Reaction des organolithiens avec l'ethoxy-5 dihydro-2,5 furannone-2. preparation de 2-2 butenolides disubstitues sur le carbone-4.

Organolithium react with 5-ethoxy 2,5 dihydro-furan-2-one $\text{1}$, easily obtained by photoxidation of furfural, leading selectively to 5-hydroxy-2,5-dihydro-furans $\text{4}$ or to 5-ethoxy-2,5 dihydro-furans $\text{5}$ disubstituted on carbon 4 ; the odcydation of the hemi-acetals $\text{4}$ transforms them with good yields in 3-2 butenolides disubstituted on carbon 4.     

Cyclohexene skeletal isomerization on AlPO4 catalysts precipitated with ammonia and promoted with sulfate ions

This study concerns the effect of SO ₄ ^2– (1–10 wt.%) on the surface acid properties of AlPO₄ catalysts obtained in aqueous ammonia and based on their catalytic activity in cyclohexene skeletal isomerization (CSI) by using the Bassett-Habgood equation for first order processes. It has been found that AlPO₄–SO ₄ ^2– catalysts are much more active in the isomerization process than AlPO₄ catalysts, indicating a strongly increased acidity. Maximum catalytic activity was reached at about 2–3 wt.% and varied with further temperature treatment. Furthermore, the sulfate loading brought about a very pronounced increase in crystallinity even at the smallest content.

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SO ₄ ^2– (1–10 .%) AlPO₄, . , - . , AlPO₄–SO ₄ ^2– , AlPO₄, . 2–3 . % . , .

     

A Simple Synthesis of a Model of the Tetracyclic Bisketal Lactone Mainframe of Saudin

The partial structure 2 of the bisketal‐type, tetracyclic saudin ( 1 ), an important natural product, was de novo synthesized. A key step was the Jones oxidation of the epoxide 3 , which gave rise to epoxide‐ring opening, followed by acetal hydrolysis, alcohol oxidation, and, finally, intramolecular acetal formation. The resulting key intermediate was finally oxidized to the target lactone 2 .     

Effect of precipitation medium and thermal treatment on structure and textural properties of chromium orthophosphates

The effect of the precipitation agent on the structure and textural properties of CrPO₄ (Cr/P=1) catalysts has been studied. Catalysts obtained in propylene oxide-aqueous ammonia exhibited higher thermal stability and, hence, greater surface areas and pore volumes than catalysts obtained in propylene oxide or aqueous ammonia exclusively. The former also showed a small surface area decrease on increasing calcination temperature up to 1073 K. Nevertheless, calcination at 1273 K develops in all cases crystalline CrPO₄, showing the -structure. Moreover, CrPO₄ catalysts thermally treated below 1273 K only exhibited hydrogen bonded hydroxy groups .     

Photoinduced swing of a diarylethene thin broad sword shaped crystal: a study on the detailed mechanism

We report a swinging motion of photochromic thin broad sword shaped crystals upon continuous irradiation with UV light. By contrast in thick crystals, photosalient phenomena were observed. The bending and swinging mechanisms are in fact due to molecular size changes as well as phase transitions. The first slight bending away from the light source is due to photocyclization-induced surface expansion, and the second dramatic bending toward UV incidence is due to single-crystal-to-single-crystal (SCSC) phase transition from the original phase I to phase II_UV. Upon visible light irradiation, the crystal returned to phase I. A similar SCSC phase transition with a similar volume decrease occurred by lowering the temperature (phase III_temp). For both photoinduced and thermal SCSC phase transitions, the symmetry of the unit cell is lowered; in phase II_UV the twisting angle of disordered phenyl groups is different between two adjacent molecules, while in phase III_temp, the population of the phenyl rotamer is different between adjacent molecules. In the case of phase II_UV, we found thickness dependent photosalient phenomena. The thin broad sword shaped crystals with a 3 μm thickness showed no photosalient phenomena, whereas photoinduced SCSC phase transition occurred. In contrast, large crystals of several tens of μm thickness showed photosalient phenomena on the irradiated surface where SCSC phase transition occurred. The results indicated that the accumulated strain, between isomerized and non-isomerized layers, gave rise to the photosalient phenomenon.

We report a swinging motion of photochromic thin broad sword shaped crystals upon continuous irradiation with UV light.     

Preparation of highly active and dispersed platinum nanoparticles on mesoporous Al-MCM-48 and their activity in the hydroisomerisation of n-octane

Platinum nanoparticles supported on Al-MCM-48 materials have been prepared. The resultant catalysts have been characterized by means of XRD, N2 physisorption experiments, scanning electron microscopy (SEM), transmission electron microscopy (TEM), temperature-programmed reduction (TPR), and diffuse reflectance infrared Fourier-transform spectroscopy (DRIFTS). The activity of these nanoparticles has been tested in relation to the hydroisomerisation of n-octane. The catalytic activities were typically 50 %, with selectivities in the isomerisation process in excess of 70 %, favouring the formation of the 3-methylheptane isomer with respect to the 2- and 4-methylheptanes.     

A class of Dantzig–Wolfe type decomposition methods for variational inequality problems

We consider a class of decomposition methods for variational inequalities, which is related to the classical Dantzig–Wolfe decomposition of linear programs. Our approach is rather general, in that it can be used with certain types of set-valued or nonmonotone operators, as well as with various kinds of approximations in the subproblems of the functions and derivatives in the single-valued case. Also, subproblems may be solved approximately. Convergence is established under reasonable assumptions. We also report numerical experiments for computing variational equilibria of the game-theoretic models of electricity markets. Our numerical results illustrate that the decomposition approach allows to solve large-scale problem instances otherwise intractable if the widely used PATH solver is applied directly, without decomposition.     

Variétés sphériques de type A

Sans résumé Manuscrit re?u le 13 juin 2000.     

Targeted smoothing parameter selection for estimating average causal effects

The non-parametric estimation of average causal effects in observational studies often relies on controlling for confounding covariates through smoothing regression methods such as kernel, splines or local polynomial regression. Such regression methods are tuned via smoothing parameters which regulates the amount of degrees of freedom used in the fit. In this paper we propose data-driven methods for selecting smoothing parameters when the targeted parameter is an average causal effect. For this purpose, we propose to estimate the exact expression of the mean squared error of the estimators. Asymptotic approximations indicate that the smoothing parameters minimizing this mean squared error converges to zero faster than the optimal smoothing parameter for the estimation of the regression functions. In a simulation study we show that the proposed data-driven methods for selecting the smoothing parameters yield lower empirical mean squared error than other methods available such as, e.g., cross-validation.