Copper-catalyzed oxaziridine-mediated oxidation of C-h bonds

The highly regio- and chemoselective oxidation of activated C-H bonds has been observed via copper-catalyzed reactions of oxaziridines. The oxidation proceeded with a variety of substrates, primarily comprising allylic and benzylic examples, as well as one example of an otherwise unactivated tertiary C-H bond. The mechanism of the reaction is proposed to involve single-electron transfer to the oxaziridines to generate a copper-bound radical anion, followed by hydrogen atom abstraction and collapse to products, with regeneration of the catalyst by a final single-electron transfer event. The involvement of allylic radical intermediates was supported by a radical-trapping experiment with TEMPO.     

Alternative Approach for Specific Tyrosinase Inhibitor Screening: Uncompetitive Inhibition of Tyrosinase by Moringa oleifera

Tyrosinase (TYR) is a type III copper oxidase present in fungi, plants and animals. The inhibitor of human TYR plays a vital role in pharmaceutical and cosmetic fields by preventing synthesis of melanin in the skin. To search for an effective TYR inhibitor from various plant extracts, a kinetic study of TYR inhibition was performed with mushroom TYR. Among Panax ginseng, Alpinia galanga, Vitis vinifera and Moringa oleifera, the extracts of V. vinifera seed, A. galanga rhizome and M. oleifera leaf reversibly inhibited TYR diphenolase activity with IC₅₀ values of 94.8 ± 0.2 µg/mL, 105.4 ± 0.2 µg/mL and 121.3 ± 0.4 µg/mL, respectively. Under the same conditions, the IC₅₀ values of the representative TYR inhibitors of ascorbic acid and kojic acid were found at 235.7 ± 1.0 and 192.3 ± 0.4 µg/mL, respectively. An inhibition kinetics study demonstrated mixed-type inhibition of TYR diphenolase by A. galanga and V. vinifera, whereas a rare uncompetitive inhibition pattern was found from M. oleifera with an inhibition constant of K_ii 73 µg/mL. Phytochemical investigation by HPLC-MS proposed luteolin as a specific TYR diphenolase ES complex inhibitor, which was confirmed by the inhibition kinetics of luteolin. The results clearly showed that studying TYR inhibition kinetics with plant extract mixtures can be utilized for the screening of specific TYR inhibitors.     

Studies on the activation energy from the ac conductivity measurements of rubber ferrite composites containing manganese zinc ferrite

Manganese zinc ferrites (MZF) have resistivities between 0.01 and 10 Ω m. Making composite materials of ferrites with either natural rubber or plastics will modify the electrical properties of ferrites. The moldability and flexibility of these composites find wide use in industrial and other scientific applications. Mixed ferrites belonging to the series Mn_(1−x)Zn_xFe₂O₄ were synthesized for different ‘x’ values in steps of 0.2, and incorporated in natural rubber matrix (RFC). From the dielectric measurements of the ceramic manganese zinc ferrite and rubber ferrite composites, ac conductivity and activation energy were evaluated. A program was developed with the aid of the LabVIEW package to automate the measurements. The ac conductivity of RFC was then correlated with that of the magnetic filler and matrix by a mixture equation which helps to tailor properties of these composites.     

Simple non-perturbative solution for MHD viscous flow due to a shrinking sheet

The magnetohydrodynamic (MHD) viscous flow due to a shrinking sheet is examined analytically. The series solution is obtained using the Adomian decomposition method (ADM) coupled with Padé approximants to handle the condition at infinity. The numerical solutions agree very well with the results by the homotopy analysis method.     

On solving the chaotic Chen system: a new time marching design for the variational iteration method using Adomian’s polynomial

This paper centres on the effectiveness of the variational iteration method and its modifications for numerically solving the chaotic Chen system, which is a three-dimensional system of ODEs with quadratic nonlinearities. This research implements the multistage variational iteration method with an emphasis on the new multistage hybrid of variational iteration method with Adomian polynomials. Numerical comparisons are made between the multistage variational iteration method, the multistage variational iteration method using the Adomian’s polynomials and the classic fourth-order Runge-Kutta method. Our work shows that the new multistage hybrid provides good accuracy and efficiency with a performance that surpasses that of the multistage variational iteration method.     

Reactivity of α-chloro-aldimines

A series of secondary N-1-(2-chloroalkylidene)amines has been prepared by condensation of disubstituted acetaldehydes with primary amines followed by chlorination with N-chlorosuccinimide in carbontetrachloride. A study of the reactivity of these N-homologues of α-chloroaldehydes is described. Treatment of the title compounds with sodium methoxide in methanol gave high yields of α,β-unsaturated aldimines. However, N-1-(2-chloro-2-methylpropylidene)amines afforded a mixture of elimination and rearrangement products, which proceeded via an aziridine intermediate. On the other hand, α-phenyl-substituted α-chloro aldimines on treatment with methoxide in methanol underwent α-substitution, consistent with an S_N1 mechanism. Powerful nucleophiles such as sodium thiophenolate in methanol and sodium azide in acetone caused α-substitution. Reaction of α-chloro aldimines with Grignard reagents produced coupling of two aldimine units or α-alkylation. Finally the reactivity of α-chloro aldimines was compared with the reactivity of the corresponding oxygen-analogues, i.e. α-chloro aldehydes.     

Synthesis and spectroscopic studies of some new substituted 6H-indolo[2,3-b]quinoxalines

Six substituted 6H-indolo[2,3-b]quinoxalines were synthesized by the interaction o-phenyl-enediamine with substituted isatins. The uv, ir and nmr spectral data for the compounds synthesized are presented and discussed.     

Probing motions between equivalent RNA domains using magnetic field induced residual dipolar couplings: accounting for correlations between motions and alignment

Approaches developed thus for extracting structural and dynamical information from RDCs have rested on the assumption that motions do not affect molecular alignment. However, it is well established that molecular alignment in ordered media is dependent on conformation, and slowly interconverting conformational substates may exhibit different alignment properties. Neglecting these correlation effects can lead to aberrations in the structural and dynamical analysis of RDCs and diminish the utility of RDCs in probing motions between domains having similar alignment propensities. Here, we introduce a new approach based on measurement of magnetic field induced residual dipolar couplings in nucleic acids which can explicitly take into account such correlations and demonstrate measurements of motions between two "magnetically equivalent" domains in the transactivation response element (TAR) RNA.     

Effects of epoxidized natural rubber as a compatibilizer in melt compounded natural rubber-organoclay nanocomposites

Nanocomposites containing natural rubber (NR) as matrix, epoxidized natural rubber (ENR) as compatibilizer and organophilic layered clay (organoclay) as filler were produced in an internal mixer and cured using a conventional sulphuric system. The effects of ENR with 25 (ENR 25) and 50 mol% epoxidation (ENR 50), respectively, were compared at 5 and 10 parts per hundred rubber (phr) concentrations. The organoclay content was fixed at 2 phr. Cure characteristics, clay dispersion, (thermo)mechanical properties of the nanocomposites were determined and discussed. Incorporation of ENR and organoclay strongly affected the parameters which could be derived from Monsanto MDR measurements. Faster cure and increased crosslink density were attributed to changes in the activation/crosslinking pathway which was, however, not studied in detail. The organoclay was mostly intercalated according to X-ray diffraction (XRD) and transmission electron microscopic (TEM) results. The best clay dispersion was achieved by adding ENR 50. This was reflected in the stiffness of the nanocomposites derived from both dynamic mechanical thermal analysis (DMTA) and tensile tests. The tensile and tear strengths of the ENR 50 containing nanocomposites were also superior to the ENR 25 compatibilized and uncompatibilized stocks.     

Curing of epoxidized natural rubber with p-phenylenediamine

The curing of epoxidized natural rubber with p-phenylenediamine catalyzed by bisphenol A was investigated. The curing reaction was found to be first order with respect to the amine and to have an activation energy of 67.5 kJ/mol. Vulcanizates of tensile strength 12.5–16.0 MPa were obtained at cure times of 30–75 min at 180°C. The amine-cured vulcanizate was observed to be more rigid than the sulfur-cured one at room temperature consistent with its relatively high T_g. Stress-strain analysis and dynamic mechanical properties indicated that the high T_g of the amine-cured vulcanizate could be attributed to the bulky amine crosslinks and hydrogen bonding effect of bisphenol A. © 1994 John Wiley & Sons, Inc.