bluuebunny/crossref_metadata_2025_split huggingface dataset:
Three‐Dimensional Structure of the Actinomycins
Authors: Lackner Helmut 
Published: December 2003

AbstractMany complicated three‐dimensional structures can be formulated for the actinomycins, red chromopeptide antibiotics distinguished by pronounced antineoplastic activity, because of their unique constitutional type. Since these structures largely determine the specific activity of the actinomycin molecules in the host cells, knowledge of the structural forms occurring naturally in solution is a decisive prerequisite for detailed insight into the mode of biological action of the actinomycins. Investigations by several methods have shown that, in spite of their deviating primary structures, all important actinomycins assume only one very characteristic, pseudo‐C2‐symmetrical, and remarkably stable three‐dimensional structural type.
Ternary Cu2+ Complexes: Stability, Structure, and Reactivity
Authors: Sigel Helmut 
Published: December 2003

AbstractThe driving forces leading to the formation of ternary Cu2+ complexes are outlined; they are (i) statistical reasons, (ii) neutralization of charge in the mixed‐ligand complexes, (iii) steric factors (bulky groups, ring size of chelates), and (iv) formation of π bonds. The last point is important for achieving a large stability, as well as for the observation of discriminating qualities. An additional stability increase may possibly be achieved through a direct interaction between the two ligands bound to the same metal ion, i.e. Schiff base formation, hydrogen bonding or a charge‐transfer relation. The latter leads to distinct structures, i.e. metal ion‐bridged charge‐transfer complexes. The relation between stability and structure on the reactivity of mixed‐ligand complexes is emphasized. The relevance of the results for mixed‐ligand complexes containing metal ions other than Cu2+ is briefly outlined.
Field Desorption Mass Spectrometry
Authors: Beckey H. D. , Schulten H.‐R. 
Published: December 2003

AbstractField desorption (FD) enables mass‐spectrometric investigation of large organic molecules without their vaporization. The present state of our theoretical understanding of the ionization of these molecules in the adsorbed state on organic emitters is described. The special problems of the technique and prospective developments in the apparatus for future analytical problems are outlined. The present progress report concentrates on analytical studies of biochemical model compounds and degradation products from environmental chemicals and drugs. The method is particularly suitable for the detection and identification of submicrogram quantities of underivatized polar substances present in complex mixtures or pre‐purified extracts from biological materials.
Ring Contractions and Expansions of Vicinally Disubstituted Cyclobutanes and Cyclopropylmethyl Compounds
Authors: Conia J. M. , Robson M. J. 
Published: December 2003

AbstractCyclobutanes and cyclopropylmethyl compounds vicinally disubstituted in appropriate manner by one electron‐donating group and one leaving or electron‐receiving group undergo extremely facile ring contractions and expansions respectively. Unlike many rearrangements of small‐ring compounds and most rearrangements of larger‐ring compounds, these reactions usually give a single product in high yield and thus possess real synthetic interest. The subject matter of this progress report is classified according to the substituents in question since the nature of these groups largely determines the conditions under which rearrangement takes place.
Organic Compounds in Nature: Limits of Our Knowledge
Authors: Blumer Max 
Published: December 2003

AbstractCurrent geochemical and environmental research reveals the presence in soils and sediments of extremely complex assemblages of organic compounds. Their recognition has long been delayed by both conceptual and analytical limitations. Even today the best techniques cannot fully resolve these mixtures into their individual components. Yet, the knowledge of their structures and abundances is needed in geochemistry, and especially for the assessment of their potential biological effect and ecological impact. The classical “natural products approach” is unlikely to provide this information; therefore, I foresee the need for a more realistic assessment of nature, that acknowledges the limitations of our present analytical powers and of our knowledge.
Methods for the Preparation of Bridgehead Olefins
Authors: Keese Reinhart 
Published: December 2003

AbstractDuring the past eight years, bridgehead olefins have attracted rapidly increasing attention. In view of their significance with regard to the stereochemistry of alkenes, the study of certain reaction mechanisms, and the nature of the double bond, detailed research into this structural type appears highly desirable. Bridgehead olefins represent connective linkages between olefins in the ground state and species which can arise in the deactivation of photochemically excited alkenes and cycloalkenes and also contribute to our understanding of the structural conditions prevailing during cis‐trans isomerization of alkenes.
Chemical Nickel Plating
Authors: Niederprüm Hans 
Published: December 2003

AbstractMetal layers can be deposited from aqueous solutions not only by electric current as in electroplating but also by strong reducing agents. Chemical deposition of nickel on metallic and nonmetallic substrates, together with the long‐known silvering of glass, are the most widespread examples. This remarkable reaction has recently found a variety of industrial applications mainly in the preparation of corrosion‐resistant, hard, and wear‐resistant protective layers, in the metallizing of plastics, and in electronics.
Synthetic Applications of Heteroaromatic Betaines with Six‐Membered Rings
Authors: Dennis Nicholas , Katritzky Alan R. , Takeuchi Yoshito 
Published: December 2003

AbstractThe cycloadditions of 3‐hydroxy‐1‐methylpyridinium betaines with electron‐deficient olefins have been extended to a variety of 1‐substituents in place of methyl. Application of resonance theory and later of quantum chemical methods led to rationalization of preceding results and prediction of new reactions. Thermal additions of 2π‐ and 6π‐electron addends at the 2,6‐positions and of 4π‐electron addends at the 2,4‐positions are described. The cycloadducts are highly interesting potential synthetic building blocks.
Sequence Analysis of Polypeptides and Proteins by Combined Gas Chromatography‐Mass Spectrometry
Authors: Nau Heinz 
Published: December 2003

AbstractA new method for determining the amino acid sequence of polypeptides consists in initial partial hydrolysis to yield a complex mixture of oligopeptides. After derivatization to enhance its volatility, the mixture is analyzed by combined gas chromatography and mass spectrometry. The sequence of the polypeptide is established by a computer from the identified oligopeptides. So far polypeptides having up to 40 amino acids have been analyzed by this method. The advantages and disadvantages of the new method compared with the stepwise procedure of the Edman degradation are considered. Since the two methods are based on fundamentally different principles they may prove to be complementary.
The Fischer‐Tropsch Synthesis: Molecular Weight Distribution of Primary Products and Reaction Mechanism
Authors: Henrici‐Olivé G. , Olivé S. 
Published: December 2003

AbstractAn evaluation of literature data concerning the production of hydrocarbons and alcohols by the Fischer‐Tropsch procedure indicates that the molecular weight distribution of the primary products can be described by the Schulz‐Flory equation (“normal” or “most probable” distribution). It follows that the highest attainable selectivity for a given molecular weight range can be calculated. A reaction mechanism is suggested.
Application of Catalyzed Reactions in Analytical Chemistry
Authors: Weisz Herbert 
Published: December 2003

AbstractCatalyzed reactions are becoming increasingly improtant in analytical chemistry, and especially in trace analysis. The acceleration of a particular reaction (indicator reaction) promoted by the catalyst provides information about the catalyst concentration. Substances which lower or raise the activity of a catalyst (inhibitors or activators, respectively) can likewise be determined in this way. The course of a reaction is usually monitored by physical measuring techniques. Various methods of conducting these kinetic‐catalytic studies are discussed in this article.—Use of catalyzed reactions for end point detection in volumetric analysis is also feasible.
The Concept of Allopolarization. Effects of Substituents on the Reactions of Ambifunctional Anions
Authors: Gompper Rudolf , Wagner Hans‐Ulrich 
Published: December 2003

AbstractThe chemistry of ambifunctional anions is of great importance in organic preparations. Planning of syntheses requires a set of rules correlating structure and selectivity. The concept of allopolarization permits a description of substituent effects on kinetically controlled reactions: the change in selectivity is a function of the change in polarity of the ambifunctional anions. The relative charge density P = lx/ly serves as “polarity index”; X and Y are the potential sites of reaction. An attempt is made to illustrate the utility of this principle by means of examples taken from various classes of compounds.
Penetration and Crosslinking of Macromolecules
Authors: Braun Dietrich 
Published: December 2003

AbstractOne of the topical problems of macromolecular chemistry is the question of the extent to which macromolecules in solution or in the amorphous solid state can engage in mutual interpenetration. The present article is concerned primarily with recent investigations on cross‐linking copolymerization and with inter‐and intramolecular reactions of polymers as a contribution to the study of molecular shape.
Molecular Complexes and Oxygen Adducts of Tetrapyrrole Pigments
Authors: Fuhrhop Jürgen‐Hinrich 
Published: December 2003

AbstractMetalloporphyrins not carrying functional groups and the π‐radicals of these species form comparably stable coplanar π‐π‐dimers by means of various types of weak interactions. Chlorophyll a and other derivatives containing carbonyl groups add water and aggregate into molecular complexes by means of hydrogen bonds. Such aggregates are of special interest in connection with a new hypothesis for the photo‐oxidation of water to oxygen during photosynthesis. Finally, Some new discoveries concerning the structure and reactivity of heme‐oxygen complexes are discussed.
Processing of Plastic Waste and Scrap Tires into Chemical Raw Materials, Especially by Pyrolysis
Authors: Sinn Hansjörg , Kaminsky Walter , Janning Jörg 
Published: December 2003

AbstractAn account of the occurrence of pyrolyzable material and the state of development of pyrolysis processes in the USA and Japan is followed by a description of laboratory and pilot‐scale experiments designed to exploit the raw material content of pyrolyzable substances rather than the heating value. Plastic waste and scrap tires can be pyrolyzed to give up to 40% by weight of aromatic compounds in fluidized sand beds which are surprisingly insensitive to the unit size of the feed material.
Hormone Receptors
Authors: Lübke Klaus , Schillinger Ekkehard , Töpert Michael 
Published: December 2003

AbstractHormone receptors are proteins located on the cell membrane or in the cell cytoplasm. Their function is to recognize and bind the respective hormone; the biological action of the hormone originates from the hormone‐receptor complex. Specific receptors have been found for all known hormones. Monitoring the equilibration between hormone, receptor, and hormone‐receptor complex (“determination of receptor binding”) permits quantitative determinations of hormones, allows the characterization of endocrinal disturbances, and contributes to the elucidation of structure‐affinity relationships.
Molecular Aspects of Sickle Cell Disease
Authors: Waterman Michael R. , Cottam G. Larry 
Published: December 2003

AbstractSickle cell hemoglobin (hemoglobin S) contains valine instead of glutamic acid in position 6 of the β‐chain. Few detectable conformational differences exist between hemoglobin S and normal adult hemoglobin (hemoglobin A). Following loss of oxygen, sickle cell hemoglobin (hemoglobin S) self associates to form a liquid crystal phase which distorts the erythrocyte into the sickle shape thereby resulting in the clinical symptomology associated with sickle cell anemia. This gel contains deoxyhemoglobin S monomers and polymers in equilibrium. The polymerization process is known to have a negative temperature coefficient, to be pH dependent, and to be extremely dependent on hemoglobin S concentration. The polymerization of deoxyhemoglobin S appears to be entropically driven and occurs in two kinetic phases, a delay period and a rapid polymerization process. The polymer consists of tubules containing six or eight strands of deoxyhemoglobin S tetramers which align with one another. Each strand is would around the tubule with a pitch of about 3000 Å, but the precise nature of the intermolecular hemoglobin S contacts is not known. Subsequent alignment of the tubules occurs and results in the tactoids observed in (S/S) erythrocytes. While many of the details of polymer formation and structure remain to be elucidated, several attempts to chemically alter the sickling phenomenon have been carried out. As yet, however, no satisfactory chemical treatment has been discovered.
Analytical Applications of Chemiluminescence
Authors: Mendenhall G. D. 
Published: December 2003

AbstractMost organic compounds react very slowly with atmospheric oxygen at room temperature. The weak chemiluminescence accompanying this autoxidation can be amplified and measured by sensitive instruments. Emission of light from many materials can also be induced mechanically and photochemically. The intensity‐time curves are so characteristic that they can be used to establish whether the composition and history of two samples are identical. Moreover, numerous physical properties can be correlated with emission and thus measured.
Stereoisomeric Quinhydrones of the [2.2](1,4)Naphthalenophane Series
Authors: Staab Heinz A. , Herz Claus P. 
Published: December 2003

AbstractCommunications are brief preliminary reports of research work in all areas of chemistry which, on account of its fundamental significance, novelty, or general applicability, should be of interest to a broad spectrum of chemists. Authors of communications are requested to state reasons of this kind justifying publication on submission of their manuscript. The same reasons should be clearly apparent from the manuscript. In cases where the editorial staff decide, after due consultation with independent referees, that these conditions are not met, manuscripts will be returned to the authors with the request to submit them for publication in a specialist journal catering for scientists working in the field concerned.
The Role of Birth Control in the Survival of the Human Race
Authors: Wiechert Rudolf 
Published: December 2003

AbstractAnalysis of the growth of the world's population shows that if no trenchant measures are taken, the number of the earth's inhabitants is likely to double to 8 billion people within as few as 35 years. Hormonal contraception is presently the best method available which would be capable of counteracting this trend. 6.5 tons of estrogen and 32 tons of progestogen would in theory be sufficient to protect the 500 million women of child‐bearing age from unwanted pregnancies for one year.—In addition, the history, industrial methods of synthesis, mode of action and side effects of steroid contraceptives as well as the possible future developments are briefly mentioned.
Electrocyclic Ring Opening Reactions of Ethylene Oxides
Authors: Huisgen Rolf 
Published: December 2003

AbstractSubstituents capable of stabilizing negative charge endow ethylene oxides with the ability to undergo electrocyclic ring opening at the CC bond; heating or irradiation generates small equilibrium concentrations of carbonyl ylides which can engage in 1,3‐dipolar cycloadditions. Apart from the normal conrotatory mode of ring opening, predicted by the Woodward‐Hoffmann rules, the disrotatory process, forbidden by orbital symmetry, also appears to occur in the case of α‐cyano‐cis‐stilbene oxide. Kinetic studies on α‐cyano‐trans ‐and ‐cis‐stilbene oxide permit construction of the energy profile for electrocyclic ring opening to give the stereoisomeric carbonyl ylides and for their recyclization and rotation.
Principles of Osmotic Desalination
Authors: Böddeker K. W. 
Published: December 2003

AbstractSalt solutions can be separated into pure water and concentrated salt solution by reverse osmosis using semipermeable membranes. The distinct features and limitations of osmotic separations are developed from a consideration of the pertinent solution properties and the conditions inherently to be met by the membranes, seen as interacting barrier phase in a process which substantially separates water from water.
Synthesis and Reactions of N‐Chlorocarbonyl Isocyanate
Authors: Hagemann Hermann 
Published: December 2003

AbstractClCONCO (N‐chlorocarbonyl isocyanate) is prepared in 90% yield by partial hydrolysis of the addition product of phosgene and cyanogen chloride. Many derivatives of the iminocarboxylic acid parent compound can be obtained from this highly reactive species. Both the acid chloride and the isocyanate group are amenable to reactions exhibiting overall selctivity, and this opens the way, inter alia, to a simple preparative synthesis of isocyanates from alcohols, phenols, thiols, and thiophenols. Combination of the two functional groups and their peculiar symmetry, which becomes evident, e.g. in the HCl adduct, facilitates ready cyclization of N‐chlorocarbonyl isocyanate under very mild conditons.
Thermal and Photochemical Cycloelimination of Nitrogen
Authors: Meier Herbert , Zeller Klaus‐Peter 
Published: December 2003

AbstractCyclic azo compounds including heteroaromatics with a NN bond of strong double‐bond character can eliminate molecular nitrogen on addition of sufficient thermal energy or on electronic excitation. Some substrates decompose spontaneously, others are moderately or even extremely resistant to N2‐elimination. From a preparative point of view this type of reaction opens a variety of interesting possibilities, e.g. the synthesis of small carbocycles or heterocycles, strained bi‐ and polycycles, as well as cage compounds. From a theoretical mechanistic viewpoint, current importance attaches to the primary fragments such as carbenes, diradicals, trimethylene, trimethylenemethane, dipoles, ylides, antiaromatic 4π‐systems, unstable cycloalkynes, and arynes.
Solid Ionic Conductors: Principles and Applications
Authors: Rickert Hans 
Published: December 2003

AbstractSolid ionic conductors, also called solid electrolytes, transport electric current by means of ions. The best known examples of these usually crystalline compounds include doped ZrO2, as well as AgI, β‐Al2O3, and CaF2. Ionic conduction in solid electrolytes reaches its maximum value if a partial lattice of a solid compound undergoes transition at elevated temperature to a quasimolten state. The ionic conductivity in such solid compounds is then as high as in molten salts. Solid electrolytes have found many scientific and technological applications; thus, they can be used to study thermodynamic and kinetic problems, and to build fuel cells, batteries, sensors, and chemotronic components.
Restriction Endonucleases
Authors: Arber Werner 
Published: December 2003

AbstractRestriction endonucleases are enzymes commonly found in bacteria which can fragment DNA. Cleavage of the DNA molecule takes place after recognition of a nucleotide sequence which is specific for each restriction endonuclease. Some of the enzymes split the DNA within the recognition sequence. This property allows these enzymes to be used for the systematic fragmentation of DNA molecules in structural and functional analyses of genetic material. The biological function of restriction endonucleases could be to provide a sort of primitive immune system which enables the cells to destroy foreign genetic material. Methylation of nucleotides lying within the recognition sequence serves to protect the cell's own DNA from being degraded by its restriction endonucleases. Restriction endonucleases were discovered during the investigation of the restriction of viral growth observed on changing hosts.
Nitrogen as Leaving Group: Dediazoniations of Aromatic Diazonium Ions
Authors: Zollinger Heinrich 
Published: December 2003

AbstractOn heating, aromatic diazonium salts eliminate nitrogen by two fundamentally different mechanistic pathways, i.e. heterolysis and homolysis. Among these pathways a series of variants are known, giving rise to a whole variety of mechanisms. This article considers the effect of specific solvation, apparently characteristic of diazonium ions, by various solvents (fluorinated alcohols, methanol, water, dimethyl sulfoxide, pyridine, and hexamethylphosphoric triamide) on the mechanism. Apart from the donor properties of the solvent, the oxidation potential and the presence of additives play a major role in determining what reaction type will predominate.
Selective Reactions with Organoaluminum Compounds
Authors: Yamamoto Hisashi , Nozaki Hitosi 
Published: December 2003

AbstractWith the aid of selected examples an overview is given of the development trends in highly discriminative reactions using organoaluminum compounds. (1) Isomerization of substituted oxiranes into allyl alcohols has been effected both stereo‐ and regioselectively by means of a new organoaluminum reagent, diethylaluminum 2,2,6,6‐tetramethylpiperidide (DATMP); this reaction which provides a new route for the regiospecific transformation of allyl alcohols into 1,3‐dienes. (2) A novel nonenzymic heterolysis of allylic phosphates of terpenoids has been effected by organoaluminum compounds and has enabled development of a new method for the selective synthesis of cyclic terpenes. (3) The reductive condensation of α‐haloketones with carbonyl compounds in the presence of diethylaluminum chloride and zinc affords β‐hydroxy‐carbonyl derivatives in good yields with exclusion of undesired self‐condensations products.
Spiroconjugation
Authors: Dürr Heinz , Gleiter Rolf 
Published: December 2003

AbstractSpiro compounds having mutually perpendicular π‐electron systems (“spiro‐polyenes”) exhibit a new kind of homoconjugation, which is designated as spiroconjugation. Spiro‐polyenes are readily accessible,e.g.by cycloaddition, by elimination of dinitrogen from 1,2‐diazaspiro‐polyenes, and by cyclization of carbenium ions or carbanions. The magnitude of the effect exerted by spiroconjugation is estimated by model calculations. It can be detected, for instance, by comparison of the electronic and photoelectron spectra of spiro‐polyenes and analogous compounds which are unable to undergo such interaction.