bluuebunny/crossref_metadata_2025_split huggingface dataset:
Preparation and Electrophilic Substitution of 1‐Trimethylsilylpentadienyllithium
Authors: Oppolzer Wolfgang , Burford Sidney C. , Marazza Fabrizio 
Published: December 2004

AbstractPentadienyllithium (16) was regioselectively and efficiently transformed to 1‐trimethylsilyl‐2,4‐pentadiene (17) by reaction with chlorotrimethylsilane (Scheme 5). Deprotonation of 17 and subsequent electrophilic attack furnished the regioisomeric products 12 and/or 13 in good yields (Schemes 5 and 6). The utility of the reaction 18→12 for the convergent assembly of the 1‐silyl‐1,3‐butadiene unit with an olefinic dienophile is further illustrated by the smooth intramolecular Diels‐Alder reaction 19→20.
Nachweis von Fragment‐Genesen im Massenspektrometer. 5. Mitteilung Aminosäure‐Sequenzanalyse: Vergleich der DADI/MIKE‐, ‘linked scan’‐ und Kollisionsaktivierungs‐Spektrometrie an aliphatischen Aminosäuren
Authors: Steinauer René , Walther Hansjörg , Schlunegger Urs Peter 
Published: December 2004

Detection of Fragment Genesis in the Mass Spectrometer. V. Amino Acid Sequence Analysis by DADI/MIKE, Linked Scan and Collisional Activation Mass Spectrometry. Aliphatic Amino AcidsThe amino acid sequence in tripeptides and tetrapeptides has been analyzed by DADI/MIKE and B/E‐‘linked scan’ spectrometry, as well as by collisional activation in the first and second field free region of a mass spectrometer embodying the inverse Nier‐Johnson geometry. Collisional activation enhances the intensity of fragment ions.
Substituierte Alkylsulfonate durch Addition von Natriumhydrogensulfit an Chalkone
Authors: Pfoertner Karl‐Heinz 
Published: December 2004

Substituted Alkyl Sulfonates by Addition of Sodium Hydrogen Sulfite to ChalconesSodium hydrogen sulfite adds to the C, C double bond of chalcones to give substituted alkyl sulfonates 1. Reaction of methyl iodide with the pyridyl substituted alkyl sulfonates 1b–1e yields the inner salts 2–4.
Etude de la fragmentation par impact électronique de dérivés du benzothiazole
Authors: Claude Saturnin , Tabacchi Raffaele , Due Laurent , Fuchs Rudolf , Boosen Karl‐Josef 
Published: December 2004

Study of Electron‐Impact Fragmentation of Benzothiazole DerivativesThe mass spectra of eighteen substituted benzothiazoles are reported and discussed. All these compounds are thermodynamically stable and give an intense molecular ion, which undergoes different types of fragmentation depending on the nature of the substituent which is rarely eliminated directly. β‐Cleavage with respect to the heterocyclic double bond is often observed. Specific 2H‐, 13C‐, 15N‐ and 34S‐labelling have been used in order to confirm the fragmentation patterns.
Synthesis, Properties, and Crystal Structure of Silyl Nitronates (Silyl Esters of aci‐Nitroalkanes): Towards the SN2 Reaction Path with Retention of Configuration at Silicon
Authors: Colvin Ernest W. , Beck Albert K. , Bastani Bahram , Seebach Dieter , Kai Yasushi , Dunitz Jack D. 
Published: December 2004

AbstractAn efficient and flexible method for the preparation of silyl nitronates is described (see 1–10). NMR. spectral investigations indicate a rapid 1,3‐silyl migration process, with an activation energy of about 10 kcal mol−1. X‐ray crystallographic studies on the silyl nitronates 3 and 8 show structures that lean towards an SN2 retention pathway at silicon.
Natural Occurrence of Enantiomeric and meso‐Astaxanthin 1. Ex Lobster Eggs (Homarus gammarus)
Authors: Rønneberg Harald , Renstrøm Britta , Aareskjold Kåre , Liaaen‐Jensen Synnøve , Vecchi Max , Leuenberger Franz J. , Müller Robert K. , Mayer Hans 
Published: December 2004

AbstractAstaxanthin (1; 3,3′‐dihydroxy‐β,β‐carotene‐4,4′‐dione) isolated from lobster eggs (Homarus gammarus) was unexpectedly found to be a mixture of all three optical isomers as determined by HPLC. analysis of the corresponding diesters of (–)‐camphanic acid. This is the first finding of meso‐astaxanthin and a meso‐carotenoid in general in nature.
Intramolekularer Carbonylgruppenaustausch in Tetracarbonyleisen‐Komplexen von Äthylen und einfachen (E)‐konfigurierten Olefinen
Authors: Von Büren Martin , Cosandey Marcel , Hansen Hans‐Jürgen 
Published: December 2004

Intramolecular Exchange of Carbonyl Group Sites in Tetracarbonyliron Complexes of Ethylene and Simple (E)‐Configurated OlefinsThe activation parameters of the intramolecular exchange of carbonyl group sites in the tetracarbonyliron complexes 2–5 of ethylene, (E)‐2‐butene, (E)‐4‐octene and (E)‐cyclooctene, respectively have been determined between 180 and 115 K by a complete line shape analysis of the 13C‐NMR, signals of the CO‐groups. The temperature dependence of the rates (k (2), k (3), k (4) and k (5), respectively) of this intramolecular exchange process can be described by the following equations (Ea in J/mol):

Further data are given in Tables 3 and 4.Furthermore, a good linear correlation between the energies of activation of the carbonyl exchange process in 1,2‐disubstituted olefins and the corresponding averaged wavenumbers ( (CO)) of the carbonyl stretching vibrations or of the corresponding chemical 13C‐shifts (δ (CO)) of the carbonyl group at fast exchange in the complexes seems to exist.
Biosynthesis of the antibiotic verrucarin E use of [1‐13C]‐, [2‐13C]‐, [1,2‐13C]‐ and [2‐13C, 2‐2H3]‐acetates. Verrucarins and roridins, 37th communication [1]
Authors: Chexal Kuldip K. , Snipes Carl , Tamm Christoph 
Published: December 2004

AbstractThe origin of the carbon skeleton of verrucarin E (1) from acetate as precursor is confirmed. Incorporation studies with [1,2‐13C]‐acetate have demonstrated that two acetoacetate units couple together as shown in pattern A (Scheme 2) and not as in B. Analysis of the deuterium distribution in both verrucarin E (1) isolated after the incorporation of [2‐13C,2‐2H3]‐acetate and in sodium acetate obtained after Kuhn‐Roth oxidation of the metabolite demonstrated that C(7) is derived from the starter unit of one of the acetoacetate moieties. The deuterium exchange in verrucarin E (1) occurring during fermentation was investigated.
On the Basicity of Triarylmethylamines in Solution
Authors: Dahn Hans , Farine Jean‐Claude , Nguyên Thi Thanh Tâm 
Published: December 2004

AbstractThe pK of 11 triarylmethylamines (tritylamines), eight of which are new, and some related bases were measured in methylcellosolve/water 80:20 (MCS); some were also measured in dioxane/water 60:40 (Dx), acetonitrile (An), nitrobenzene (Nb) and acetic acid (Ac). (1) The influence of the aryl groups on the basicity is essentially additive; (2) In different solvents, similar linear free energy relationships were found, with differences in p* characteristic of solvation; (3) The influence on basicity of substituents of the aryl groups follows Hammett's relationship. These results indicate a preponderance of inductive effects. N, N‐Dimethyltritylamine (3b) (pK=3.40) shows a marked crowding effect, absent in the isomeric tertiary amines 13 and 14. Tri‐p‐nitrotritylamine (10a) (pK=3.10), N, N‐dimethyl‐tri‐p‐nitrotritylamine (10b) (pK=0.50), and 3b are particularly weak bases; the base‐weakening effect of the trinitrotrityl group is similar to that of the cyanomethyl and trifluoroethyl groups.
The Synthesis of 3,5‐Diamino‐1,2,4‐oxadiazoles. 2nd Communication
Authors: Tilley Jefferson W. , Ramuz Henri , Levitan Paul , Blount John F. 
Published: December 2004

AbstractThe 5‐amino‐3‐arylamino‐1,2,4‐oxadiazoles 2 are conveniently prepared by oxidative cyclization of the arylamidinoureas 10. The process is also capable of producing a variety of the 5‐substituted‐amino analogs 32 when the appropriately substituted guanidine 31 is employed as the substrate. Two different types of rearrangement leading to triazol‐3‐ones accompany cyclization depending on the choice of starting material. The structures of the rearranged products were established by X‐ray crystallographic analysis and the reaction mechanisms leading to these unexpected products are discussed.
The Photoelectron Spectra of Ni, Pd, Pt‐Diallyl
Authors: Böhm Michael C. , Gleiter Rolf , Batich Christopher D. 
Published: December 2004

AbstractThe He (I) photoelectron (PE.) spectra of bis (π‐allyl)palladium (2a), bis (π‐allyl)platinum (3a), bis (π‐methallyl)palladium (2b) and bis (π‐methallyl)‐platinum (3b) have been recorded and compared with the PE. spectra of bis (π‐allyl)nickel (1a) and bis (π‐methallyl)nickel (1b). By use of the He (II) PE. spectra 2a, 2b and 3b and correlations between the PE. spectra of 1–3 it is possible to assign the first seven to eight transitions in the PE. spectra of 1–3. In contrast to previous assignments it is shown that the first band in the PE. spectrum of 1a corresponds to the ejection of an electron from 7au, a pure ligand orbital. The assignment proposed is supported by semiempirical calculations of the INDO‐type by considering the relaxation effects explicitly using the ΔSCF and transition operator method.
Der Einbau von Magnesium in Liganden der Chlorophyll‐Reihe mit (2,6‐Di‐t‐butyl‐4‐methylphenoxy)magnesiumjodid
Authors: Zass Engelbert , Isenring Hans Peter , Etter Rolf , Eschenmoser Albert 
Published: December 2004

Introduction of Magnesium into Ligands of the Chlorophyll Series by (2,6‐Di‐t‐butyl‐4‐methylphenoxy)magnesium IodideExperimental details are given for the new method of introducing magnesium into porphinoid ligands by (2,6‐di‐t‐butyl‐4‐methylphenoxy)magnesium iodide (1), previously published in preliminary form [1]. Besides magnesium octaethylporphyrinate (14), methyl pyrochlorophyllide a (10), methyl chlorophyllide a (8), and methyl bacteriochlorophyllide a (12), the complexation of pheophytin a (2) to chlorophyll a (3) and of pheophytin b (4) to chlorophyll b (5) are described.
Photochemistry of Biliverdin IX δ as a model for the study of the photoproducts from natural biliverdin IXγ (pterobilin)
Authors: Bois‐Choussy Michèle , Barbier Michel 
Published: December 2004

AbstractThe configurations of biliverdin‐IXγ and ‐IXδ dimethyl esters 1 and 2 in solutions, have been studied using Nuclear‐Overhauser‐Effect (NOE) experiments. Irradiation (500‐700 nm) of biliverdin IXδ gave syn‐Z → anti‐E isomerization of the central methine bridge and in aerated polar solutions, four new cyclized pigments were isolated for which structures 3, 4, 5 and 6 are proposed. Neobiliverdin IXδ 3 is also formed in degassed solution (Φ=4.10−5) but pigments 4, 5 and 6 arise from photo‐oxidation with O2. Biliverdin IXδ appears to be a good model for the study of photo‐reactions occurring on the vinyl groups of the natural biliverdin IXγ (pterobilin).
Synthesen von 3‐Hydroxy‐4‐methyl‐3‐cyclobuten‐1,2‐dion (Methylmoniliformin)
Authors: Bellus Daniel , Martin Pierre , Sauter Hanspeter , Winkler Tammo 
Published: December 2004

Syntheses of 3‐Hydroxy‐4‐methyl‐3‐cyclobutene‐1,2‐dione (Methylmoniliformin)New routes to 3‐hydroxy‐4‐methyl‐3‐cyclobutene‐1,2‐dione(9), the lowest homologue of the mycotoxine moniliformin are described. A common feature of all pathways is the synthesis of methylcyclobutanes having the oxidation level 6. Precursors, which are easily transformed to9by acid catalyzed hydrolysis, include [2+2]‐cycloadducts ofin situgenerated methyl ketene to tetraethoxyethylene and [2+2]‐photocycloadducts of dichlorovinylenecarbonate with 1,1‐dichloro‐1‐propene. The acid hydrolysis of [2+2]‐cycloadducts of chlorotrifluoroethylene toN, N‐diethyl‐1‐propynylamine yields the diethylamide of9 (=22)in 50% overall yield. In addition, a convenient one‐pot‐two‐steps synthesis of a new electronrich ethylene, 1,1,2‐triethoxy‐2‐trimethylsilyloxy‐ethylene(11), is described.
Stoffwechselprodukte von Mikroorganismen 198. Mitteilung Die Eurekansäure, ein Abbauprodukt von Flambamycin und Avilamycin: Konfigurationsbestimmung durch Röntgenstrukturanalyse und Abbau
Authors: Kupfer Ernst , Neupert‐Laves Katarina , Dobler Max , Keller‐Schierlein Walter 
Published: December 2004

Eurekanic Acid, a Degradation Product of Flambamycin and Avilamycin: Determination of the Configuration by X‐Ray Structural Analysis and DegradationThe relative configuration of methyl O‐acetyleurekanate (6), obtained by mild methanolysis of avilamycin A (2) and subsequent acetylation, was determined by an X‐ray structural investigation. Methyl dihydroeurekanate (7), similarly obtained from avilamycin C (3) and from reduced avilamycin A methyl ether, was degraded to L‐threonolactone (14) which allowed to determine the chirality of eurekanic acid (4).
The Photochemistry of 5,6‐Dimethylidene‐2‐norborananone. Synthesis and Diels‐Alder Reactivity of 2,3‐Dimethylidenebicyclo‐[2.1.1]hexane
Authors: Schwager Luis , Vogel Pierre 
Published: December 2004

Abstract2,3‐Dimethylidenebicyclo [2.1.1]hexane (4) was isolated from direct irradiation (253.7 nm) of 5,6‐dimethylidene‐2‐norbornanone (3). Quenching experiments at 253.7 nm, as well as direct and sensitized irradiations at >300 nm suggested that a high vibrationally excited S1‐ or a S2‐state is required for the photodecarbonylation of 3 in contrast with other β, γ‐unsaturated ketones for which α‐cleavage occurs with lower excitation‐energy. The new diene 4 reacted toward tetracyano‐ethylene (k (1mol−1 s−1))=(3.1±0.34) · 10−3) in toluene and (6.2±0.11) · 10−3 in benzene only 60 times more slowly than 2,3‐dimethylidenenorbornane (5) and ca. 850 times as fast as 2,3‐dimethylidene‐syn1,4,5,6‐tetramethylbicyclo[2.1.1]‐hexane (9).
Préparation de dérivés de sucres acétyléniques terminaux et d'acides ynuroniques par réaction de Wittig. Note de laboratoire
Authors: Tronchet Jean M. J. , Bonenfant Alain P. , Perret Françoise , Gonzalez Alberto , Zumwald Jean‐Bernard , Martinez Ernesto M. , Baehler Bruno 
Published: December 2004

Synthesis of Terminal Acetylenic Sugars Derivatives and Ynuronic Acids Derivatives by Use of a Wittig ReactionThe method described for the preparation of terminal acetylenic sugars presents two advantages over earlier procedures: no new asymmetric center is created and the chain can be extended by one or more C‐atoms. The method also allows preparation of ynuronic acids. The aldehydosugars derivatives 1–7 gave in good to excellent yields the corresponding gem‐dibromoenoses 8–14 from which either the terminal acetylenic sugars derivatives 15–21 or the ynuronic acids 22–24 were easily prepared. A few examples of 1,3‐dipolar cycloadditions (leading to 28–30) with these acetylenic sugar derivatives are also described.
On the Use of N, N′‐Dipyrrolylmethane in Heterocyclic Synthesis. Dipyrrolo [1,2c:2′, 1′e]‐2 H‐imidazole and its Aromatic Anion
Authors: Burger Ulrich , Dreier Francine 
Published: December 2004

AbstractThe pyrrolate ion (5) is shown to produce N, N′‐dipyrrolyl methane (4) when reacted with dichloromethane under conditions of weak counter‐ion association. 1‐Azoniafulvene ion (13) is suggested to be the key intermediate in this reaction. N, N′‐Dipyrrolyl methane (4) undergoes intramolecular oxidative coupling when treated with butyllithium and Cu(II)chloride, yielding the novel ring system of dipyrrolo [1,2c: 2′, 1′e]‐2 H‐imidazole (3). The latter upon reaction with methyl lithium forms the Hückel‐aromatic anion 2. Solutions of the lithium salt are stable for several days. Reaction of the dianion of 2,2′‐bipyrrole (11) with dichloro‐methane does not produce 3, but a pyrrolophane‐type dimer (12) having its two anticoplanar N, N′‐bipyrrolediyl subunits arranged in a double layer and joined by a methylene group on each side.
Wasserstoff/Deuterium‐Isotopeneffekte bei Europium‐Verschiebungsexperimenten and Allencarbonsäureestern
Authors: Lang Robert W. , Hansen Hans‐Jürgen 
Published: December 2004

Hydrogen/Deuterium Isotope Effects on Europium Induced Chemical Shifts of Allenic EstersIt is shown by 1H‐NMR. experiments, that in the presence of europium shift reagents, mixtures of allenic esters and their partially deuterated counterparts give rise to α‐, β‐, γ, and δ‐H/D isotope effects (cf. Table and Fig. as well as formula 6 for assignment). The measured effects (0.4 to 2.2%) are mainly attributable to C, H/C, D bond length differences which influence the complexation constant between the shift reagent and the substrate as well as the s‐cis/s‐trans conformational equilibrium of the allenic esters.
Halochrome Molekeln 2. Mitteilung [1] Synthese und acidobasisches Verhalten substituierter 6,6‐Diphenyl‐6H‐chromeno [4,3‐b]indole
Authors: Gunzenhauser Sigmund , Balli Heinz 
Published: December 2004

Halochromic Molecules, Synthesis and Acidobasic Behaviour of Substituted 6,6‐Diphenyl‐6H‐chromeno [4,3‐b]indolesSubstituted 6,6‐diphenyl‐6H‐chromeno [4,3‐b]indoles are forming with acid ring‐opened, intensely coloured diphenyl‐indol‐3‐yl‐carbenium salts. The preparation of these products is described and the UV./VIS.‐spectra are discussed. An investigation of the halochromic properties is made by spectrophotometric determination of ϵpH*‐ and ϵH0*‐curves in buffered methanol/water solutions. pK*‐ and pK*‐values respectively are calculated and the suitability of substituted 6 H‐ chromeno [4, 3‐b]indoles as chromogens in self‐duplicating stationery is discussed.
The Adamantane Rearrangement of 1,2endo‐Trimethylenenorbornane. Preliminary communication
Authors: Klester Alfred Michael , Josef Jäggi Franz , Ganter Camille 
Published: December 2004

Abstract1,2endo‐Trimethylenenorbornane (1) in the presence of aluminium bromide in carbon disulfide at −60° isomerizes at a much higher rate than its 2exo‐isomer 2 to 2endo, 6endo‐trimethylenenorbornane (3) as the sole product. By consequence, the hydrocarbon 2 being the next intermediate in the sequence of the adamantane rearrangement of 1 seems to be very unlikely.
Laser‐Induced Luminescence of Flavinium Salts
Authors: Visser Antonie J. W. G. , Van Hoek Arie , Müller Franz 
Published: December 2004

AbstractFlavinium salts dissolved in an ethanolic glass exhibit blue fluorescence and orange‐red phosphorescence upon excitation with a UV. line from an argon ion laser equipped with UV. optics. This arrangement enables the wavelength distribution and the time‐dependence of the phosphorescence to be measured in a relatively short time. Four cationic flavins were investigated. In spite of the small difference in the chemical structure of the compounds studied, large differences in the spectral shapes and in the ratio of quantum efficiencies of phosphorescence and fluorescence became apparent. The phosphorescence lifetimes were of the same order of magnitude, which indicated a similar rate of depletion of the lowest excited triplet state for all four cations. However, the efficiency of triplet formation (intersystem crossing) is affected by slight structural modifications in the pyrimidine subnucleus of the flavinium salt. The results point to a possible role of vibronic spin‐orbit coupling in the intersystem crossing.
Broad horizons – SETI, SF and education
Authors: Griffiths Martin 
Published: December 2004

Science fiction (SF) is often perceived as a ‘fringe’ form of entertainment that excites the socially challenged. This misperception detracts from the critical, scientific and interpretive nature of the genre which can be directed into science teaching at school and university levels as an innovative way of exploring the cultural background, politics, leitmotif and themes of society, science and their operation. One example is the ‘alien’ theme in SF; it is perceptually one of the driving factors in the search for extraterrestrial intelligence (SETI). Such a topic can become an introduction to current technology, the motives and politics of science and the sociological implications inherent in a confrontation with the ideal of man's uniqueness in the cosmos. When applied to the SETI, SF engenders a constructive convergence in studies such as biological determinism, the evolution of life, communication, interstellar travel and methods of contact, thus enriching the consideration of possible life in the cosmos. Adopting elements of SF in lifelong learning therefore enables informed, imaginative reflection and debate that educates, trains and instructs, broadening the potential of students and their future roles by invoking an analysis of vital public, scientific and humanistic fields.Imagination lies at the basis of all discoveryOctavio Paz
Interactive effects of drought and shade on three arid zone chenopod shrubs with contrasting distributions in relation to tree canopies
Authors: Prider J. N. , Facelli J. M. 
Published: December 2004

Summary

Plants that grow beneath trees in arid systems may frequently experience both water and light limitation, although protection from high radiation loads during drought may compensate for a loss of productivity due to reduced light availability when water is plentiful.
We examined the effects of shading, during an imposed water deficit, on the carbon gain, stomatal conductance (gs) and shoot water potential (Ψs) of seedlings of three shrubs: Atriplex vesicaria (Heward ex Benth.), a C4 species, and Enchylaena tomentosa (R. Br.) and Rhagodia spinescens (R. Br.), which are restricted to shaded sites beneath trees.
Under conditions of limiting water, photosynthetic rates measured at saturating light (Amax) were negative in high‐light grown Enchylaena plants but remained positive in shade‐grown plants. When water was not limiting, Amax was reduced in shade‐grown Atriplex but shade did not affect carbon gain in the other two species.
AtriplexΨs was higher in shaded than in unshaded plants, but in unshaded plants positive carbon gain was maintained at Ψs below −10 MPa. Stomatal conductance and Amax decreased more slowly with increasing water deficit in shaded conditions in all species.
Atriplex was tolerant of a broader range of light and soil moisture conditions than Enchylaena, with Rhagodia intermediate between these two species. The interactive effect between drought and shade and the ecophysiological tolerances of these three species have consequences for their field distributions.
Confidence intervals on catch estimates from a recreational fishing survey: a comparison of bootstrap methods
Authors: Hoyle S. D. , Cameron D. S. 
Published: December 2004

AbstractBootstrap methods are often used for confidence intervals on recreational fish catch estimates, because they are relatively robust and straightforward to implement. Such data are typically highly skewed and zero‐inflated, presenting difficulties for many estimation methods. However, bootstrap performance in many situations is not well understood. Inaccurate confidence intervals can cause management errors, and biased intervals can promote errors in one direction. Although the analyses originate from recreational fisheries data, the conclusions are generally applicable to similarly distributed data from other sources. Using simulation, non‐parametric bootstrap confidence intervals (bootstrap normal, bootstrap percentile, hybrid, bootstrap‐t, BC, and BCa) on catch rate and total catch estimates from a recreational fishing survey were compared. The intervals' coverage (proportion of times the ‘true’ mean fell within the confidence intervals) and relative bias were also compared. The bootstrap‐t, using a resample size of slightly less than n/2, provided confidence intervals with the most correct coverage for both parameters. Intervals were biased, usually substantially, for all other methods, with the commonly used bootstrap percentile among the more biased methods.