bluuebunny/crossref_metadata_2025 huggingface dataset:
Peptide Conformation from Coupling Constants: Scalar Couplings as Restraints in MD Simulations
Authors: Eberstadt Matthias , Mierke Dale F. , Köck Matthias , Kessler Horst
Published: December 2004

The question of how far one can go in the determination of conformation with the sole use of coupling constants as restraints in MD simulations was addressed. Couplings are being used ever more frequently as constraints as measuring heteronuclear long‐range coupling constants becomes easier. For this investigation, cyclosporin A, which has previously been extensively examined with NOE‐restrained simulations, is used as a model system. Many additional one‐and three‐bond coupling constants have been measured. The MD simulations were carried out with the addition of a potential‐energy penalty function based directly on the Karplus curve. It is shown that, for dihedral angles with more than one coupling, the restraints are very efficient, in agreement with the structure observed from NOEs. However, it turned out that the structure of CsA is not adequately described, when only J couplings are used.
Synthesis and reactivity of naphthalene sandwich complexes of molybdenum. X‐ray structure of [Mo(η6‐naphthalene) {P(OMe)3}3] and [Mo(H)(η6‐naphthalene){P(OMe)3}3][BF4]
Authors: Thi Ngoc Phan Do , Spichiger Serge , Paglia Patrick , Bernardinelli Gérald , Kündig E. Peter , Timms Peter L.
Published: December 2004

The sandwich complexes bis(η6‐naphthalene)molybdenum(0) (1), bis(η6‐1‐methylnaphthalene)molybdenum(0) (2), and bis(η6‐1,4‐dimethylnaphthalene)molybdenum(0) (3) are synthesized by cocondensation of Mo‐atoms with the naphthalene ligands. Complexes 1–3 are also obtained by reduction of MoCl5 or MoCl4. 2THF with highly activated Mg in the presence of the naphthalene ligands. Mg was activated by sublimation of the metal in a simple rotating solution reactor. Complex 2 exists as a mixture of regio‐ and stereoisomers. Three regioisomers, 3a–c, are formed in reactions of Mo‐atoms with 1,4‐dimethylnaphthalene, whereas 3a, the isomer with the Mo‐atom coordinated to the unsubstituted rings, is formed selectively via the reductive method. The ligands in 1–3 are highly labile. CO displaces both naphthalene rings in 2 and 3 to give [Mo(CO)6], while PF3, P(OMe)3, and PMe3 displace only one coordinated naphthalene in 1 to yield the [Mo(η6‐naphthalene)L3] complexes 4–6. In toluene, arene exchange is a competitive process in reactions of 1 with PF3. Complexes 5 (L = P(OMe)3) and 6 (L = PMe3) react with HBF4 to give the cationic metal hydride complexes 8 and 9. The X‐ray crystal structures of [Mo(η6‐naphthalene) {P(OMe)3}3] (5) and [Mo(H)(η6‐naphthalene) {P(OMe)3}3][BF4] (8) are reported.
Structural Chemistry of Chiral Complexes. NMR and X‐ray studies on Rh1 compounds of (S)‐1‐[bis(p‐methylphenyl)phosphino]‐2‐[(p‐methoxybenzyl)amino]‐3‐methylbutane
Authors: Berger Heinrich , Nesper Reihnard , Pregosin Paul S. , Rüegger Heinz , Wörle Michael
Published: December 2004

The Rh1(diolefin)complexes [Rh(nbd)(2)][PF6] [Rh(1,5‐cod)(2)][PF6], and [Rh((Z)‐α ‐acetamidocinnamic acid)(2)][PF6] (2 = the chiral P,N‐ligand (S)‐1‐[bis(p‐methylphenyl)phospino]‐2‐[p‐methoxybenzyl)amino]‐3‐methylbutane have been prepared and characterized. These complexes exit as a mixture of isomers arising from different five‐membered‐ring conformations and diastereoisomers due to both the prochiral nitrogen and olefin ligands. The three‐dimensional solutions structures of these complexes have been studied with the specific aim of understanding how the chiral pocket is built. Aspects of the exchange dynamics and their possible relevance to homogeneous hydrogenation are discussed The solid‐state structure for the nbd complex, [Rh(nbd)(2)][PF6], as well as detailed one‐ and two‐dimensional 31P‐, 13C‐, and 1H‐NMR results are presented.
Degradation of Cephaloridine on Alkaline Hydrolysis
Authors: Vilanova Bartolomé , Muñoz Francisco , Donoso Josefa , Garcia‐Blanco Francisco
Published: December 2004

A kinetic study on the alkaline hydrolysis of cephaloridine (1) at pH 10.5 and 37° was carried out using ion‐pair reversed‐phase HPLC. The main resulting degradation products, the 7‐epimer 2 of 1, the Δ2‐isomer 3 of 1, and the 3‐methylidene compound 4 were identified. The presence of a pyridinio group at C(31) results in a slightly increased formation constant for the 3‐methylidene compound 4 and the 7‐epimer 2, and introduces a new reaction: the isomerization of the double bond at C(3) in the dihydrothiazine ring to C(2).
Introduction of a New Class of Ligands for the Metal‐Catalyzed Enantioselective Synthesis
Authors: Spescha Maurus
Published: December 2004

Protected thiosugars were prepared as ligands for the metal‐catalyzed enantioselective synthesis. The protecting groups in these ligands were varied to test a proposed new concept for the metal‐catalyzed enantioselective synthesis. This new concept centres on the use of a stair‐like ligand with a large substituent on one side and a small substitutent on the other rather than the commonly employed ligands which have C2 symmetry (see Fig.3). In such a ligand, both substituents should have a major influence on the coordination of a prochiral substrate. To test this proposal, 3‐thio‐α‐D‐glucofuranose derivatives with the following substituents were synthesized: 1,2‐O‐isopropylidene‐5,6‐O‐methylidene (see 24), 1,2:5,6‐di‐O‐isopropylidene (see 2), 5,6‐O‐cyclohexylidene‐1,2‐O‐isopropylidene (see 23), 1,2‐O‐cyclohexylidene‐5,6‐O‐isopropylidene (see 14), 1,2:5,6‐di‐O‐cyclohexylidene (see 13), 5,6‐O‐(adamantan‐2‐ylidene)‐1,2‐O‐isopropylidene (see 21), and 1,2:5,6‐di‐O‐(adamantan‐2‐ylidene) (see 25, Table 2). As a representative of the allofuranoses, 1,2:5,6‐di‐O‐isopropylidene‐3‐thio‐α‐D‐allofuranose (6) was chosen. The following derivatives of 1,2‐O‐isopropylidene‐α‐D‐xylofuranose were also synthesized: 1,2‐O‐isopropylidene‐5‐deoxy‐3‐thio‐α‐D‐xylofuranose (29), 1,2‐O‐isopropylidene‐3‐thio‐α‐D‐xylofuranose (28) and 5‐O‐[(tert‐butyl)‐diphenylsilyl]‐1,2‐O‐isopropylidene‐3‐thio‐α‐D‐xylofuranose (15, see Table 2). The proposed concept was tested using the copper‐catalyzed 1,4‐addition of BuMgCl to cyclohex‐2‐en‐1‐one. The enantioselectivity was very dependent on the ligand used and was up to 58%.
‘One‐Pot’ Synthesis of Raumacline from Ajmaline
Authors: Endress Susanne , Stöckigt Joachim
Published: December 2004

For the alkaloid raumacline (2), which is a biotransformation product of ajmaline (1) in Rauwolfia serpentina cell cultures, an efficient ‘one‐pot’ synthesis was developed using a NaBH4/riboflavin/light‐mediated transformation of 1 into 2 with a total yield of 86%.
Solution Equilibria in Trialkyl‐Phosphite Derivatives of [Ir4(CO)12]. Crystal Structure of [Ir4(CO)11{P(OCH2)3CEt}]
Authors: Besançon Katya , Laurenczy Gábor , Lumini Tito , Roulet Raymond , Gervasio Giuliana
Published: December 2004

The monosubstituted [Ir4(CO)11L] clusters (L = P(OPh)3, 1; L = P(OMe)3, 2; L = P(OCH2)3CEt, 3) were obtained in good yields by the reaction of [Ir4(CO)11I]− with the corresponding phosphite. In the solid state, cluster 3 has a Cs geometry with all terminal ligands as shown by an X‐ray analysis. Three isomers are present in solution: one with terminal ligands (A) and two with three edge‐bridging CO's and with L in axial (B) or radial (C) position (see Scheme). The thermodynamic and kinetic parameters of isomerisations B ⇌ A and A ⇌ C were determined by simulation of the variable‐temperature 31P‐NMR spectra. The three isomers correspond to three minima on the kinetic pathway of CO scrambling, whose relative energies vary independently within a small range (1–9 kJ mol−1 at 298 K). At low temperature, isomer C is always the least stable and is not observed for 1 which bears the most bulky phosphite ligand. The isomerisations are due to two intramolecular merry‐go‐rounds of CO groups about two unequivalent faces of the unbridged species A.
Synthesis and Spectroscopic Characterization of N2‐(N‐Acetyl‐6‐O‐stearoyl‐α‐D‐muramoyl)‐L‐alanyl‐D‐isoglutamine
Authors: Dick Fritz , Jenny Titus A.
Published: December 2004

Title compound 1 was synthesized by a published route which had to be modified (seven steps from readily obtainable starting materials). Characterization of 1 was achieved by spectroscopic means (FAB‐MS, 1H‐NMR, including 2D‐COSY). Furthermore, commercially available reference material purchased for comparison, was unequivocally established to be 10, i.e. incompletely deprotected 1.
Simultaneous Formation of 8H‐Isoquino[2,1‐b][2,7]naphthyridin‐8‐ones and 13H‐Pyrido[4′,3′:3,4]pyrrolo[2,1‐b][3]benzazepin‐13‐ones, a Novel Potential Alkaloidal System
Authors: Nagarajan Kuppuswamy , Rodrigues Patrick J. , Nethaji Munirathinam , Vöhler Markus , von Philipsborn Wolfgang
Published: December 2004

Condensation of 3,4‐dihydro‐6,7‐dimethoxyisoquinoline (4) with 4‐methylnicotinoyl chloride (12) in refluxing pyridine gives 5,6,13,13a‐tetrahydro‐2,3‐dimethoxy‐8H‐isoquino[2,1‐b][2,7] naphthyridin‐8‐one (11), along with some of its 13,13a‐didehydro derivative 7. A similar reaction of 4 with 4‐(chloromethyl)nicotinoyl chloride (14) affords, in addition to 7, the isomeric product 10,11‐dihydro‐7,8‐dimethoxy‐13H‐pyrido[4′,3′:3,4]pyrrolo[2,1‐ b][3]benzazepin‐13‐one (3). Analogous pairs of products are obtained from 3,4‐dihydro‐6,7‐(methylenedioxy)‐ and 3,4‐dihydro‐6,7,8‐trimethoxy‐isoquinolines (15 and 18, resp.). The structure of 3 was established by extensive NMR data and confirmed by single‐crystal X‐ray studies. Structure 7 has the ring system of the Alangium alkaloids like alangimarinc (1), while the isomeric ring system 3 is predicted to be present in nature on biogenetic reasoning.
Azidinium‐Salze. 25. Mitteilung. Symmetrische Azopyridinon‐Farbsalze: Synthese, spektrophotometrische und acidobasische Eigenschaften, Metallkomplex‐Bildung und kinetische Untersuchung der Azo‐Kupplung
Authors: Balli Heinz , Chaquime Lucy , Hellrung Bruno , Misun Marian
Published: December 2004

Symmetrical Azopyridinone Dyes: Synthesis, Spectrophotometrical and Acidobasic Properties, Metal‐Complex Formation and Kinetical Investigation of the Azo‐Dye Formation

The 13 substituted symmetrical azopyridinone dyes 2a–n were synthesized and their VIS spectra measured. The pK*a values of some dyes and of pyridinone coupling components were determined in MeOH/H2O 64:36. The metal‐complex formation of the dyes with Cd2+, Zn2+, Co2+, Ni2+, and Cu2+ was investigated, and complex‐formation constants of the 1:1 complexes were determined in H2O for 2k (Table 2) and in dimethylformamide/H2O 1:1 for some other azopyridinone dyes (Table 3). The mechanism of the azo‐dye formation was investigated and found to be much more complicated than expected. A mechanism of the coupling reaction was developed (see Scheme 4) and fitted by least‐squares calculations.
Nucleosides. Part LV. Efficient synthesis of arabinoguanosine building blocks
Authors: Resmini Matthias , Pfleiderer Wolfgang
Published: December 2004

From guanosine (1) as starting molecule, protected arabinoguanosine derivatives such as phosphoramidite precursors and arabinoguanosine (18) itself were prepared in high yields. Inversion of the configuration at C(2′) was achieved by introduction of the (trifluoromethyl)sulfonyl residue and subsequent displacement by nucleophiles like acetate, bromide, and azide. The guanine moiety was protected at the amide function by the 2‐(4‐nitrophenyl)ethyl (npe) group on O6 and at the NH2 function by the 2‐(4‐nitrophenyl)ethoxycarbonyl (npeoc) group.
Metabolic Products of Microorganisms. Part 269. 5‐Phenylpentadienoic‐acid derivatives from Streptomyces sp.
Authors: Potterat Olivier , Zähner Hans , Metzger Jörg W. , Freund Stefan
Published: December 2004

Two new phenylpentadienamides isolated from the culture filtrate of Streptomyces sp. were assigned the structures 5‐(4‐aminophenyl)penta‐2,4‐dienamide (2) and N2‐[5‐(4‐aminophenyl)penta‐2,4‐dienoyl]‐L‐glutamine (3). In addition, 5‐(4‐aminophenyl)penta‐2,4‐dienoic acid (1) has been isolated, and its spectroscopic characteristics are reported for the first time. Compounds 1–3 exist in both the (2E,4E)‐ and (2E,4Z)‐configurations. Electrospray and tandem MS, and HPLC/MS proved to be particularly suitable for the characterization of these metabolites.
Synthese des Pyrrolizidin‐Alkaloides (±)‐Trachelanthamidin
Authors: Horni Albert , Hubáček Ivana , Hesse Manfred
Published: December 2004

Synthesis of the Pyrrolizidine Alkaloid (±)‐Trachelanthamidine

The important intermediate in the synthesis of the title compound 8 is the diastereoisomer mixture of ethyl 2‐[2‐(1,3 dioxolan‐2‐yl)ethyl]‐5‐oxopyrrolidine‐3‐carboxylate (3a/3b) which was prepared from nitromethane, acrylaldehyde, and diethyl fumarate (Scheme). Its reduction (NaBH4, t‐BuOH, MeOH) gave exclusively the trans‐alcohol 4a, which was converted to the protected pyrrolidine derivative 6. The deprotection and reduction of 6 gave the pyrrolizidine alkaloid 8, characterized as its hydrochloride.
α,α‐Disubstituted Allyl Sulfones: An approach to the synthesis of vinyl‐branched pheromone analogues
Authors: Hoskovec Michal , Koutek Bohumír , Lazar Josef , Streinz Ludvík , Brožová Eva , Kalinová Blanka , Vrkoč Jan
Published: December 2004

The two‐step alkylation of phenyl prop‐2‐enyl sulfone (1) with protected ω‐bromoalkanols and 1‐iodoalkanes (→3; see Scheme 1) followed by a Pd‐catalyzed desulfonylation with LiBH4 affords a 96:4 mixture of vinylbranched, protected alcohols and corresponding ethylidene‐branched isomers (see Scheme 2; 4 and 5, respectively). By utilizing the large difference in reactivity of mono‐ and trisubstituted CC bonds towards singlet oxygen, the ethylidene derivatives are easily removed from the mixture by photo‐oxygenation. The vinyl‐branched compounds are inert to this reaction and can be conveniently isolated in highly pure form (99.5%) and ca. 45% overall yield.
Glycosylidene Carbenes. Part 18. Insertion of glycosylidene carbenes into the SnH bond of tributyl‐ and triphenylstannane: A synthesis of stannoglycosides
Authors: Uhlmann Peter , Nanz Daniel , Bozó Eva , Vasella Andrea
Published: December 2004

Insertion of the glycosylidene carbenes, derived from the gluco‐ and the manno‐diazirines 1 and 2, into the SnH bond of R3SnH (R = Bu or Ph) leads to the fully substituted stannoglycosides 3–8 (53–77%). The 1,2‐cis‐configurated products are formed preferentially (α‐D/β ‐D ranges from 2.5:1 to 5.1:1 with 1, and 1:1.3 to 1:4.2 with 2). Relative to CH2Cl2, THF favors formation of the equatorial Sn‐glycosides. The stannylated (benzyloxy)glucals 9 and 10 were isolated as side products. The reaction of 1 with (Bu3Sn)2 yielded 9 (17% in CH2Cl2; 36% in CCl4) together with the azines 11 and the benzyloxyglucal 12. NMR Data of the Sn‐glycosides 3–8 show evidence for an anomeric effect, 1J(C(1),H) being larger in the axial and 1J(Sn,C(1)) larger in the equatorial anomers.
Crystal Structure and Packing of 1‐(4, 6‐Diamino‐1, 3, 5‐triazin‐2‐yl)guanidinium nitrate
Authors: Venugopalan Paloth , Bürgi Hans Beat
Published: December 2004

The title compound is a condensation product of four guanidinium ions formed during crystallization of guanidinium nitrate. The crystal structure consists of infinite molecular ribbons stacked into layers which are rotated 58.5° with respect to each other and connected by two different networks of hydrogen bonds.
Strukturen und Molekül‐Eigenschaften ladungsgestörter Moleküle. 52. Mitteilung. 2, 3‐Diphenylchinoxalin‐Radikalanionen in Lösung und im Kristall
Authors: Bock Hans , John Andreas , Näther Christian , Ruppert Klaus
Published: December 2004

Structures and Molecular Properties of Charge‐Pertubed Molecules. 2, 3‐Diphenylquinoxaline Radical Anions in Solution and in Crystals

The Na⊕ and K⊕ radical‐ion salts of 2, 3‐diphenylquinoxaline seem to be (according to a structural database search) among the first ones of N‐heterocyclic radical anions in crystals. The one‐electron reduction in aprotic 1, 2‐dimethoxyethan (DME) solution at metal mirrors and the crystallization under Ar have been preceded by cyclovoltammetric (CV) and ESR/ENDOR measurement. The first electron insertion at −1.63 V proves to be reversible, whereas the irreversible second step, which is accompanied by an overcrossing of the CV line, can be rationalized by an ‘ECE‐DISP’ mechanism via a dianion redox disproportionation. The ENDOR spectrum resolves four 1H couplings and allows to simulate the ESR spectrum including the 14N hyperfine splittings. Both dark‐blue single crystals of the radical ion salts \documentclass{article}\pagestyle{empty}\begin{document}$[2,3{\rm - diphenylquinoxaline}^{{.} \ominus} {\rm Met}^ \oplus ({\rm DME})]^.$\end{document} show unexpected similarities for Met⊕ = Na⊕, K⊕ despite the 36‐pm difference in their ionic radii. The largest structural changes inflicted by the one‐electron reduction of the N‐heterocyclic molecule are observed in the vicinity of the N‐centers bearing the highest effective nuclear charge. The DME‐chelated metal cations coordinate at the N electron pairs and form Met⊕(DME)‐bridged polymer chains of the radical anion, which are differently ondulated in the Na⊕ and K⊕ radical anion salts. The take‐home lesson suggests that many more N‐heterocyclic molecules might be analogously reduced under optimized conditions and isolated as single crystals.
Enantioselectivity and cis/trans‐Selectivity in Dirhodium(II)‐Catalyzed addition of diazoacetates to olefins
Authors: Müller Paul , Baud Corine , Ené Doïna , Motallebi Sharokh , Doyle Michael P. , Brandes Bridget D. , Dyatkin Alexey B. , See Marjorie M.
Published: December 2004

The Rh11‐catalyzed carbenoid addition of diazoacetates to olefins was investigated with [Rh2{(4S)‐phox}4] (1;phox = tetrakis[(4S)‐tetrahydro‐4‐phenyloxazol‐2‐one]), [Rh2{(2S)‐mepy}4] (2; mepy = tetrakis[methyl (2S)‐tetrahydro‐5‐oxopyrrole‐2‐carboxylate]), and [Rh2(OAc)4] (3). While catalysis with 2 and 3 afford preferentially trans‐cyclopropanecarboxylates, the cis‐isomers are the major products with 1. In general, the enantioselectivities achieved with 1 and 2 are comparable. Additions catalyzed by 1 are strongly sensitive to steric effects. Highly substituted olefins afford cyclopropanes in only poor yield. The preferential cis‐selectivity observed in reactions catalyzed by 1 is attributed to dominant interactions between the ligand of the catalyst and the substituents of both olefin and diazoacetate, which overrule the steric interactions between olefin and diazoacetate in the transition state for carbene transfer.
Metal Complexes with Macrocyclic Ligands. Part XXXVIII. Steric effects in the copper(II) and nickel(II) complexes with tetra‐N‐alkylated 1,4,8,11‐tetraazacyclotetradecanes
Authors: Oberholzer Martin R. , Neuburger Markus , Zehnder Margareta , Kaden Thomas A.
Published: December 2004

A series of tetra‐N‐alkylated 1,4,8,11‐tetraazacyclotetradecanes have been synthesized and their complexation potential towards Ni2+ and Cu2+ studied. In the case of sterically demanding alkyl substituents, such as i‐Pr, PhCH2, or 2‐MeC6H4CH2, no metal complexes are formed, whereas for substituents such as Me, Et, and Pr, the metal ion is incorporated into the macrocycle. The spectroscopic properties of the Ni2+ and Cu2+ complexes in aqueous solution indicate that, depending on the sterical hindrance of the N‐substituents, the complexes are either square planar or pentacoordinated. All these Ni2+ and Cu2+ complexes react with N to give ternary species, the stability of which have been determined by spectrophotometric titrations. The tendency to bind N decreases with increasing steric hindrance of the alkyl substituents. The X‐ray studies of the Ni2+ complex with the macrocycle 11, substituted by two Me and two Pr groups, and that of the Cu2+ complex with the tetraethyl derivative 8 show that in the solid state, the metal ions exhibit square planar coordination with a small distortion towards tetrahedral geometry.
Synthesis and 15N‐ and 17O‐NMR Spectra of 5‐Methyl(15N2)[O2,O4‐17O2]uridine (= (15N2)[O2,O4‐17O2]Ribosylthymine)
Authors: Amantea Antonio , Walser Markus , Séquin Urs , Strazewski Peter
Published: December 2004

The 5‐methyl(15N2)[O2,O4‐17O2]uridine (= (15N2)[O2,O4‐17O 2]ribosylthymine; 15) was synthesized and analyzed by 15N‐ and 17O‐NMR spectroscopy. (15N2)Urea was condensed with 2,3‐dibromo‐2‐methylpropanoyl chloride (3) and cyclized to form (15N2)thymine (5). After glycosidation, the 17O isotopes were introduced in two separate steps: hydrolytic ring opening of 2,5′‐anhydro derivative 9 and hydrolysis of 3‐nitro‐1H‐1,2,4‐triazole derivative 12 with labelled water in the presence of a strong base. The 15N‐ and 17O‐NMR spectra (Fig.) of 15 in phosphate‐buffered water serve as references for heteronuclear NMR spectra of labelled RNA fragments.
Solution‐Structure and Aggregation Behavior of Two Dilithiostyrene Derivatives
Authors: Günther Harald , Eppers Oswald , Hausmann Heike , Hüls Dietmar , Mons Hans‐Egbert , Klein Klaus‐Dieter , Maercker Adalbert
Published: December 2004

The solution structure and the aggregation behavior of (E)‐2‐lithio‐1‐(2‐lithiophenyl)‐1‐phenylpent‐1‐ene (1) and (Z)‐2‐lithio‐1‐(2‐lithiophenyl)ethene (2) were investigated by one‐ and two‐dimensional 1H‐, 13C‐, and 6Li‐NMR spectroscopy. In Et2O, both systems form dimers which show homonuclear scalar 6Li,6Li spin‐spin coupling. In the case of 2, extensive 6Li,1H coupling is observed. In tetrahdrofuran and in the presence of 2 mol of N,N,N′,N′‐tetramethylethylylenediamine (tmeda), the dimeric structure of 1 coexists with a monomer. The activation parameters for intra‐aggregate exchange in the dimers of 1 and 2 (1 (Et2O): ΔH≠ = 62.6 ± 13.9 kJ/mol, ΔS≠ = 5.8 ± 14.0 J/mol K, ΔG≠(263) = 61.1 kJ/mol; 2 (dimethoxyethane): ΔH≠ = 36.9 ± 6.5 kJ/mol, ΔS≠ = −61 ± 25 J/mol K, ΔG≠(263) = 54.0 kJ/mol) and the thermodynamic parameters for the dimer‐monomer equilibrium for 1 (ΔH°; = 26.7 ± 5.5 kJ/mol, ΔS° = 63 ± 27 J/mol K), where the monomer is favored at low temperature, were determined by dynamic NMR studies.
Ein neues 3‐Amino‐2H‐azirin als Aib‐Pro‐Baustein: Synthese des C‐terminalen Nonapeptids von Trichovirin I 1B
Authors: Luykx Roeland , Bucher Christoph B. , Linden Anthony , Heimgartner Heinz
Published: December 2004

A New 3‐Amino‐2H‐azirine as an Aib‐Pro Synthon: Synthesis of the C‐Terminal Nonapeptide of Trichovirin I 1B

The synthesis of methyl N‐(2,2‐dimethyl‐2H‐azirin‐3‐yl)‐L‐prolinate (3), a novel 3‐amino‐2H‐azirine, is described (Scheme 2). It is shown that the reaction of COCl2 with thioamide 5 is remarkably faster than with the corresponding amide 4, and the yield of 3 is much better in the synthesis starting with 5. The 3‐amino‐2H‐azirine 3 has been used as a building block of the dipeptide moieties Aib‐Pro in the synthesis of nonapeptide 17 (Schemes 4 and 5), the C‐terminal 6–14 segment of the peptaibole trichovirin I 1B. The structure of 17 was established by single‐crystal X‐ray crystallography (Figs.1 and 2).
Extrem aufgeweitete Tetrathiafulvalene mit Polyen‐Spacern. Carotinoide Tetrathiafulvalene. Polymethin‐tetracyanotetrathiafulvalen‐Radikalkationen, eine neue Klasse von Violenen
Authors: Märkl Gottfried , Pöll Andreas , Aschenbrenner Norbert G. , Schmaus Corinna , Troll Theodor , Kreitmeier Peter , Nöth Heinrich , Schmidt Martin
Published: December 2004

Extremely Expanded Tetrathiafulvalenes with Polyene Spacers. Carotinoid Tetrathiafulvalenes. Polymethin‐Tetracyanotetrathiafulvalene Radical Cations, a New Class of Violenes

The synthesis of extended tetrathiafulvalenes11with di‐, tetra‐, hexa‐, octa‐, deca‐ and dodecamethine spacers is described by a PPh3‐inducedWittig‐reaction‐like condensation of the corresponding polyenedials10with 2‐thio‐1,3‐dithiole‐4,5‐dicarbonitrile (9). By the same procedure, the dimethyloctamethine‐ and the tetramethyl‐hexadecamethine‐tetrathiafulvalenes14and15, respectively, were obtained. The extended tetrathiafulvalenes represent multistep vinylogous redox systems of the ‘violene type’. They can be oxidized to give the cyanine‐like radical cations,e.g.11sem,14sem, and15sem, and the dications,e.g.11ox,14ox, and15ox; their UV/VIS/NIR spectra are reported. The crystal and molecular structure of (all‐E)‐2,2′‐(octa‐2,4,6‐trien‐1,8‐diylidene)bis[1,3‐dithiole‐4,5‐dicarbonitrile] (11e) was determined: it is a rod‐like, planar molecule; in the crystal, it forms staples along the longest molecule axis. The CV measurements confirm that the redox potentials of11, 14, and15decrease asymptotically with the increasing length of the spacer. Because of the close relationship of the extended tetrathiafulvalenes (ETTF's) to the carotinoids, they are named ‘caroviologenes’; they formally belong to the class of molecular wires.
Optisch aktive 3‐Amino‐2H‐azirine als Bausteine für enantiomerenreine αα‐disubstituierte α‐Aminosäuren: Synthese von Isovalin‐Synthonen und Einbau in ein Trichotoxin‐A‐50‐Segment
Authors: Bucher Christoph B. , Heimgartner Heinz
Published: December 2004

Optically Active 3‐Amino‐2H‐azirines as Synthons for Enantiomerically Pure αα‐Disubstiuted α‐Amino Acids: Syntheses of Isovaline Synthons and a Segment of Trichotoxin A‐50

The synthesis of a novel 3‐amino‐2‐methyl‐2‐[2‐(phenylsulfonyl)ethyl]‐2H‐azirine derivative 12 with a chiral substituent at the amino group is described. Chromatographic separation of the diastereoisomer mixture gave pure diastereoisomers which, after an electrochemical cleavage of the phenylsulfonyl group, yielded the (S)‐ and (R)‐isovalin (Iva) synthons 13a and 13b, respectively. The absolute configuration of the precursor molecule 12b was established by X‐ray crystallography. The Iva synthons were successfully used in the synthesis of the C‐terminal pentapeptide Z‐Leu‐Aib‐(R)‐Iva‐Gln‐Valol of the peptaibole Trichotoxin A‐50 and its epimer.
Synthesis of Monodisperse Macromolecular Bicyclic and dendritic compounds from (R)‐3‐hydroxybutanoic acid and benzene‐1,3,5‐tricarboxylic acid and analysis by fragmenting MALDI‐TOF mass spectroscopy
Authors: Seebach Dieter , Herrmann Guide F. , Lengweiler Urs D. , Amrein Walter
Published: December 2004

As previously shown, oligo‐ and poly(β‐hydroxyalkanoates) have a high tendency to form lamellar crystallites with ca. 50‐Å thickness which corresponds to chain lengths of 16 units (Fig. 1). To have monodisperse model compounds, we have now prepared bicyclic derivatives with three parallel (27–29) or two parallel and an antiparallel chain (68–70) consisting of up to 16 3‐hydroxybutanoate (3‐HB) units. We also prepared dendritic compounds (71–75, 82–85) containing oligo(3‐HB) chains which cannot possibly be arranged as in the lamellae; the branching units were prepared from trimesic acid (= benzene‐1,3,5‐tricarboxylic acid). So far, none of the prepared samples formed crystals or contained crystalline domains which would have been suitable for single‐crystal or powder‐diffraction X‐ray analysis. The terminally deprotected dendrimers (74, 75, and 85) are multi‐anionic (up to 12 peripheral CO2H groups) and biodegradable. The macromolecular HB derivatives (molecular weight up to 10150 Da) have been fully characterized by IR, 1H‐ and 13C‐NMR, [α]D, and elemental analysis. Especially important is the analysis by mass spectrometry with the MALDI‐TOF technique (Fig. 2), proving that the products are monodisperse; application of a new variation of this MS method (post source decay = PSD or fragment analysis by structural time of flight = FASTTM) allows for the observation of metastable fragment ions and, thus, is a tool for structural oligomer analysis (Fig. 3).
Organonickel(II) Complexes with Anionic N‐Donor Ligands. Hydration of coordinated nitriles at a nickel(II) site
Authors: Sánchez Gregorio , Ruiz Francisco , Garcia Joaquín , Carmen de Arellano M. Ramírez , López Gregorio
Published: December 2004

The hydroxo complex (Bu4N)2[Ni2(C6F5)4(μ‐OH)2]reacts with 2,3,4,5,6‐pentafluoro benzenamine (C6F5‐NH2), 1,3‐diaryltriaz‐1‐enes (ArNHN=NAr, Ar = Ph, 4‐MeC6H4, 4‐MeOC6H4), 7‐aza‐1H‐indole (= 1H‐pyrrolo[2.3‐b]pyridine; Hazind),N‐phenylpyridin‐2‐amine(pyNHPh), andN‐phenylpyridine‐2‐carboxamide (py‐CONHPh) at room temperature in acetone to give the binuclear complexes (Bu4N)2[Ni2(C6F5)4(μ‐C6F5NH)2] (1) and (Bu4N)2[{Ni(C6F5)2} 2(μ‐OH)(μ‐azind)] (2) and the mononuclear complexes Bu4N[Ni(C6F5)2(ArN3Ar)] (3–5), Bu4N[Ni(C6F5)2(pyNPh)] (6), and Bu4N[Ni(C6F5)2(pyCONPh)] (7). The hydroxo.complex (Bu4N)2[{Ni(C6F5)2‐(μ‐OH)}2] promotes the nucleophilic addition of water to pyridine‐2‐carbonitrile, 2‐aminoacetonitrile, and 2‐(dimethylamino)acetonitrile, and complexes8–10containing pyridine‐2‐carboxamidato, 2‐aminoacetamidato and 2‐(dimethylamino)acetamidato ligands are formed. Analytical (C, H, N) and spectroscopic (IR,1H and19F‐NMR, and FAB‐MS) data were used for structural assignments. A single‐crystal X‐ray diffraction study of (Bu4N)2[{Ni(C6F5)2}2(μ‐OH)(μ‐azind)] (2) established the binuclear nature of the anion; the two Ni‐atoms are bridged by an OH group and a 7‐aza‐7H‐indol‐7‐yl group, but the central NiONiNCN ring is not planar, the dihedral angle between the NiONi and NiNCNNi planes being 84.4°.
Synthesis of N‐{[(9H‐Fluoren‐9‐yl)methoxy]carbonyl}‐Protected (Fmoc) β‐Amino Acids (= homo‐α‐amino acids) by direct homologation
Authors: Ellmerer‐Müller Ernst P. , Brössner Dagmar , Maslouh Najib , Takó Andreas
Published: December 2004

The successful application of the Arndt‐Eistert protocol starting from commercially available N‐{[(9H‐fluoren‐9‐yl)methoxy]carbonyl}‐protected (Fmoc) α‐amino acids leading to enantiomerically pure N‐Fmoc‐protected β‐amino acids in only two steps and with high yield is reported.
Stability and Lability of Dicopper Double‐Stranded Helicates in Solution
Authors: Carina Riccardo F. , Williams Alan F. , Piguet Claude
Published: December 2004

Stability constants have been measured for a series of ligands based on a 2,2′‐(pyridine‐2,6‐diyl)bis[1H‐benzimidazole] unit which forms dinuclear double‐stranded helical complexes with copper(I). Variation of different structural parameters confirms the importance of the coordinate bond, the stacking interactions, and the weakly bridging pyridine units observed by X‐ray crystallography. The stabilities of the complexes depend strongly on the solvent, and in MeCN, which is a good solvent for copper(I), the complexes are less stable and assemble in a stepwise manner. The interconversion of the enantiomers may be followed by 1H‐NMR and takes place on a millisecond time scale around room temperature. The trends in lability are similar to those found for the stability of the complexes.
Diastereo‐ and Regioisomeric Bicyclic Thiohydantions from Chiral 1,3‐Thiazolidine‐2,4‐dicarboxylic acids
Authors: Miskolczi István , Zékány András , Rantal Ferenc , Linden Anthony , Kövér Katalin E. , Györgydeák Zoltán
Published: December 2004

Bicyclic thiohydantoins were synthesized in a stereoselective manner by reacting (2R)/(2S)‐diastereoisomer mixtures of 1,3‐thiazolidine‐2,4‐dicarboxylic acids or their dimethyl diesters with PhNCS. 5,5‐Dimethyl‐1,3‐thiazolidine‐2,4‐dicarboxylic acid with PhNCS led to a cyclization involving the CO group at the C(2) center of the thiazolidine ring, while the acid's dimethyl diester gave cyclization involving the CO group at C(4). In contrast, reactions involving unsubstituted 1,3‐thiazolidine‐2,4‐dicarboxylic acid or its dimethyl diester led to thiohydantoins in which the ring closure had taken place only with the COO group at C(4). Independently of the direction of the ring closure, all reactions produce exclusively products with the (R)‐configuration at C(2). The configurational assignments were based on1H‐ and13C‐NMR studies, and confirmed by X‐ray crystallographic analyses.
Enantioselektive Synthese von α‐substituiertenN‐Methyl‐sulfonamiden. Vorläufige Mitteilung
Authors: Enders Dieter , Thomas Christian R. , Raabe Gerhard , Runsink Jan
Published: December 2004

The first asymmetric α‐alkylations of lithiated sulfonamides bearing the chirality information within the amine moiety under high asymmetric inductions (de 83–95%) are described. Racemization‐free acidic hydrolysis led to the title compounds11in acceptable overall yields and with high enantiomeric purity (ee 91− ≥ 98%;Scheme 2). As a novel chiral auxiliary, the primary amine (S,S)‐or (R,R)‐2was synthesized employing the classicalErlenmeyerphenylserine synthesis (Scheme 1).
Analysis of biomaterials deposited on soft contact lenses
Authors: Wedler Frederick C.
Published: December 2004

Formation of opaque deposits on the anterior (air) surface of hydrophilic soft contact lenses is a problem worthy of investigation by all concerned. These deposits have been analyzed for biomaterials by chemical, biochemical, electrophoretic, and immunological techniques. Qualitative and quantitative chemical colorimetric tests revealed the presence of variable amounts of protein (5–10 μg/lens), carbohydrate (1.0–1.2 μg/lens), and phospholipids (0.01–0.05 μmole/lens). Cholesterol and glucose were not present at detectable levels. Fluorescent antibody tests with appropriate controls gave positive tests for albumin, lysozyme, γ‐G‐globulin, and α1‐lipoprotein in the deposits, all proteins presents in tear fluid. Deposits were most effectively removed from the lenses by the combination of heat, sodium dodecyl sulfate (SDS) detergent, and the thiol reagent dithiothreitol (DTT). SDS‐denatured protein migrated on polyacrylamide gels with electrophoretic patterns corresponding to molecular weights for those proteins detected by the above antibody tests. The nature of the bonding interactions of biomaterials to the lenses was probed by chemical reagents used to remove them, employed singly and in all possible combinations. Urea, guanidine hydrochloride, potassium thiocyanate, potassium perchlorate, hydroxylamine, and EDTA were much less effective than SDS and DTT. These data suggest that apolar interactions plus disulfide bonds may be important in stabilizing the deposit structure, and point to improved cleaning procedures.
In vitro clot formation on the polyelectrolyte complexes of sodium dextran sulfate with chitosan
Authors: Fukuda Hiroshi , Kikuchi Yasuo
Published: December 2004

The solution of charged sodium dextran sulfate (molecular weight 6000) co‐reacted with that of oppositely charged chitosan to form a water‐insoluble hydrous complex, whose clot‐inhibiting property was sensitive to such reaction conditions as the order of mixing, the kind of acid species used for the adjustment of pH, and the concentration of hydrogen ion at which the preparation of polyelectrolyte complex was carried out, although all the polyelectrolyte complexes were prepared from the same starting materials. In order to clarify the relationship between the molecular conformation of the polyelectrolyte complex and its clot‐inhibiting property, ir spectroscopic studies, in vitro blood‐clotting tests, color reaction with toluidine blue, elemental analyses, and so on, were performed. Consequently, it was suggested that the polyelectrolyte complexes consisting of sodium dextran sulfate of lower molecular weight are more clot‐inhibiting than those with higher molecular weight. It was also found that polyelectrolyte complexes prepared under the mixing order of sodium dextran sulfate solution added to chitosan solution are also more clot‐inhibiting than those prepared under the reverse mixing order, chitosan solution added to sodium dextran sulfate solution. Furthermore, it has become apparent that the anticlotting polyelectrolyte complexes have some characteristic ir absorption bands, and that they displayed color reaction with toluidine blue. A new possibility may be anticipated from our experimental results, providing important information relating to realizing a functional polymer, antithrombogenic material, whose property may be ordered.
The effect of surface charge on arterial thrombosis
Authors: Helmus Michael N. , Gibbons Donald F. , Jones Richard D.
Published: December 2004

The amount of thrombus formed on the surface of implants of random copolymers of (L‐glutamic acid co‐L‐leucine) implanted in the femoral and carotid arteries of dogs is related to composition and the degree of ionization. When the initial surface concentration of unionized glutamic acid is greater than 10%, the surface is completely covered with thrombus; the thrombus is, however, limited in thickness to ∼600 μ. For surface concentration of unionized glutamic acid less than 10%, the amount of thrombus is a linear function of the degree of ionization. When 10% of the total surface sites consists of ionized glutamic acid residues, there is no thrombus, and only formed elements adhere to the surface. Thrombus covered surfaces tended to show decreased thrombus formation by seven days post implantation. The presence of thrombus enhanced the rate of development of an endothelial‐like cell layer on the surface, while on thrombus‐free implants, large‐diameter flattened monocytes, some of which formed confluent patches, covered only small areas of the implant surface.
Morphological assessment of hepatoma cells (HepG2) microencapsulated in a HEMA‐MMA copolymer with and without Matrigel
Authors: Babensee Julia E. , De Boni Umberto , Sefton Michael V.
Published: December 2004

Hepatoma cells (HepG2), an anchorage‐dependent cell line, were microencapsulated in a HEMA‐MMA polyacrylate membrane to which the cells do not ad here. This environment was altered by the coencapsulation of Matrigel, a reconstituted extracellular matrix derived from the Engelbreth‐Holm‐Swarm (EHS) mouse tumor basement membrane, to provide sites for cell attachment. The effect on the cells of these two capsule microenvironments during a 2‐week in vitro culture period was assessed by examining the spatial arrangement, morphology, and viability of the cells using light microscopy and scanning electron microscopy (SEM). In preparation for microscopy, dissolution of the polymer was prevented by the use of frozen sections embedded in a water‐soluble compound. Similarly, freeze cleavage of conductively stained capsules permitted SEM observation of the capsule interior along with ultrastructural detail of the cells. In the absence of Matrigel, cells in HEMA‐MMA capsules were found to form aggregates in intracapsular pockets with central necrosis occurring at day 7 in large aggregates. The coencapsulation of HepG2 cells with Matrigel, resulted in an initially uniform distribution of essentially individual cells with aggregates appearing later within the Matrigel. Many cells within these capsules had remained viable when examined up to day 14 with only limited cellular necrosis, implying a favorable environment for microencapsulated HepG2 cells. © 1992 John Wiley & Sons, Inc.
Parallel processing on minicomputers: A powerful tool for quantum chemistry
Authors: Seeger Rolf
Published: December 2004

It is shown that performance of quantum chemistry program systems on minicomputers may be enhanced by a factor between 2.5 and 2.8 utilizing microprogramming techniques and a computer architecture allowing four processor activities to operate in parallel, i.e., memory access, basic arithmetic operations, and two high‐speed 64‐bit floating‐point arithmetic units.
A comparison of selected force constants derived from ab initioSCF calculations on β‐hydroxyacrolein. Acrolein, performic acid, and formic acid
Authors: Bock Charles W. , Trachtman Mendel , George Philip
Published: December 2004

Harmonic force fields have been calculated for the planar hydrogen‐bonded ring conformer of β‐hydroxyacrolein, cCc, which is the most stable, and the chain conformer, cCt, generated by 180° rotation of OH about the CO bond axis. The equilibrium structure obtained using the 4‐31 G basis set with full geometry optimization was employed in each case. Selected force constants for the bonds directly concerned in the formation of the ring from the chain structure, and the increments in going from the one to the other, are compared with the values for the corresponding conformers of performic and formic acids. As the ring size increases from four in trans‐formic acid, to five in cis–cis‐performic acid and to six in the cCc conformer of β‐hydroxyacrolein there is a successive increase in the mechanical strength of the hydrogen‐bridging unit. The energy changes for the chain → ring conversion do not follow this progression: performic acid is out of order. But, since a force constant is a localized bond property, whereas the energy changes are determined not only by interactions specific to the hydrogendonor and hydrogen‐acceptor groups but also by interactions involving more distant parts of the molecule, the force constants for the bonds directly concerned in the formation of the hydrogen bridge provide a less ambiguous basis for comparing the strength of the intramolecular hydrogen bonding.
The imposition of linear constraints upon eigenvalue problems
Authors: Handler George S. , Trindle Carl
Published: December 2004

A simple method of imposing linear constraints upon an eigenvalue problem is described that reduces the dimension of the problem by the number of constraints imposed. Several applications are outlined.
Planar and nonplanar vibrations of cytosine
Authors: Putnam B. F. , Van Zandt L. L.
Published: December 2004

The formalism developed in the preceding article is used in a normal coordinate analysis of the pyrimidine base cytosine. The results of both planar and nonplanar vibrations are reported. The model yields 66 frequencies with an average error of about 6.5 cm−1 (ca. 0.5%). The vibrational modes are compared with experimental data and discussed in terms of potential energy distributions and Cartesian displacements. Two isolated low‐frequency (near 200 cm−1) out‐of‐plane modes are predicted in the vicinity of where two such modes are believed to occur. In addition, the model has taken into account the observed coalescence of the torsional and wagging modes of the amino group upon deuteration of the amino group. Recent data from sulfur‐substituted cytosine (2‐thiocytosine) were useful in making assignments.
Koopmans' theorem ionization potentials of methyl and silyl halides, chalcogenides, amines, and phosphines: An ab InitioSCF study
Authors: Glidewell Christopher , Thomson Colin
Published: December 2004

Koopmans' theorem ionization potentials have been calculated for a series of hydrides, methyls, and silyls HnX, (CH3)nX, and (SiH3)nX (X = F, Cl, n = 1; X = O, S, n = 2; X = N, P, n = 3), together with some mixed species (MH3)nXH3‐n (X = N, P; M = C, Si) using ab initio SCF methods. The calculated values give excellent agreement with experimental values without the inclusion of d functions. For the chlorides, HCl, CH3Cl, and SiH3Cl, the values vary rather little over a wide range of basis sets, and are unaffected by the inclusion of d functions.
A unified treatment of valence and bond order from density operators
Authors: Jug Karl
Published: December 2004

A generalization of the quantum chemical definition of valence of atoms in molecules is suggested. Valence is considered as expectation value of diatomic parts of density operators. It appears as a sum of contributions from occupied orbitals of all atomic pairs that contain the reference atom. This definition is applicable on self‐consistent‐field (SCF) and configuration interaction (CI) level in any atomic orbitals (AO) basis. Its usefulness is demonstrated in an application to special molecules. Photoelectron spectroscopy and reactivity is discussed in this context.
Molecular associations: Values of the expansion parameters for new classes of atoms
Authors: Sordo J. A. , Fraga S.
Published: December 2004

Expansion parameters, for use in a 1/R expansion developed at this laboratory for the study of molecular interactions, have been determined for new classes of atoms. Test calculations with these new parameters have yielded very satisfactory results for the glycine zwitterion, in dimer form and solvated, confirming the goodness of the parameters and the appropriateness of the formulation.
Computational modeling of the mechanisms of the free radical‐chain reaction of alkanes with oxygen. The oxidation of isobutane, n‐butane, and isopentane
Authors: Allara D. L. , Edelson D. , Irwin K. C.
Published: December 2004

A general computational method for obtaining complete solutions of time‐dependent kinetic equations has been developed and applied to free radical‐initiated reactions of alkanes with oxygen. The method has been applied to the low‐temperature, peroxide‐initiated oxidations of isobutane and isopentane. Using available independently measured and estimated values for the rate constants and activation parameters for each of the 20 proposed reaction steps for the oxidation of isobutane, the rates and products have been calculated for both the liquid phase and gas phase in the range of 100°–155°C. The calculated rates and products of oxidation agree with published experimental values. The oxidation of isopentane was examined by a 32‐reaction model. The rate constants were estimated using values for the appropriate rate steps in the oxidation of n‐butane and isobutane. The calculation of the oxidation rate and products agree with our experiments.
Free‐radical chain dechlorination of chloroethanes in liquid triethylsilane
Authors: Aloni R. , Rajbenbach L. A. , Horowitz A.
Published: December 2004

The kinetics and mechanism of the free‐radical chain dechlorination of C2Cl6, C2Cl5H, and sym‐C2Cl4H2 in Et3SiH wereinvestigated over a wide temperature range. The propagation step of the dechlorination of chloroethanes (C2Clx H6−x) proceeds by the following reactions:
equation image
Analysis of the temperature dependence of product formation gave the Arrhenius expressions for k4/k3 which in turn were utilized for the estimation of the absolute Arrhenius parameters for hydrogen abstraction from Et3SiH. Our results show that the values of Eabs are lower in Et3SiH than in c‐C6H12 by about 2–3 kcal/mol, while the A factors are almost equal. In competitive studies k2 was determined versus Br abstraction from n‐C5H11Br. The relative Arrhenius parameters determined by this method show that variations in both A factors and activationenergies are responsible for the reactivity trends observed in the Cl transfer reactions of Et3Si radicals.
über die Herstellung von Legierungen nach dem Tränkverfahren
Authors: Anders Heinz
Published: December 2004

Die Entwicklung der Fertigungsverfahren fü Preßteile aus Sintermetallen und die Verbesserung ihrer Gebrauchs‐eigenschaften hat besonders im Ausland beachtliche Fortschritte erzielt und den pulvermetallurgischen Erzeugnissen neue Anwendungsgebiete erschlossen. Eine wesentliche Neuerung in der Herstellung von Maschinenpreßteilen bedeutet die Anwendung des Tränkverfahrens, bei dem zunächst ein poriges Sintereisen‐ oder Sinterstahlpreßteil angefertigt wird, des‐sen Poren man mit einem geeigneten Tränkmetall ausfüllt.

Der Bericht behandelt die in den letzten Jahren erzielten Ergebnisse und soll an‐regen, die Vorteile der Pulvermetallurgie und die des Tränkverfahrens in noch stärkerem Maße als bisher in Anwendung zu bringen, zumal derartig hergestellte Werkstoffe korrosionstechnisch von aus‐gesprochen guter Beständigkeit sind.
Intrinsically photosensitive polyimides: Approaches to fluorinated homologs
Authors: Moore J. A. , Gamble Donald R.
Published: December 2004

Cyclobutane tetracarboxylic acid dianhydrides, which were prepared via photodimerization of maleic anhydride or methylmaleic anhydride, gave high molecular weight film‐forming polyimides with oxydianiline. The resulting polyimides were found to be photosensitive, presumably because of retrocyclization of the cyclobutane ring. Despite the presence of the cyclobutane ring, the polyimides show good thermal stability. In an effort to enhance the thermal stability and lower the dielectric constant of these polyimides, an approach was made to prepare the fluorinated analogs of the cyclobutane dianhydride monomers via the preparation and photodimerization of fluorinated maleic anhydride derivatives. Thus, both fluoromaleic and difluoromaleic anhydrides were photolyzed to give the target monomers. The proposed application of these monomers, however, has yielded disappointing results due to complications in their synthesis and attempted polymerization.
Oxydationswirkungen des Eisenchlorids im Sonnenlicht
Authors: Benrath Alfred
Published: December 2004

Im Folgenden soll noch einmal kurz zusammengestellt werden, welche Substanzen bei den einzelnen Versuchen mit einander reagierten, und welche Produkte dabei entstanden.