An Efficient One-Pot Approach to the Construction of Chiral Nitrogen-Containing Heterocycles under Mild Conditions

A new, general, and practical procedure for the highly enantioselective synthesis of functionalized nitrogen-containing heterocycles has been developed. The simple cyclic hemiaminals were directly catalyzed for the first time as nucleophiles in an enamine-based asymmetric conjugate addition reaction. The practical approach recycles the catalyst and solvent which make it possible for large-scale and diversity-oriented chemical production

An Efficient One-Pot Approach to the Construction of Chiral Nitrogen-Containing Heterocycles under Mild Conditions

A new, general, and practical procedure for the highly enantioselective synthesis of functionalized nitrogen-containing heterocycles has been developed. The simple cyclic hemiaminals were directly catalyzed for the first time as nucleophiles in an enamine-based asymmetric conjugate addition reaction. The practical approach recycles the catalyst and solvent which make it possible for large-scale and diversity-oriented chemical production

Neocucurbitacin D, a novel lactone-type norcucurbitacin as xanthine oxidase inhibitor from <i>Herpetospermum pedunculosum</i>

A novel lactone-type norcucurbitacin, designated as neocucurbitacin D (1), together with five known cucurbitane triterpenes were isolated from traditional Tibetan medicine “Se Ji Mei Duo”, which is the seed of Herpetospermum pedunculosum (Ser.) C.B. Clarke. The structure of neocucurbitacin D was elucidated by spectroscopic analysis, including 2D NMR and X-ray techniques. Compounds 1–6 were screened for their xanthine oxidase (XOD) inhibitory activity. Compound 1, 2 and 4 exhibited significant XOD inhibition with IC50 values ranging from 10.16 to 18.41 μM. The absolute stereochemistry and XOD inhibitiory activity of lactone-type norcucurbitacins was reported firstly.</p

Probing Surface Saturation Conditions in Alternating Layer Growth of CdSe/CdS Core/Shell Quantum Dots

We monitor effective band gap energy shifts and free reagent concentration during the formation of CdS shells on CdSe nanocrystals to test the hypothesis that alternating addition of stoichiometric doses of precursors can effectively saturate surface sites and thereby enforce conformal shell growth. The selective ionic layer addition and reaction (SILAR) mechanism has been proposed to describe growth under such conditions, and the method is attractive because of the opportunity to (1) avoid cross-reaction of precursors in growing binary films in solution and (2) enforce conformal growth, rather than regioselective growth, by saturating all available surface sites in a self-limiting manner in each half-cycle. The strong red shift that takes place when CdS shells are grown on CdSe cores provides a convenient process monitoring tool that complements scanning transmission electron microscopy imaging and analytical measurements of free reagent concentration. We find that, under commonly used conditions, a cadmium oleate precursor reacts incompletely at chalcogenide-saturated nanocrystal surfaces. Although approximately spherical particles are obtained, the growth does not proceed via saturating cycles, as described in the SILAR mechanism. This has implications for the rational control of conformal and regioselective growth of epilayers on nanocrystal quantum dots and higher-dimensional chalcogenide semiconductor nanostructures via solution processes

Quantum Yield Regeneration: Influence of Neutral Ligand Binding on Photophysical Properties in Colloidal Core/Shell Quantum Dots

This article describes an experiment designed to identify the role of specific molecular ligands in maintaining the high photoluminescence (PL) quantum yield (QY) observed in as-synthesized CdSe/CdZnS and CdSe/CdS quantum dots (QDs). Although it has been possible for many years to prepare core/shell quantum dots with near-unity quantum yield through high-temperature colloidal synthesis, purification of such colloidal particles is frequently accompanied by a reduction in quantum yield. Here, a recently established gel permeation chromatography (GPC) technique is used to remove weakly associated ligands without a change in solvent: a decrease in ensemble QY and average PL lifetime is observed. Minor components of the initial mixture that were removed by GPC are then added separately to purified QD samples to determine whether reintroduction of these components can restore the photophysical properties of the initial sample. We show that among these putative ligands trioctylphosphine and cadmium oleate can regenerate the initial high QY of all samples, but only the “L-type” ligands (trioctyphosphine and oleylamine) can restore the QY without changing the shapes of the optical spectra. On the basis of the PL decay analysis, we confirm that quenching in GPC-purified samples and regeneration in ligand-introduced samples are associated chiefly with changes in the relative population fraction of QDs with different decay rates. The reversibility of the QY regeneration process has also been studied; the introduction and removal of trioctylphosphine and oleylamine tend to be reversible, while cadmium oleate is not. Finally, isothermal titration calorimetry has been used to study the relationship between the binding strength of the neutral ligands to the surface and photophysical property changes in QD samples to which they are added

Susceptibility of Autonomous Driving Agents to Learning-Based Action-Space Attacks

Intelligent vehicles with increasing complexity face cybersecurity threats. This paper studies action-space attacks on autonomous driving agents that make decisions using either a traditional modular processing pipeline or the recently proposed end-to-end driving model obtained via deep reinforcement learning (DRL). Such attacks alter the actuation signal and pose direct risks to the vehicle’s state. We formulate the attack construction as a DRL problem based on the input from either an extra camera or inertial measurement unit deployed. The attacks are designed to lurk until a safety-critical moment arises and cause a side collision upon activation. We analyze the behavioral differences between two driving agents when subjected to action-space attacks and demonstrate the superior resilience of the modular processing pipeline. We further investigate the performance and limitations of two enhancement methods, i.e., adversarial training through fine-tuning and progressive neural networks. The result offers valuable insights into vehicle safety from the viewpoints of both the assailant and the defender and informs the future design of autonomous driving systems.</p

Smart Chemical Oxidative Polymerization Strategy To Construct Au@PPy Core–Shell Nanoparticles for Cancer Diagnosis and Imaging-Guided Photothermal Therapy

Imaging-guided photothermal therapy (PTT) in a single nanoscale platform has aroused extensive research interest in precision medicine, yet only a few methods have gained wide acceptance. Thus, it remained an urgent need to facilely develop biocompatible and green probes with excellent theranostic capacity for superior biomedical applications. In this study, a smart chemical oxidative polymerization strategy was successfully developed for the synthesis of Au@PPy core–shell nanoparticles with polyvinyl alcohol (PVA) as the hydrophile. In the reaction, the reactant tetrachloroauric acid (HAuCl4) was reduced by pyrrole to fabricate a gold (Au) core, and pyrrole was oxidized to deposit around the Au core to form a polypyrrole (PPy) shell. The as-synthesized Au@PPy nanoparticles showed a regular core–shell morphology and good colloidal stability. Relying on the high X-ray attenuation of Au and strong near-infrared (NIR) absorbance of PPy and Au, Au@PPy nanoparticles exhibited excellent performance in blood pool/tumor imaging and PTT treatment by a series of in vivo experiments, in which tumor could be precisely positioned and thoroughly eradicated. Hence, the facile chemical oxidative polymerization strategy for constructing monodisperse Au@PPy core–shell nanoparticles with potential for cancer diagnosis and imaging-guided photothermal therapy shed light on an innovative design concept for the facile fabrication of biomedical materials

Discrete Miktoarm Star Block Copolymers with Tailored Molecular Architecture

Molecular architecture is a critical factor in regulating phase behaviors of the block copolymer and prompting the formation of unconventional nanostructures. This work meticulously designed a library of isomeric miktoarm star polymers with an architectural evolution from the linear-branched block copolymer to the miktoarm star block copolymer and to the star-like block copolymer (i.e., 3AB → 3(AB1)B2 → 3(AB)). All of the polymers have precise chemical composition and uniform chain length, eliminating inherent molecular uncertainties such as chain length distribution or architectural defects. The self-assembly behaviors were systematically studied and compared. Gradually increasing the relative length of the branched B1 block regulates the ratio between the bridge and loop configuration and effectively releases packing frustration in the formation of the spherical or cylindrical structures, leading to a substantial deflection of phase boundaries. Complex structures, such as Frank–Kasper phases, were captured at a surprisingly higher volume fraction. Rationally regulating the molecular architecture offers rich possibilities to tune the packing symmetry of block copolymers

Antineoplastic Agents. 488. Isolation and Structure of Yukonin from a Yukon Territory Fungus

Cancer cell line bioassay-guided separation of an extract from a Yukon Territory fungus resulted in the isolation of a new C16-terpene dilactone designated yukonin (1) accompanied by two previously known, structurally related constituents (2 and 3). The structure of each was determined by X-ray crystallographic analysis. Dilactone 2 was found to correspond to LL-Z1271α isolated from fungi in the genera Oidiodendron, Acrostalagmus, and Holwaya, while dilactone 3 had earlier been prepared by reduction of an α-lactol derivative. Each of the dilactones was found to inhibit growth of human cancer cell lines (pancreas, breast, CNS, lung, colon, and prostate) and some pathogenic fungi