bluuebunny/crossref_metadata_2025_split huggingface dataset:
Reactions of Vinyl Azides
Authors: L'abbé Gerrit 
Published: December 2003

AbstractThis review is designed to demonstrate the versatility of vinyl azides in organic reactions. The reactive azide function is susceptible to thermolysis, photolysis, cycloadditions, and attack by nucleophiles and electrophiles. The neighboring double bond accentuates the reactivity of the azide function and provides additional intramolecular pathways for reaction. Last but not least, the presence of an appreciable electron density at the β‐vinyl carbon makes this class of compounds comparable with enamines in their reactions with electrophiles and 1,3‐dipoles.
Biomimetic Polyene Cyclizations
Authors: Johnson William S. 
Published: December 2003

AbstractCertain polyenic substances having trans olefinic bonds in a 1,5 relationship can be induced to undergo stereospecific, non‐enzymic, cationic cyclization to give polycyclic products with all‐trans (“natural”) configuration. These transformations appear to mimic in principle the biogenetic conversion of squalene into polycyclic triterpenoids. Acetal and allylic alcohol functions are effective initiators for these cyclizations, and methylacetylenic end groups are particularly useful terminators since they lead to five‐membered rings. Thus a tetraenic acetal having no chiral centers has been converted in a single step into a tetracyclic product having seven such centers. The process is highly stereoselective, giving only two of 64 possible racemates. Systems have also been developed for effecting the total synthesis of the steroid nucleus in a single step starting from a substrate containing only one ring. The mechanism of these biomimetic cyclizations and also that of their enzymic counterparts is still unknown, but the balance of the evidence favors a synchronous process.
Bacteriorhodopsin as an Example of a Light‐Driven Proton Pump
Authors: Oesterhelt Dieter 
Published: December 2003

AbstractApart from the long known visual pigments, another retinal protein complex exists in nature, viz. bacteriorhodopsin from halobacteria. In contrast to the visual pigments such as the rhodopsins, which act as light sensors in the eye, bacteriorhodopsin actually transforms light energy. This energy conversion is connected with the asymmetric incorporation of bacteriorhodopsin in the lattice structure of the purple membrane which forms patches on the cell surface of halobacteria. Alongside the chlorophyll system, the purple membrane system represents the second light energy conversion principle to be discovered in living nature. Bacteriorhodopsin acts as a light‐driven proton pump or as the main component of such a pump system. Absorption of light triggers off a cycle of reactions coupled with the spatially oriented uptake and release of a proton. In the intact cell an electrochemical gradient is thus built up across the cell membrane of the bacterium in which part of the absorbed light energy is stored and which is not dependent upon redox processes as in the case of respiration or photosynthesis. This electrochemical gradient can supply the energy required for ATP synthesis in the cell; a reversible proton‐translocating ATPase serves as catalyst system.
Far‐Infrared Fourier Spectroscopy as a Method for Structure Determination in Chemistry
Authors: Knözinger Erich 
Published: December 2003

AbstractCouched in the often confusing term “Fourier Spectroscopy” is the combined exploitation of a fundamental physical concept, a simple mathematical principle, and use of modern data technology. Introduction of this relatively young method into instrument technology has given great impetus to nuclear magnetic resonance and infrared spectroscopy. The infrared spectroscopic investigation of low frequency molecular vibrations has especially profited; in this connection one speaks of far infrared (FIR) Fourier Spectroscopy, which has opened up numerous interesting applications in organic, inorganic, and physical chemistry.
Biosynthesis of the Purple Membrane of Halobacteria
Authors: Sumper Manfred , Reitmeier Heribert , Oesterhelt Dieter 
Published: December 2003

AbstractHalobacteria are extremely specialized organisms. They live exclusively in saturated solutions of common salt. The cell membrane of these bacteria exhibit insular regions which can be isolated by membrane fractionation. These regions consist of a lipid matrix containing bateriorhodopsin molecules in a hexagonal crystalline arrangement. Bacteriorhodopsin is a deep purple retinal‐protein complex (“purple membrane”). The purple membrane functions as a light energy converter.—How can such a differentiated membrane region arise? In vivo studies on the biosynthesis of the purple membrane showed another cell membrane fraction, the so‐called brown membrane, to be a biosynthetic precursor. Bacterioopsin (the retinal‐free protein) is initially incorporated into the brown membrane and can only form the purple membrane by crystallization in an energy‐dependent reaction after prior reaction with retinal. This reaction is reversible. Removal of the retinal by formation of retinal oxime causes the purple membrane regions to disappear. Reconstitution of the bacteriorhodopsin by addition of retinal regenerates the purple membrane.
Neuron Models
Authors: Hamprecht Bernd 
Published: December 2003

AbstractThe nervous system is made up of several kinds of cells which should ideally be examined separately in biochemical studies. Since we are not yet able to isolate the different kinds of cells, use is made of cell lines derived from a single cell which are isolated from tumors of the nervous system and which can be reproduced at will in the same way as bacteria. They include cell lines isolated from a mouse neuroblastoma which exhibit some of the properties of nerve cells. Hybrid cells obtained by fusion of mouse neuroblastoma cells with rat glioma cells display even more characteristics of nerve cells. This article considers properties which justify our regarding the hybrid cells as neuron models. Application of the cells to problems of the nervous system is then demonstrated: (1) processing of information arriving in the form of mutually opposing hormonal signals; (2) mode of action of morphine.
Chemical Ecology—A Chapter of Modern Natural Products Chemistry
Authors: Schildknecht Hermann 
Published: December 2003

AbstractThe chemical reaction of arthropods to their environment, i.e. their chemical ecology, can be studied particularly well with water beetles. Stenus comma, an aquatic beetle weighing only 2.5 mg, saves itself from drowning with the aid of an alkaloid, and the water beetle Ilybius fenestratus defends itself against small mammalian predators with a compound belonging to the same class. The water beetle Platambus maculatus employs a diterpene for precisely the same purpose and the whirligig beetle a norsesquiterpene, which also offers protection against troublesome microorganisms. As chemical artists, the ants can hardly be surpassed. In particular, the myrmecine ants guarantee their food supplies with plant growth substances. Since these compounds, depending upon concentration, can also act as inhibitors, we are confronted with an excellent example of an ecological equilibrium being established with the aid of organic chemicals. Even the little parasitic bombardier beetle Paussus favieri is tolerated, on account of its defensive chemistry, in the nest of the myrmecine ant Pheidole. In contrast, inorganic compounds are largely responsible for the stability of spiders' webs.
Nitrogen as Leaving Group: Aliphatic Diazonium Ions
Authors: Kirmse Wolfgang 
Published: December 2003

AbstractNitrogen is one of the best known leaving groups. Decomposition of aliphatic diazonium ions has rendered carbocations (carbon cations) accessible in cases where the solvolysis of halides or sulfonic esters follows different pathways. The limits of unassisted decomposition are seen in primary diazonium ions, where the participation of external nucleophiles and of neighboring groups in the elimination of nitrogen becomes noticeable. Cyclopropanediazonium ions occupy a position intermediate between aliphatic and aromatic diazonium ions. New versatile methods of generating aliphatic diazonium ions facilitate their preparative use.
Eutrophication and Wastewater Purification
Authors: Gleisberg Dietrich , Kandler Joachim , Ulrich Hansjörg , Hartz Peter 
Published: December 2003

AbstractOne of the nuisance conditions brought about by the impact of civilization on some regions is the accelerated eutrophication of receiving waters, especially of stagnant waters, owing to the increased supply of plant nutrients from industrial and municipal wastes. To counteract this situation it is recommended that effective measures be taken to reduce the supply of phosphorus and, in part, nitrogen to such waters. Insofar as the supply of these elements stems from communal wastewaters this objective can be achieved by extensive purification. While the most suitable method for removal of nitrogen is nitrification and denitrification, that which recommends itself for the removal of phosphorus is the method of phosphate precipitation, a well established, economic technique which can be employed in existing plant. The success of the measures taken, however, depends to a large extent on achieving wastewater having a very low phosphorus content.
Collisional Activation Mass Spectrometry—A New Probe for Determining the Structure of Ions in the Gas Phase
Authors: Levsen Karsten , Schwarz Helmut 
Published: December 2003

AbstractOrganic ions with high translational energy colliding inelastically with neutral atoms or molecules become excited electronically at the expense of their translational energy. The excitation energy enables a wide range of dissociation reactions to occur and the intensity relationships yield information about the structure of the ions concerned and also permit conclusions to be drawn about the mechanism of their formation.
Chemical Methods of Studying Ribosome Structure
Authors: Küchler Ernst 
Published: December 2003

AbstractThe use of chemical methods, and in particular affinity labeling, has greatly advanced our knowledge about the structure of the active sites of ribosomes. Experiments have shown that ribosomal proteins and ribosomal ribonucleic acids participate in the structure of the binding sites for messenger ribonucleic acid and transfer ribonucleic acid.
Detoxification of Automotive Exhaust Gases with Multifunctional Catalysts
Authors: Weigert Wolfgang M. , Koberstein Edgar 
Published: December 2003

AbstractCarbon monoxide, hydrocarbons, and nitrogen oxides can be simultaneously transformed into inoffensive substances on multifunctional catalysts. Consisting of noble metals or base metal oxides deposited on an inert support, these catalysts can be used in monolithic form or as granular packings. Compared with other methods of automotive exhaust detoxification, multifunctional catalysts are distinguished by incurring fuel savings.
Catalyzed Addition of Aldehydes to Activated Double Bonds—A New Synthetic Approach
Authors: Stetter Hermann 
Published: December 2003

AbstractIn the presence of cyanide ions as catalyst, aromatic and heterocyclic aldehydes can be smoothly added to α,β‐unsaturated ketones, esters, and nitriles in aprotic solvents to form γ‐diketones, 4‐oxo carboxylic esters, and 4‐oxo nitriles. Thiazolium salts in the presence of bases are also suitable catalysts; they permit not only addition of aromatic and heterocyclic aldehydes but also the addition of aliphatic aldehydes.
Gaseous Chloride Complexes with Halogen Bridges—Homo‐Complexes and Hetero‐Complexes
Authors: Schäfer Harald 
Published: December 2003

AbstractThe chemistry of the gaseous halide complexes which have been observed in large numbers in recent years is discussed for the example of chlorides.Homo‐complexes—dimeric or polymeric chloride molecules containing chlorine‐bridge linkages [e.g.(NaCl)n,n= 2—4; (CuCl)n,n= 2—5; (BeCl2)n,n= 2—4; Pd6Cl12]—occur through broad areas of the Periodic Table. Comparison of the dissociation enthalpy of such molecules with the vaporization enthalpy of the liquid chlorides is particularly informative.Hetero‐complexesare formed from different chloride molecules by linkage through Cl bridges. Not only 1:1 complexes (e.g.NaAlCl4, TIPbCl3, KThCl5, CdPbCl4, AlUCl8) but also larger molecules (e.g.CoAl2Cl8, CrAl3Cl12, Cu2UCl6, In2UCl10) are known. Types of formulas, structures, and above all the formation enthalpies of such complexes are discussed critically. Hetero‐complexes are useful in chemical transport reactions, as aids in syntheses, and in gas‐chromatographic separations (lanthanoids, actinoids). They also play a part in many industrial processes (chlorination of ores) and in recently developed types of high‐pressure discharge tubes.
New Varieties of Sandwich Complexes
Authors: Werner Helmut 
Published: December 2003

AbstractMetal sandwich complexes of the classical type M(CnHn)2 and related structural analogs are characterized by a sandwich‐like coordination of one central atom by its ligands. This structural principle has been extended in recent years now that di‐ and tri‐nuclear sandwich‐type complexes have become accessible. The best‐known representatives are the triple‐decker sandwich complexes. A survey of this topical field is presented in this report, which also underlines the utility of kinetic and mechanistic studies for the development of new synthetic pathways in organometallic chemistry.
Intramolecular [4+2] and [3+2] Cycloadditions in Organic Synthesis
Authors: Oppolzer Wolfgang 
Published: December 2003

AbstractNumerous examples of intramolecular cycloaddition of 1,3‐dienes, nitrones, and azomethine imines attest the preparative value of this variant for regioselective and stereoselective synthesis of annelated and bridged ring systems. The common features, differences, and limitations of these types of reaction are systematically reviewed.
Secondary and Tertiary Structure of Polysaccharides in Solutions and Gels
Authors: Rees David A. , Welsh E. Jane 
Published: December 2003

AbstractMany polysaccharide chains can adopt ordered helical and ribbon‐like secondary structures. It seems however that these chains are often so stiff and extended that the cooperative interactions necessary for stability in the solvent environment can only be achieved when inter‐chain as well as intra‐chain interactions are favorable. Hence we commonly find two‐or more‐stranded associations of helices, of ribbons, or of helices with ribbons. These can be regarded as tertiary and higher levels of structure. The ordered secondary structure characteristically requires a regular repeating sequence of sugar residues, and the termination of this sequence by insertion of a residue of different type may also terminate the secondary structure and hence the association in which it is involved. This is the mechanism by which native polysaccharides may link up to form three dimensional networks, or gels, in which state they perform their natural roles in maintaining the hydration and integrity of biological tissues. For several polysaccharides there is evidence that the mechanism of biological control over the fine topology and properties of the gel network is mediated by enzymes which modify sugar residues at the polymer level to change the pattern of “interrupting” sugar residues.
Efficiency and Evolution of Enzyme Catalysis
Authors: Albery W. John , Knowles Jeremy R. 
Published: December 2003

AbstractA new function derived from kinetic data, the efficiency function, can be used to quantify the efficiency of a catalyst. For freely diffusing species the maximum efficiency is unity. The enzyme triose phosphate isomerase has an efficiency of 0.6 and is thus almost a perfect catalyst. The efficiency of the acetate ion as catalyst for the same reaction is 2.5 × 10−11. This increase in catalytic efficiency is discussed in terms of three types of evolutionary improvement: the uniform binding of the substrate to the enzyme, changes in the internal thermodynamics of the bound states, and more effective catalysis of elementary steps. These concepts are illustrated for triose phosphate isomerase.
The Pauli Principle and Chemical Bonding in Molecules and Solids
Authors: Jansen Laurens , Block Ruud 
Published: December 2003

AbstractThe starting point of this contribution is a discussion of the concept of exchange interactions and its relation to chemical bonding and valence. Then, a class of phenomena characterized by weak chemical bonding (≲1eV) in solids and molecules is analyzed in more detail, with “superexchange” in insulating solids with paramagnetic 3d‐cations serving as prototype. A model of “effective electrons” is developed for weak bonding on the basis of exchange perturbation theory, taking full account of the Pauli principle. The model is applied to: (i) magnetic interactions in solids (interaction energy 10−2 to 10−4 eV); (ii) stability of noble‐gas halides (binding energy ≈ 1eV); and (iii) rotational barriers in simple molecules (barrier heights of several 0.1 eV).
Flash Thermolysis of Organic Compounds
Authors: Seybold Günther 
Published: December 2003

AbstractFlash thermolysis is a special form of gas‐phase thermolysis that is particularly suitable for the production and isolation of thermolabile reaction products. The theoretical basis and the instrumental requirements of the method are first given, and the preparative importance of flash thermolysis for the synthesis of thioketenes, allenes, arynes, highly strained ring systems, quinodimethanes, and other reactive compounds is then demonstrated with the aid of examples.
Steric Interactions in Organic Chemistry: Spatial Requirements of Substituents
Authors: Förster Hans , Vögtle Fritz 
Published: December 2003

AbstractFrom the very extensive factual material available on steric interactions in organic chemistry, the present paper reviews the investigations that provide information about the “size” (“spatial requirement”) of the substituents. After an introductory section on the results obtained with the aid of spectroscopic data and by analysis of chemical reactions, attention is turned to conformational processes, including hindered rotation in ethanes, in the biphenyl system, in butadienes, multiply substituted arenes, molecular propellers, and triptycenes. The main emphasis is placed on the explanation of sterically hindered ring inversions in bridged arenes, a class of compounds particularly suitable for the study of steric interactions. The results obtained by variation of the parameters in this system are also discussed with respect to the spatial requirements of organic substituents.
Entropy, Binding Energy, and Enzymic Catalysis
Authors: Page M. I. 
Published: December 2003

AbstractWhy are enzyme‐catalyzed reactions so much faster than uncatalyzed reactions, and why are enzymes so specific? What is the effect of mere approximation of enzyme and substrate, and what is the influence of the strain energy? Attempts to answer these questions have led to comparisons between entropy changes in intermolecular and intramolecular reactions, and to determinations of the intrinsic energy of the bond arising by non‐covalent interaction between enzyme and substrate.
“Organic Metals”—The Intermolecular Migration of Aromaticity
Authors: Perlstein Jerome H. 
Published: December 2003

AbstractThe electrical conductivity of a wide variety of organic materials is reviewed and a comparison made between those materials which transport charge efficiently and those that do not. This comparison reveals that the most efficient conductors contain molecules (a) whose radical ions form a new aromatic sextet upon one‐electron oxidation or reduction and (b) whose aromaticity can migrate by mixed‐valence interaction. The electrical data are presented in light of the chemistry of these materials and the importance of chemical bonding interactions in the electron transport process. Several new examples are discussed and directions for further research explored.
Macrolides. Recent Progress in Chemistry and Biochemistry
Authors: Masamune Satoru , Bates Gordon S. , Corcoran John W. 
Published: December 2003

AbstractThe progress of macrolide chemistry and biochemistry over the past several years has indeed been remarkable. This review attempts to cover the important contributions which have been made in the fields of structural and synthetic chemistry as well as in the biosynthesis and mode of antimicrobial activity of the “polyoxo” macrolide antibiotics. Emphasis has been placed on the recent synthetic achievements in this field.
The Electro‐Optical Kerr Effect in Conformational Analysis. New analytical methods (10)
Authors: Aroney Manuel James 
Published: December 2003

AbstractBirefringence induced in a material by application of an electric field is known as the Kerr effect. The great sensitivity of this property to variations of molecular geometry and environment has made it the basis of a potent method for studying the conformations of molecules. It has, in addition, been effectively applied to investigations of the geometry of solvation and of solute aggregation. An outline is presented of the general technique of measurement of the Kerr effect, the extraction of directed effective polarizabilities of solute molecules and of structural groups, the calculation of theoretical Kerr constants for candidate structures of molecules, and the procedures used to determine solute conformations. The scope and limitations of the method are illustrated by specific conformational problems drawn from the literature. In the case of highly flexible molecules analysis of the measured Kerr constant is inhibited by uncertainties in conformational energy data. This paper is concerned mainly with the specification of detailed conformational energy data. This paper is concerned mainly with the specification of detailed conformational features of molecules. Reference is made, however, to the utilization of pulsed orienting fields to study shapes and dimensions of biological macromolecules.
Oxometallates with Planar Coordination
Authors: Müller‐Buschbaum Hanskarl 
Published: December 2003

AbstractOxo compounds whose coordination polyhedra have a planar structure are a rarity in solid state chemistry. The present state of this research area provides a wealth of interesting principles concerning one‐, two‐, and three‐dimensional linkage of square‐planar structural units. In certain cases the environment of these units in solids requires new conceptual models for the chemical bonding.
Fluorinated Isocyanates and Their Derivatives as Intermediates for Biologically Active Compounds
Authors: Kühle Engelbert , Klauke Erich 
Published: December 2003

AbstractFluorinated aromatic isocyanates are valuable intermediates for the production of herbicides and insecticides. Unexpected rearrangements during the synthesis of 2‐(trifluoromethyl)phenyl isocyanates provide an entry to N‐trifluoromethylaminobenzoyl fluorides, a novel class of compounds having fungicidal properties. Carbamoyl fluorides, the products of HF addition to isocyanates, undergo ready sulfenylation. Use of these intermediates for the synthesis of N‐sulfenylcarbamate insecticides led to the discovery of the new principle of detoxification of carbamates by sulfenylation.
Titanium Tetrachloride in Organic Synthesis [New synthetic methods (21)]
Authors: Mukaiyama Teruaki 
Published: December 2003

AbstractTitanium tetrachloride can accelerate numerous organic reactions. Valuable syntheses of, e.g., allyl sulfides, amides, enamines, and ketones are based upon transformations of functional groups with TiCl4. Particular mention should also be made of carbon‐carbon linkage with TiCl4 which permits the synthesis of hydroxy ketones and carbonyl compounds of the Michael adduct type. TiCl4 reduced in situ is suitable for the reduction of chloroarenes or the linkage of two aldehyde molecules to give an alkene.