Showing papers in "Angewandte Chemie in 1982"
TL;DR: In this article, a lecture dedicated to the late Robert B. Woodward, a supreme patterner of chaos, was given. And it is our collaboration on orbital symmetry conservation, the electronic factors which govern the course of chemical reactions, which is recognized by half of the 1981 Nobel Prize in Chemistry.
Abstract: Robert B. Woodward, a supreme patterner of chaos, was one of my teachers. I dedicate this lecture to him, for it is our collaboration on orbital symmetry conservation, the electronic factors which govern the course of chemical reactions, which is recognized by half of the 1981 Nobel Prize in Chemistry. From Woodward I learned much: the significance of the experimental stimulus to theory, the craft of constructing explanations, and the importance of asethetics in science. I will try to show you how these characteristics of chemical theory may be applied to the construction of conceptual bridges between inorganic and organic chemistry.
1,126 citations
TL;DR: A wide variety of complex oligosaccharides has now been made accessible as a result of methodological improvements in the sphere of chemical synthesis, which can be used for the study of conformations and interactions with protein-receptor molecules.
Abstract: A wide variety of complex oligosaccharides has now been made accessible as a result of methodological improvements in the sphere of chemical synthesis, which can be used for the study of conformations and interactions with protein-receptor molecules. Such work is of particular importance because the oligosaccharide chains of glycoproteins and glycolipids anchored to the plasma membrane are important in cell-cell interactions and are receptors for enzymes, hormones, proteins, and viruses; furthermore, they determine the antigen properties of cells. The methods of synthesis available for the production of selective linkages in oligosaccharides are dealt with, particular attention being paid to conversion of saccharides into oligosaccharides, which are important from the biological point of view.
885 citations
TL;DR: In this paper, the authors proposed a model for peptide conformation analysis using NMR spectroscopic methods, including many recent advances, in relation to their ability to contribute to peptide conformational analysis.
Abstract: Cyclic peptides containing biologically active hormone sequences are suitable models for studying conformation-activity relationships. In such models the usual flexibility of peptide chains is reduced by their cyclic arrangement. However, conformational analysis of such systems by experimental means is possible only if a single conformer predominates at equilibrium, and criteria for this are put forward. NMR spectroscopic methods, including many recent advances, are discussed in relation to their ability to contribute to peptide conformational analysis.
776 citations
678 citations
TL;DR: From Configurational Notation of Stereoisomers to the Conceptual Basis of Stereo Chemistry as mentioned in this paper is the title of a lecture [V. Prelog, ACS Symp. Ser. No. 12 (1975) 179] in which the chronologically and alphabetically third member of the CIP-triumvirate discusses the history of the system.
Abstract: “From Configurational Notation of Stereoisomers to the Conceptual Basis of Stereochemistry” is the title of a lecture [V. Prelog, ACS Symp. Ser. No. 12 (1975) 179] in which the chronologically and alphabetically third member of the CIP-triumvirate discusses the history of the CIP-system. The experience accumulated in the meantime, particularly the efforts to teach computers to use the system as described in 1966 by Cahn, Ingold and Prelog, has shown that certain of its aspects must be more strictly formulated and others modified. Questions dealing with stereoisomers (and stereotopicity), which previously have been treated predominantly in a pragmatic manner, are now discussed in a systematic way.
513 citations
399 citations
TL;DR: In this paper, the relative importance of bond strength, steric effects, and polarity in determining the rate and orientation of free radical subsitution and free radical addition reaction is considered.
Abstract: The relative importance of bond strength, steric effects, and polarity in determining the rate and orientation of free radical subsitution and free radical addition reaction is considered. The factors which control substitution reaction (radical transfer reaction) are gathered together as five “rules”, and a similar five “rules” are proposed for addition rections. These “rules” are shown to be special cases of two “laws” which govern all free radical reactions.
325 citations
TL;DR: In this article, the relative topicity of approach of reactants is defined as like {Ik} and unlike (ul) if the corresponding descriptor pairs are Re*,Re*. or R*,Re*, and Re*,Si* or R*)Si*, respectively.
Abstract: uous specification of the steric course and the product configuration of diastereoselective reactions. The reflection-invariant relative topicity of approach of reactants is defined as like {Ik) and unlike (ul) if the corresponding descriptor pairs are Re*,Re*. or R*,Re*, and Re*,Si* or R*,Si*, respectively. The descriptor pair notations (Ik and uc) of reactants disclose related steric courses of reactions more often than do the relative configurations of their products, for which the configurational notation I= R*,R* and u= R*,S* is proposed. The advantage of specifying the relative topicity is demonstrated by means of a series of recent examples of importance to the synthetic organic chemist taken from the literature and from our own work. 1. Introduction-The Problem Complex natural products such as macrolides, ionophores, and related compounds have recently become target molecules for synthetic organic chemists[’]. The often formidable array of asymmetric carbon atoms in the structures of these compounds[21 has stimulated the development of highly stereoselective asymmetric syntheses. Foremost amongst these are the ald~l[~-~], nitr~aldoF“~], and Michael additions[81. In all these reactions two planar trigonal carbon atoms are linked together to create a pair of asymmetric tetrahedral carbon atoms [route (a) in Scheme 11. Stereoselective additions to 01efins[~] and to trigonal carbon atoms a to an asymmetric carbon [routes (b) and (c), respectively, in Scheme 11, give the same type of products 1L7.9-121
318 citations
TL;DR: In this article, the authors proposed a stereoselective CC bond forming reaction to obtain a diastereomeric adduct from E- or Z-crotyl compounds.
Abstract: In an ideal synthesis, the formation of the CC bonds as well as the creation of the chiral centers and of the final functionality would be carried out simultaneously in each step. Accordingly, stereoselective CC bond forming reactions are required. Instead of a pair of diastereomers, a synthesis should yield only one of the two possible diastereomers. Such selectivity is often achieved in the addition of crotylboron, -aluminum, -tin, -titanium, and -chromium compounds to aldehydes. Depending on whether one starts from E- or Z-crotyl compounds, one diastereomeric adduct is formed preferentially. In this case the newly formed CC bond carries a methyl substituent. However, the synthetic goal is often a compound carrying an oxygen function at the site of the new CC bond. Compounds of this type can be obtained through addition of heteroatom-substituted allylboronic acid esters or analogous reagents to aldehydes.
295 citations
TL;DR: The four-component condensation of amines and carbonyl compounds with isocyanides11 and suitable acid components (water, thiosulfuric acid, hydrogen selenide, hydrogen azide, cyanic acid, thiocyanic acid and carboxylic acids, methyl hydrogen carbonate) to form α-amino acid derivatives was discovered in 1959 as discussed by the authors.
Abstract: The four-component condensation of amines and carbonyl compounds with isocyanides11 and suitable acid components (water, thiosulfuric acid, hydrogen selenide, hydrogen azide, cyanic acid, thiocyanic acid, carboxylic acids, methyl hydrogen carbonate) to form α-amino acid derivatives was discovered in 1959. This reaction principle shares some features with the Strecker synthesis and the Passerini reaction. The four-component condensation affords easy and effective one-pot synthesis of complex molecules from simple building blocks. Only in recent years, however, have the preparative advantages of the four-component condensation been exploited by numerous authors in the synthesis of diverse natural products and related compounds, although many of the possibilities opened by this principle were recognized more than two decades ago. In this progress report some instructive syntheses of various antibiotics are reviewed. The design of each of these syntheses involves a four-component condensation as key step, by means of which lengthy sequences of reactions are avoided, which otherwise would be required to achieve the synthetic goal.
294 citations
TL;DR: In this article, the significance of typical model reactions, both with regard to some fundamental aspects of synthesis gas chemistry, and in comparison with previous views concerning the mechanism of the Fischer-Tropsch synthesis, is discussed.
Abstract: The Fischer-Tropsch Synthesis counts among the industrial-scale processes having a versatile and broad product range, and has for decades offered the most attractive possibility for the use of coal as a source of heating oil and fuels. This conceivably simple reaction, the catalytic hydrogenation of carbon monoxide, generally leads to simple hydrocarbons as well (i.e. short chain olefins) that have been sought as chemical feedstocks since the oil crisis of the seventies, but fails to provide the large-scale, economic process required, due in large part to the minimal selectivity of traditional Fischer-Tropsch processes. In an effort to solve this problem current research in this sector is concerned not only with the optimization of old and the development of new catalytic systems, but also increasingly with the elucidation of numerous relevant reaction mechanisms. This article will discuss, from the viewpoint of an organometallic chemist, the significance of typical model reactions, both with regard to some fundamental aspects of synthesis gas chemistry, and in comparison with previous views concerning the mechanism of the Fischer-Tropsch Synthesis. The importance of various unique classes of complexes that have been studied in the context of Fischer-Tropsch chemistry is also evaluated with regard to their importance in the synthesis of hydrocarbons from carbon monoxide and hydrogen. It emerges that the primary steps of the reductive oligomerization of carbon monoxide are best described by the carbide/methylene mechanism, as originally proposed by Hans Fischer and Franz Tropsch.
TL;DR: In this paper, an alternative mechanism involving dioxiranes as the reactive species was proposed, which is relevant to catalytic oxidations using O2, H2O2 or ROOH.
Abstract: In this progress report an attempt is made to rationalize, from a mechanistic point of view, the different ways in which oxygen is transferred from inorganic and organic peroxides to nucleophilic substrates, particularly olefins. Oxygen transfer from transition-metal peroxides, which is relevant to catalytic oxidations using O2, H2O2 or ROOH, occurs via a cyclic or “pseudocyclic” peroxymetalation in which a dioxametallacycle is formed. Owing to the wide discrepancy between peroxymetalation and the conventional oxidation mechanism, i.e. nucleophilic attack of the substrate at the electrophilic “active oxygen”, we propose an alternative mechanism involving dioxiranes as the reactive species. The generation of dioxiranes appears to be a common denominator in the reactions of most organic peroxides e.g. peroxy acids, the reaction of electrophilic ketones with H2O2, or ozonizations. Oxygen transfer from dioxirane reagents probably involves the formation of a charge-transfer π-complex between the substrate and the carbon atom of the dioxirane, and the subsequent formation of a cyclic peroxidic intermediate.
TL;DR: In this paper, the authors proposed a methodology for α-tert-alkylation of aldehydes, ketones and esters using silyl enol ethers.
Abstract: Carbonyl compounds undergo α-alkylation via the corresponding silyl enol ethers using SN1 active alkyl halides or acetates in the presence of Lewis acids. This methodology extends the scope of carbonyl chemistry considerably, since SN1 active alkylating agents are generally base sensitive and therefore unsuitable for reactions with enolate anions or nitrogen analogs. A prime example is the α-tert-alkylation of aldehydes, ketones and esters.
TL;DR: In contrast to the thoroughly studied plarity properties of pure liquids, only little is known about the polarity of mixtures of liquids, although the majority of machanistic and preparative work is not carried out in pure phases as mentioned in this paper.
Abstract: In contrast to the thoroughly studied plarity properties of pure liquids, only little is known about the polarity of mixtures of liquids, although the majority of machanistic and preparative work is not carried out in pure phases. Using a widely applicable two-parameter equation, polar behavior of binary liquid mixtues can be described quantitatively as a function of their composition. Based on this equation, satisfactory explanations are found for deviations observed for binary solvent mixtures from the linear correaltion of polarity scales, as well as for the unusual activation parameters estimated by Winstein for solvolysis of tert-butyl chloride. Applications of the equation range from a rapid test for determining water contents of solvents, the study of reaction mechanisms, to polymer chemistry.
TL;DR: In this paper, the authors investigated the properties of supercooled aqueous solutions and compared them with normal liquids, and concluded that the anomalies of water, for instance the density maximum of the liquid phase and the initial decrease in viscosity with pressure, are confined to the pT-range: T < 300 K, p ≲ 200 MPa.
Abstract: Water at low temperatures (T < 300 K) reveals a multitude of unusual physico-chemical properties, which originate from the strong directivity of the hydrogen bonds. This directivity forces the molecules into a tetrahedral arrangement of first neighbors and thus leads to a poor packing efficiency. All anomalies of the liquid become more pronounced in the metastable range below the melting pressure curve. In the following article, methods for the investigation of the supercooled range are presented. A few anomalous static and dynamic properties are described and compared with the properties of normal liquids. The experimental results are discussed in the context of computer simulations and the more recent theories on cold water. It can be concluded that the anomalies of water, for instance the density maximum of the liquid phase and the initial decrease in viscosity with pressure, are confined to the pT-range: T < 300 K, p ≲ 200 MPa. At these pressures, lowering the temperature leads to an unexpected behavior of almost all physical properties. They seem to approach a phase transition at T ≃ (TM − 50 K), the nature of which has not yet been fully characterized. This phenomenon has hitherto been observed only in supercooled water.—In the future it will be possible to use supercooled aqueous solutions for kinetic studies, thus expanding the available dynamic range for the investigation of aqueous solutions considerably.
TL;DR: In this article, it was shown that the condensation of o-hydroxyacetophenone with aliphatic aldehydes and ketones in the presence of pyrrolidine leads to 4-chromanones in good yields.
Abstract: Many natural products, such as the plant insecticides rotenone and ageratochromene (precocene), some fungitoxic phytoalexins, hashish constituents, vitamin E, and plant pigments, contain the chroman ring system as common structural unit. Some years ago we discovered a particularly simple entry to this class of heterocyclic compounds: The condensation of o-hydroxyacetophenones with aliphatic aldehydes and ketones in the presence of pyrrolidine leads to 4-chromanones in good yields. The scope of application and limitations as well as modification of this versatile synthesis are outlined; the accompanying tables give a good indication of the wealth and variety of possible substituents. The reaction of o-hydroxyacetophenone, e. g. with glyoxylic acid or with α,β-unsaturated ketones, proceeds atypically. The majority of the new chromanones can be used, inter alia, for the preparation of chromones, chromones and chromons, and for the synthesis of tricyclic and larger ring systems.
TL;DR: A review of the syntheses and structures of these complexes as well as a unified, albeit qualitative, approach to their bonding and properties can be found in this article, where the metal atom is bonded to the three phosphorus atoms of the np3-ligand and in addition to one P atom of the intact P4 molecule.
Abstract: Salts of 3d, 4d, and 5d metals in the presence of the ligands 1,1,1,-tris(diphenylphosphinomethyl)ethane (triphos) or tris (2-diphenylphosphinoethyl) amine (np3) react with white phosphorus P4 (or yellow As4) to produce several mononuclear sandwich and dinuclear triple-decker sandwich complexes, which contain the unprecedented cyclo-triphosphorus (or cyclo-triarsenic) unit acting as a trihapto-ligand. In these complexes the metal atoms are bonded to the there phosphorus atoms of the phosphane ligand and to the three atoms of the cyclo-P3 or cyclo-As3 unit. The complexes are diamagnetic or have μeff-values corresponding to one or two unpaired electrons. The cyclo-P3 ligand is coordinatively unsaturated as proved by the fact that the mononuclear sandwich compounds may form Lewis-base adducts with electron-acceptor fragments. Reaction of the complexes (np)3M (M = Ni, Pd) with white P4 leads to formation of diamagnetic compounds [(np3)M(η1-P4)], in which the metal atom is bonded to the three phosphorus atoms of the np3-ligand and in addition to one P atom of the intact P4 molecule, which behaves as a monohapto-ligand. This article contains a review of the syntheses and structures of these complexes as well as a unified, albeit qualitative, approach to their bonding and properties.
TL;DR: The complete manuscript of as discussed by the authors appears in: Angew. Chem. Suppl. 1982, 147. DOI:10.1002/anie.198201470, p.
Abstract: The complete manuscript of this communication appears in: Angew. Chem. Suppl. 1982, 147. DOI:10.1002/anie.198201470
TL;DR: The tricyclo[3.3.0.2] octan-3-one as mentioned in this paper is a simple building block that can be used to obtain a wide range of cyclopentanoid structures.
Abstract: Bi- and tricyclopentanoid natural products have been in the focus of synthetic interest in recent years. With widely applicable building blocks lacking, most work has been restricted in the past to specific approaches to single racemic target structures. A novel concept is now presented which uses for the first time a simple building block, tricyclo[3.3.0.02,8]octan-3-one, providing enantiospecific access to diverse cyclopentanoid structures. Tricyclooctanone is obtained in a few steps and in high yield from benzene, via the triplet-sensitized oxadi-π-methane rearrangement of bicyclo[2.2.2]octenone as a photochemical key reaction. Smooth preparations of both enantiomers are possible by resolving the racemic mixture and of the bicyclooctenone. Owing to its structural properties the tricyclooctanone undergoes a wide array of regio- and stereoselective transformations in high yields, in compliance with the expectations for a synthetic key compound of general applicability. The total syntheses of boschnialactone, allodolicholactone, irido- and isoiridomyrmecin, and loganin aglucone 6-acetate provide the first examples of successful applications. Promising entries to coriolin, 6a-carbaprostacyclin and homologs, and 6,11-dehydroestrone complement this review of the potential of tricyclooctanone as a versatile building block for the enantiospecific total synthesis of polycyclopentanoids and related compounds.
TL;DR: The present review surveys the development of the chemistry of the acyl cyanides during the last 25 years, and the syntheses and most important reaction possibilities of RCOCN are outlined in four large schemes.
Abstract: Through improved and novel syntheses, acyl cyanides—especially aliphatic acyl cyanides—have become so readily accessible that numerous synthetic applications present themselves. The present review surveys the development of the chemistry of the acyl cyanides during the last 25 years. The syntheses and most important reaction possibilities of RCOCN are outlined in four large schemes: reactions of the CO- and CN-groups—in each case with preservation of the carbon skeletal framework—as well as acylations with cleavage of cyanide ions.
TL;DR: In particular, it was learned that asymmetric membrane structures comprise a very thin consolidated barrier layer (5000 A or less for membranes with economically practical filtration rates) supported by an integral but less dense substrate which does not participate in the transport process.
Abstract: After a long period of dormancy, membrane separation processes have begun to emerge as technically significant and commercially relevant unit operations. Prior to the mid-sixties, synthetic membranes were employed for those few specialized laboratory applications which could tolerate low permeability and poor selectivity or in electrochemical applications excluding, e. g., batteries, fuel cells, chloride-alkali electrolysis, where marginal chemical stability remained a severe limitation. Within the framework of a broad R & D program started in the US in the mid-fifties and devoted to the production of fresh water from brackish and seawater, developments of more suitable membranes arose out of the application of the principles of physical chemistry, modern polymer chemistry (especially surface or interfacial polymerization and polycondensation technology), and electron microscopy. In particular, it was learned that asymmetric membrane structures comprise a very thin consolidated barrier layer (5000 A or less for membranes with economically practical filtration rates) supported by an integral but less dense substrate which does not participate in the transport process. Later and after much effort, composite membranes were developed in which the salt-rejecting skin (still only 5000 A thick) was placed atop a supporting matrix formed from a more chemically and mechanically stable polymer.—The main desalination research effort led to several spin-off developments in related membrane fields, e.g. the successful preparation and commercialization of ultrafiltration technology in the automobile, food, and chemical industries. Also, ion-exchange membranes prepared from perfluorinated polymers offered the electrochemical industry much better chemical stability than the earlier phenolic-resin-based ion-exchange membranes.—Current efforts are aimed at the improved selectivity and stability required for very specific separation processes (e.g. separation of heavy metal salts from waste water or selective enrichment of gases). In the future, the mechanisms of biological processes will have to be exploited for successful development of synthetic membranes suitable for more sophisticated separations.
TL;DR: Amino acids have been used more and more frequently as auxiliary agents or educts in asymmetric syntheses as discussed by the authors, but only about 20% have so far been used in sterically pure form.
Abstract: Since the stereoisomers of molecules with one or more asymmetric centers often exhibit different biological activities (e.g. thalidomide, pheromones), stereoselective synthesis as a method of preparative chemistry is rapidly attaining importance. Of the numerous drugs prepared by total synthesis that contain at least one asymmetric center, only about 20% have so far been used in sterically pure form. Amino acids constitute the greatest “chiral pool” of compounds whose enantiomers can be obtained commercially in large amounts; they are gradually being used more and more frequently as auxiliary agents or educts in asymmetric syntheses.
TL;DR: The complete manuscript of this communication appears in: Angew. Chem. Suppl. 1982, 1739 as discussed by the authors, and the full manuscript of the complete manuscript is available in: http://www.anie.org.
Abstract: The complete manuscript of this communication appears in: Angew. Chem. Suppl. 1982, 1739. DOI:10.1002/anie.198217390
TL;DR: The biologically porphyrins important thus available permit a mechanistic explanation of the light-induced dermatoses in porphyria diseases and suggest promising medical applications in diagnosis and therapy.
Abstract: Uroporphyrinogen III plays a key role in the biosynthesis of heme, the red pigment of blood. In vivo studies with specifically 14C- and 3H-labeled precursors have revealed that the formation uroporphyrinogen III in the organism follows several primary and subsidiary pathways. Model experiments on the pattern of biosynthesis have led to simple and effective methods of synthesizing uroporphyrin analogs and have shown that their production is strongly favored thermodynamically. The biologically porphyrins important thus available permit a mechanistic explanation of the light-induced dermatoses in porphyria diseases and suggest promising medical applications in diagnosis and therapy.
TL;DR: The complete manuscript of as discussed by the authors appears in: Angew. Chem. Suppl. 1982, 1473. DOI:10.1002/anie.198214730].
Abstract: The complete manuscript of this communication appears in: Angew. Chem. Suppl. 1982, 1473. DOI:10.1002/anie.198214730