Search

The strengths and weaknesses of EXAFS in determining the radial distribution of the first nearest neighbours in coordination compounds are well known. We point out here how local information given by EXAFS upon the first coordination sphere can be used to characterize short or long range order or disorder in low dimensional materials. Low temperature measurements, use of several absorbing species in the same compound, comparison between seemingly contradictory X-Ray diffraction and EXAFS data allow to solve three dimensional structural problems related to physical properties. The materials studied are insulating uni- or bimetallic magnetic chains and pure or intercalated MPS3 layers. [ABSTRACT FROM AUTHOR]

Current views on the structure and function of biological membranes imply a great variety of dynamical aspects. These range from the dynamic phase behaviour of phospholipids and conformational variations of membrane proteins during membrane-associated processes, to the interactions between membranes in fusion and pore formation. The structural description of these processes, many of which imply the possibility of intermediate structures, calls for fast time-resolved diffraction methods, i.e. the cinematographic approach. The present article gives first an overview on the specific problems and the theories for their solution in X-ray diffraction on membranes, and then reviews the present possibilities of time-resolved structural studies using synchrotron radiation on phospholipid model systems and functional membranes (sarcoplasmic reticulum membrane). It is shown that such studies bear great promise in entering the millisecond time domain and bridging the existing gap between static structural information and the wealth of dynamic data derived from spectroscopic methods. [ABSTRACT FROM AUTHOR]

Small angle X-ray scattering and diffraction methods can provide information about chromatin structure at the levels of nucleosomes, chromatin fibre and whole nuclei. Making use of the high flux and point focusing achieved on synchrotron radiation cameras it has been possible to carry out systematic static and kinetic investigations of chromatin structure with minimum damage or perturbation on fresh samples. The results provide a strong evidence for a preformed 3-D zigzag structure at low ionic strength which folds instantaneously into the "30 nm filament" upon increasing the ionic strength. The extent of folding at a given ionic strength depends on the nature of the cations and can be influenced by other substances which bind to DNA. Models of chromatin fibre structure are discussed in the context of various experimental results. [ABSTRACT FROM AUTHOR]

The use of anomalous X-ray scattering of light elements like sulfur and phosphorus is of particular interest in biological structure research. These elements serve as native labels in proteins, nucleic acids and membranes. Their medium scattering power is drastically changed at their K absorption edges at wavelengths between 5 and 6 Å where X-ray absorption excludes the use of open air diffractometers. The construction of a new diffractometer tunable to wavelengths between 1.2 and 7 Å is presented. First results of anomalous scattering from sulfur in bacteriorhodopsin near the K absorption edge have been obtained recently. Their possible impact on crystallography will be considered. A comparison with nuclear spin dependent neutron scattering is given. [ABSTRACT FROM AUTHOR]

Applications of X-ray absorption spectroscopy in two areas of liquid state studies using synchrotron radiation are reviewed. One area concerns with the determination of the local structure, bonding and dynamics at the metal site of transition metal complexes using Extended X-Ray Absorption Fine Structures (EXAFS) and X-Ray Absorption Near Edge Structures (XANES). Emphasis are placed on the implications of the derived parameters to the reactivity of the complexes in exchange reactions (ligand and electron) in solution and to the subtle electronic and structural change of the complexe upon dissolution. The other area deals with the principles and practices of X-ray induced photo-conductivity of hydrocarbon solutions. The conductivity measurements of EXAFS of organometallics in hydrocarbon solution is described. The special feature of the technique. EXAFS measurement under the condition of total X-ray absorption, is discussed in some details. Current developments is related areas are also noted. [ABSTRACT FROM AUTHOR]

The first observations of an increase of the reactivity between gases in the presence of a metal were made, almost one hundred years ago, by chemists like Davy or Thenard. This pioneer work led Berzelius and Mitscherlich to define the concept of "decomposition of species by contact under a catalytic force". Since then much work has been done in order to understand the behavior of these small transition metal clusters which are able to promote some chemical reactions. In fact, catalysis has become a field of enormous economical interest. It spans a wide variety of areas from oil reforming to the preparation of synthetic fibers or fertilizers. Theoretical research as well as chemical engineering are therefore deeply involved and many current studies deal with the knowledge of such materials. It is not always easy to characterize the electronic and crystallographic structures of very small aggregates. Their size (a few nanometers) is due to the fact that as many atoms as possible must be active and therefore must be at the surface. Moreover, the analysis has to be done in situ, under the true reaction conditions, in order to build a physical model for the role of the catalyst. Then, many experimental techniques have been used, including most recently electron microscopy and X-ray absorption. We focus our attention here on the EXAFS (Extended X-ray Absorption Fine Structure) technique and its possibilities for the study of supported metal catalysts. Most of the examples come from a collaboration between LURE and some public CNRS laboratories (Strasbourg, Meudon) and a private one (IFP — Rueil Malmaison). We begin with some generalities about the technique and the type of catalysts studied, then move to several examples of application. [ABSTRACT FROM AUTHOR]

In this short review of recent advances in XANES we have discussed the use of this spectroscopy to probe the geometry of local structure via multiple scattering effects in the real space and and the local partial unoccupied density of states in metals via the band structure approach. Several important aspects of x-ray absorption spectroscopy have not be considered such as the excitonic effects, the many body infrared singularities in free electron metals and the many body final state configurations in solids with high electronic correlation such as the valence fluctuating metals and correlated oxides as CeO2 and the new high Tc oxide superconsuctors. These effects require to go beyond the one-electron aproximation discussed here but give important information on the local electronic structure of materials. [ABSTRACT FROM AUTHOR]

This paper aims at showing the interest of synchrotron radiation for various applications in the field of Inorganic Chemistry. After a short survey of the main properties of this light source and some characteristics of the storage rings, two main applications are reviewed. The first one concerns X-ray scattering techniques. The study of very tiny single crystals and the improvements in the field of powder structure investigations are underlined. The second field of applications deals with X-ray absorption spectroscopies. After a short recall of the theory, remarkably enlightened during the last years, the discussion is mainly centered on the EXAFS applications in the various fields of inorganic chemistry. Some examples of XANES results are also given. [ABSTRACT FROM AUTHOR]

A series of azophenol-dyed macrocycles, which accommodate selectively a given metal or ammonium ion in their cavity and undergo a concurrent color change, were designed and synthesized in order to study molecular recognition in biological systems. Selective coloration of some azophenol hosts and selective fluorescent emission of benzothiazolyl crowns for metal cations are observed to be useful for lithium analysis. Amine selective coloration with some azophenol hosts including enantiomeric discrimination are reported and X-ray crystal structures of sec.-amine inclusion complexes are illustrated. Selective coloration with two acerands is capable of discriminating the substitution pattern of alkylamines. [ABSTRACT FROM AUTHOR]

A review is given of the recent developments of the chemistry of cyclotriveratrylenes and cryptophanes, and of their use in the study of host-guest interactions. Experiments dealing with the complexation of tetrahedral species including neutral molecules (halogenomethanes and alkanes) and organic soft cations (tri- and tetramethyl ammonium cations, acetylcholine) are discussed. Prospective applications of cyclotriveratrylenes and cryptophanes to the design of ferroelectric liquid crystals and of three-dimensional charge tranfer materials are presented. [ABSTRACT FROM AUTHOR]

An increasing appreciation of molecular architecture and design will provide a greater impetus to tackle the exciting and elaborate structural targets that will confront chemistry in the twenty-first Century. It is, therefore, of the upmost importance that methods by which outwardly complex chemical systems can be assembled selectively, rapidly, and efficiently, are devised. Towards this goal, a trebly diastereoselective tandem Diels-Alder reaction sequence has been developed. Subsequently, we have demonstrated how this synthetic methodology can be utilized for the synthesis of novel macropolycyclic molecular structures. In each cycloaddition, the stereoelectronic characteristics, that are inherent in part of the rigid bicyclic frameworks of both bisdienophile and bisdiene building blocks, are used to dictate their respective modes of reaction. The outcome of this substrate-directed approach to organic synthesis allows the rapid and highly-controlled assembly of an increasing range of apparently complex molecular structures. The diastereoselectivities that are witnessed in each cycloaddition are rationalized as involving kinetically-controlled transition-state effects. The rationalization is based upon maximal vicinal staggering and the subsequent minimization of torsional strain within the rigid bicyclic framework in the transition state. In this survey, the application of the repetitive Diels-Alder approach for the synthesis of a wide range of macropolycyclic molecular structures is examined. The potential of these compounds as ideal precursors in the preparation of intriguing hydrocarbons is revealed. The pivotal role played by the sequential use of mild and forcing conditions — most notably the use of thermally- and high pressure-promoted Diels-Alder reactions — to produce oligomeric compounds in a predictable and stepwise manner is documented. Throughout these discussions, special attention has been given to the methods employed for the purification and characterization of this new class of unnatural products. [ABSTRACT FROM AUTHOR]

Structure-activity relationships for biologically active congeneric compounds were examined by using free-energy-related physicochemical parameters and regression technique. The steric effects involved in such biological activities were expressible and separable from other factors as steric parameters depending upon the situations. In this article, examples for enzyme reactions, enzyme inhibitions, insecticides, fungicides, herbicides, plant growth regulators and synthetic sweetners, mostly from our own laboratory are reviewed. The steric parameters used here are Taft Es, Hancock Esc, Verloop STERIMOL and van der Waals molar volume. [ABSTRACT FROM AUTHOR]

This section reviews the molecular shape descriptors developed by Amoore, Allinger, Simon et al. and Testa and Purcell. The illustrative examples discussed refer to the odour similarity and cardiotoxic aglycones. One has stressed the methods based on the reference structure because, correctly formulated, these methods seem to offer promising perspectives to model the steric effects in biological systems. Finally, a short discussion of possible connections between steric and other substituent constants (relevant in the context of multicollinearity in QSAR) is included. [ABSTRACT FROM AUTHOR]

The definition of steric parameters initially based on Van der Waals raddi (rv) is described. Tables of various sets of rv are given. These different types of rv are shown to be interrelated. The rv values of Bondi are chosen as the standard set. Substituents are classified according to the degree of conformational dependence of their steric effect. Values of the steric parameters v are obtained directly from rv whenever possible. For symmetric MZn groups they are obtained from calculated rv values. For planar π-bonded groups equations are derived which permit the calculation of both v and the corresponding delocalized electrical effect parameters. Values of v for other grups are based on kinetic data scaled by correlation with groups for which v is known. Over 300 values of v are reported. [ABSTRACT FROM AUTHOR]

This chapter deals with the description of the main topological indices and of related indicatros for molecular constitution used in structure-activity relationships (QSAR). The topological indices are numerical quantities based on various invariants or characteristics of molecular graphs. For the convenience of the discussion, these indices are classified according to their logical derivation from topological invariants, rather than according to their chronological development. [ABSTRACT FROM AUTHOR]

Sufficient data are now available to study trends in the spectra of molecules on substitution with progressively heavier atoms down the periodic table. We have chosen the carbonyls and ketenes for review of the effect of replacement of the oxygen by sulfur or selenium and the nitriles for which there is information on substitution of nitrogen by phosphorus. The basic spectroscopy of these chromophores is reviewed for the ground and electronic excited states, correlating observed trends and differences. Using this information, conclusions are drawn concerning the spectroscopic effects of substitution by heavy atoms. In particular, it is observed that such substitutions have predictable effects on the ground state but often surprising and interesting changes are found in the excited state. In most cases, more information can be gleaned from the electronic spectra of the heavy atom substituted chromophores due to lower transition energies, stronger spin-orbit coupling effects and favorable changes in excited state geometries. Future advances in our understanding of molecular spectroscopy are likely to come from the pursuit of the spectra of other chromophores substituted with heavy atoms. [ABSTRACT FROM AUTHOR]

Interdisciplinary barriers separating the "organic", "inorganic" and "solid state" subdisciplines of chemistry exist because there is no conceptual apparatus which clearly delineates how the rules of bonding depend on the identity of the atoms comprising a molecule and how they change as we move from the domain to another. We present a theory which is based on a fusion of MO and VB concepts which enables one to do the following: Dissect a molecule into fragments, apportion the valence electrons in the fragment orbitals, and then make "bonds" connecting the fragments. In this way two key elements of the bonding problem become transparent:(a)The interplay between fragment excitation (investment) and "bond" making (return) in engendering net stabilization (profit) of the system.(b)The mechanism of bond-making. Nonmetals bind primarily by spin-pairing plus one-electron delocalization due to overlap while metals bind mainly by two-electron delocalization in which overlap is assisted by an overlap-independent mechamism of bonding. The recognition that the mode of electron delocation depends on the identity of an atom sets up the stage for a differentiation between structural and reactivity properties expected from organic and inorganic molecules. This point is illustrated by diverse applications and reference to existing experimental and computational data. The article itself can be viewed as a first step towards a reexamination of solid state physics phenomena (most notably conductivity and superconductivity) from a chemist's vantage point after it has been demonstrated that a self-consistent, qualitative understanding of molecular electronics across the Periodic Table is now at hand. [ABSTRACT FROM AUTHOR]

After a brief introduction "carbanion" and "carbanion" radical rearrangements are reviewed with special concern to the literature of the last ten years. Radical "anion" systems are included because of the increasing importance of electron transfer reactions. The first topic deals with the cyclopropyl-allyl and the intimately related allyl "anion" rearrangement. Next, electron transfer induced rearrangements of cyclopropanes (stereoisomerization and formation of ring-opened derivatives via trimethylene radical "anions") are reviewed. In relation with this subject the question is discussed whether the rearrangement of cyclopropanes in the presence of a base is initiated by proton or electron transfer. Section 3 deals with electron transfer induced rearrangements of the generalized cyclobutene-butadiene radical "anion" system, followed by the cyclization of 5-hexenyl "anions". Because of the widespread use of 5-hexenyl radical cyclizations as a mechanistic tool it is important to separate the radical from the "anion" rearrangement. Section 5 deals with the isomerization of α-substituted vinyllithium compounds. Section 6 describes how the structure of a "carbanion" is strongly influenced by a variation of the gegenion, the solvent, complexing agents and the temperature. Electron transfer valence isomerizations are treated next. The complexity of "carbanion" structures in solution, as shown in sections 6 and 7, is also observed in the solid state as demonstrated by the polymorphism of organolithium compounds. Section 9 is devoted to some results concerning rearrangements of and within alkyllithium aggregates, and the final topic refers to "carbanion"-accelerated rearrangements. [ABSTRACT FROM AUTHOR]

Complex eliminations proceed with rearrangements. They'occur in most of the different branches of preparative organic chemistry. By relative notations of positions, they are ordered and named in a classifying way, independent from the respective reaction mechanism. Structural peculiarities of particular types are also included and distinguished. If only one or two groups or bonds migrate, the classifications are distinct and unequivocal. Nevertheless, in case of two migrating residues it is occasionally necessary to evaluate by labelling, which type the reaction belongs to. In the case of multiple migrations, it is necessary to divide the reaction into partial steps via (several) postulated or proved intermediates. The presented material shows synthetical potential and points out that most types can be realized according to different reaction mechanisms and under most different reaction conditions. [ABSTRACT FROM AUTHOR]

This paper focusses on stereoselective syntheses of deoxy oligosaccharides which are an integral part of a number of antibiotics and cytostatics. Following a short review on the biosynthesis of these sugars, the general techniques including classical methods as well as recent stereospecific procedures for the preparation of deoxy glycosides are outlined. For these oligosaccharide-containing pharmaceuticals, brief explanations of their current employment in medical applications are given. The key problems of glycosylations and detailed discussions of selectively adjusted methods will be addressed. [ABSTRACT FROM AUTHOR]

"Pseudo-sugar" is the name of a class of compounds in which a ring-oxygen of a hexopyranoid sugar is replaced by a methylene group. There are thirty-two theoretically possible stereoisomers in the pseudo-sugar family, including anomer-like compounds. The first pseudo-sugar was synthesized by G. E. McCasland and his coworkers in 1966. The most accessible starting material for the synthesis of pseudo-sugars was the Diels-Alder adduct of furan and acrylic acid. The Diels-Alder adduct was readily resolved by means of optically active amines as resolution reagents into the two antipodes, which were then used as starting compounds for the synthesis of enantiomeric pseudo-sugars. Also, a chiral synthesis from true sugars has been used for the preparation of enantiomeric pseudo-sugars. [ABSTRACT FROM AUTHOR]

Methods now available for the partial protection of carbohydrate hydroxyl groups by alkylation are critically evaluated. Of the numerous alkyl protecting groups known, only those widely used in synthetic chemistry of saccharides are discussed. Benzyl, allyl, triphenylmethyl and other arylalkyl ethers, trimethylsilyl, tert-butyldimethylsilyl, and tert-butyldiphenylsilyl ethers are the main topics of the article. [ABSTRACT FROM AUTHOR]

The three dimensional structure of oligosaccharides determines their interaction with receptors and hence is important for their biological activity. Conformational analysis of oligosaccharides makes the three dimensional structure available. The analysis of the conformation of oligosaccharides is usually determined by a combination of computational methods and experimental techniques. NMR spectroscopy is the most important experimental tool. The calculational techniques cover a wide range with most emphasis put into force field calculations. Conformational flexibility plays an important role in many though not in all oligosaccharide structures. Glycosidic linkages to a side chain of a pyranose ring are more flexible than are linkages to the pyranose ring. The major attempts are described to determine the three dimensional structure of oligosaccharides with the exception of homooligomers. This review covers conformational analyses of blood group antigens of N-linked and of O-linked oligosaccharide chains, of glycolipids, of oligosaccharides related to O-specific polysaccharides of bacteria, and of oligosaccharides related to proteoglycans. [ABSTRACT FROM AUTHOR]

The chemical syntheses of the carbohydrate portions of the naturally occurring glycolipids derived from the sphingosine bases (as well as the syntheses of the complete glycosphingolipids) and of myoinositol-containing lipids derived from esters of glycerol, which were reported between 1977 and 1987, are reviewed. This article therefore brings up to date a previous review published in Chemistry and Physics of Lipids (1980) 26: 287. Since this is a very active field of research, most of the new synthetic techniques introduced into carbohydrate chemistry in the last decade are discussed. The syntheses of sphingosine bases and glycerol derivatives from carbohydrate precursors acting as ‘chiral templates' are also reviewed with a more historical perspective. [ABSTRACT FROM AUTHOR]

Synthetic photochemical applications to carbohydrates are described by photosubstitution, photoaddition, photoizomerisation, photoreduction, and photoxidation reactions. The use of photoremovable protecting groups in osidic syntheses is also evoked. The recent development of such radical reactions using photoinitiators is mainly due to their regio and stereoselectivities interpreted in terms of stabilized conformations of carbohydrate radicals by stereoelectronic factors. The preferential α-attack of anomeric radicals is a well documented example of such transformations. Some new methodologies using single electron transfer reactions are also indicated in this review. [ABSTRACT FROM AUTHOR]

The O-polysaccharides of bacterial lipopolysaccharides are in general regular, periodic polymers with diverse structures that contain, in many instances, comparatively rare monosaccharides. The oligosaccharides that constitute the repeating units of these polysaccharide antigens provide a demanding challenge in terms of glycoside synthesis, an objective which is particularly important since these structures act as antigenic determinants which are valuable markers of bacterial infection. Advances in glycoside synthesis together with the ancillary techniques of chromatographic separation and high resolution NMR spectroscopy have permitted rational synthesis of such oligosaccharides to be planned, successfully completed and have in a limited number of instances, even allowed small polysaccharides, representative of the O-polysaccharide, to be synthesized via polymerization of synthetic repeating units. The most intensive synthetic efforts have focused on the O-antigens of Salmonella, Shigella, and E. coli, although increasing attention is being given to the synthesis of antigenic determinants of other Gram-negative pathogens, including those of Brucella. [ABSTRACT FROM AUTHOR]

Although the use of cyclopropanes in synthesis has been an area of very considerable interest for many years, much less attention has been drawn to the potential and actual uses of cyclopropenes. This may perhaps be a reflection of the perceived difficulty in handling such species, but probably also results from the rather limited routes to cyclopropenes, and in particular those bearing other functional groups, compared to the variety available which lead to the saturated analogues. However, the development of more flexible routes to cyclopropenes makes available not only the exploitation of those reactions which are peculiar to the unsaturated system but also, through stereocontrolled addition to the double bond, provides more efficient routes to cyclopropanes of particular stereochemistry. This review will attempt to identify the progress which has been made in the synthesis of cyclopropenes, to show the rich variety of their synthetic transformations, and to point to areas where future progress may occur. It will not cover purely structural or spectroscopic aspects of cyclopropene chemistry, which were dealt with in most excellent fashion by Closs 1), and in several later reviews 2). Nor will the photochemical or metal induced reactions be dealt with in great detail, the former area having been covered in reviews by Padwa3) and the latter in an article in the present series by Binger and Buch4). [ABSTRACT FROM AUTHOR]

Syntheses and ring opening reactions of vicinally donor-acceptor-substituted cyclopropanes are treated in this review. Special attention is directed to methyl 2-siloxy cyclopropanecarboxylates which display an particularly rich and versatile chemistry leading to synthesis of polyfunctional compounds, certain carbocycles, and heterocyclic systems. Other classes of donor-acceptor-substituted cyclopropanes are also regarded emphasizing new developments. Although preparative aspects are strongly accentuated, interesting mechanistic features, characteristic for this type of three-membered rings, will also be discussed. [ABSTRACT FROM AUTHOR]

Substituted on the same carbon by an electron donating group and an adjacent multiple bond, cyclopropanes provide building blocks of unprecedented synthetic potential. They are readily available along different routes involving: cyclopropanone hemiacetals, oxaspiropentanes, alkylidenecyclopropanes, 1-heterosubstituted lithiocyclopropanes, α-enone silyl enol ethers, 1,3-dichloroacetone and 1-hydroxycyclopropylcarbonyl derivatives as main sources. First of all, they undergo acid-induced C3→C4 ring expansion to four-membered rings, in particular to the 2-vinylcyclobutanone system, which is an efficient precursor of C5, C6 and C8 homologous rings by subsequent acid- and base-induced, thermal, or photolytic ring enlargements. On the other hand, they undergo thermal C3→C5 ring expansion providing cyclopentanone enol ethers or derivatives with high chemo-, regio- and stereoselectivity. [ABSTRACT FROM AUTHOR]