An oxidative coupling for the synthesis of arylated quaternary stereocentres and its application in the total synthesis of powelline and buphanidrine

Catechol derivatives directly bonded to all-carbon quaternary stereocentres are prevalent in nature. An oxidative coupling strategy for the synthesis of this motif is described. Pivoting on the base-catalysed Michael addition of carbon-centred pro-nucleophiles to in situ generated ortho-benzoquinones, the method is broad in scope, high yielding and provides remarkably simple access to this challenging motif. The application of this methodology in the total synthesis of the crinane-type amaryllidaceae alkaloids (±)-powelline and (±)-buphanidrine is demonstrated and our efforts towards an enantioselective synthesis described.     

Isotope-induced desymmetrization can mimic isotopic perturbation of equilibria. On the symmetry of bromonium ions and hydrogen bonds

The desymmetrizing effect of isotopic substitution on the geometry of otherwise symmetrical structures can be surprisingly large when there is anharmonic coupling between a desymmetrizing mode and modes whose zero-point energies are strongly isotope-dependent. This phenomenon is found to account for experimental observations that had been previously interpreted as implying asymmetry in bromonium ions and in hydrogen phthalate ions in aprotic solvents. The unlabeled structures are concluded to have the higher symmetry found in computed structures. Great care must be taken in applying isotopic perturbation as a test of asymmetry.     

Single shot spatial and temporal coherence properties of the SLAC Linac Coherent Light Source in the hard x-ray regime

We measured the transverse and longitudinal coherence properties of the Linac Coherent Light Source (LCLS) at SLAC in the hard x-ray regime at 9 keV photon energy on a single shot basis. Speckle patterns recorded in the forward direction from colloidal nanoparticles yielded the transverse coherence properties of the focused LCLS beam. Speckle patterns from a gold nanopowder recorded with atomic resolution allowed us to measure the shot-to-shot variations of the spectral properties of the x-ray beam. The focused beam is in the transverse direction fully coherent with a mode number close to 1. The average number of longitudinal modes behind the Si(111) monochromator is about 14.5 and the average coherence time τ(c)=(2.0±1.0) fc. The data suggest a mean x-ray pulse duration of (29±14) fs behind the monochromator for (100±14) fc electron pulses.     

Paschen curve approach to investigate electron density and deposition rate of Cu in magnetron sputtering system

In this work, Paschen curve for argon gas was obtained during copper deposition using a DC magnetron sputtering system. Five process parameters of Paschen curve were used to obtain the electron density and deposition rate of the deposited nanostructured thin films. Plasma parameter such as electron density was correlated with the deposition rate. It is observed that a minimum deposition rate was obtained for the plasma process parameter corresponding to the Paschen minimum. This investigation helps to understand and optimize the quality of nanostructured thin films depending on the process parameters.     

Electrical characteristics of etched ion-tracks in polyimide filled with silver nanoparticles

Silicon ions, of energy 150?MeV and fluence ～10¹²?ions/cm², were used to register latent tracks in 40?µm thick polyimide samples. Different sizes of tracks were obtained by etching the ion irradiated polyimide samples, in chemical solutions, by varying the temperature and etching period. Silver nanoparticles were diffused into the etched tracks by immersing the polyimide samples in silver solution and then irradiating with 6.5?MeV electrons at different fluences varying from 1?×?10¹⁵ to 5?×?10¹⁵?cm^?2. Results of morphological and elemental analysis, carried out by Scanning Electron Microscopy and Energy Dispersive X-ray. Analysis revealed that the conical tracks could be fully filled with silver nanoparticles at electron fluence of 5?×?10¹⁵?cm^?2. The minimum d. c. resistance of an array of tracks, filled with silver nanoparticles and measured across the polyimide film, was orders of magnitude higher as compared to that of silver wires of equivalent sizes connected in parallel. In addition, these silver nanoparticles filled tracks exhibited rectifying I–V behavior and frequency dependent a. c. resistance, characteristic of metal–polymer nano-composites. Possible mechanisms have been discussed, which can justify the asymmetric current–voltage characteristics in such nano-composites.     

Thermal reactions of 8-methylbicyclo[4.2.0]oct-2-enes: competitive diradical-mediated [1,3] sigmatropic, stereomutation, and fragmentation processes

[reaction: see text] At 275 degrees C, 8-exo-methylbicyclo[4.2.0]oct-2-ene (1a) undergoes a [1,3] sigmatropic rearrangement to 5-methylbicyclo[2.2.2]oct-2-enes, of which the orbital symmetry-allowed si product is only marginally favored over the forbidden sr product; that is, si/sr is 2.4. Accompanying the [1,3] shift are significant amounts of epimerization and fragmentation. The 8-endo epimer 1b, which yields no [1,3] product, experiences primarily direct fragmentation and secondarily epimerization. A diradical intermediate can account for all such observations.     

An organocatalytic oxidative coupling strategy for the direct synthesis of arylated quaternary stereogenic centers

A broadly applicable oxidative coupling strategy of 3-substituted catechols and carbon-centered pro-nucleophiles for the construction of arylated quaternary stereogenic centers has been developed. Pivoting on a base-catalyzed addition of a carbon-centered acid to an in situ generated o-benzoquinone, the method is general and atom-economical and provides remarkably efficient access to one of the most challenging structural motifs. Furthermore, use of chiral bifunctional organocatalysts allows the process to be rendered asymmetric (up to 81% ee).     

The Effect of Storage Carryover on Plant Expansion Planning

This paper considers the expansion of a river abstraction plant which is used conjunctively with a storage reservoir to supply water. Because of the uncertainty of inflows to the reservoir, the problem is stochastic and because of storage carryover from year to year, decisions on the plant size have a persistent effect. By assuming that the storage carryover effects last for a finite time only, the problem can be formulated in terms of a recurrence relation which can be solved by a version of dynamic programming. The optimality of the solution obtained is investigated by sensitivity testing.