2OTf gives good yields of the highly insoluble homoleptic Ln(II) complexes, Ln(Tp(Me2))2 (Ln = Sm (1a), Yb (2a), Eu (3a)). Use of the additionally 4-ethyl substituted Tp(Me2,4Et) ligand produces the analogous, but soluble Ln(Tp(Me2,4Et))2 (1-3b) complexes. Soluble compounds are also obtained with the Tp(Ph) and Tp(Tn) ligands (Tn = thienyl), Ln(Tp(Ph))2 (Ln = Sm, 1c; Yb, 2c) and Ln(Tp(Tn))2 (Ln = Sm, 1d; Yb, 2d). To provide benchmark parameters for structural comparison the series of Sm(Tp(Me2))2X complexes (X = F, 1e; Cl, 1f; Br, 1g; I, 1h; BPh4, 1j) were prepared either via oxidation of the Sm(Tp(Me2))2 or salt metathesis from SmX3 (X = Cl, Br, I). The solid-state structures of 1-3a, 1b, 1-2c and 1e, 1f, 1h, and 1j were determined by single-crystal X-ray diffraction. The homoleptic bis-Tp complexes are all six-coordinate with trigonal antiprismatic geometries, planes of the kappa(3)-Tp ligands are parallel to one another. In the series of Sm(Tp(Me2))2X complexes the structure changes from seven-coordinate molecular compounds, with intact Sm-X bonds, for X = F, Cl, to six-coordinate ionic structures [Sm(Tp(Me2))2]X (X = I, BPh4), suitable crystals of the bromide compound could not be obtained. The dependence of the structures on the size of X is understandable in terms of the interplay between the size of the cleft that the [Sm(Tp(Me2))2](+) fragment can make available and the donor ability of the anionic group toward the hard Sm(III) center.     

EXAFS and XANES Studies of Silica-Titania Sol-Gel Films

A series of SiO₂-TiO₂ sol-gel films with and without heat treatment were analyzed by EXAFS and XANES spectroscopies. Both techniques indicate that essentially all Ti⁴⁺ ions remain four-fold coordinated, with a Ti–O bond distance between 1.82–1.85 Å. In the glassy films produced by heat treatment at 900° C, a gradual phase separation may occur at the nanoscale, as the TiO₂ concentration of the films increases.     

The effects of ZnGa2O4 formation on structural and optical properties of ZnO:Ga powders

Gallium-doped zinc oxide (ZnO:Ga 1, 2, 3, 4 and 5 at%) samples were prepared in powder form by modifying the Pechini method. The formation of zinc gallate (ZnGa₂O₄) with the spinel crystal structure was observed even in ZnO:Ga 1 at% by X-ray diffraction. The presence of ZnGa₂O₄ in ZnO:Ga samples was also evidenced by luminescence spectroscopy through its blue emission at 430 nm, assigned to charge transfer between Ga³⁺ at regular octahedral symmetry and its surrounding O²⁻ ions. The amount of ZnGa₂O₄ increases as the dopant concentration increases, as observed by the quantitative phase analysis by the Rietveld method.     

Titanium and zirconium ketimide complexes: synthesis and ethylene polymerisation catalysis

The syntheses of ketimide titanium complexes of the type Ti(NC^tBu₂)₃X (X = Cl, Cp, Ind), Ti(NC^tBu₂)₄ and the zirconium complex CpZr(NC^tBu₂)₂Cl are described. When activated by MAO, all compounds are ethylene polymerisation catalysts. In the conditions studied, the most active catalyst is CpZr(NC^tBu₂)₂Cl, with an activity of 2.7 × 10⁵ kg/(molZr [E] h). Titanium complexes are less active by about two orders of magnitude. The polyethylene produced is linear, as determined by NMR spectroscopy. Molecular structures of Ti(NC^tBu₂)₃X (X = Cl, Cp, Ind) and Ti(NC^tBu₂)₄ were determined by X-ray single crystal diffraction.