Photochemical Reactions 127th CommunicationPhotochemistry of Epoxy-enones: Intramolecular Trapping of a Carbonyl Ylide

On ¹n, π* -excitation 5, 6-epoxy-2-hydroxy-5, 6-dihydro-β-ionone ((E)-4) shows the typical behaviour of a, β-unsaturated γ, δ-epoxy ketones undergoing primarily C(γ), O-cleavage of the oxiran. However, ¹π, π*-excitation of (E)? 4 leads to enol ether 10 which is formed by intramolecular insertion of the hydroxyl group of the ylide c .     

Separation of polycyclic aromatic hydrocarbons with a C60 bonded silica phase in microcolumn liquid chromatography

A chemically bonded C₆₀ silica phase was synthesized as a stationary phase for liquid chromatography (LC) and its retention behavior evaluated for various polycyclic aromatic hydrocarbons (PAHs) using microcolumn LC. The results indicate that the C₆₀ bonded phase offers selectivity different from that of octadecylsilica (ODS) bonded phases in the separation of isomeric PAHs. With the C₆₀ phase, PAH molecules having a partial structure similar to that of the C₆₀ molecule, e.g. triphenylene and perylene, were retained longer than with ordinary ODS stationary phases. The results also show that good correlation exists between the retention data with this C₆₀ bonded phase and with C₆₀ itself as the stationary phase.     

Water formation reaction on Pt(111): role of the proton transfer

The catalytic water formation reaction on Pt(111) was investigated by kinetic Monte Carlo simulations, where the interaction energy between reaction species and the high mobility of H(2)O molecule was considered. Results obtained clearly reproduce the scanning tunneling microscopy images which show that the reaction proceeds via traveling the reaction fronts on the O-covered Pt(111) surface by creating H(2)O islands backwards. The reaction front is a mixed layer of OH and H(2)O with a (square root 3 x square root 3)R30(o) structure. Coverage change during the reaction is also reproduced in which the reaction consists of three characteristic processes, as observed by the previous experiments. The simulation also revealed that the proton transfer from H(2)O to OH plays an important role to propagate the water formation.     

Effect of sodium cation on the electrochemical reduction of CO<Subscript>2</Subscript> at a copper electrode in methanol

The electrochemical reduction of CO₂ with a Cu electrode in methanol was investigated with sodium hydroxide supporting salt. A divided H-type cell was employed; the supporting electrolytes were 80 mmol dm⁻³ sodium hydroxide in methanol (catholyte) and 300 mmol dm⁻³ potassium hydroxide in methanol (anolyte). The main products from CO₂ were methane, ethylene, carbon monoxide, and formic acid. The maximum current efficiency for hydrocarbons (methane and ethylene) was 80.6%, at −4.0 V vs Ag/AgCl, saturated KCl. The ratio of current efficiency for methane/ethylene, r _f(CH₄)/r _f(C₂H₄), was similar to those obtained in LiOH/methanol-based electrolyte and larger relative to those in methanol using KOH, RbOH, and CsOH supporting salts. In NaOH/methanol-based electrolyte, the efficiency of hydrogen formation, a competing reaction of CO₂ reduction, was suppressed to below 4%. The electrochemical CO₂ reduction to methane may be able to proceed efficiently in a hydrophilic environment near the electrode surface provided by sodium cation.     

Determination of palladium by a new preconcentration method (Mg-W cell)-electrothermal atomic absorption spectrometry

A new concentration method for palladium using Mg-W cell-electrodeposition has been developed. The method was combined with electrothermal atomic absorption spectrometry (ETAAS) with a tungsten tube atomizer. The detection limit of palladium by this method was 0.37 ng ml^–1 (3 S/N). The severe interferences on the AAS signal of palladium caused by large amounts of Al, Ca, Cu, Fe, K, Na, Pb and Zn were eliminated by the Mg-W cell-electrodeposition method. The method was adapted for the determination of palladium in environmental samples. The recovery of palladium spiked environmental samples was in the range of 102 to 114%. Received: 12 August 1996 / Revised: 16 October 1996 / Accepted: 7 November     

A microscopic theory for solution chemical reactions: Introduction of reactant and medium structures into generalized langevin equation formalism

A microscopic formulation of solution chemical reactions, taking reactants and medium structures into consideration, is presented on the basis of microscopic understandings obtained by recent quantum chemical methods (i.e., ab initio molecular orbital theory, etc.). Assuming thermal equilibrium of the medium bath, an effective internal Hamiltonian is derived, and, further, its derivative with respect to internal normal coordinates is proved explicitly to give the same force field as is provided by the free-energy surface or potential of mean force. The free-energy surface can be expressed in the composite normal coordinate system (CNCS ) consisting of some normal coordinate systems of isolated reactants and surrounding solvent molecules (i.e., medium solvent molecules). In CNCS , in use of diagonal elements obtained in the Hessian matrix of the free-energy surface, effective normal-mode frequencies, which reflect the equilibrium solvent effect, are estimated. Furthermore, on the generalized Langevin equation (GLE ) treatment, a closed expression of the time-dependent frictional coefficient is derived on a microscopic basis, reflecting the reactant and solvent structures. The nonequilibrium effect is estimated by an analytical expression similar to that in the Grote–Hynes theory. The rate constant is evaluated for a typical model system and it is shown that the equilibrium rate constants should be reduced by a factor 0.997. Finally, it is concluded that the present microscopic theory is reasonably applicable to the estimation of chemical reaction rate constants in solution. © 1994 John Wiley & Sons, Inc.     

The synthesis of pyrazolo[3,4-d]pyridazines. Photochemical cyclization to pyrazolo[3,4-d]pyridazin-4(5H)-ones with subsequent functionalization

The synthesis of 1,3-disubstituted and 1,3,5-trisubstituted 1H-pyrazolo[3,4-d]pyridazin-4(5H)-ones is conveniently performed by photochemical cyclization. Functionalization of the former compounds leading to the formation of 1,4-disubstituted and 1,3,4-trisubstituted 1H-pyrazolo[3,4-d]pyridazines is smoothly effected through pertinent nucleophilic substitutions.     

Buxus alkaloids. XII. Benzamide alkaloids from Buxus sempervirens L

The isolation and structural elucidation of ten new alkaloids from the “additional weak bases” fraction of Buxus sempervirens L. are reported. A structural feature common to eight of the new alkaloids is the presence of a secondary benzamide function at the C-3 position. Evidence is presented for assignment of structures for N-benzoylcycloxobuxine-F (I), N-benzoylcycloxobuxidine-F (Xa), N-benzoyldihydrocyclomicrophylline-F (XII), N-benzoylcycloxobuxoline-F (XF), N-benzoyl-O-acetylcycloxobuxoline-F (XVI), N-benzoylbuxidienine-F (XIX), N-benzoylcycloprotobuxoline-D (XXIIIa), N-benzoylcycloprotobuxoline-C (XXXIIIb), tigloycyclovirobuxeine-B (XXIXb), and N-acetylcyclopropotobuxine-D (XXXc).     

A CoMFA analysis with conformational propensity: An attempt to analyze the SAR of a set of molecules with different conformational flexibility using a 3D-QSAR method

CoMFA analysis, a widely used 3D-QSAR method, has limitations to handle a set of SAR data containing diverse conformational flexibility since it does not explicitly include the conformational entropic effects into the analysis. Here, we present an attempt to incorporate the conformational entropy effects of a molecule into a 3D-QSAR analysis. Our attempt is based on the assumption that the conformational entropic loss of a ligand upon making a ligand-receptor complex is small if the ligand in an unbound state has a conformational propensity to adopt an active conformation in a complex state. For a QSAR analysis, this assumption was interpreted as follows: a potent ligand should have a higher conformational propensity to adopt an `active-conformation'-like structure in an unbound state than an inactive one. The conformational propensity value was defined as the populational ratio, N<sub>active</sub>/N<sub>stable</sub>, of the number of energetically stable conformers, N<sub>stable</sub>, to the number of `active-conformation'-like structures, N_active. The latter number was calculated by counting the number of conformers that satisfied the structural parameters deduced from the active conformation. A set of SAR data of imidazoleglycerol phosphate dehydratase inhibitors containing 20 molecules with different conformational flexibility was used as a training set for developing a 3D structure-activity relationship by a CoMFA analysis with the conformational propensity value. This resulted in a cross-validated squared correlation coefficient of the CoMFA model with the conformational propensity value (R ² _cross = 0.640) higher than that of the standard CoMFA model (R ² _cross = 0.431). Then we evaluated the quality of the CoMFA models by predicting the inhibitory activity for a new molecule.