bluuebunny/crossref_metadata_2025_split huggingface dataset:
Main Group Element Analogues of Carbenes, Olefins, and Small Rings
Authors: Driess Matthias , Grützmacher Hansjörg 
Published: January 2004

AbstractFor a long time methylene (:CH2), ethylene (H2CCH2), and cyclopropane [(CH2)3] were considered as compounds whose higher main group homologues or analogues could not be synthesized. One consequence of the failure of earlier experiments was, for example, the “double bond rule” which was introduced over twenty years ago and is still often quoted. Although it does not dispute the existence of multiple bonding between elements, it states that it leads to very kinetically unstable species. This interpretation of the double bond rule has long since been surpassed due to improved experimental and quantum chemical methods which have allowed the synthesis of main group analogues of methylene, ethylene, and cyclopropane that are kinetically stable in solution and the solid state at room temperature. In the following review recent advances in the synthesis of these compounds will be referenced, and the concepts, which were developed by a systematic study, will be laid out. In doing so, a detailed description of the complex reactivity of the individual classes of compounds will be avoided in order to highlight the basic principles. These can then be used to approximately predict reactivity and characteristics of new compounds. There are significant differences between formally and even structurally analogous compounds with elements from the higher rows as compared to their “lighter” homologues. For example, while every chemist is familiar with the planar D2h symmetrical structure of H2CCH2 as the only “significant” isomer, it must be accepted that Sn2H4 has four isomers that are separated by very small energy differences. For cis/trans isomerism of olefins only the rotation of the CC double bond needs to be considered, but there are three possible mechanisms for the disilene derivatives R1RSiSiR1R. In the last century cyclopropane was considered to be not producible and this provided the impetus for Baeyer's strain theory. Investigations into small silicon cycles, which were discovered much later, show that this theory has more validity for these rings. As a result of the collected data, it is slowly becoming clearer that precisely the carbon compounds once seen as mere models are in reality the “exotic” compounds. Rules with broader applications can be developed by studying the compounds made up of main group elements.
Glycals in Organic Synthesis: The Evolution of Comprehensive Strategies for the Assembly of Oligosaccharides and Glycoconjugates of Biological Consequence
Authors: Danishefsky Samuel J. , Bilodeau Mark T. 
Published: January 2004

AbstractThis review provides a personal account of the explorations of a research group in oligosaccharide and glycoconjugate construction. The journey began twenty years ago with the study of Diels–Alder reactions of complex dienes. By extending this methodology to aldehydo‐type heterodienophile equivalents, access to unnatural glycals was gained (LACDAC reaction). From this point a broad‐ranging investigation of the use of glycals in the synthesis of oligosaccharides and other glycoconjugates was begun. Mobilization of glycals both as glycosyl donors and glycosyl acceptors led to the strategy of glycal assembly. Several new glycosylation techniques were developed to provide practical underpinning for this logic of glycal assembly. Glycal‐based paradigms have been shown to be nicely adaptable to solid phase supported synthesis. Moreover, glycal assembly—both in solution and on solid phases—has been used to gain relatively concise and efficient entry to a variety of biologically interesting and potentially valuable constructs. Some of these syntheses, particularly in the field of tumor antigens, have led to novel compounds which are in the final stages of preclinical assessment. This review presents an account of the chemical reasoning at the center of the program.
Structural Aspects of Nucleic Acid Analogs and Antisense Oligonucleotides
Authors: Egli Martin 
Published: January 2004

AbstractHybridization of complementary oligonucleotides is essential to highly valuable research tools in many fields including genetics, molecular biology, and cell biology. For example, an antisense molecule for a particular segment of sense messenger RNA allows gene expression to be selectively turned off, and the polymerase chain reaction requires complementary primers in order to proceed. It is hoped that the antisense approach may lead to therapeutics for treatment of various diseases including cancer. Areas of active research in the antisense field focus on the mechanisms of cellular uptake of antisense molecules and their delivery to specific cell sites, an improved understanding of how these molecules inhibit the production of proteins, as well as the optimization of the chemical stability of antisense molecules and the thermodynamic stability of the duplexes they form with the mRNA targets. The last two issues in particular have prompted chemists to launch an extensive search for oligonucleotide analogs with improved binding properties for hybridization with RNA and higher resistance toward nuclease degradation. During the last years this research has resulted in a flurry of new chemical analogs of DNA and RNA with modifications in the sugar–phosphate backbone as well as in the nucleobase sites. However, to date little effort has been directed toward uncovering the exact origins of the gain or loss in stability when nucleic acid analogs bind to RNA. Although large amounts of thermodynamic data have been collected, the structural perturbations induced by the modifications in hybrid duplexes are only poorly understood. For many modified oligonucleotides the compatibility of protection, coupling, and deprotection chemistry with standard DNA and RNA synthesis protocols makes it now possible to generate modified nucleic acid fragments or mixed oligonucleotides containing modifications at selected sites in quantities suitable for three‐dimensional structure investigations. Such studies should reveal the structural origins of the observed changes in affinity and specificity of binding for particular modifications and may guide the development of second‐and third‐generation antisense molecules. In addition, the availability of a previously unimaginable variety of modified building blocks and the investigation of their structures provides the basis for a deeper understanding of the native DNA and RNA structures. This contribution will summarize the results of X‐ray crystallographic structure determinations of modified nucleic acid fragments conducted in our laboratory during the last three years and the insights gained from them.
Two‐Metal Ion Catalysis in Enzymatic Acyl‐ and Phosphoryl‐Transfer Reactions
Authors: Sträter Norbert , Lipscomb William N. , Klabunde Thomas , Krebs Bernt 
Published: January 2004

AbstractNumerous studies, both in enzymatic and nonenzymatic catalysis, have been undertaken to understand the way by which metal ions, especially zinc ions, promote the hydrolysis of phosphate ester and amide bonds. Hydrolases containing one metal ion in the active site, termed mononuclear metallohydrolases, such as carboxypeptidase. A and thermolysin were among the first enzymes to have their structures unraveled by X‐ray crystallography. In recent years an increasing number of metalloenzymes have been identified that use two or more adjacent metal ions in the catalysis of phosphoryl‐transfer reactions (R‐OPO3 + R′‐OH → R′‐OPO3 + R‐OH; in the case of the phosphatase reaction R′‐OH is a water molecule) and carbonyl‐transfer reactions, for example, in peptidases or other amidases. These dinuclear metalloenzymes catalyze a great variety of these reactions, including hydrolytic cleavage of phosphomono‐, ‐di‐ and ‐triester bonds, phosphoanhydride bonds as well as of peptide bonds or urea. In addition, the formation of the phosphodiester bond of RNA and DNA by polymerases is catalyzed by a two‐metal ion mechanism. A remarkable diversity is also seen in the structures of the active sites of these di‐ and trinuclear metalloenzymes, even for enzymes that catalyze very similar reactions. The determination of the structure of a substrate, product, stable intermediate, or a reaction coordinate analogue compound bound to an active or inactivated enzyme is a powerful approach to investigate mechanistic details of enzyme action. Such studies have been applied to several of the metalloenzymes reviewed in this article; together with many other biochemical studies they provide a growing body of information on how the two (or more) metal ions cooperate to achieve efficient catalysis.
The Application of Analytical Methods to the Detection of Hidden Explosives and Explosive Devices
Authors: Kolla Peter 
Published: January 2004

AbstractThe detection of hidden explosives has undergone an enormous development due to an increased desire for safety and the increased terrorist attacks in the last few years. This development was made possible in particular by the rapid advances in the development of powerful analytical techniques in general. These technologies, however, must be specially adapted for the problems of explosives detection. These problems encompass, for example, the large variety of different explosives, the camouflage of explosive devices, and the complexity of the composition of suspicious objects. Frequent air‐travelers have most certainly already been confronted with a so‐called explosives detection apparatus. Baggage controls at airports are a very important and well‐known example of the application of detection technologies. This example also serves to demonstrate the high technological requirements, such as the variability of the object to be examined and a control procedure for a sealed object that must be completed with high reliability in a short period of time. The search for explosive devices or weapons cannot, however, be limited to the recognition of an external appearance with the help of X‐ray imaging. These days, explosive devices, in particular, can readily be installed and hidden in objects of daily life by the use of tiny electric and electronic elements. Therefore, in addition to the application of X‐ray imaging, the use of other technologies becomes necessary. The following article describes and discusses methods and scientific limits of explosives detection under the precondition of possible use at security checkpoints.For corrigendum see DOI:10.1002/anie.199713711
Compounds Containing Planar‐Tetracoordinate Carbon
Authors: Röttger Dirk , Erker Gerhard 
Published: January 2004

AbstractIt is a great challenge to synthesize compounds containing planar‐tetracoordinate carbon that are stable enough to be isolated, investigated, and handled under ambient conditions. There have therefore been many attempts to construct hydrocarbon frameworks that can incorporate and stabilize tetravalent carbon in this “unnatural” planar coordination geometry, for example, by steric constraints. However, success is apparently more readily achieved if the “square Planar” carbon atom is stabilized electronically. Planar‐tetracoordinate carbon is sp2‐hybridized and has an electron deficient σ system. The p orbital orthogonal to the bonding plane is occupied by two electrons. Therefore, σ‐donor/π‐accptor substituents—that is, many metals—are in principle able to stabilize this “unnatural” geometry of tetravalent carbon. Meanwhile, well‐established procedures for synthesizing very stable “anti‐van't Hoff/LeBel compounds” have been devised, in which the planar‐tetracoordinate carbon atom is stabilized, usually by the combined action of two directly bonded metals. In most cases, the square‐planar carbon is part of a double bond system. Typical stabilizing metal combinations are Zr/Al or Zr/B, but there are also examples containing two transition metals from the right‐hand side of the periodic table. Reliable estimates for the energy of stabilization of square planar carbon in such compounds are now available. Both theoretical and experimental investigations show that square planar carbon atoms that are stabilized by Zr/Zr+ and Zr/Al in dimetallic compounds are favored by about 12 and 40 kcalmol−1, respectively, over the trigonal planar alternatives. Square planar coordinated carbon is thus no longer an unusual feature in organometallic chemistry and must now be more frequently considered as an alternative structural possibility.
A New Synthetic Strategy towards Molecules with Mechanical Bonds: Nonionic Template Synthesis of Amide‐Linked Catenanes and Rotaxanes
Authors: Jäger Ralf , Vögtle Fritz 
Published: January 2004

AbstractAfter Sauvage et al. and Stoddart et al. had developed the now commonly used and well‐established catenane and rotaxane synthesis utilizing phenanthroline/Cu+and bipyridinium/crown units, a new way to synthesize mechanical bonds was discovered in 1992—the synthesis of amide‐linked catenanes and rotaxanes around a neutral template. The formation of the template, which leads to the host–guest bond, does not proceed by covalent or ionic interaction, but by weak supramolecular interactions, such as hydrogen bonding, π‐stacking, π‐donor–π‐acceptor interaction, and steric complementarity. The simple synthetic units (arene dicarboxylic acid dichlorides and diamines) can be varied in an astonishing number of ways, few steps are required to obtain the target molecules, and the yields of these syntheses approach those of ordinary organic syntheses. After the preparation of [2]catenanes, it took only a few years to prepare [2]‐, [3]‐, and bis[2]‐rotaxanes of the amide type. The introduction of sulfonamide groups enabled alkylation of the sulfonamide nitrogen atom, and this made chemical reactions with catenanes and rotaxanes feasible. Intra‐ and intermolecular connections of catenanes and rotaxanes were then possible, and a molecule with the topology of a “pretzel” has recently been prepared. This allowed new insights into the molecular recognition processes of neutral guest and host substances, as well as further understanding of these processes in template synthesis, isomerism in catenanes, and the regioselectivity of the formation of catenane isomers. Competitive binding studies of guest molecules in the cavity of concave receptors with more than one binding site can be perfomed. These studies might result in higher synthetic yields of molecules with mechanical bonds and lead to further progress in the development of supramolecular nanostructures and molecular switches.
Superconductivity and Chemistry
Authors: Simon Arndt 
Published: January 2004

AbstractThe microscopic theory of superconductivity worked out by Bardeen, Cooper, and Schrieffer led to a fundamental understanding of the phenomenon. However, this theory does not provide a materials aspect. Starting from the special situation of chemical bonding in carbides and carbide halides of the rare earth metals a general view of the origin of superconductivity is developed, based on a tendency for (pairwise) localization of conduction electrons. This approach is tested in terms of special features in the electronic band structure and compared with existing physical models of electron pairing in real space. Its applicability to high‐temperature superconductors discovered during the last decade is discussed.
Chemistry and Molecular Biology in the Search for New LHRH Antagonists
Authors: Kutscher Bernhard , Bernd Michael , Beckers Thomas , Polymeropoulos Emmanuel E. , Engel Jürgen 
Published: January 2004

AbstractHormones—and in particular, the sex hormones—were the first growth factors discovered to be involuntary helpers of cancer. Female breast cancer and male prostate cancer are the best known examples of tumors acknowledged to be hormone‐dependent. A look at cancer statistics shows that breast cancer is still the most frequent cancer in women; in men, prostate cancer plays a similarly dominant role with increasing age. Shutting down the main production site of the sex hormones estrogen and testosterone either by removing the ovaries or by castration is a well‐known and often effective therapy; however, these procedures can be problematic due to the concomittant psychological stress. Modern hormone therapy for advanced breast cancer and prostate cancer attempts to spare the patient such irreversible operative procedures for as long as possible, by using hormone antagonists, such as the LHRH antagonists, which hinder deployment of the hormone itself and thus its growth‐promoting activity.
Effects of calcium carbonate and its purity on crystallization and melting behavior, mechanical properties, and processability of syndiotactic polypropylene
Authors: Supaphol Pitt , Harnsiri Wipasiri , Junkasem Jirawut 
Published: February 2004

AbstractCalcium carbonate‐filled syndiotactic poly(propylene) (CaCO3‐filled s‐PP) was prepared in a self‐wiping, co‐rotating twin‐screw extruder. The effects of CaCO3 of varying particle size (1.9, 2.8 and 10.5 μm), content (0–40 wt %), and type of surface modification (uncoated, stearic acid‐coated, and paraffin‐coated) on the crystallization and melting behavior, mechanical properties, and processability of CaCO3‐filled s‐PP were investigated. Non‐isothermal crystallization studies indicate that CaCO3 acts as a good nucleating agent for s‐PP. The nucleating efficiency of CaCO3 for s‐PP was found to depend strongly on its purity, type of surface treatment, and average particle size. Tensile strength was found to decrease, while Young's modulus increased, with increasing CaCO3 content. Both types of surface treatment on CaCO3 particles reduced tensile strength and Young's modulus, but improved impact resistance. Scanning electron microscopy (SEM) observations of the fracture surfaces for selected CaCO3‐filled s‐PP samples revealed an improvement in CaCO3 dispersion as a result of surface treatment. Finally, steady‐state shear viscosity of CaCO3‐filled s‐PP was found to increase with increasing CaCO3 content and decreasing particle size. © 2004 Wiley Periodicals, Inc. J Appl Polym Sci 92: 201–212, 2004
Synthesis and characterization of novel aromatic condensation polymers containing rigid benzoxazole pendent groups
Authors: Kim Ji‐Heung , Kim Yong Hwan , Kim Young Jun , Won Jong Chan , Choi Kil‐Young 
Published: February 2004

AbstractA new aromatic diamine monomer containing benzoxazole substituents was prepared by a multistep synthesis starting from 1,4‐dibromo‐2,5‐difluorobenzene. The diamine was polymerized with commercial aromatic dianhydride or dicarboxylic acid chloride monomers to provide several different poly(amic acid)s and polyamides with their inherent viscosities in the range of 0.24–0.46 dL/g. Thermal properties of these polymers including thermal imidization of poly(amic acid)s into polyimides were investigated by using FTIR, DSC, and TGA. © 2004 Wiley Periodicals, Inc. J Appl Polym Sci 92: 178–185, 2004
Polyaniline–nylon 6 composite fabric for ammonia gas sensor
Authors: Hong Kyung Hwa , Oh Kyung Wha , Kang Tae Jin 
Published: February 2004

AbstractConducting polymers constitute a new class of sensing materials, which can be prepared by a simple oxidative polymerization method. An ammonia gas sensor was prepared by polymerizing aniline on the surface of nylon 6 fabrics and doped with various acids in this study. The fabricated polyaniline (PANI)–nylon 6 composite fabrics showed high sensitivity and fast response for NH3 gas. In particular, the PANI–nylon 6 composite fabrics doped by monocarboxylic acids (such as formic acid, acrylic acid, and trichloroacetic acid) displayed excellent sensitivity and fast response. Moreover, no significant changes in the Tm and crystallinity were found in these composite fabrics. Hence the PANI–nylon 6 composite fabrics are attractive alternatives for use in sensing applications because of their excellent sensing ability, low cost, and ease of preparation. © 2004 Wiley Periodicals, Inc. J Appl Polym Sci 92: 37–42, 2004
Cure properties of methacrylate‐type prepolymer that include cyclohexane moiety
Authors: Kim Whan Gun , Lee Jun Young 
Published: February 2004

AbstractNew methacrylate‐type prepolymers including a cyclohexane moiety, which may be used as binder resins for the color filter resist of liquid crystal display, were synthesized by the reaction with methacrylic acid (MA) and epoxy resins. Their photo‐ and thermal cure properties were investigated using Fourier transform infrared spectroscopy and DSC, respectively. Their photocure reaction rates and the extent of reaction conversion increased with the concentration of photoinitiator and intensity of UV irradiation. An increase of photocure reaction temperature was attributed to the increase of photocure reaction rate and the degree of reaction conversion for EHPE‐3150‐MA prepolymer. Thermal stability was studied by observing changes in the transmittance of the photocured polymer film upon heating. The polymer was extremely stable, showing almost no transmittance change in the visible range even after being heated at 250°C for 1 h. We also investigated thermal cure reaction using a DSC technique. An autocatalytic kinetic reaction occurs in these systems, and the kinetic parameters of all systems were reported in terms of a generalized kinetic equation that considered the diffusion term. It can be shown that the reaction conversion rate of NC‐9110‐MA is faster than that of EHPE‐3150‐MA, regardless of the kind of thermal initiator. © 2004 Wiley Periodicals, Inc. J Appl Polym Sci 92: 43–52, 2004
Acetylation of sugarcane bagasse hemicelluloses under mild reaction conditions by using NBS as a catalyst
Authors: Sun Xaio‐Feng , Sun Run‐Cang , Zhao Li , Sun Jing‐Xia 
Published: February 2004

AbstractSugarcane bagasse hemicelluloses were partially acetylated with acetic anhydride using N‐bromosuccinimide (NBS) as a catalyst under mild conditions in an almost solvent‐free system. The overall yield and degree of substitution (DS) were varied from 66.2 and 83.5% and between 0.27 and 1.15, respectively, by changing the reaction temperature and duration. It was found that the yield and DS increased with reaction temperature from 18 to 80°C and reaction time between 0.5 and 5.0 h. The results showed that treatment of the native hemicelluloses with acetic anhydride using NBS as a catalyst conveniently provided the corresponding biopolymer esters. The products were characterized by FTIR, 13C‐NMR spectroscopy, and thermal analysis. The new biopolymer acetates were thermally stable to over 200°C but underwent significant and rapid thermal degradation when heated above the onset of thermal degradation. © 2004 Wiley Periodicals, Inc. J Appl Polym Sci 92: 53–61, 2004