Different reaction routes found in acid-catalyzed glycosylation of endo- and exo-glycals: competition between Ferrier rearrangement and protonation

Acid-mediated glycosylations of endo- and exo-glycals have been carried out in good to excellent yields, in which a mixture of two products is often obtained resulting from Ferrier rearrangement and protonation. The former reaction exclusively takes place with the t-butyl carbonate or hydroxyl substituent at the C3 position of endo-glycals, while the latter mainly occurs in the glycosylation of exo-glycals with allyl benzyl ether or acetate. In addition to the substituent effect, protecting groups are critical to determine the activity and favored reaction pathway. Furthermore, the method is applicable to O-, C-, and N-nucleophiles.     

In Situ Fabrication of ZnS Semiconductor Nanoparticles in Layered Organic-inorganic Solid Template

Ordered ZnS semiconductor nanoparticles were in situ synthesized in metal halide perovskite organic/inorganic layered hybrids (CnH2n 1NH3)2ZnCl4 (n=10 and 12) by reaction of their spin-casting films with H2S gas. Transmission electron microscopy, UV-vis spectroscopy and small-angle X-ray diffraction were used to characterize the morphology and the structure of formed nanoparticles. Obtained results indicate an effective way to incorporate functional inorganic nanoparticles into structured organic matrices.     

Probing the Effects of Reagent Gas Pressure and Ion Source Temperature for Dimethyl Ether Chemical Ionization of Tricyclic Antidepressants in an External Source Ion Trap Mass Spectrometer

This study examines the reagent gas pressure and ion source temperature dependence for dimethyl ether chemical ionization (DME CI) mass spectra recorded with an external source ion trap mass spectrometer (ITMS). Information for better controls of the reagent gas pressure in order to obtain fair CI spectra is provided. The origin of M^+? ions observed in DME CIMS is discussed in detail. Furthermore, the ion source temperature effect on the DME CI is also investigated.     

Electron Paramagnetic Resonance Study of Radical Pairs and Isolated Radicals Trapped in Irradiated Long-Chain Hydrocarbons at Low Temperatures

The intrinsic characteristics of radical pairs produced in squalane and in cetane receiving high gamma-dose are extensively studied with the EPR technique at temperatures from 77°K up to 150°K. The spectra of the paired radicals occur at g=4 with a very low transition probability in contrast to that of isolated radicals which appear at g=2 A well-resolved hyperfine spectrum corresponding to the species (CH₃CH₂.CH₂CH₃) is observed in cetane. The isothermal decay rates of radical pairs in cetane below 100°K are significantly slow; however, the decay kinetics at 150°K is first order with rate constant=1.86 min^?1. A relatively slower decay rate is obtained for isolated radicals suggesting that the decay mechanism of paired radicals is through geminate recombination. The relative inter-radical distance in radical pairs is known from a decay curve as a function of temperature. The yields of radical pairs are low in both matrices, only few percents of those of isolated radicals. The formation mechanisms of paired radicals with direct radiolytic bond scission process are discussed in connection with the experimental observations.     

H2S-induced reorganization of mixed monolayer of carboxylic derivatives on silver surface

The structure of self-assembled monolayers ofp-terphenyl-4-carboxylic acid and the mixed monolayers of this acid with n-hexadecanoic acid on silver surface were studied by reflection-IR spectroscopy, near-edge X-ray absorption fine structure (NEXAFS) measurement, X-ray photoemission electron microscopy (X-PEEM), and atomic force microscopy. Exposure of the p-terphenyl-4-carboxylate monolayer to H2S vapor resulted in reorganization of the film structure into clusters of the corresponding free acids, in tens of nanometer dimension. Exposure of the mixed monolayer to H2S resulted in reorganization of the mixed monolayer film into phase-separated clusters of respective component molecules. The saturated aliphatic acid formed clusters of submicrometer size, whereas the p-terphenyl-4-carboxylic acid formed clusters of tens of nanometer size, presumably due to different surface mobility and/or intermolecular interaction of the two types of molecule. Restoration of the monolayer film from the clusters, driven by the reaction between the free acid molecules and the basic surface sites, proceeded at different speeds for the two types of molecules. The saturated acid monolayer was restored much faster than the p-terphenyl-4-carboxylic acid monolayer. A domain-separated monolayer in several micrometers scale was obtained. The process was imaged by tapping mode atomic force microscopy.     

Synthesis of (+)-zeylenone from shikimic acid

Starting from shikimic acid, the total synthesis of zeylenone was studied. The product was proved to be the (+)antipode of zeylenone through analysis and comparison of their respective spectra (including NMR, MS, IR and CD) and optical data. The absolute configuration of the natural product was thus determined to be (1S,2S,3R).     

Mechanism of hydrodynamic separation of biological objects in microchannel devices

In this study, the separation mechanism employed in hydrodynamic chromatography in microchannel devices is analyzed. The main purpose of this work is to provide a methodology to develop a predictive model for hydrodynamic chromatography for biological macromolecules in microchannels and to assess the importance of various phenomenological coefficients. A theoretical model for the hydrodynamic chromatography of particles in a microchannel is investigated herein. A fully developed concentration profile for non-reactive particles in a microchannel was obtained to elucidate the hydrodynamic chromatography of these particles. The external forces acting on the particles considered in this model include the van der Waals attractive force, double-layer force as well as the gravitational force. The surface forces, such as van der Waals attractive force as well as the double-layer repulsive force, can either enhance or hinder the average velocity of the macromolecular particles. The average velocity of the particles decreases with the molecular radius because the van der Waals attractive force increases the concentration of the particles near the channel surface, which is the low-velocity region. The transport velocity of the particles is dominated by the gravity and the higher density enlarges the effect caused by gravity.     

Probing stereoselective inhibition of the acyl binding site of cholesterol esterase with four diastereomers of 2'-<Emphasis Type="Italic">N</Emphasis>-α-methylbenzylcarbamyl-1, 1'-bi-2-naphthol

Background  

Recently there has been increased interest in pancreatic cholesterol esterase due to correlation between enzymatic activity in vivo and absorption of dietary cholesterol. Cholesterol esterase plays a role in digestive lipid absorption in the upper intestinal tract, though its role in cholesterol absorption in particular is controversial. Serine lipases, acetylcholinesterase, butyrylcholinesterase, and cholesterol esterase belong to a large family of proteins called the α/β-hydrolase fold, and they share the same catalytic machinery as serine proteases in that they have an active site serine residue which, with a histidine and an aspartic or glutamic acid, forms a catalytic triad. The aim of this work is to study the stereoselectivity of the acyl chain binding site of the enzyme for four diastereomers of an inhibitor.     

Systematic Study of Ti‐Distribution in Titanosilicate *BEA Zeolites via Symmetry‐Adapted Enumeration

The high performance of titanosilicate zeolites in various industrial oxidation reactions is highly affected by the distribution of Ti atoms in their frameworks. Because of their structural complexity, previous theoretical studies mainly focus on the preferential location of single Ti atoms within the unit cells of titanosilicate zeolites. When multiple Ti atoms are required, conventional approaches consider only symmetrically related T sites to reduce the computation complexity. Such symmetry‐constrained approaches obviously overlooked many possible configurations. Herein, we conduct a systematic study on the distribution of two Ti atoms in the unit cell of titanosilicate zeolite *BEA . Different from conventional symmetry‐constrained approaches, we introduce two Ti atoms simultaneously without any constraint and adopt a symmetry‐adapted algorithm to enumerate all possible configurations for double‐Ti introduction. We generate a total of 273 distinct configurations and analyze the Ti‐distribution via Boltzmann statistics. We find that many of the configurations overlooked by conventional symmetry‐constrained approaches indeed exhibit more feasible energies, which may lead to different Ti‐distributions. Our study indicates the necessity of unconstrained introduction of Ti atoms when multiple‐Ti atoms are considered for calculations.     

杂多阴离子的有机金属化学(Ⅲ)——缺位Keggin阴离子PWO9-34的酯基锡杂多配合物的合成及性质

利用酯基锡与缺位Keggin结构杂多阴离子PW9O9-34反应,合成了6种新的杂多阴离子--有机金属配合物M9[(R'OOCCHR"CH2SnOH2)3(PW9O34)2]·xH2O(M=(CH3)4N+,K+;R'=CH3-,CH3CH2-;R"=H,CH3-),通过元素分析、IR光谱、紫外电子光谱、1H NMR、31P NMP、183w NMR和TGA-DSC热分析等测试手段对标题配合物进行了表征和性质研究,确定该系列配合物为A-β-PW9 型夹心配合物结构.