bluuebunny/crossref_metadata_2025_split huggingface dataset:
1,4‐Dihydropyridines: Effects of Chirality and Conformation on the Calcium Antagonist and Calcium Agonist Activities
Authors: Goldmann Siegfried , Stoltefuss Jürgen 
Published: December 2003

Abstract4‐Aryl‐l,4‐dihydropyridine‐3,5‐dicarboxylic diesters of the nifedipine type have become almost indispensable for the treatment of cardiovascular diseases since they first appeared on the market in 1975. There are some twenty derivatives currently under clinical development worldwide and work in this area is continuing undiminished. The 1,4‐dihydropyridines are the most effective of the calcium antagonists or calcium channel blockers. They are valued not only for their pharmacological effect, but also as a tool for the investigation of the calcium channel, particularly since the discovery that this class also includes compounds that have exactly the opposite action profile and are known as calcium agonists. There are even instances in which this reversal of activity is found between enantiomers. In view of the importance of chirality to pharmacological activity, the present article will describe methods for the separation of enantiomers, point out the structural differences between calcium antagonists and calcium agonists, and attempt to explain the difference in their behavior.
Silicon Nitride—From Powder Synthesis to Ceramic Materials
Authors: Lange Horst , Wötting Gerhard , Winter Gerhard 
Published: December 2003

AbstractSilicon nitride is a ceramic material of great interest to advanced engine construction and mechanical engineering owing to an outstanding combination of favorable properties like high mechanical strength at high temperatures, corrosion and wear resistance, great hardness, and low density. The material is based on high‐quality Si3N4 powders, which are shaped and sintered to the ceramic component. This overview outlines the properties required for Si3N4 powders suitable for advanced ceramics. Processes in commerical use and those under development for the production of high quality Si3N4 powders are discussed as well as material manufacturing processes and material properties. By steadily improving powder quality, material properties, and the economy of powder and component production, chemistry and chemical technology play a major role in recent efforts to create a solid fundament for broad applications of silicon nitride ceramics.“If one would surrender to geological phantasies, one could imagine that during the formation of our planet, when elements combined to the compounds making up its crust and mountain ranges, silicon reacted with nitrogen, and the still red‐hot nitrogen‐silicon, on contact with water, may have decomposed to silicic acid and ammonia. Thus, ammonia may have been formed originally and nitrogen thereby introduced into the forming organic compounds when living nature first started to appear.”H. Sainte‐Claire Deville and F. Wöhler“Ueber die directe Bildung des Stickstoffsiliciums” in: Ann. Chem. Pharm. 34 (1859) 248: “If one would surrender to geological phantasies, one could imagine that during the formation of our planet, when elements combined to the compounds making up its crust and mountain ranges, silicon reacted with nitrogen, and the still red‐hot nitrogen‐silicon, on contact with water, may have decomposed to silicic acid and ammonia. Thus, ammonia may have been formed originally and nitrogen thereby introduced into the forming organic compounds when living nature first started to appear.”
Routes to New Aromatic Polycarbonates with Special Material Properties
Authors: Freitag Dieter , Fengler Gerd , Morbitzer Leo 
Published: December 2003

AbstractPolycarbonate together with the acrylonitrile/butadiene/styrene copolymers, and the poly amides constitute the leading groups of engineering thermoplastics; for polycarbonate in particular, continued dynamic growth is prophesized.The underlying reason for this lies not only in its outstanding combination of technical properties and excellent price/performance balance, but also in the chemical and physical potential inherent in the basic structure of polycarbonate. The following review demonstrates with examples how this potential can be used in the development of new polycarbonates through the incorporation of alternative monomers, changes in the linear structure, end group variation, addition of special additives, and blending. The main emphasis of this articles lies in the development of polycarbonates resistant to high temperature, with a good balance of technologically valuable properties. For scientific and practical interests a new criterion is offered for polycarbonates having both high‐temperature stability and impact strength.“One must not only make a discovery, but also realize that a discovery has been made.”Hermann Schnell [1]
Glycolipids as Immunomodulators: Syntheses and Properties
Authors: Lockhoff Oswald 
Published: December 2003

AbstractAlthough the most successful vaccines still consist of attenuated or killed pathogens, it is evident that genetic engineering will eventually provide pure antigens in sufficient quantities. To achieve a protective effect, however, the immune response to these structures, which are only weakly immunogenic, must be activated by adjuvants. The discovery of chemically defined, well‐tolerated. synthetic adjuvants is therefore especially important in effective immunoprophylaxis. A very promising class of compounds in this respect are synthetic glycosylamides, with two long alkyl chains consisting of alkylamines and fatty acids. Unlike natural glycolipids, which exert an immunosuppressive effect, the new glycolipid analogues described here show marked immunostimulation. In vaccination experiments with various antigens they provide efficient protection. Their mode of action, which differs from that of known adjuvants, makes glycolipid analogues particularly interesting for immunization of patients with defective T lymphocyte function, for example AIDS patients.
Herbicides in Photosynthesis Research
Authors: Draber Wilfried , Tietjen Klaus , Kluth Joachim F. , Trebst Achim 
Published: December 2003

AbstractPhotosynthesis and herbicide research have a long common history. Soon after the introduction of a new group of highly effective herbicides in 1956, the substituted aryl ureas, their mode of action was recognized to be the inhibition of light‐driven electron flow and photosynthetic oxygen evolution. Many subsequent highly successful commercial herbicides, including those from other chemical classes, act by this mechanism. The study of their interaction with the photosynthetic system at the molecular level is a prime example of how rapidly advances in unraveling the mechanism of photosynthesis can be translated into practical uses. At the same time the herbicides continued to be valuable and efficient tools in photosynthesis research. In the past herbicides were instrumental in establishing many principal features of the biochemistry and biophysics of photosynthesis, in particular the role of plastoquinone, which is displaced from its binding pocket in one of the protein subunits of the photosystem II. Their further use in elucidating the molecular biology of photosynthesis is illustrated by the recent importance of herbicide‐tolerant mutants for determining structural aspects of photosystem II. We will review the past and present state of the interaction of herbicide and photosynthesis research and will provide a model for the orientation of herbicides within the three dimensional structure of their target, the D1 protein of photosystem II.
Organic and Organometallic Molecular Magnetic Materials—Designer Magnets
Authors: Miller Joel S. , Epstein Arthur J. 
Published: December 2003

AbstractMagnets composed of molecular species or polymers and prepared by relatively low‐temperature organic synthetic methodologies are a focus of contemporary materials science research. The anticipated properties of such molecular‐species‐based magnetic materials, particularly in combination with other properties associated with molecules and polymers, may enable their use in future generations of electronic, magnetic, and/or photonic/photronic devices ranging from information storage and magnetic imaging to static and low‐frequency magnetic shielding. A tutorial of typical magnetic behavior of molecular materials is presented. The three distinct models (intramolecular spin coupling through orthogonal orbitals in the same spatial region within a molecule/ion, intermolecular spin coupling through pairwise “configuration interaction” between spin‐containing moieties, and dipole—dipole, through‐space interactions) which enable the design of new molecular‐based magnetic materials are discussed. To achieve the required spin couplings for bulk ferro‐ or ferrimagnetic behavior it is crucial to prepare materials with the necessary primary, secondary, and tertiary structures akin to proteins. Selected results from the worldwide effort aimed at preparing molecular‐based magnetic materials by these mechanisms are described. Some organometallic solids comprised of linear chains of alternating metallocenium donors (D) and cyanocarbon acceptors (A) that is, …︁D•+ A•− D•+ A•−…︁, exhibit cooperative magnetic phenomena. Bulk ferromagnetic behavior was first observed below the critical (Curie) temperature Tc of 4.8 K for [FeIII(C5Me5)2]•+ [TCNE]•− (Me = methyl; TCNE = tetracyanoethylene). Replacement of FeIII with MnIII leads to a ferromagnet with a Tc of 8.8 K in agreement with mean‐field models developed for this class of materials. Replacement with CrIII, however, leads to a ferromagnet with a Tc lowered to 3.65 K which is at variance with this model. Extension to the reaction of a vanadium(o) complex with TCNE leads to the isolation of a magnet with a Tc ≈ 400 K, which exceeds the thermal decomposition temperature of the material. This demonstrates that a magnetic material with a Tc substantially above room temperature is achievable in a molecule/organic/polymeric material. Finally, a new class of one‐dimensional ferrimagnetic materials based on metalloporphins is discussed.
Nitrile Imines: From Matrix Characterization to Stable Compounds
Authors: Bertrand Guy , Wentrup Curt 
Published: December 2003

AbstractThe concept of 1,3‐dipolar cycloaddition was developed, starting in the late 1950s, largely by Rolf Huisgen and his students in Munich, and it has led to one of the most versatile methods for the construction of five‐membered ring heterocycles. Although first known only as transient intermediates, nitrile imines have been at the heart of mechanistic studies of this type of cycloaddition reactions. Hundreds of mechanistic papers appeared in 1960s and 1970s; reliable spectroscopic observations were achieved in the early 1980s both at low temperatures and in the gas phase; finally, the first crystalline nitrile imine was reported in 1988. The unusual structures found by X‐ray analyses as well as the facile rearrangements observed experimentally have fostered a new interplay between experiment and theory. The story of nitrile imines, from matrix characterization to stable compounds, nicely illustrates the role that main group elements can play in organic chemistry.
Chiral Aziridines—Their Synthesis and Use in Stereoselective Transformations
Authors: Tanner David 
Published: December 2003

AbstractThe preparation of enantiomerically pure compounds is one of the major areas of organic chemistry. Much emphasis is placed on the elaboration of naturally occurring starting materials and on the development of techniques for enantio‐selective transformations of achiral substrates. In this field, chiral aziridines form an attractive class of compounds, since they are available in enantiomerically pure (or highly enriched) form by a variety of procedures and can be used for asymmetric synthesis in a number of different ways. The chemistry of aziridines is dominated by ring‐opening reactions, the driving force for which is relief of ring strain. By suitable choice of sub‐stituents on the carbon and nitrogen atoms, excellent stereo‐ and regiocontrol can be attained in ring‐opening reactions with a wide variety of nucleophiles, including organometallic reagents; this makes chiral aziridines useful as substrates for the synthesis of important biologically active species including alkaloids, amino acids, and /Mactam antibiotics. Substrate‐controlled diastereo‐selective synthesis is also possible by use of aziridines as removable chiral auxiliaries, while metalation at a ring carbon atom allows aziridines to be used as chiral reagents for asymmetric synthesis. Chiral bisaziridines can act as ligands for transition metals, and applications in the challenging field of enantioselective catalysis can be envisioned. Today, the exclusion of three‐membered carbo‐ and heterocycles from the arsenal of the organic chemist is inconceivable.H. Heimgartner, Angew. Chem. 1991, 103, 271; Angew. Chem. Int. Ed. Engl. 1991, 30, 238.
Theoretical Definition of a Functional Group and the Molecular Orbital Paradigm
Authors: Bader Richard Frederick William , Popelier Paul Lode Albert , Keith Todd Alan 
Published: December 2003

AbstractIt is the purpose of this review to demonstrate that the empirical classification of the observations of chemistry in terms of the properties assigned to functional groups is a consequence of and is predicted by physics. This is accomplished by showing that the atoms and functional groups of chemistry can be identified with bounded space‐filling objects whose properties are defined by quantum mechanics. The quantum mechanical definition of a group is combined with a new pictorial representation of its form to obtain a unified picture which should make it eminently recognizable to chemists. This picture, when combined with the demonstrated ability of these groups to recover the measured properties of atoms in molecules, is offered as one which meets the expectations a chemist associates with the concept of a functional group. The manner in which this physical definition of a group differs fundamentally from models of functional groups based upon molecular orbital theory is discussed.
Determination of Reaction Paths for Pentacoordinate Metal Complexes with the Structure Correlation Method
Authors: der Heyde Thomas Auf 
Published: December 2003

AbstractCrystal structures potentially deliver far more information than is present in the average structural communication—if sufficient structural data on closely related molecules or molecular fragments are available, it may be possible to infer details of geometric changes occurring along certain reaction pathways for the species of interest. This geometric information is extrapolated from an analysis of the similarities between the structures of the fragment in the various crystalline environments, by a method that is now known as structure correlation analysis. Since it was first proposed twenty years ago, the method has been applied to a large variety of chemical systems, but none have received as much attention as the class of five‐coordinate compounds. Comparative analyses of the structures of pentacoordinate complexes have yielded information about the intimate mechanisms of substitution and addition/elimination reactions at tetrahedral and square‐planar complexes, and about intramolecular isomerizations of five‐coordinate compounds. Since its inception, the structure correlation method has gradually adapted techniques from other branches of science, in particular group‐theoretical and multivariate statistical techniques, which have been shown to be enormously powerful tools for probing geometrically complex systems. This review traces the development of the method of structure correlation and the evolution of these co‐opted techniques, with a specific emphasis on studies of five‐coordinate metal complexes.
Ligands as “Compass Needles”: How Orientations of Alkene, Alkyne, and Alkylidene Ligands Reveal π‐Bonding Features in Tetrahedral Transition Metal Complexes
Authors: Gibson Vernon C. 
Published: December 2003

AbstractThis article outlines the strikingly simple structural pattern to which transition metal complexes containing π‐bonding ligands conform, and suggests simple rules (and ligand classifications) by which the pattern can be understood, and by which individual structures can be rationalized or predicted. Alkenes, alkynes, and alkylidene ligands adopt distinct orientations with respect to other ligands attached to a transition metal because of competition between the various ligand types for metal‐ligand π bonds, whatever the coordination geometry. However, a particularly interesting feature of the tetrahedral geometry is that “single‐faced” π ligands can function as “sensors” of relative ligand π‐donor capacities for all other groups attached to the metal. Here, using simple “triad representations” of tetrahedral complexes, a simple qualitative analysis is presented that builds on calculations by Hoffmann et al. and which allows preferred ligand orientations and relative ligand π‐bond strengths to be deduced for a wide variety of these transition metal complexes.
Sterically Stabilized Vesicles
Authors: Lasic Danilo D. 
Published: December 2003

AbstractAfter significant developments in liquid crystal and polymer research, scientists became interested in lyotropic systems containing polymers. These studies investigated, for instance, phase behavior and stability characteristics of suspensions of colloidal particles containing water‐soluble or surface‐adsorbed polymers or block copolymers. The most frequently studied were micelles, latex prticles, and lipid vesicles (liposomes). Liposomes aggregate and fuse in the presence of hydrophilic polymers but their properties were difficult to explain when block copolymers were adsorbed or surfactants with larger polymeric polar heads were inserted into the liposome membrane, because such systems are inherently ill defined. Liposomes containing diacyl surfactants with covalently linked, longer polymer chains display many new properties with very important consequences for both basic and applied research. They stimulate fundamental studies on phase behavior and polymer conformation, scaling laws, colloidal and surface properties, and cell function: applications deal predominantly with liposomes as drug delivery systems. While in basic research theory is currently more advanced than experiment, in medical applications theoretical understanding lags behind experimental achievements. It was discovered only relatively late that liposomes with an appropriate polymer coating are significantly more stable in a biological milieu, a necessary condition for their utility as drug carriers. In particular in medical applications, this practice has rejuvenated the field of anticancer therapy and targeted drug deliviery. All these advances were made possible by an effective and synergistic overlap of many different disciplines.
Ligand Effects on the Chemoselectivity of Transition Metal Catalyzed Reactions of α‐Diazo Carbonyl Compounds
Authors: Padwa Albert , Austin David J. 
Published: December 2003

AbstractThe transition metal catalyzed reaction of α‐diazo carbonyl compounds has found numerous applications in organic synthesis, and its use in either heterocyclic or carbocyclic ring formation is well precedented. Early work in this area made use of insoluble copper catalysts. Although these catalysts are still employed today, their use has decreased significantly with the advent of homogeneous copper catalysts and catalysts based on other metals. The discovery that RhII carboxylates facilitate nitrogen loss from diazo compounds rekindled significant interest in the field of diazo/carbenoid chemistry. Since the realization that RhII carboxylates are superior catalysts for the generation of transient electrophilic metal carbenoids from α‐diazo carbonyl compounds, intramolecular carbenoid addition and insertion reactions have assumed strategic importance in CC bond‐forming reactions in organic synthesis. In contrast to other catalysts that are suitable for carbenoid reactions of diazo compounds, those constructed with the dirhodium(II) framework are most amenable to ligand modifications that, in turn, can influence reaction selectivity. This article will emphasize the chemical behavior of transition metal carbenoid complexes that are greatly affected by the nature of the ligand groups attached to the metal center. Much of the discussion will center on the ability of the dirhodium(II) ligands to determine reaction preference toward different functional groups on the same molecule.
Toxicity of Chlorinated Organic Compounds: Effects of the Introduction of Chlorine in Organic Molecules
Authors: Henschler Dietrich 
Published: December 2003

AbstractIndustrial “chlorine chemistry” is accused of polluting the environment with its products and having adverse effects on health. The accusation has been followed by the demand that this ill‐defined branch of chemistry be simply abolished, a demand which has been repeated vigorously now for years. It grew out of misgivings such as those summarized here: 1. A number of polychlorinated cyclic compounds are very stable and highly lipophilic so that they accumulate in the environment and affect mankind through the food chain. Prominent examples are chlorinated pesticides, biphenyls, dibenzodioxins, and dibenzofurans. 2. Many chlorinated compounds are toxic for man and animals. Their genotoxicity (mutagenicity and carcinogenicity) plays a role in risk assessment; more and more chlorinated organic compounds are being shown to be carcinogenic and classified accordingly.To date there are no comprehensive reviews of the toxicology of chlorinated organic compounds. The present treatise is the result of an attempt to analyze the situation systematically. It makes generalizations as to the kind and degree of toxicity of the most important classes of chlorinated organic compounds and elaborates recommendations for reduction and avoidance of risk during production, occupational handling, and general use.
Otto Roelen, Pioneer in Industrial Homogeneous Catalysis
Authors: Cornils Boy , Herrmann Wolfgang A. , Rasch Manfred 
Published: December 2003

AbstractOtto Roelen discovered the oxo synthesis (hydroformylation) in 1938, and despite all the problems created by the war years he was able to explore successfully the fundamental aspects related to its application up to the point of building the first plant. At the same time he laid the groundwork for industrial utilization of homogeneous organometallic catalysts. Almost simultaneously, his contemporary, Walter Hieber, was investigating the basic chemistry of the same catalysts, but with no knowledge whatsoever of their potential application. Hydroformylation today constitutes one of the most important industrial examples of a homogeneous catalytic process.For corrigendum see DOI:10.1002/anie.199423481