Azetidines are four-membered nitrogen-containing heterocycles that hold great promise in current medicinal chemistry due to their desirable pharmacokinetic effects. However, a lack of efficient ...synthetic methods to access functionalized azetidines has hampered their incorporation into pharmaceutical lead structures. As a 2+2 cycloaddition reaction between imines and alkenes, the aza Paternò-Büchi reaction arguably represents the most direct approach to functionalized azetidines. Hampered by competing reaction paths accessible upon photochemical excitation of the substrates, the current synthetic utility of these transformations is greatly restricted. We herein report the development of a visible light-enabled aza Paternò-Büchi reaction that surmounts existing limitations and represents a mild solution for the direct formation of functionalized azetidines from imine and alkene containing precursors.
Laser wakefield acceleration (LWFA) and its particle-driven counterpart, particle or plasma wakefield acceleration (PWFA), are commonly treated as separate, though related, branches of high-gradient ...plasma-based acceleration. However, novel proposed schemes are increasingly residing at the interface of both concepts where the understanding of their interplay becomes crucial. Here, we present a comprehensive study of this regime, which we may term laser-plasma wakefields. Using datasets of hundreds of shots, we demonstrate the influence of beam loading on the spectral shape of electron bunches. Similar results are obtained using both 100-TW-class and few-cycle lasers, highlighting the scale invariance of the involved physical processes. Furthermore, we probe the interplay of dual electron bunches in the same or in two subsequent plasma periods under the influence of beam loading. We show that, with decreasing laser intensity, beam loading transitions to a beam-dominated regime, where the first bunch acts as the main driver of the wakefield. This transition is evidenced experimentally by a varying acceleration of a low-energy witness beam with respect to the charge of a high-energy drive beam in a spatially separate gas target. Our results also present an important step in the development of LWFA using controlled injection in a shock front. The electron beams in this study reach record performance in terms of laser-to-beam energy transfer efficiency (up to 10%), spectral charge density (regularly exceeding10pCMeV−1), and angular charge density (beyond300pCμsr−1at 220 MeV). We provide an experimental scaling for the accelerated charge per terawatt (TW) of laser power, which approaches 2 nC at 300 TW. With the expanding availability of petawatt-class (PW) lasers, these beam parameters will become widely accessible. Thus, the physics of laser-plasma wakefields is expected to become increasingly relevant, as it provides new paths toward low-emittance beam generation for future plasma-based colliders or light sources.
A New Twist on Cooperative Catalysis Schindler, Corinna S.; Jacobsen, Eric N.
Science (American Association for the Advancement of Science),
05/2013, Letnik:
340, Številka:
6136
Journal Article
Recenzirano
Two independent catalysts working synergistically allow the controlled synthesis of all four stereoisomers in a reaction that forms carbon-carbon bonds.
Also see Research Article by
Krautwald
et al.
...The stereochemical configuration of an organic compound is often linked directly to its physical and biological properties. Larger molecules can contain multiple stereochemical elements, and chiral catalysts are often used to control each of these defined spatial arrangements in the efficient synthesis of a complex target (
1
). Many powerful enantioselective catalysts, which control the handedness or absolute configuration of a product, have been developed (
2
), but full control of the relative configuration in compounds can be a greater challenge if multiple stereocenters are generated; high diastereoselectivity, where one relative configuration is established, may be achieved, but access to the complementary diastereomeric product usually requires major redesign of the catalyst, substrate, and/or reaction conditions, and in some cases is not possible at all (see the figure, panel A) (
3
). On page 1065 of this issue, Krautwald
et al.
(
4
) demonstrate an elegant solution to this challenge based on the cooperative action of multiple chiral catalysts in a single reaction.
Azetines, four-membered unsaturated nitrogen-containing heterocycles, hold great potential for drug design and development but remain underexplored due to challenges associated with their synthesis. ...We report an efficient, visible light-mediated approach toward 1- and 2-azetines relying on alkynes and the unique triplet state reactivity of oximes, specifically 2-isoxazolines. While 2-azetine products are accessible upon intermolecular 2 + 2-cycloaddition via triplet energy transfer from a commercially available iridium photocatalyst, the selective formation of 1-azetines proceeds upon a second, consecutive, energy transfer process. Mechanistic studies are consistent with a stepwise reaction mechanism via
-
bond homolysis following the second energy transfer event to result in the formation of 1-azetine products. Characteristic for this method is its operational simplicity, mild conditions, and modular approach that allow for the synthesis of functionalized azetines and tetrahydrofurans (via
hydrolysis) from readily available precursors.
Aims. We investigate the dependence of star formation and the distribution of the components of galaxies on the strength of ram pressure. Several mock observations in X-ray, Hα and HI wavelength for ...different ram-pressure scenarios are presented. Methods. By applying a combined N-body/hydrodynamic description (GADGET-2) with radiative cooling and a recipe for star formation and stellar feedback 12 different ram-pressure stripping scenarios for disc galaxies were calculated. Special emphasis was put on the gas within the disc and in the surroundings. All gas particles within the computational domain having the same mass resolution. The relative velocity was varied from 100 km s-1 to 1000 km s-1 in different surrounding gas densities in the range from 1 $\times$ 10-28 to 5 $\times$ 10-27 g/cm3. The temperature of the surrounding gas was initially 1 $\times$ 107 K. Results. The star formation of a galaxy is enhanced by more than a magnitude in the simulation with a high ram-pressure (5 $\times$ 10-11 dyn/cm2) in comparison to the same system evolving in isolation. The enhancement of the star formation depends more on the surrounding gas density than on the relative velocity. Up to 95% of all newly formed stars can be found in the wake of the galaxy out to distances of more than 350 kpc behind the stellar disc. Continuously stars fall back to the old stellar disc, building up a bulge-like structure. Young stars can be found throughout the stripped wake with surface densities locally comparable to values in the inner stellar disc. Ram-pressure stripping can shift the location of star formation from the disc into the wake on very short timescales. As the gas in a galaxy has a complex velocity pattern due to the rotation and spiral arms, the superposition of the internal velocity field and the ram pressure causes complex structures in the gaseous wake which survive dynamically up to several 100 Myr. Finally we provide simulated X-ray, Hα and HI observations to be able to compare our results with observations in these wavebands. These simulated observations show many features which depend strongly both on the strength and the duration of the external ram pressure.
We study the case of a bright (L > L
) barred spiral galaxy from the rich cluster A3558 in the Shapley supercluster core (z = 0.05) undergoing ram-pressure stripping. Integral-field spectroscopy with ...Wide Field Spectrograph (WiFeS) at the 2.3-m Australian National University telescope, complemented by imaging in ultraviolet (GALEX), B and R European Southern Observatory (ESO) 2.2-m WFI, Hα (Magellan), K (United Kingdom Infrared Telescope), 24 and 70 μm (Spitzer), allows us to reveal the impact of ram pressure on the interstellar medium. With these data we study in detail the kinematics and the physical conditions of the ionized gas and the properties of the stellar populations. We observe one-sided extraplanar ionized gas along the full extent of the galaxy disc, extending ∼13 kpc in projection from it. Narrow-band Hα imaging resolves this outflow into a complex of knots and filaments, similar to those seen in other cluster galaxies undergoing ram-pressure stripping. The gas velocity field is complex with the extraplanar gas showing signature of rotation, while the stellar velocity field is regular and the K-band image shows a symmetric stellar distribution. We use line-ratio diagnostics to ascertain the origin of the observed emission. In all parts of the galaxy, we find a significant contribution from shock excitation, as well as emission powered by star formation. Shock-ionized gas is associated with the turbulent gas outflow and highly attenuated by dust (A
v
= 1.5-2.3 mag). All these findings cover the whole phenomenology of early-stage ram-pressure stripping. Intense, highly obscured star formation is taking place in the nucleus, probably related to the bar, and in a region 12 kpc south-west (SW) from the centre. These two regions account for half of the total star formation in the galaxy, which overall amounts to 7.2 ± 2.2 M yr−1. In the SW region we identify a starburst characterized by a ∼5× increase in the star formation rate over the last ∼100 Myr, possibly related to the compression of the interstellar gas by the ram pressure. The scenario suggested by the observations is supported and refined by ad hoc N-body/hydrodynamical simulations which identify a rather narrow temporal range for the onset of ram-pressure stripping around t ∼ 60 Myr ago, and an angle between the galaxy rotation axis and the intracluster medium wind of ∼45°. The ram pressure is therefore acting at an intermediate angle between face-on and edge-on. Taking into account that the galaxy is found ∼1 Mpc from the cluster centre in a relatively low density region, this study shows that ram-pressure stripping still acts efficiently on massive galaxies well outside the cluster cores, as also recently observed in the Virgo cluster.
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