bluuebunny/crossref_metadata_2025_split huggingface dataset:
A Quarter Century of Thin‐Layer Chromatography—An Interim Report
Authors: Stahl Egon 
Published: December 2003

AbstractThin‐layer chromatography (TLC) has been in general use since 1958. The prerequisites were the use of very small particle size material in the layers and the demonstration that not only lipophilic mixtures but also mixtures of cardiac glycosides, alkaloids, sugars, and amino acids could be separated quickly and simply on silica gel layers. The impressive separations obtained using the method later encouraged attempts to use adsorbants having such, narrow‐range small particles in column chromatography. In order to achieve sufficient flow rates the use of high pressures was necessary. Thus, high pressure or high performance liquid chromatography (HPLC) was born, whose godfather, as far as instrumental development is concerned, was gas chromatography (GC). Today, chromatography is classified according to the nature of the transport phase: liquid, gas, or “electro” chromatography. Sample preparation often requires more time than the actual separation. rapid thermal separation and application techniques (TAS, TFG) have brought advantages to TLC. As before, the art of chromatography consists in choosing the appropriate mobile phase, stationary phase, and separation technique. Gradient TLC offers new possibilities; for example, gradient layers make it possible to obtain selective information on the individual components of a mixture and also allow significant focussing effects to be achieved. Sequential and centrifugal TLC have paved the way for further advances. The questions arise whether the standard conditions, formulated 25 years ago, are still applicable and how the variations which have been introduced should be judged. Group‐ and substance specific methods of detection are discussed in addition to universal methods. Quantitation is critically discussed and it is concluded that HPLC has the better chance in the furture. Combination and coupling techniques ease the positive identification of individual substances. Finally, it is discussed why TLC is the simplest, most versatile, most cost effective, and most widely used chromatographic method and reasons are proposed as to why it will remain so for the foreseeable future.
2‐Phosphaallyl Cations by Formal Insertion of P⊕ into the CC Double Bond
Authors: Schmidpeter Alfred , Lochschmidt Siegfried , Willhalm Angela 
Published: December 2003

AbstractThe complete manuscript of this communication appears in: Angew. Chem. Suppl. 1983, 710. DOI:10.1002/anie.198307100
Synthesis of Acylpolyamines: Acetylspermidines and C12:0‐Acarnidine
Authors: Boukouvalas John , Golding Bernard T. , McCabe Richard W. , Slaich Pritpal K. 
Published: December 2003

AbstractThe complete manuscript of this communication appears in: Angew. Chem. Suppl. 1983, 860. DOI:10.1002/anie.198308600
η6‐Bonded Alkylbenzenes as Novel Bridging Ligands in Dinuclear Complexes of Iron and Cobalt
Authors: Jonas Klaus , Koepe Gerd , Schieferstein Ludwig , Mynott Richard , Krüger Carl , Tsay Yi‐Hung 
Published: December 2003

AbstractThe complete manuscript of this communication appears in: Angew. Chem. Suppl. 1983, 920. DOI:10.1002/anie.198309200
Stereoselective Synthesis of Enantiomerically Pure Natural Products—Estrone as Example
Authors: Quinkert Gerhard , Stark Herbert 
Published: December 2003

AbstractNatural products have been synthesized for billions of years in animals, plants, and microorganisms. As a rule they occur enantiomerically pure. Their chiral character corroborates their use in metabolism or as biologically active agents. Natural products may be insufficient in quality or quantity. They have recently begun to become accessible, either unchanged or modified, by biological synthesis; here, too, they are obtained enantiomerically pure. In the last twenty years chemical synthesis has become a major concern of organic chemists. Their target compounds are primarily enantiomerically pure natural products or biologically active variants thereof.
Synthesis of the Chiral Synthetic Building Block (1R, 5S)‐3‐Phenyl‐4‐oxa‐2,6‐diazabicyclo[3.2.0]hept‐2‐en‐7‐one from 6‐Aminopenicillanic Acid
Authors: Häbich Dieter 
Published: December 2003

AbstractThe complete manuscript of this communication appears in: Angew. Chem. Suppl. 1983, 1035. DOI:10.1002/anie.198310350
The Photooxygenation of Estrogens: a New Synthesis of 19‐Norsteroids
Authors: Lupón Pilar , Gómez José , Bonet Juan‐Julio 
Published: December 2003

AbstractThe complete manuscript of this communication appears in: Angew. Chem. Suppl. 1983, 1025. DOI:10.1002/anie.198310250
Photochemistry of Previtamin D3 at 92 K; Formation of an Unstable Tachysterol3 Rotamer
Authors: Maessen Paul A. , Jacobs Harry J. C. , Cornelisse Jan , Havinga Egbert 
Published: December 2003

AbstractThe complete manuscript of this communication appears in: Angew. Chem. Suppl. 1983, 994. DOI:10.1002/anie.198309940
Synthesis and Structure of a Linearly anti‐Annelated Tetracycloundecane. A Potential Precussor for a Coriolin Synthesis
Authors: Demuth Martin , Cánovas Antonio , Weigt Eva , Krüger Carl , Tsay Yi‐Hung 
Published: December 2003

AbstractThe complete manuscript of this communication appears in: Angew. Chem. Suppl. 1983, 1053. DOI:10.1002/anie.198310530
Reversal of the Refioselectivity in the Zirconocene‐induced Coupling of a ketone with Isoprene
Authors: Erker Gerhard , Dorf Ulrich 
Published: December 2003

AbstractThe complete manuscript of this communication appears in: Angew. Chem. Suppl. 1983, 1120. DOI:10.1002/anie.198311200
Diels‐Alder Reactions of Dienes with Alkynes Catalyzed by Diazadieneiron
Authors: tom Dieck Heindirk , Diercks Reiner 
Published: December 2003

AbstractThe complete manuscript of this communication appears in: Angew. Chem. Suppl. 1983, 1138. DOI:10.1002/anie.198311380
Thermolysis and Reactions of 4,5‐Benzotricyclo[4.1.0.01,3]hept‐4‐ene and o‐(Propadienyl)styrene
Authors: Brinker Udo H. , Wilk Gabriele , Gomann Klaus 
Published: December 2003

AbstractThe complete manuscript of this communication appears in: Angew. Chem. Suppl. 1983, 1228. DOI:10.1002/anie.198312280
SbF2+ as a Lewis Acid in Na(SbF)PO4·nH2O (n = 2–4) and NH4(SbF)PO4·H2O
Authors: Mattes Rainer , Holz Klaus 
Published: December 2003

AbstractThe complete manuscript of this communication appears in: Angew. Chem. Suppl. 1983, 1315. DOI:10.1002/anie.198313150
First Proof of di‐, tri‐, and all‐cis‐Conformations in Sterically Hindered Trimethinecyanines (Carbocyanines) of the Indoline and Benzothiazole Series
Authors: Allmann Rudolf , Anis Hans‐Joachim , Benn Reinhard , Grahn Walter , Olejnek Stanislaw , Waśkowska Alicja 
Published: December 2003

AbstractThe complete manuscript of this communication appears in:Angew. Chem. Suppl. 1983, 1147. DOI:10.1002/anie.198311470
Synthesis and Structure of a Macrocycle with a Skeletal Framework of Arsenic, Carbon, Oxygen, and Nitrogen Atoms
Authors: Roesky Herbert W. , Djarrah Habibollah , Lucas Joachim , Noltemeyer Mathias , Sheldrick George M. 
Published: December 2003

AbstractThe complete manuscript of this communication appears in: Angew. Chem. Suppl. 1983, 1424. DOI:10.1002/anie.198314240
Biochemical Aspects of Cholinergic Excitation
Authors: Maelicke Alfred 
Published: December 2003

AbstractA prerequisite for every biological system to develop and to continue to function (“to live”) is an effective communication between its components, i.e. its cells. This intercellular communication is essentially of a chemical nature: It employs neurotransmitters and hormones as messengers, and receptors as the receivers of transmitted signals. As is typical for all communication systems, biological signal processes usually also utilize only relatively small amounts of material. This general rule, however, does not apply to some synaptic communication systems. One typical exception, for instance, is the nerve‐muscle synapse and, in particular, its special form, the nerve‐electroplaque synapse of electric fish. These systems, therefore, lend themselves to biochemical studies permitting investigation of the molecular basis of biological communication processes. Thus, the acetylcholine receptor of the plasma membrane of the postsynaptic cell was established as a structurally and functionally rather complicated “transducer system” responsible for both the reception of the chemical message and its conversion into an electrical activity of the receiving cell.
Analysis of Enzyme Structure and Activity by Protein Engineering
Authors: Fersht Alan R. , Shi Jian‐Ping , Wilkinson Anthony J. , Blow David M. , Carter Paul , Waye Mary M. Y. , Winter Greg P. 
Published: December 2003

AbstractStructure‐activity relationships of enzymes can now be analyzed for the first time by the systematic alteration of protein structure. Recent developments in the chemical synthesis of DNA fragments and recombinant DNA technology enable the facile modification of proteins by highly specific mutagenesis of their genes. Kinetic analysis of the mutant enzymes combined with high‐resolution structural data from protein X‐ray crystallography allow direct measurements on the relationships between structure and function. In particular, the strength and nature of enzyme‐substrate interactions and their detailed roles in catalysis and specificity can now be studied. We have developed such analysis of enzyme structure‐function by site‐directed mutagenesis of the tyrosyl‐tRNA synthetase from Bacillus stearothermophilus, concentrating so far on the subtle role of hydrogen bonding in both substrate specificity and catalysis. We find that the energetics of tyrosine and ATP binding must be analyzed in terms of an exchange reaction with solvent water. Based on this idea and structural data, we have engineered an enzyme of improved enzyme‐substrate affinity, and there thus appear to be real prospects of engineering proteins of new specificities, activities, and structural properties. We are also using protein engineering to gather direct information on the nature of enzyme catalysis. For example, we find the catalysis of formation of Tyr‐AMP from Tyr and ATP is due largely to electrostatic and hydrogen bonding interactions that are stronger in the transition state than in the ground state—a “strain” mechanism rather than acid‐base or covalent catalysis.
Organolanthanoid Compounds
Authors: Schumann Herbert 
Published: December 2003

AbstractUp to little more than a decade ago organolanthanoid compounds were still a curiosity. Apart from the description of an isolated number of cyclopentadienyl and indenyl derivatives, very few significant contributions had been made to this interesting sector of organometallic chemistry. However, subsequent systematic studies using modern preparative and analytical techniques, together with X‐ray single crystal structure determinations, enabled the isolation and characterization of a large number of very interesting homoleptic and heteroleptic compounds in which the lanthanoid is bound to hydrogen, to substituted or unsubstituted cyclopentadienyl groups, to allyl or alkynyl groups, or even to phosphorus ylides, trimethylsilyl, and carbonylmetal groups. These compounds, which are all extremely sensitive to oxygen and water, open up new possibilities in the field of catalysis and have great potential in organic synthesis—as recent studies with pentamethylcyclopentadienyl derivatives, organolanthanoid(II) compounds, and hexamethyllanthanoid complexes have already shown.
Mercury(II)‐ and Palladium(II)‐Catalyzed [3,3]‐Sigmatropic Rearrangements [New Synthetic Methods (46)]
Authors: Overman Larry E. 
Published: December 2003

AbstractMercury(II) and palladium(II) salts have found broad applications as catalysts for [3,3]‐sigmatropic rearrangements leading to formation of CO, CN, CS, and CC σ bonds. Increases in reaction rate are often very large (1010 – 1014 at 1 M catalyst concentration) and allow many previously difficult transformations to be conducted at or near room temperature, often with attendant increases in stereoselectivity and decreases in by‐product formation. The mechanism of these catalyzed transformations is briefly discussed, and evidence is summarized to suggest that many follow a cyclization‐induced rearrangement mechanism.
Gas Chromatographic Separation of Enantiomers on Optically Active Metal‐Complex‐Free Stationary Phases. New Analytical Methods (24)
Authors: Schurig Volker 
Published: December 2003

AbstractIf a stationary phase A employed in gas chromatography possesses a chemical affinity for substance B, which is to be separated, then the retention behavior is not only determined by the normal physical equilibrium between the gas and liquid phases but also by the chemical equilibrium A + B ⇋ AB. If A and B are chiral and A is present in optically active form while B is a racemic mixture, then it is possible to achieve a gas chromatographic enantiomer resolution without the isolation of diastereomers: the energetically different diastereomeric associates AR BR and AR BS are formed rapidly and reversibly. This enantiospecific resolution principle was first demonstrated in 1966 by the quantitative resolution of racemic amino acid derivatives on optically active peptide phases in analogy to the well‐known stereospecificity of enzymes. The anchoring of the chiral resolving agent to thermally stable polysiloxanes together with the employment of high resolution capillary columns and the use of appropriate derivatization strategies has led to the development of enantiomer resolution into a routine modern method for many classes of substances. The demonstration of enantiospecificity in the gas chromatographic separation process is of fundamental interest, and its systematic study can result in a significant contribution to the understanding of the molecular mechanism of “chiral recognition”. The gas chromatographic separation of enantiomers has also proven to be an accurate and sensitive method for the determination of the enantiomeric composition of natural products and products of enantioselective transformations (asymmetric syntheses, “chiral pool” transformations, kinetic resolutions, biomimetic reactions) and for the quantification of racemization, e.g. in the synthesis and hydrolysis of peptides. In any research program devoted to the phenomenon of chirality, the gas chromatographic separation of the enantiomers of volatile compounds constitutes an indispensable modern instrumental technique.
The Homoaldol Reaction, or How to Overcome Problems of Regio‐ and Stereo‐selectivity
Authors: Hoppe Dieter 
Published: December 2003

AbstractEven today, there are conspicuous gaps in the register of organic synthetic methods. Thus, numerous types of homoenolate reagents are listed; yet no generally applicable method for the homologue of the aldol addition, viz. the homoaldol addition, is to be found. The methods documented up to 1980 offer only singular solutions, and, moreover, are not stereoselective. The present article gives an overview of the existing possibilities and then describes how the gap can be closed by using a new class of homoenolate reagents: 2‐alkenyl esters ofN,N‐diisopropylcarbamic acid are lithiated and the resulting allyllithium compounds tailored to high regio‐ and diastereoselectivity in addition reactions to the carbonyl group via metal exchange. Altogether, the homoaldol reactions now offer a general and reliable, highly stereoselective entry to γ‐hydroxycarbonyl compounds, whereby the C(β)C(γ) bond is formed. Furthermore, it is demonstrated how problems associated with the application of allyl anions as synthetic reagents can be solved today.
Double Asymmetric Synthesis and a New Strategy for Stereochemical Control in Organic Synthesis
Authors: Masamune Satoru , Choy William , Petersen John S. , Sita Lawrence R. 
Published: December 2003

AbstractThis account examines double asymmetric induction from theoretical and practical viewpoints. In the context of four major organic reactions‐the aldol, Diels‐Alder, catalytic hydrogenation, and epoxidation‐it is shown that a double asymmetric induction can be analyzed in terms of the single asymmetric reactions of each of the two chiral reactants. A rule which qualitatively relates the results of these single asymmetric reactions with the outcome of the double asymmetric reaction is proposed. A powerful new strategy based on this rule for the predictable creation of new chiral centers is discussed and the use of this strategy for the synthesis of sugars and macrolides is presented.
Oxidation of Organometallic Compounds
Authors: Halpern Jack 
Published: December 2003

AbstractStable organometallic compounds, notably of the later transition metals (groups VI–VIII), usually are characterized by closed shell electron configurations (typically 18‐electron valence shells) which are destabilized by electron addition or removal. One‐electron oxidation of such compounds results in the formation of unstable radical ions, whose characteristic reactivity patterns include susceptibility to nucleophilic attack, disproportionation, and metal‐carbon bond dissociation. Two‐electron oxidation may result in dissociation or oxidation of the organic ligand. In this review studies on the chemical and electrochemical oxidations of metal carbonyls, metal‐olefin complexes, and alkyl transition‐metal compounds are described. The studies encompass the following themes: (1) The kinetics and thermodynamics of the initial redox steps; (2) The characterization and reactivity patterns of the resulting oxidation products; (3) The synthetic and catalytic applications of organometallic redox processes.
The 3′‐End of tRNA and Its Role in Protein Biosynthesis
Authors: Chládek Stanislav , Sprinzl Mathias 
Published: December 2003

AbstractIn the course of protein biosynthesis, the 3′‐ends of aminoacyl‐tRNA (aa‐tRNA) and peptidyl‐tRNA specifically interact with macromolecules of the protein biosynthesis machinery. The 3′‐end of tRNA consists of an invariant C‐C‐A single strand. Interaction of the aminoacyl‐tRNA 3′‐end with elongation factor Tu (EF‐Tu) containing bound GTP is necessary for the formation of the aa‐tRNA·EF‐Tu·GTP complex and, after the complex binds to the ribosome, for the GTP hydrolysis. This process is followed by the specific binding of the aminoacyl‐tRNA 3′‐end to the aminoacyl (A) site of the ribosome. In this review, a model is proposed that involves Watson‐Crick base pairing of the CC sequence of the aminoacyl‐tRNA 3′‐end with a specific GG sequence of the ribosomal 23S RNA. Similarly, peptidyl‐tRNA binds with its 3′‐end to the peptidyl (P) site of the ribosome. This binding may also involve Watson‐Crick base pairing of the C‐C‐A sequence with a complementary sequence of 23S RNA. It is proposed that peptide bond formation is catalyzed by a functional site of the 23S RNA located near the 3′‐ends of aminoacyl‐tRNA and peptidyl‐tRNA. A model is suggested in which two loops of the 23S RNA, brought into close proximity via folding, are involved both in binding the 3′‐ends of the tRNAs and in catalyzing peptide bond formation. This model presumes a dynamic structure for ribosomal RNA, which is modulated by interaction with elongation factors and ribosomal proteins.
Combination of Chromatographic Separation Methods with Mass Spectrometry—a Modern Technique for Studying Metabolism
Authors: Spiteller Gerhard 
Published: December 2003

AbstractPowerful separation methods developed during the past few decades, in particular glass capillary gas chromatography (GC) and high performance liquid chromatography (HPLC), can be combined with sensitive methods of detection such as mass spectrometry (MS) to provide new insights into metabolic processes. Research in a field that was long thought to be exhausted has thus been revitalized. New perspective have been opened up for studying the metabolism of foods, spices and flavorings, drugs, and compounds produced in the body as well as for establishing the way in which the organism rids itself of harmful substances or how and when these compounds can interfere with the metabolic processes occurring in the body.
Host/Guest Chemistry of Organic Onium Compounds—Clathrates, Crystalline Complexes, and Molecular Inclusion Compounds in Aqueous Solution
Authors: Vögtle Fritz , Löhr Hans‐Gerd , Franke Joachim , Worsch Detlev 
Published: December 2003

AbstractThe importance of organic onium compounds for host/guest interactions in biological processes has been long recognized. In contrast, the versatility of synthetic onium compounds as host molecules in abiotic chemistry has only been investigated in detail for the last few years. Due to their inclusion ability and selectivities, organic ammonium, pyridinium, and phosphonium salts and other organic salts with specific structural features are interesting as clathrate‐forming molecules. The crystal structures of the clathrates afford insight into the conformational adaptability of onium clathrands with limited flexibility to the geometric requirements of the guest molecules. Optically active onium hosts can be used for enantiomeric enrichment. In crystalline tetracyanoquinodimethane(TCNQ) complexes and radical anion salts, the structure of the organic heterocation has a marked effect on the electrical conductivity. Macrocyclic onium compounds of the phane type with “exohydrophilic” and “endolipophilic” cavities are suitable as models of enzymes and biological receptors. They can also be used to transfer aromatic and aliphatic guest molecules selectively into aqueous solution. Catalytic activity (including H/D exchange) has already been obtained with synthetic compounds having large cavities. The further development of such host/guest systems may contribute to the understanding of biological systems and the improvement of technical processes (catalysis in molecular cavities).
Dinuclear Complexes with Predictable Magnetic Properties
Authors: Kahn Olivier 
Published: December 2003

AbstractWhen two paramagnetic transition metal ions are present in the same molecular entity, the magnetic properties can be totally different from the sum of the magnetic properties of each ion surrounded by its nearest neighbors. These new properties depend on the nature and the magnitude of the interaction between the metal ions through the bridging ligands. If both ions have an unpaired electron (e.g. Cu2+ ions), then the molecular state of lowest energy is either a spin singlet or a spin triplet. In the former case, the interaction is said to be antiferromagnetic, in the latter case ferromagnetic. The nature and the order of magnitude of the interaction can be engineered by judiciously choosing the interacting metal ions and the bridging and terminal ligands, and, thus, by the symmetry and the delocalization of the orbitals centered on the metal ions and occupied by the unpaired electrons (magnetic orbitals). The first success in this “molecular engineering” of bimetallic compounds was in the synthesis of a Cu2+VO2+ heterobimetallic complex in which the interaction is purely ferro‐magnetic. The same strategy could be utilized for designing molecular ferromagnets, one of the major challenges in the area of molecular materials. Another striking result is the possibility of tuning the magnitude of the interaction through a given bridging network by modifying the nature of the terminal ligands, which, in some way, play the role of “adjusting screws”. By careful selection of the bridging and terminal ligands, a very large antiferro‐magnetic interaction can be achieved, even if the metal ions are far away from each other. Some sulfur‐containing bridges are especially suitable in this respect.