bluuebunny/crossref_metadata_2025_split huggingface dataset:
Chemical Bonding in Higher Main Group Elements
Authors: Kutzelnigg Werner 
Published: December 2003

AbstractMany concepts used for a qualitative description of chemical bonding that originated in the early days of theoretical chemistry have been vindicated recently by quantum chemical calculations, at least as far as first row elements are concerned. However, many concepts that have been justified for first row elements (Li to Ne) cannot—contrary to widespread belief—be generalized to the higher main group elements. This applies particularly to the concept of hybridization, which should be viewed with considerable caution. The essential difference between the atoms of the first and higher rows is that the cores of the former contain only s‐AOs, whereas the cores of the latter include at least s‐ and p‐AOs. As a consequence, the s and p valence AOs of first row atoms are localized in roughly the same region of space, while the p valence AOs of higher row atoms are much more extended in space. This has the consequence that for the light main group elements both lone‐pair repulsion and isovalent hybridization play a greater role than for the heavy main group elements. Furthermore, this implies that single bonds between first row elements are weak and multiple bonds are strong, whereas for the second or higher row elements single bonds are strong and multiple bonds weak. The “extended valence” (violation of the octet rule) observed in compounds of higher main group elements has very little to do with the availability of d‐AOs but is due rather to the size of these atoms and thus to the reduced steric hindrance between ligands and, to a lesser extent, also to the lower electronegativity of the heavy atoms. A model based on the concept of electron‐rich multicenter bonds is certainly closer to reality than one involving hybrids with the participation of d‐AOs. The XO bonds in phosphane oxides, sulfoxides, oxo acids and related compounds are better formulated as semipolar rather than as true double bonds, even if they behave in some respects like double bonds.—The growing interest of theorists in compounds of higher main group elements parallels new and, in some instances, spectacular results of experimental research on the chemistry of these elements.
Pyrylium Mediated Transformations of Primary Amino Groups into Other Functional Groups. New Synthetic Methods (41)
Authors: Katritzky Alan R. , Marson Charles M. 
Published: December 2003

AbstractThe first pyrylium salt was isolated some 80 years ago, yet up to the 1950s only moderate interest was taken in the preparation, properties and uses of such salts. However, the past thirty years has seen a phenomenal growth in the literature pertinent to this area of chemistry: the importance of pyrylium salts as intermediates has been realized. They are readily prepared by a variety of generally applicable routes, and they are highly reactive towards nucleophiles. Together, this enables the convenient synthesis of a great variety of acyclic and heterocyclic compounds. We have used highly substituted pyrylium salts for the two‐step conversion of the amino group in alkylamines RNH2 into numerous other functionalities. In the first step, the pyrylium salts are converted with the amines into N‐substituted pyridinium salts, which, in the second step, react with Nu⊖ to give the desired products RNu. In some cases the R moiety is also changed, e.g. by elimination. Studies of the reactions of these pyridinium salts have allowed interesting insights into the mechanisms of nucleophilic substitution, in addition to rendering aliphatic amines important synthetic intermediates. Thus, the method complements the diazotization procedure for the transformation of arylamines.
Mycotoxins from Mold Fungi — Weapons of Uninivited Fellow‐Boarders of Man and Animal: Structures, Biological Activity, Biosynthesis, and Precautions
Authors: Franck Burchard 
Published: December 2003

AbstractIn addition to the antibiotics, mycotoxins represent a second large group of biologically active substances of microbial origin. Over 300 mycotoxins are so far known and can be subdivided into 25 structural types. They are formed by approximately 350 molds of which there are 10 000 different strains. As mycotoxin‐producing molds tend to spread aggressively on foodstuffs, they can cause epidemic mycotoxicoses.—The mycotoxins have been isolated, their structures determined, and their biological activities investigated; their biosynthesis has now also largely been elucidated. Accordingly, mycotoxins are derived from only three groups of key biogenetic building blocks: polyketides, isopentenyl pyrophosphate, and amino acids. Mycotoxins are mainly non‐polar, chemically stable, are formed in high concentrations, and do not contain—unlike antibiotics—any sugars. Mycotoxins display an enormous variety of toxic‐pharmacological activities, e.g. liver degradation, hemorrhages, carcinomas. The identification of mycotoxicoses is impeded by their long incubation periods and the fact that the mycotoxins tend to accumulate in the mammalian organism, which means that even very low concentrations in food are a potential danger. The particularly dangerous aflatoxin B1 1 may not exceed a limit in food of 5 ppb (5 μg/kg), which is difficult to control.
Cobalt‐Mediated [2 + 2 + 2]‐Cycloadditions: A Maturing Synthetic Strategy [New Synthetic Methods (43)]
Authors: Vollhardt K. Peter C. 
Published: December 2003

AbstractDicarbonyl (η5‐cyclopentadienyl)cobalt functions as a matrix on which a variety of unsaturated organic substrates undergo mutual bond formation. In this way α,ω‐diynes cocyclize with monoalkynes to give annelated benzenes, while o‐diethynylbenzenes furnish biphenylenes, and α,ω‐enynes lead to the formation of complexed bi‐and tricyclic dienes. Nitriles cocyclize with two alkynyl groups to give pyridines and other heterocycles, isocyanates allow access to annelated 2‐pyridones, and incorporation of carbon monoxide provides complexed cyclopentadienones. In many cases remarkable chemo‐, regio‐, and stereoselectivity are observed, partially facilitated by use of the trimethylsilyl substituent as a controlling group. The scope and level of maturity of the method are demonstrated by the synthesis of a series of hitherto inaccessible, novel, and theoretically interesting molecules, and by its utilization in several unique approaches to a variety of natural products, e.g. protoberberines, steroids, vitamin B6, and camptothecin.
Chelation or Non‐Chelation Control in Addition Reactions of Chiral α‐ and β‐ Alkoxy Carbonyl Compounds [New Synthetic Methods (44)]
Authors: Reetz Manfred T. 
Published: December 2003

AbstractThe addition of C‐nucleophiles such as Grignard reagents or enolates to chiral α‐ or β‐alkoxy aldehydes or ketones creates a new center of chirality and is therefore diastereogenic. In order to control stereoselectivity, two strategies have been developed: (1) Use of Lewisacidic reagents which form intermediate chelates, these being attacked stereoselectively from the less hindered side (chelation control); (2) use of reagents incapable of chelation, stereoselective attack being governed by electronic and/or steric factors (non‐chelation control). Generally, the two methods lead to the opposite sense of diastereoselectivity. It is possible to predict the outcome by careful choice of organometallic reagents containing elements such as Li, Mg, B, Si, Sn, Cu, Zn, or Ti.For corrigendum see DOI:10.1002/anie.198407461
Organic Polymeric and Non‐Polymeric Materials with Large Optical Nonlinearities
Authors: Williams David J. 
Published: December 2003

AbstractNonlinear optical properties are a sensitive probe of the electronic and solid‐state structure of organic compounds and as a consequence find various applications in many areas of optoelectronics including optical communications, laser scanning and control functions, and integrated optics technology. Because of their strongly delocalized π electronic systems, polymeric and non‐polymeric aromatic compounds show highly nonlinear optical effects. Nowadays, polymer chemists are able to tailor specific materials properties for various applications. Some organic substances with π electronic systems exhibit the largest known nonlinear coefficients, often considerably larger than those of the more conventional inorganic dielectrics and semiconductors, and thus show promise for thin‐film fabrication, allowing the enormous function and cost advantages of integrated electronic circuitry. The electronic origins of nonlinear optical effects in organic π electronic systems are reviewed, with special emphasis being given to second‐order nonlinear optical effects. Methods for measuring nonlinear optical responses are outlined, and the critical relationships of the propagation characteristics of light to observed nonlinear optical effects and to solid‐state structure are discussed. Finally, the synthesis and characterization of organic crystals and polymer films with large second‐order optical nonlinearities are summarized.
Chiral Macrocycles as Reagents and Catalysts
Authors: Kellogg Richard M. 
Published: December 2003

AbstractIonophores, whether of natural or synthetic origin, encapsulate their ionic “guests” using noncovalent bonding. This encapsulation process resembles, at least superficially, the bonding of a substrate by an enzyme‐active site. The analogy to enzymes can be extended further if the ionophore is provided with functional groups that can react with a suitable guest molecule bound in the cavity of the ionophore. We have embedded in the periphery of a macrocycle a 1,4‐dihydropyridine, a mimic of the coenzyme NADH. The macrocycle, in addition to having (weak) ionophoric properties, is chiral. The strategy has led to compounds that react as artificial hydrogenases and which are capable of distinguishing, in a predictable fashion, between the prochiral faces of suitable carbonyl substrates. Ancillary developments from this approach have been many. A remarkably general method for the preparation of a wide variety of macrocycles has been developed which depends on some remarkable chemical idiosyncrasies of the cesium ion. In attempts to exploit the chemical possibilities of these macrocycles, unusual chemistry, possibly relevant to the action of the enzyme, 3‐phosphoglyceraldehyde dehydrogenase, has been uncovered. In a similar vein, study of macrocycles has led to variants of the aldol condensation on chiral templates. Finally, catalytic CC bond formation mediated by transition metals is revealed to be an area in which chiral macrocycles can play a useful role by acting as chiral ligands for the transition metal.
Prostaglandin Syntheses by Three‐Component Coupling. New Synthetic Methods (49)
Authors: Noyori Ryoji , Suzuki Masaaki 
Published: December 2003

AbstractThe role played by prostaglandins (PGs) in the human body is bewitching. Following the elucidation of their structures in the early 1960s, tremendous efforts were made for the realization of an efficient chemical synthesis, since a sufficient supply of such very rare, naturally occurring local hormones relies solely on their total synthesis. Of the many synthetic routes described in the literature, the majority of which are linear or sequential in nature, the Corey synthesis is perhaps the most versatile; so far more than 5000 prostaglandin analogues have been prepared and have been tested biologically. Some natural prostaglandins and their analogues are already in clinical use. In view of the continuing expansion of this field, we have been fascinated with the concept of developing a highly convergent entry to prostaglandins based on a new strategy. The discovery of a highly enantioselective method for the reduction of prochiral ketones and the elaboration of a procedure for double vicinal CC‐coupling with enones have enabled a one‐pot synthesis of the complete prostaglandin skeleton by linking a chiral 4‐oxygenated 2‐cyclopentenone unit and two side‐chain blocks. This approach is efficient and flexible and allows the direct production of all the naturally occuring prostaglandins and a wide spectrum of analogues.
Synthesis and Structure of the 7‐Trifluoromethyl‐1,3,5,2,4,6,8‐trithiatetrazocine Cation
Authors: Höfs Hans‐Ulrich , Hartmann Gabriele , Mews Rüdiger , Sheldrick George M. 
Published: December 2003

AbstractFor corrigendum see DOI:10.1002/anie.198500761
Synthesis of Cyclosporine and Analogues: Structural Requirements for Immunosuppressive Activity
Authors: Wenger Roland Maurice 
Published: December 2003

AbstractCyclosporine is a new immunosuppressive drug which was first marketed in 1983 under the trade name Sandimmune®. This compound, an innovation in selective immune modulation, was isolated from a fungal culture and characterized as a cyclic undecapeptide containing a novel amino acid together with several N‐methylated amino acids. The new amino acid (4R)‐4‐[(E)‐2‐butenyl]‐4,N‐dimethyl‐L‐threonine (MeBmt) was the only unknown amino acid of cyclosporine and there had previously been no means for its isolation. For this reason and because it seemed possible that MeBmt could play a significant role in determining the pharmacological activity of cyclosporine, its synthesis in enantiomerically pure form was undertaken. The next step was the development of a total synthesis of cycloporine, which appeared attractive, not only for its intrinsic worth, but also as an important tool for investigating the relationships between structure and immunosuppressive activity. Essential for the immunosuppression are the amino acids MeBmt, Abu, Sar, and MeVal in the positions 1, 2, 3 and 11, but probably also still larger parts of the molecule. Such information could be valuable for finding new chemical leads or drugs with a new activity profile.
Synthesis and Biological Activity of α‐Methylene‐γ‐butyrolactones
Authors: Hoffmann H. M. R. , Rabe Jürgen 
Published: December 2003

Abstractα‐Methylene‐γ‐butyrolactones [dihydro‐3‐methylene‐2(3H)furanones] constitute an important group of natural products and possess wide‐ranging biological activities. Progress in the synthesis of the heterocycle and the classification of the synthetic methods are not only of practical interest, but also fundamentally important as a current example for the construction of an unusual 1,4‐functionality distance and an α‐substituted acrylic ester moiety which is susceptible to nucleophilic attack.
Monoclonal Antibodies: Their Chemistry, Functions, and Possible Uses
Authors: Seiler Friedrich Robert , Gronski Peter , Kurrle Roland , Lüben Gerhard , Harthus Hans‐Peter , Ax Wolfgang , Bosslet Klaus , Schwick Hans‐Gerhard 
Published: December 2003

AbstractIn December of last year the Nobel Prize for Medicine or Physiology was awarded to the natural scientists N. K. Jerne, C. Milstein, and G. Köhler. These three immunologists have contributed fundamentally to our understanding of antibody synthesis by the individual immune cell. They confirmed the rule, established in the sixties, that only one antibody comes from each type of immune cell, thus giving it the status of a dogma. Jerne worked out the basic understanding of the great variety of antibodies, and his Jerne‐Plaque‐Test (1963) has enabled us to describe and analyze a single antibody‐producing immune cell in vitro. Thus, the study of immune cell interaction and the cooperation of lymphocyte subpopulations, which leads to the formation of antibodies, received decisive impulses and important methodological prerequisites. In 1975 Köhler and Milstein succeeded in permanently stabilizing, i.e., immortalizing, the antibody production of an already immunologically specifically primed mortal immune cell by means of cell fusion (hybridization) with an immortal, cancerous immune cell (myeloma cell). The hybrid cell and the hybrid cell clone grown from it in cell culture produced a homogeneous antibody, having the desired constant binding specificity, in a practically unlimited fashion and independently of an animal organism, a monoclonal antibody. This scientific breakthrough was the beginning of a tremendously stimulating new biotechnological development, which very quickly spread to laboratories in diverse fields and which today enables us to take up, and in part even solve, scientific, technical, and medical problems that would otherwise not have been analyzable at present.
Hexafluoropropene Oxide — A Key Compound in Organofluorine Chemistry
Authors: Millauer Hans , Schwertfeger Werner , Siegemund Günter 
Published: December 2003

AbstractOf the important C3‐compounds in organofluorine chemistry, namely hexafluoropropene, hexafluoroacetone and hexafluoropropene oxide, the latter is chemically the most versatile compound. Hexafluoropropene oxide provides another example of the frequently observed change in reactivity when hydrocarbon compounds are converted into their perfluorinated derivatives. The overwhelming majority of the known reactions of hexafluoropropene oxide are initiated through attack by a nucleophile. The conversion of hexafluoropropene oxide into hexafluoroacetone in the presence of Lewis acids is the basis of further development of the chemistry of this synthetic chemical. Hexafluoropropene oxide is also regarded as a convenient source of difluorocarbene. In industrial chemistry it now plays a significant part in the manufacture of high‐grade organofluorine products.
Recent Developments in the Field of β‐Lactam Antibiotics
Authors: Dürckheimer Walter , Blumbach Jürgen , Lattrell Rudolf , Scheunemann Karl Heinz 
Published: December 2003

AbstractThe era of β‐lactam antibiotics, which represent the most important class of drugs against infectious diseases caused by bacteria, began more than fifty years ago with the discovery of penicillin G. Further improvements by isolation and structure elucidation of new natural compounds, and systematic chemical modification of these, is a striking example of to what extent chemistry can contribute to the progress of drug therapy. The complex relationship between structure and activity requires, even today, a largely empirical approach. The minimum structural unit for antibiotic activity had to be revised several times over decades. Both the activated β‐lactam ring with an acidic group and the nature and spatial arrangement of the other substituents and rings decisively affect the potency, antibacterial spectrum, pharmacokinetics, and toxicity. Totally synthetic mono‐ and bicyclic compounds from the series of monobactams, penems, carbapenems, 1‐oxacephems, and 1‐carbacephems are increasingly joining the classic groups obtained by semisynthesis from 6‐amino‐penicillanic acid and 7‐aminophalosporanic acid.
Organocobalt Compounds in the Synthesis of Pyridines–An Example of Structure‐Effectivity Relationships in Homogeneous Catalýsis
Authors: Bönnemann Helmut 
Published: December 2003

AbstractThe cocyclization of alkynes with cyano compounds using organocobalt catalysts of the type [YCoL] has evolved into a versatile and technically useful method for synthesizing pyridine and its derivatives. An important advance came with the realization that the organo group Y remains attached to the cobalt throughout the catalytic cycle. This opened up the possibility of optimizing the catalyst by varying the controlling ligand Y.
Catalytic Synthesis of Organolithium and Organomagnesium Compounds and of Lithium and Magnesium Hydrides—Applications in Organic Synthesis and Hydrogen Storage
Authors: Bogdanović Borislav 
Published: December 2003

AbstractA recent development in homogeneous catalysis is the discovery of catalysts that are active for the lithiation of 1‐alkenes to alkenyllithium compounds and lithium hydride as well as for the hydrogenation of lithium and magnesium under mild conditions. The catalytically prepared magnesium hydride is highly reactive and adds to 1‐alkenes to give diorganomagnesium compounds and can also be used in the preparation of, for example, silane and “active” magnesium. The use of metal hydrides in hydrogen storage is discussed: hydrogenation/dehydrogenation experiments show that the catalytically prepared magnesium hydride (which can be doped with a second metal) can be used as a high‐temperature hydrogen storage material.
Radwaste Immobilization by Structural Modification—the Crystallochemical Properties of SYNROC, a Titanate Ceramic
Authors: White Timothy J. , Segall Robert L. , Turner Peter S. 
Published: December 2003

AbstractNew studies utilizing electron microscopic techniques have revealed the immobilization of radionuclides in crystalline ceramic waste forms to be not merely a question of substituting waste elements into the appropriate crystallographic sites of a host matrix. Rather than entering the ceramic as a “continuous” solid solution it has been found that the incorporation of highly radioactive waste elements is commonly accompanied by structural modification. This may take the form of cation ordering, crystallographic shear, or twinning on a unit cell scale. Such mechanisms considerably enhance the capacity of a ceramic to immobilize (simulated) radwaste and impart to it the flexibility to respond to inevitable variations in wastestream composition.
Chiral Compounds Synthesized by Biocatalytic Reductions [New Synthetic Methods (51)]
Authors: Simon Helmut , Bader Johann , Günther Helmut , Neumann Stefan , Thanos Jordanes 
Published: December 2003

AbstractIt has been known for many decades that chiral compounds can be obtained by stereospecific biocatalytic reduction. Further significant methodological developments in this field have, however, only been made during the past ten years; they include the application of previously unused microorganisms and electron donors, the discovery of additional substrates for the known reductases, the development of methods for regenerating reduced pyridine nucleotides, and the discovery of new reductases which were sought for specific preparative purposes. Many chiral compounds can now be synthesized by microbial hydrogenation using H2 and hydrogenase‐containing microorganisms as well as by electromicrobial or electroenzymatic reduction. In the two latter methods, anaerobic or aerobic organisms are supplied with electrons from electrochemically reduced, artificial mediators, e.g., methyl viologen. Reductases that do not require pyridine nucleotides and can accept electrons directly from reduced viologens are especially useful. Two examples of this type of enzyme are described which are of preparative interest. Many cells contain methyl viologen‐dependent NAD(P) reductases, a large number of which have still not been characterized. A productivity number is proposed which allows different methods of bioconversion with microorganisms to be compared. The productivity numbers of compounds synthesized by the methods described in this review are often 10‐ to 100‐fold higher than those of substances obtained by conventional techniques.