bluuebunny/crossref_metadata_2025_split huggingface dataset:
Developments in Arylnitrene Chemistry: Syntheses and Mechanisms [New synthetic methods (31)]
Authors: Iddon B. , Meth‐Cohn O. , Scriven E. F. V. , Suschitzky H. , Gallagher P. T. 
Published: December 2003

AbstractAryl‐ and heteroaryl‐nitrenes can take part in intra‐and intermolecular reactions in both of their possible electronic states (triplet and singlet). In this review we have endeavored to high‐light recent synthetic uses of these reactive intermediates as well as draw attention to avenues open to further exploration in this field. Singlet arylnitrenes will interact with suitable ortho‐positioned substituents to give a variety of fused azoles, some in excellent yield. In suitable solvents and in presence of amines and alcohols, phenylnitrenes undergo ring expansion to azepines which can also occur in nitrenes of certain fused bicyclic aromatics (naphthalene, quinoline, isoquinoline, benzo[b]thiophene). The latter nitrenes may also give rise to o‐diamines which are useful starters for further heterocyclic synthesis. Triplet arylnitrenes (usually regarded as having only a nuisance effect in synthetic work) may also be utilized in practicable heterocyclic syntheses within a suitable molecular framework. Decomposition of aryl azides in a mixture of an organic and polyphosphoric acid leads to fused oxazoles. The mechanism is discussed for all the reactions considered.
Capto‐dative Substituent Effects in Syntheses with Radicals and Radicophiles [New synthetic methods (32)]
Authors: Viehe Heinz Günter , Merényi Robert , Stella Lucien , Janousek Zdenek 
Published: December 2003

AbstractThe 100 yers old Wurster's salts have long been recognized as compounds with redical cations. Their unusual stabilization derives partly form capto‐dative (cd) substitution. This principle is now discussed as one factor of radical stabilization and it is applied to simple methine derivatives. cd‐Substitution has synthetically useful applications: cd‐substituents on a carbon atom allow its selective dehydrodimerization. Olefines with geminal and thus cross‐conjugated cd‐substituents are “radicophilic” and permit twofold carbon radical addition. cd‐Substituted olefines are useful antioxidants, polymerization inhibitors and are promising agents in the control of biological radical reactions. Generally, many reactions of cd‐substituted molecules appear to involve radicals.
Reaction Paths on Multidimensional Energy Hypersurfaces
Authors: Müller Klaus 
Published: December 2003

AbstractTheoretical investigation of a chemical reaction requires detailed knowledge of the potential energy of the molecular system. As a first step in such a study, minimum‐energy reaction paths have to be mapped on a generally multidimensional molecular potential surface. Even at this stage problems are encountered that have only recently been solved satisfactorily. The difficulties involved and various ways to handle them are discussed in connection with a two‐parametric model potential. Three chemical reactions examined illustrate the usefulness of such theoretical investigations as well as their current limitations.
A Model of Hydrogen‐Bonded Liquids
Authors: Luck Werner A. P. 
Published: December 2003

AbstractThe orientation defect model can be used for quantitative estimates and for understanding the properties of H‐bonded liquids, such as water and alcohol. The defect concentrations can be determined by vibrational spectroscopy, and the applicability of the approximation procedure derives from considering H‐bonds as chemical equilibria. Possible extensions of the simple model are critically discussed.
Chemistry and Function of Human Plasma Proteins
Authors: Schwick Hans‐Gerhard , Haupt Heinz 
Published: December 2003

AbstractHuman blood plasma contains a large number of proteins. New analytical and preparative techniques have so far permitted the isolation of more than one hundred such proteins; about ninety percent of these were first characterized in the last 30 years. The plasma proteins include components of the clotting and complement systems as well as proteinase inhibitors, immunoglobulins, lipoproteins, and carrier proteins. The biological function of some of the plasma proteins is not yet known. Deficiency of one or more plasma proteins usually causes serious health disorders, the best known example being hemophilia. Several biologically active proteins can be gained from blood plasma; these proteins—for example the blood clotting factors—are of great prophylactic and therapeutic importance and allow a better exploitation of the valuable plasma than can be obtained by means of transfusion.
Electrophilic Reagents—Recent Developments and Their Preparative Application
Authors: Effenberger Franz 
Published: December 2003

AbstractThe most widely known electrophilic agents are protic acids and compounds with an electronsextet partial structure. Recent research has aimed at the development of new electrophilic reagents, with greater reactivity on the one hand and higher selectivity on the other, which would largely obviate the addition of Lewis acids (Friedel‐Crafts catalysts), and also allow control of the isomer ratio in reactions with ambivalent substrates. Compounds with “super‐leaving groups”, such as trifluoromethanesulfonate and fluorosulfate, have been demonstrated as most advantageous in this respect since they are sufficiently polarized or dissociated for reactions to take place even in the absence of Friedel‐Crafts catalysts. Heterocycles such as pyridones or imidazole are likewise suitable leaving groups; they are employed for their high selectivity, and also because they allow working under non‐aqueous conditions.
Viroids: A Class of Subviral Pathogens
Authors: Gross Hans J. , Riesner Detlev 
Published: December 2003

AbstractInfectious RNA molecules lacking a protein coat have recently been shown to be the cause of several diseases in higher plants. These molecules, termed viroids, have been characterized as circular chains composed of about 360 nucleotide residues. They therefore comprise a class of infectious pathogens even smaller than viruses or bacteriophages. As a result of work on viroids it is now possible for the first time to give a complete description of the structure of a eukaryotic pathogen. Viroids possess structural and dynamic properties which have not as yet been observed with other nucleic acids. This article summarizes and discusses the results of predominantly biochemical and physicochemical studies on viroids.
The Kinetic and Mechanistic Evaluation of NMR Spectra. New analytical methods (18)
Authors: Binsch Gerhard , Kessler Horst 
Published: December 2003

AbstractIn addition to the static parameters of the chemical shifts and coupling constants, which serve as a source of knowledge for molecular structure and stereochemistry, an NMR spectrum can frequently furnish dynamic quantities characterizing relaxation and exchange phenomena. The information about nuclear switching processes has proved to be particularly useful in practice for the detection of internal molecular motions and for the estimation or determination of the corresponding energy barriers. A plethora of studies of this nature has in the past been performed on simple proton spectra. Methodological developments of recent years have led to a significant reduction of the effort required for the quantitative dynamic evaluation of NMR spectra arising from complex spin systems or involving other nuclei. In many cases it has, moreover, become possible to extract detailed mechanistic information inaccessible by other means. The practical execution of such analyses will be explained and illustrated by a selected number of applications.
Towards an Understanding of the Carbon‐Carbon Bond
Authors: Rüchardt Christoph , Beckhaus Hans‐Dieter 
Published: December 2003

AbstractIn this essay, the classical question of “the influence of the number and kind of substituents on the strength of the CC bond”, is pursued with the modern tools of contemporary physical organic chemistry. Based on the work of Karl Ziegler, the products and kinetics of thermolysis of a large number of highly branched aliphatic hydrocarbons and phenyl‐ or cyano‐substituted derivatives were investigated. For each class of compounds, a linear relationship was found between the free enthalpy of activation of the homolytic cleavage of the weakest CC bond and the strain energy in the ground state. These relationships permitted a quantitative separation of steric and electronic effects on the cleavage of CC bonds. The influence of the size of the substituent groups on bond angles, bond lengths, and the conformational behavior of model compounds was studied by means of experimental structure determinations and force field calculations. CC bond lengths up to 164 pm, bond angles at tetracoordinate carbon as large as 126°, and unusual eclipsed and gauche preferred conformations were found.
High‐Resolution Transmission Electron Microscopy Re‐examined as a Tool in Solid State Chemistry
Authors: Gruehn Reginald , Mertin Wilhelm 
Published: December 2003

AbstractTransmission electron microscopy (TEM) can be used with crystalline solids to obtain direct images of small structural groups comprising a few coordination polyhedra with resolution nearly down to atomic scale (“lattice imaging”). More exact knowledge of the conditions required for direct imaging, as well as improvements in the instruments themselves, have now made it possible to examine very small defect regions (microdomains), faults in the stacking sequence of structural groups or atom layers (planar or Wadsley defects), and isolated defects in narrowly delimited areas that may actually be below the dimensions of the unit cell. The structural principle of the very smallest ordered regions can even be determined when X‐ray structure analysis proves unable to do this. “Block structures” are particularly suitable as models for the testing and further development of the high‐resolution method; the detection of three‐dimensional, two‐dimensional, and one‐dimensional defects has been studied on such structures.
Liquid Metals and Liquid Semiconductors
Authors: Hensel Friedrich 
Published: December 2003

AbstractNumerous liquid systems have electrical properties which resemble those of crystalline and amorphous semiconductors. The existence of “semiconducting” behavior in these liquids is mostly related to a continuous transition from a metallic to a “semiconducting” state when a thermodynamic variable such as temperature, density or concentration is changed. Changes in the nature of the chemical interaction and the associated changes in the structure of the liquid are of fundamental importance for the transition to a “semiconducting” state. This will be demonstrated for the ionic liquid CsAu, for covalent liquid selenium, and for expanded liquid metals.
Structure and Function of the Energy‐Converting System of Mitochondria
Authors: von Jagow Gebhard , Engel Wolf Dieter 
Published: December 2003

AbstractThe main energy source for all endergonic processes occurring in living organisms is the phosphate bond energy of nucleoside triphosphates, especially adenosine triphosphate (ATP). In aerobic organisms, as for instance in mammals, more than 90% of ATP is formed during the process called oxidative phosphorylation. In this process, similar to that of muscle contraction and nerve excitation, nature works with vectorial processes taking place at a membrane separating distinct spaces from each other. The present article deals with the operation of a set of water‐insoluble membrane proteins and enzymes vectorially transporting electrons, protons and other ions, which finally leads to the formation of ATP. This machinery transforming substrate oxidation energy into chemical energy in the form of the phosphoric anhydride bond of ATP operates with a very high efficiency.The structure and function of the machinery of mitochondrial oxidative phosphorylation are described. It consists of the electron transfer chain, the ATP‐synthetase, the adenine nucleotide translocase and the phosphate carrier. The electron transfer chain can be resolved into multiprotein complexes—at three of them energy conversion takes place—and into the electron carriers ubiquinone and cytochrome c. The substrate oxidation energy is converted into the chemical energy of ATP with an electrochemical proton gradient as intermediary form. The energetic aspects of the processes are analyzed by linear irreversible thermodynamics. Great success has been gained during the past few years on the structural characterization of the participating proteins. The function of the various systems is partially elucidated on the molecular level; this concerns especially the mechanism of proton and adenine nucleotide translocation, as well as ATP formation.
The Structure of Simple Molecular Liquids
Authors: Zeidler Manfred D. 
Published: December 2003

AbstractLiquid structure is described by the pair‐correlation function. If the liquied particles are molecules then the respective correlation function depends on distance and several angular variables; hence, for the sake of simplicity a symmetry‐adapted representation must be used. The most important structural parameters of this molecular pair‐correlation function can be determined approximately by a combination of several scattering experiments, such as neutron (employing different isotopes), X‐ray and electron diffraction. Light scattering, dielectric, and nuclear magnetic relaxation experiments provide further, albeit less direct, access to such determinations. Some results obtained by measurements on chloroform are reported. Theoretical computations of these structural parameters are also discussed.
Recent Progress in Oxo‐ and Fluorometalate Chemistry
Authors: Hoppe Rudolf 
Published: December 2003

AbstractAbout a quarter of a century ago a review article having almost the same title appeared in this journal[1]. Since then many hundreds of new fluorides and oxides of metals have been synthesized, and repeatedly subjected to detailed investigation. Why, and to what end are such compounds still studied[2]? Has our knowledge been not only widened but also deepened? What advances have been made in synthetic chemistry in this sector? Have new ideas led to unforeseen results and have unexpected findings forced the revision of tested concepts? This area of research belongs to solid state chemistry, and in the meantime has become almost unsurveyable even for a committed researcher. In this paper, therefore, an attempt is made to outline any relevant advances that have been made and to present open questions and new aspects using selected examples, mainly from the chemistry of the first row of the transition metal series. Those not directly involved in this area may be surprised to find that even substances with a simple composition are also cited. They might ask whether such compounds mentioned in text books are not already understood. Although it is a widely‐held view that such compounds are well known, this is incorrect: Probably no‐one has ever prepared a sample of CrF2 or Na2O whose composition “adequately” exactly corresponded to the quoted formula[3]. Typical examples which demonstrate the considerable effort necessary for finally proving what others long ago already assumed to know, can be taken from the area of inorganic chemistry (e.g.: As2O) as well as from organic chemistry (e.g. C4[C(CH3)3]).
Irritant and Defense Substances of Higher Plants—A Chemical Herbarium
Authors: Schildknecht Hermann 
Published: December 2003

AbstractAll living organisms respond to stimulation, reacting more or less sensitively and more or less typically to a wide variety of energy forms such as light, heat, gravity, pressure (sound), electricity—and chemicals. A living organism responds to an irritation by releasing irritants which—as potential defense substances—are directed against the attacker or assist the organism endogenously in an intrinsic defense reaction. Often very small energy changes perceived by the plant are enough to induce a series of physiological processes ultimately manifested as a glandular reaction or even movement. The irritants involved in these processes act on membranes as defense substances in the presence of an attacker, or as endogenous factors in their own cellular environment. These chemically very diverse low‐molecular active principles have been found in many parts of plants and in many plant families. For this reason alone we could speak of a chemical herbarium, but the case is even stronger because, in this botanical documentation, not only the individual chemicals are considered in context but also whole sets of interacting substances, since it is only in these sets that optimal activity is found (just as one considers not only the parts of a plant but also the whole plant in botany).
Target Directed Drug Synthesis: The Aminoacyl‐tRNA Synthetases as Possible Targets
Authors: von der Haar Friedrich , Gabius Hans‐Joachim , Cramer Friedrich 
Published: December 2003

AbstractThe “tailor‐made” pharmaceutical has been an old dream ever since Paracelsus' days. His saying “Dosis sola facit venenum” is still valid. The ideal pharmacon should inhibit the pathological process or the parasitical organism to a maximum while causing as little harm as possible to the human organism. In order to achieve this goal one must try to make use of metabolic differences between the metabolism of the pathological organism and normal human metabolism in a rational way. With today's improved knowledge of enzymatic processes this seems to be a possible and highly promising approach. The pharmaceutical should act upon a process of central importance, such as the process of protein biosynthesis, where the required highly accurate construction of the macromolecules is achieved by a “proofreading” process. It is shown that this “proofreading” mechanism exhibits specific difference in different species.
Half a Century of Polystyrene—A Survey of the Chemistry and Physics of a Pioneering Material
Authors: Echte Adolf , Haaf Franz , Hambrecht Jürgen 
Published: December 2003

AbstractHermann Staudinger, the founder of polymer chemistry, would have been 100 years old this year. One of the key materials with which he elucidated the structure of high polymers was polystyrene.—Polystyrene, one of the most important thermoplastics, has now been manufactured industrially for some 50 years. Not only has it simplified our daily lives in a variety of ways, it has also been a model substance for the development and expansion of polymer chemistry and polymer physics, and it was the pioneering material for handling polymer solutions and melts. This article surveys the development and present state of knowledge of the polymerization of styrene and of the relationships between structure and properties.
Polymers with Metal‐Like Conductivity—A Review of their Synthesis, Structure and Properties
Authors: Wegner Gerhard 
Published: December 2003

AbstractPolymers such as polyacetylene, which have an extended π‐electron system in their backbone, or like poly(p‐phenylene) consist of a sequence of aromatic rings are excellent insulators in their native state and can be transformed by oxidation or reduction in the solid state into conductive CT‐complexes which exhibit metal‐like conduction characteristics. The chemical and physical processes involved and the reasons for the observed quasimetallic conductivity are not yet fully understood. The real structure of these materials in chemical and physical terms, i. e. their complicated morphology and texture, as well as the results available on the structure‐property relationships of the “organic metals” must be considered when discussing their properties. In other words, a discussion of conductive polymers should be based on what is known of the highly conducting CT‐complexes of low‐molecular weight compounds. The discovery of the highly conducting polymer complexes has opened up a new interdisciplinary field of research which borders on polymer science, solid‐state and semiconductor physics and on organic solid‐state chemistry. It is hoped that this area will lead to numerous novel materials and technical applications.
Structure and Bonding in Cyclic Sulfur‐Nitrogen Compounds—Molecular Orbital Considerations
Authors: Gleiter Rolf 
Published: December 2003

AbstractThe molecular structures of monocyclic sulfur‐nitrogen ring systems, such as S2N2, S3N, S4N and S5N, can be considered as examples of electron rich (4n + 2)π systems. The structures of S4N4, S4N, P4S4, As4S4 and the bicyclic structures S4N, S4N as well as S5N6 can be rationalized on the basis of a planar tetrasulfur tetranitride with 12π electrons.
Methods of Fluorination in Organic Chemistry
Authors: Gerstenberger Michael R. C. , Haas Alois 
Published: December 2003

AbstractSince the days of the “Manhattan Project” our knowledge of the chemistry of fluorine‐con‐taining compounds has increased tremendously. At first only considered out of curiosity, this area of research bordering between inorganic and organic chemistry has developed from modest beginnings to such a complexity of interesting ramifications that it would be very difficult to review all aspects in detail. The motivating force behind the developments in this field were, above all, the unusual properties the numerous compounds acquire on introduction of fluorine substituents. The spectrum of unusual modes of behavior extend from extreme stabilization in fluorine‐containing polymers and blood‐substitutes to drastic increases in reactivity in pharmacologically and phytomedicinally active substances and in dyes. Moreover, one physical peculiarity of the fluorine atom, its magnetic moment, opens a new route to the decyphering of metabolic processes.—Parallel to the ever increasing number of fluorinated compounds there has been a concomitant increase in the variety of methods of synthesis and reagents, which is frequently difficult to overview in entirety, even for the specialists. Older, simple methods such as halogen metathesis with metal fluorides and electrofluorination are being refined as far as higher selectivity is concerned, and completely new, very reactive substances such as hypofluorites and noble‐gas fluorides enable fascinating reactions on complex substrates,e. g.steroids and nucleobases. The present review is an attempted documentation and classification of the methods as well as a critical appraisal, not least from the standpoint of efficiency and economic aspects.
Chemistry and Biochemistry of Microbial α‐Glucosidase Inhibitors
Authors: Truscheit Ernst , Frommer Werner , Junge Bodo , Müller Lutz , Schmidt Delf D. , Wingender Winfried 
Published: December 2003

Abstractα‐Glucosidases are among the most important carbohydrate‐splitting enzymes. They catalyze the hydrolysis of α‐glucosidic linkages. Their substrates are—depending on their specificity—oligo‐ and polysaccharides. Microbial inhibitors of α‐amylases and other mammalian intestinal carbohydrate‐splitting enzymes studied during the last few years have aroused medical interest in the treatment of metabolic diseases such as diabetes. Moreover, they extend the spectrum of microbial secondary metabolites which comprises an enormous variety of structures. They also contribute considerably to a better understanding of the mechanism of action of α‐glucosidases. These inhibitors belong to different classes of substances. Those studied most thoroughly are microbial α‐glucosidase inhibitors which are members of a homologous series of pseudooligosaccharides of the general formula (4). They all have a core in common which is essential for their inhibitory action, a pseudodisaccharide residue consisting of an unsaturated cyclitol unit, and a 4‐amino‐4,6‐dideoxy‐ glucose unit. The—in many respects—most interesting representative of this homologous series is acarbose (5), a pseudotetrasaccharide exhibiting a very pronounced inhibitory effect on intestinal α‐glucosidases such as sucrase, maltase and glucoamylase. The present paper will review this new field of microbial α‐glucosidase inhibitors which has been studied with particular intensity during the past ten years.