Asymmetric catalysis is a rapidly growing field in modern organic chemistry and has been indispensable for the synthesis of enantioenriched materials to meet demands from the academies to ...pharmaceutical industries. Asymmetric dearomative cyclization catalyzed by transition metals has been a hot research area in the last decade. Fascinated by its ability to construct sterically hindered quaternary stereogenic center(s) through dearomatization and simultaneously forging new ring structure(s) through cyclization, palladium-catalyzed asymmetric dearomative cyclization has been applied to the synthesis of structurally complicated natural products and it is increasingly prevalent in the literature. In particular, the resultant product from dearomative cyclization, which usually carries one or more unsaturated C-C bond(s), allows further functional group transformations. Previously reported applications of palladium-catalyzed asymmetric dearomative cyclization in natural product synthesis are presented here and discussed in depth.
This review discusses palladium-catalyzed asymmetric dearomative cyclization, which is invaluable for producing enantioenriched materials, for complex natural product synthesis.
Infrared brightness temperature data are used to investigate convective evolution during tropical cyclone (TC) formation in a quasi-Lagrangian framework. More than 150 named Atlantic storms during ...1989–2010 were examined. It is found that both convective intensity and convective frequency increase with time in the inner pouch region but change little, or even weaken slightly, in the outer pouch region. Convection thus appears to concentrate toward the circulation center as genesis is approached. However, large variability is found from storm to storm in convective intensity, area, and duration, and the convective evolution of individual storms does not resemble the composite mean. Further analysis suggests that the composite mean or the median represents the probability of occurrence of convection instead of a recurrent pattern. Three distinct spatial patterns of convection are identified using cluster analysis. Substantial differences in convection intensity and area are found among the clusters and can be attributed to the impacts of environmental conditions. These differences suggest that convection intensity or area is not a key feature of convection for tropical cyclogenesis. In particular, a small and weak convective system is not necessarily associated with a weak vortex. A simple proxy of the radial gradient of convection is found to be similar among the clusters. Furthermore, convection is most effective in strengthening the TC protovortex when its maximum occurs near the pouch center. These findings suggest that organized convection near the pouch center is a key feature of convection for tropical cyclogenesis and that emphasizing convective intensity or frequency without considering the spatial pattern may be misleading.
Celotno besedilo
Dostopno za:
DOBA, IZUM, KILJ, NUK, PILJ, PNG, SAZU, SIK, UILJ, UKNU, UL, UM, UPUK
Asymmetric catalysis for chiral compound synthesis is a rapidly growing field in modern organic chemistry and provides enantioselective materials to meet the demands of various fields. However, the ...construction of all-carbon quaternary stereocenters poses a distinct challenge in organic synthesis. The development of catalytic asymmetric conjugate additions that require only a catalytic amount of a transition metal with a chiral ligand or organocatalyst has provided an efficient approach to the preparative-scale synthesis of enantioselective and/or diastereoselective conjugate adducts. Such reactions have been used in various synthetic applications such as natural product synthesis and reports of the use of this approach are becoming increasingly common in the literature. In particular, tandem copper-catalyzed asymmetric conjugate addition/enolate trapping by a carbon electrophile enables diastereoselective synthesis of α,β-substituted ketones with contiguous stereogenic centers, which is still an intricate task in organic synthesis. In this review, the use of asymmetric conjugate addition in natural product synthesis is described and discussed in depth.
Many natural products have intriguing biological properties that arise from their fascinating chemical structures. Numerous synthetic methods/strategies have been invented to improve the efficiency ...of natural-product synthesis. A broad group of compounds with diverse functionalities for various applications (
e.g.
, development of new drugs) are available. However, the intrinsic complexity of the structural skeleton and the reactive functional groups on natural products pose tremendous challenges to chemical syntheses. Hypervalent(
iii
) iodine has demonstrated its synthetic versatility by enabling various transformations (
e.g.
, skeletal rearrangement, cyclization and/or cycloaddition and sp
3
C-H bond functionalization), which have been utilized widely in the synthesis of natural products. Here, recent innovations of hypervalent(
iii
) iodine in natural-product synthesis are described and discussed.
The focus article discusses the innovation of hypervalent(
iii
) iodine regarding skeletal rearrangement, cycloaddition and cyclization, and sp
3
C-H functionalization in natural product synthesis.
Abstract
The thermodynamic aspects of tropical cyclone (TC) formation near the center of the wave pouch, a region of approximately closed Lagrangian circulation within the wave critical layer, are ...examined through diagnoses of a high-resolution numerical simulation and dropsonde data from a recent field campaign. It is found that the meso-β area near the pouch center is characterized by high saturation fraction, small difference in equivalent potential temperature θe between the surface and the middle troposphere, and a short incubation time scale. Updrafts tend to be more vigorous in this region, presumably because of reduced dry air entrainment, while downdrafts are not suppressed. The thermodynamic conditions near the pouch center are thus critically important for TC formation.
The balanced responses to convective and stratiform heating at the pregenesis stage are examined using the Sawyer–Eliassen equation. Deep convection is concentrated near the pouch center. The strong radial and vertical gradients of latent heat release effectively force the transverse circulation and spin up a surface protovortex near the pouch center. Stratiform heating induces modest midlevel inflow and very weak low-level outflow, which contributes to the midlevel spinup without substantially spinning down the low-level circulation.
The analysis of dropsonde data shows that the midlevel θe increases significantly near the pouch center one to two days prior to genesis but changes little away from the pouch center. This may indicate convective organization and the impending TC genesis. It also suggests that the critical information of TC genesis near the pouch center may be masked out if a spatial average is taken over the pouch scale.
Celotno besedilo
Dostopno za:
DOBA, IZUM, KILJ, NUK, PILJ, PNG, SAZU, SIK, UILJ, UKNU, UL, UM, UPUK
The role of cumulus congestus (shallow and congestus convection) in tropical cyclone (TC) formation is examined in a high-resolution simulation of Tropical Cyclone Fay (2008). It is found that ...cumulus congestus plays a dominant role in moistening the lower to middle troposphere and spinning up the near-surface circulation prior to genesis, while deep convection plays a key role in moistening the upper troposphere and intensifying the cyclonic circulation over a deep layer. The transition from the tropical wave stage to the TC stage is marked by a substantial increase in net condensation and potential vorticity generation by deep convection in the inner wave pouch region.
Celotno besedilo
Dostopno za:
DOBA, IZUM, KILJ, NUK, PILJ, PNG, SAZU, SIK, UILJ, UKNU, UL, UM, UPUK
Supramolecular polymers have attracted increasing attention in recent years due to their perfect combination of supramolecular chemistry and traditional polymer chemistry. The design and synthesis of ...macrocycles have driven the rapid development of supramolecular chemistry and polymer science. Pillarnarenes, a new generation of macrocyclic compounds possessing unique pillar‐shaped structures, nano‐sized cavities, multi‐functionalized groups, and excellent host–guest complexation abilities, are promising candidates to construct supramolecular polymer materials with enhanced properties and functionalities. This review summarizes recent progress in the design and synthesis of pillararene‐based supramolecular polymers (PSPs) and illustrates their diverse applications as adsorption and separation materials. All performances are evaluated and analyzed in terms of efficiency, selectivity, and recyclability. Typically, PSPs can be categorized into three typical types according to their topologies, including linear, cross‐linked, and hybrid structures. The advances made in the area of functional supramolecular polymeric adsorbents formed by new pillararene derivatives are also described in detail. Finally, the remaining challenges and future perspectives of PSPs for separation‐based materials science are discussed. This review will inspire researchers in different fields and stimulate creative designs of supramolecular polymeric materials based on pillararenes and other macrocycles for effective adsorption and separation of a variety of targets.
Supramolecular polymer materials have attracted significant attention for wide potential applications in numerous fields, particularly in adsorption and separation, due to the elegant combination of supramolecular chemistry with polymer science. This review highlights the recent advancements in pillararene‐based supramolecular polymers for adsorption and separation applications, with an emphasis on their synthetic strategies, adsorption mechanisms, and separation performances.
This paper presents a machine-learning classifier where computations are performed in a standard 6T SRAM array, which stores the machine-learning model. Peripheral circuits implement mixed-signal ...weak classifiers via columns of the SRAM, and a training algorithm enables a strong classifier through boosting and also overcomes circuit nonidealities, by combining multiple columns. A prototype 128 × 128 SRAM array, implemented in a 130-nm CMOS process, demonstrates ten-way classification of MNIST images (using image-pixel features downsampled from 28 × 28 = 784 to 9 × 9 = 81, which yields a baseline accuracy of 90%). In SRAM mode (bit-cell read/write), the prototype operates up to 300 MHz, and in classify mode, it operates at 50 MHz, generating a classification every cycle. With accuracy equivalent to a discrete SRAM/digital-MAC system, the system achieves ten-way classification at an energy of 630 pJ per decision, 113× lower than a discrete system with standard training algorithm and 13× lower than a discrete system with the proposed training algorithm.
Aiming to deal with the irrelevant or redundant features, this paper proposes eight kinds of feature selection methods. The first seven feature selection methods include CART and Random Forests ...(CART-RF), CHIAD and Random Forests (CHIAD-RF), SVM and Random Forests (SVM-RF), Bayesian Network and Random Forests (BN-RF), neural Network and Random Forests (NN-RF), K-Means and Random Forests (K-Means-RF) and Kohonen and Random Forests (Kohonen-RF). These methods use CART, CHAID, SVM, BN, NN, K-Means and Kohonen to evaluate the importance and ranking of features, and then obtain feature subsets through RF algorithm. The eighth method is named hybrid integration methods and random forests (Integrate-RF). Integrate-RF uses the average importance of the seven methods and the optimal features subset can be selected based on the OOB data classification error rate. Experimental results indicate that feature selection methods proposed in this article can effectively select features and reduce the data dimension.
Rechargeable zinc-manganese dioxide batteries that use mild aqueous electrolytes are attracting extensive attention due to high energy density and environmental friendliness. Unfortunately, manganese ...dioxide suffers from substantial phase changes (e.g., from initial α-, β-, or γ-phase to a layered structure and subsequent structural collapse) during cycling, leading to very poor stability at high charge/discharge depth. Herein, cyclability is improved by the design of a polyaniline-intercalated layered manganese dioxide, in which the polymer-strengthened layered structure and nanoscale size of manganese dioxide serves to eliminate phase changes and facilitate charge storage. Accordingly, an unprecedented stability of 200 cycles with at a high capacity of 280 mA h g
(i.e., 90% utilization of the theoretical capacity of manganese dioxide) is achieved, as well as a long-term stability of 5000 cycles at a utilization of 40%. The encouraging performance sheds light on the design of advanced cathodes for aqueous zinc-ion batteries.
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