bluuebunny/crossref_metadata_2025_split huggingface dataset:
Biochemistry of Natural Proteinase Inhibitors
Authors: Tschesche Harald 
Published: December 2003

AbstractThe natural inhibitors of proteolytic enzymes are proteins. These inhibitors associate reversibly with the enzymes to form stoichiometric protein‐protein complexes, in which substrate‐analogous association at the active center of the enzyme results in competitive inhibition of all catalytic functions. The very widespread occurrence of inhibitors in the animal and plant kingdoms underlines their biological importance in the intermediate metabolism, which can be understood as an extension of the possibilities for temporary and local limitations of enzyme activities. Existing knowledge includes a series of covalent structures, detailed kinetic data on the reversible protein‐protein interaction, the processes involved in inactivation, and chemical methods for the modification of these proteins. Early X‐ray structural data for an inhibitor and its enzyme complex provide an insight into the molecular structure of the latter and the interactions involved in the association to form the complex.
Activation of Molecular Oxygen
Authors: Henrici‐Olivé G. , Olivé S. 
Published: December 2003

AbstractMolecular oxygen as a ligand in transition metal complexes is of interest for biochemistry, and for inorganic and organometallic chemistry. Several examples of O2 complexes, as well as the kinetics and mechanism of their formation are presented. The influence of other ligands on the activation of O2 is discussed on the basis of a MO description of the electronic structure of the oxygen molecule and of the bonding within the metal‐oxygen complexes. Reactions of coordinated oxygen are described.
Results and Problems of O‐Glycoside Synthesis
Authors: Wulff Günter , Röhle Gerbard 
Published: December 2003

AbstractConsiderable problems attend the synthesis of biologically important natural products whose carbohydrate components display a greater or lesser degree of complexity. The present progress report surveys recent developments in the formation of O‐glycosidic bonds starting from 1‐halo sugars and from sugar 1,2‐orthoesters. Particular emphasis is placed on attempts to reach a better understanding of the steric course of glycosylation reactions on the basis of mechanistic considerations. Studies on the silver‐salt dependence of the Koenigs‐Knorr synthesis led to improvements in preparative applications and to new concepts concerning the reaction course. The introduction of sugar 1,2‐orthoesters as glycosylation components has greatly enriched our preparative armoury for the synthesis of trans glycosides. Remarkable progress has also been made in the synthesis of cis glycosides, in part by exploiting novel neighbouring group effects.
Chemically Produced Excited States
Authors: White Emil H. , Miano Jeffrey D. , Watkins Carol J. , Breaux Eves J. 
Published: December 2003

AbstractElectronically excited states of organic molecules are formed in many chemical reactions. Such chemically produced excited states are (with one exception) identical to light produced excited states, and they undergo the molecular transformations expected of such states (“photochemistry without light”). The excited states can also be used in energy transfer experiments. This review covers the generation of chemically produced excited states, the chemical reactions they undergo, and the possible role of chemically produced excited states in biology.
Mass Spectral Interaction between the Functional Groups of Multiply Substituted Alkanes
Authors: Bosshardt Herbert , Hesse Manfred 
Published: December 2003

AbstractThe mass spectral decomposition of di‐ and polyfunctional alkanes is characterized essentially by two, fundamentally different reactions: On the one hand the structures of the fragment ions are determined by separated degradation of each functional group; on the other hand, however, a relatively large number of fragment ions result from the combined action of two or even more functional groups. This special type of fragmentation has become an important topic in today's mass spectral research. In the present progress report an attempt has been made to summarize the results obtained so far.
Beech Lignin—Proposal of a Constitutional Scheme
Authors: Nimz Horst 
Published: December 2003

AbstractThe high molecular weight material lignin consists of phenylpropane units linked together by a variety of bond types. During the past eight years, two newly developed degradation procedures have permitted the first direct determinations of the nature of these bonds. The first procedure affords a very mild partial hydrolysis of benzyl ether bonds. Eleven dimeric, trimeric, and tetrameric degradation products were obtained in this way from spruce and beech lignin: they exhibited three different kinds of bonds between the C9 structural units, and their structures have all been elucidated. In the second procedure, the most important kind of bond in lignin, i. e. the arylglycerol‐β‐aryl ether bond, can be subjected to directed cleavage under mild conditions after introduction of a suitable neighboring group. On application to beech lignin, 91 % of the material was degraded giving monomeric to tetrameric phenols. Complete structural elucidation of the twenty dimeric phenols isolated and a knowledge of their relative yields and the yields of the other fractions obtained by gel filtration permitted a structural scheme to be set up for beech lignin in which the C9 structural units are linked together by no less then ten different kinds of bonds. The structural scheme, which can be readily explained biogenetically, has the same elemental composition as natural beech lignin. Further support for the structural scheme comes from a comparison of the 13C‐NMR spectrum of natural beech lignin and a 13C‐NMR spectrum calculated for the proposed structure on the basis of about fifty lignin model substances.
Mass Transport in Solids
Authors: Frischat Günther Heinz 
Published: December 2003

AbstractTwo aspects are especially important in connection with mass transport in solids; these are the phenomenological description of the diffusion process and the discussion of the transport mechanism. In addition to the self‐diffusion of the constituent ions or atoms of a solid, which can be followed only with the aid of radioactive or stable isotopes, other known types of diffusion are the diffusion of trace elements, which can in principle be treated in the same way as self‐diffusion, and chemical diffusion, in which the diffusion partners differ in their chemical composition. Processes of this type are being increasingly studied with the electron beam microprobe. Regardless of the crystallinity and the type of bonding, problems concerned with diffusion in solids can be discussed from a single standpoint by slight adaptation to suit the particular situation in question.
The Cyclobutadiene Problem
Authors: Maier Günther 
Published: December 2003

AbstractCyclobutadiene has recently become the center of reawakened interest. By use of the matrix isolation technique, cyclobutadienes can now be prepared as monomers frozen in matrices. Thus both the unsubstituted parent compound and its alkyl‐substituted derivatives are amenable to study. The present progress report surveys the entire field; however, particular emphasis is placed on the author's own results from investigations concerning this classical problem of organic chemistry.
Equilibrium and Transport Properties of Low‐Melting Salts
Authors: Richter Joachim 
Published: December 2003

AbstractAn understanding of the physical and chemical phenomena and the structure of salt melts calls not only for kinetic and spectroscopic data but also for systematic studies on the equilibrium and transport properties of individual systems. The present article deals with the activity coefficients, transport numbers, and self‐diffusion and interdiffusion coefficients of salt melts, and the results of consistent measurements of these quantities are reported.
Transition to Nonmetallic Behavior in Expanded Liquid and Gaseous Metals
Authors: Hensel Friedrich 
Published: December 2003

AbstractAt high temperatures and pressures, liquid and gaseous metals exhibit a steady transition from metallic to nonmetallic behavior when the density is sufficiently reduced. All the properties are influenced by this transition, as is shown by measurement (on mercury, cesium, and potassium) of the electric conductivity, the density, the thermoelectric power, and in some cases the Hall effect, as well as the optical absorption. The continuous variation of the density over a wide range is achieved by compression or expansion above the critical temperatures of the metals, i. e. 14900C for mercury, 17500C for cesium, and 19250C for potassium.
1,3‐Anionic Cycloadditions of Organolithium Compounds: An Initial Survey. New synthetic methods (4)
Authors: Kauffmann Thomas 
Published: December 2003

AbstractIn the field of [3 + 2]‐cycloaddition reactions, 1,3‐anionic cycloadditions have now joined 1,3‐dipolar cycloadditions as a second reaction type of major general importance. The cycloaddition in question has been found to occur with 2‐azaallyllithium compounds, allyllithium compounds (with an electron acceptor located on C2), and 1,3‐diphenylpropargyllithium. Compounds with CC, CN, NN, and CS double bonds and those with CC and CN triple bonds were found to be 1,3‐anionophilic. 1,3‐Anionic cycloaddition opens new routes to 1‐aza‐, 1,2‐diaza‐, 1,3‐diaza‐, 1,2,4‐triaza‐, and 1‐thia‐3‐azacyclopentane derivatives, to pyrroles and imidazoles, as well as (and this appears to be of special importance) to cyclopentane derivatives. Currently available data attribute stereospecificity to the 1,3‐anionic cycloadditions, which occupy an interesting intermediate position between 1,3‐dipolar cycloadditions and transition‐metal‐catalyzed cyclizations. A two‐step mechanism has been demonstrated in one case.
The Theory of Pericyclic Reactions
Authors: Epiotis Nicoloas D. 
Published: December 2003

AbstractPericyclic reactions include thermal and photochemical cycloadditions and cycloreversions, electrocyclic reactions and sigmatropic shifts. Configuration interaction analysis can be used for making predictions about preferred reaction pathways, about reaction rates, about the nature of possible intermediate steps, and above all about the stereochemistry of such reactions.
Mechanisms of Photochemical Air Pollution
Authors: Pitts J. N. , Finlayson B. J. 
Published: December 2003

AbstractSelected aspects of the chemistry of photochemical air pollution is discussed and some important, unresolved problems dilineated. The reactive species considered include NO2, O3, O(3P), O(1D), O2(1Δg), OH and HO2. Both the kinetics and mechanicsms of the reactions constituting the major tropospheric sources and sinks of these species are treated where available. The application of this information in both computer and smog chamber simulations of photochemical smog is discussed.
The Wolff Rearrangement of α‐Diazo Carbonyl Compounds
Authors: Meier Herbert , Zeller Klaus‐Peter 
Published: December 2003

AbstractThe readily accessible α‐diazo carbonyl compounds are distinguished by their high reactivity, which opens up a variety of preparative applications under modified conditions. Wolff rearrangements of these compounds, induced thermally, photochemically, or catalytically, afford ketenes. Free and complexed carbenes, 1,3‐dipoles, 1,3‐diradicals, and the antiaromatic oxirenes have been considered as intermediates or transition states. The present progress report attempts to integrate preparative and theoretical aspects.
Preparation, Structure, and Reactions of Halomalondialehdes
Authors: Reichardt Christian , Halbritter Klaus 
Published: December 2003

AbstractThe present progress report provides a survey of the synthesis, structure, and reactions of halomalondialdehydes, including “cyanomalondialdehyde” (diformylacetonitrile), the only pseudohalogen compound so far known in this series. Halomalondialdehydes are reactive compounds, applicable in a variety of ways, and are characterized particularly by their cyclizations with bifunctional compounds. Many of the reactions discussed below can be applied to substituted malondialdehydes other than those specified.
Formation of Ad‐Layers and Clusters on Condensation of Metal Vapors on Solid Surfaces
Authors: Niedermayer Rolf 
Published: December 2003

AbstractSynthesis of organometallic materials can be accomplished in many cases by cocondensation of metal atoms and organic molecules at low temperatures. The reaction kinetics is determined by the competition between metal cluster growth and formation of the organometallic compound. Interesting compounds may contain one or more metal atoms; the latter type could be obtained by reaction between a cluster containing the desired number of metal atoms and an organic molecule. A precise knowledge of the events occurring on condensation of metal atoms and cluster formation can therefore be of value in the control of chemical synthesis. These phenomena have been investigated in connection with the study of the growth of thin metallic films, both experimentally and theoretically. Direct observation of the formation of very small clusters is difficult. The good agreement between experimental results and recent calculations for the development of large clusters, however, allows reliable theoretical conclusions for the first stages of adsorption and cluster formation. The present contribution describes experimental work on film growth and relevant theoretical concepts, and an attempt is made to develop applications to organometallic synthesis.
Sulfur Atoms as Ligands in Metal Complexes
Authors: Vahrenkamp Heinrich 
Published: December 2003

AbstractThe types of sulfur bonding—as sulfane or sulfide—encountered in the molecules of maingroup elements are almost unknown in the chemistry of metal complexes, where the sulfur atoms function instead as two‐electron donors by bridging two metal atoms, as four‐electron donors by bridging three or four metal atoms, or as six‐electron donors by incorporation between four metal atoms. In such complexes, the metal‐metal bond can be modified over a wide range by chemical or electrochemical variation of the number of electrons present. The readiness with which polynuclear complexes containing metals and sulfur undergo redox reactions is also utilized by Nature in the active sites of some redox proteins.
D‐Penicillamine—Production and Properties
Authors: Weigert Wolfgang M. , Offermanns Heribert , Degussa Paul Scherberich 
Published: December 2003

AbstractThe non‐naturally occurring amino acidD‐penicillamine, originally only of interest as a key substance in the structural elucidation and total synthesis of penicillins is gaining increasing importance as a therapeutic, particularly in the long‐term treatment of rheumatoid arthritis.D‐Penicillamine which previously was obtained semi‐synthetically by degradation of penicillins can now be totally synthesized using a new process which starts from isobutyraldehyde, sulfur, ammonia, and hydrogen cyanide and yields the racemate which is then selectively resolved. The biochemical behavior ofD‐penicillamine—chelate formation with heavy metal ions, cleavage of disulfide bridges, and condensation with aldehyde groups—affords information related to its therapeutic activity.
Properties of Sulfur‐Sulfur Bonds
Authors: Steudel Ralf 
Published: December 2003

AbstractSS bonds are extraordinarily flexible and have properties that are observed only on isolated occasions for other homonuclear bonds: the bond lengths very between 1.8 and 3.0Å, the bond angles between 90 and 180° and the dihedral angles between 0 and 180°; the bond energies amount to up to 430 kJ/mol. The SS stretching frequencies can appear over the range 177–820 cm−1 and force constants of 1.4 to 6.3 mdyne/Å have been calculated. This variability is illustrated with examples containing isolated and cumulated SS bonds.
Detection of Fragment Genesis in the Mass Spectrometer: DADI Mass Spectrometry as an Aid in the Structure Analysis of Organic Compounds
Authors: Schlunegger Urs Peter 
Published: December 2003

Abstract“Direct Analysis of Daughter Ions” (DADI) can be carried out with commercial mass spectrometers embodying the Nier‐Johnson inverse geometry. DADI measurements permit experimental detection of the consecutive formation of molecule fragments (fragment genesis). Knowledge of fragment genesis enables the chemist to clarify the fragmentation processes of molecule‐ions and provides information on the structure of fragments formed in the mass spectrometer. In combination with classical mass spectrometry such information makes it easier to determine the structure of compounds, to analyze mixtures, to determine the sequence in periodically constructed molecules, for example oligopeptides, and to study rearrangement reactions occurring in the mass spectrometer.
Substitution Reactions Which Proceed via Radical Anion Intermediates
Authors: Kornblum Nathan 
Published: December 2003

AbstractA new type of substition process at a saturated carbon atom is described. These reactions, which proceed via a chain sequence in which radical anions and free radicals are intermediates, are noteworthy for providing novel and powerful means of synthesis: they occur readily under mild conditions, they give excellent yields of pure products, and, in contrast to SN2 displacements, they are rather insensitive to steric hindrance. As a consequence, radical anion processes are especially valuable for the preparation of highly branched structures. Many inorganic and organic anions readily enter into these displacements and, indeed, amines are also effective. Systems which undergo substitutions via this electron transfer mechanism include benzylic, cumylic, strictly aliphatic, and heterocyclic molecules. It is of interest that a number of groups which do not behave as leaving groups in SN2 displacements are readily displaced at room temperature from a satureted carbon atom via the radical anion‐free radical pathway, e.g., nitro, azide, sulfone, and ether groups.
Mechanism of Ozonolysis
Authors: Criegee Rudolf 
Published: December 2003

AbstractThe formation of ozonides (1,2,4‐trioxolanes) from alkenes and ozone can be described as a succession of three [2+3] cycloadditions or cycloreversions involving primary ozonides (1,2,3‐trioxolanes) and aldehyde or ketone oxides as decisive intermediates, all of which have finite lifetimes. There is no warranted experimental basis for assuming an alternative mechanism.
Intramolecular 1,3‐Dipolar Cycloaddition Reactions
Authors: Padwa Albert 
Published: December 2003

AbstractThe intramolecular 1,3‐dipolar cycloaddition reaction of suitably functionalized 1,3‐dipoles represents a general scheme for the synthesis of novel fused ring heterocycles. Such reactions of a number of 1,3‐dipoles are summarized and the general outline and potential analogies for these reactions noted. While the immediate aim of this review is to survey and correlate published work, it is hoped that general and specific points in need of study will be revealed and will stimulate further work in this fertile field.
Mechanistic Aspects of Olefin Metathesis
Authors: Calderon N. , Ofstead E. A. , Judy W. A. 
Published: December 2003

AbstractIn this paper views regarding the nature of the transition state in olefin metathesis are discussed. Earlier concepts that assumed a pairwise scrambling mechanism have been recently challenged. Metathesis studies involving acyclic as well as cyclic olefins have been reported which strongly suggest a chain process that may involve a carbene‐to‐metallocycle interconversion as the pathway responsible for transalkylidenation. Catalytic systems that display a low apparent metathesis activity on terminal olefins display a unique specificity in cross reactions with internal olefins. Critical experiments indicate that terminal olefins rapidly undergo regenerative metathesis preferentially. Results suggest that in the transient metallocycle, the most stringent steric requirements occur at the β‐carbon, although the origin of this steric demand is not obvious.
New Developments in the Field of Vitamin B12: Reactions of the Cobalt Atom in Corrins and in Vitamin B12 Model Compounds
Authors: Schrauzer G. N. 
Published: December 2003

AbstractStudies with vitamin B12 model compounds such as the cobaloximes provide a basis for the understanding of the mode of action of corrinoid coenzymes in enzymatic reactions. They also widen our knowledge of the properties and reactions of organocobalt compounds. The present article outlines the most important nonenzymatic reactions of cobalt in vitamin B12 and in model compounds of the cobaloxime type.
Vinamidines and Vinamidinium Salts—Examples of Stabilized Push‐Pull Alkenes
Authors: Lloyd Douglas , McNab Hamish 
Published: December 2003

AbstractThe name vinamidines has been proposed for 1,5‐diazapentadienes: these compounds can be regarded as vinylogous amidines and at the same time as push‐pull substituted (and thereby stabilized) alkenes. Vinamidine and vinamidinium structural elements may also form part of a ring system. Characteristic of this class of compounds are their reactivity toward electrophiles (at Cβ) and nucleophiles (Cα) and their regenerative character,i.e.their tendency to undergo substitution instead of addition reactions. Reactions of vinamidinium salts with nucleophiles are of special preparative value: they leade.g.to pyrazoles, oxazoles, pyrimidines, diazepines, quinolines, and quinolizines.
Orbital Correspondence Analysis in Maximum Symmetry
Authors: Halevi E. Amitai 
Published: December 2003

AbstractThe use of Orbital Correspondence Analysis in Maximum Symmetry (OCAMS) for determining the allowed course of a reaction is described in terms of the familiar “A, S” notation. Several well known thermal and photochemical reactions are used as examples: Polyene cyclization; the fragmentation and dimerization of cyclobutadiene; [π4 + π2] cycloaddition; the isomerization of Dewar‐benzene. The relation of OCAMS to the classical correlation procedures of Woodward and Hoffmann and of Longuet‐Higgins and Abrahamson is brought out, as are the sources of the occasionally contradictory predictions of OCAMS and of the classical procedures.
Is Gold Chemistry a Topical Field of Study?
Authors: Schmidbaur Hubert 
Published: December 2003

AbstractThe question raised in the title of this progress report is answered point for point with an emphatic “yes”: Ranging from metallic gold,viaclusters of gold atoms in low valence states, compounds of AuI, AuII, AuIII, and AuV, organogold derivatives, ylide and carbene complexes, to the catalytic activity of gold it embraces a long‐neglected area of chemistry holding promise of many a discovery.