bluuebunny/crossref_metadata_2025_split huggingface dataset:
Synthesis, Separation, and Characterization of Optically Pure C76 Mono‐Adducts
Authors: Herrmann Andreas , Diederich François 
Published: December 2004

AbstractNucleophilic Bingel cyclopropanation of D2‐C76 with bis[(S)‐1‐phenylbutyl] 2‐bromomalonate in toluene in the presence of base yielded three constitutionally isomeric pairs of diastereoisomeric mono‐adducts together with one other constitutional isomer. All seven mono‐adducts were isolated in optically pure form by prep. HPLC on a (S,S)‐Whelk‐O1 chiral stationary phase. They represent the first optically pure adducts of an inherently chiral fullerene. Characterization by UV/VIS, CD, 13C‐ and 1H‐NMR spectroscopy allowed identification of pairs of stereoisomers and symmetry assignments: the two pairs of diastereoisomers which were isolated as the major product possess C1 symmetry, whereas the third pair of diastereoisomers, which is a minor product, is C2‐symmetrical. The circular dichroism spectra of the optically active C76‐adducts showed very pronounced Cotton effects resulting from strong chiroptical contributions of the chiral fullerene chromophore with the maximum observed Δε values being twice as high than those previously measured for optically active adducts of achiral fullerenes with a chiral addition pattern. Whereas the regioselectivity of mono‐additions to C70 correlates with the degree of local bond curvature and the regioselectivity of multiple Bingel cyclopropanations of C60 with electronic parameters such as coefficients of the lowest unoccupied molecular orbital (LUMO), no such simple predictive correlations exist for the nucleophilic addition to C76. Despite full spectral characterization, an unambiguous structural assignment of the isolated compounds was not possible, except for the two C2‐symmetrical isomers. Based on considerations of local bond curvature and the previous experiences with the chemistry of C70, the structures of the C2‐symmetrical stereoisomers were assigned as (S,S,fC)‐3 and (S,S,fA)‐3, resulting from addition to the polar α‐type C(1)C(6) bond.
Stereoselektive Synthese von 2′‐O‐(2‐Methoxyethyl)ribonucleosiden: Nachbargruppenbeteiligung der Methoxyethoxy‐Gruppe bei der Ribosylierung von Heterocyclen
Authors: Martin Pierre 
Published: December 2004

Stereoselective Synthesis of 2′‐O‐(2‐Methoxythyl)ribonucleosides: Neighboring‐Group Participation of the Methoxythoxy Group in the Ribosylation StepA new access to 2′‐O‐(2‐methoxyethyl)ribonucleosides, building blocks for second‐generation antisense oligonucleotides, is presented. The influence of various reaction parameters on the coupling reaction of 2‐O‐(2‐methoxyethyl)‐D‐ribose derivatives with heteroaromatic bases as the key step was investigated, and reaction conditions were optimized with regard to formation of the desired β‐D‐anomers. With Sn2+ salts as promotors in polar solvents, these β‐D‐anomers were formed with a high degree of steroselectivity.
Total Synthesis of (−)‐(2R)‐Dihydromyricoidine
Authors: Häusermann Ursula A. , Linden Anthony , Song Jiangao , Hesse Manfred 
Published: December 2004

AbstractAn asymmetric synthesis of the spermidine alkaloid (−)‐(2R)‐dihydromyricoidine (5) was performed by employing two ring‐enlargement reactions. The chiral center was introduced by a diastereoselective Michael addition of perhydropyridazine (7) to the α,β‐unsaturated ester 6. The (Z)‐CC bond was obtained by a selective Wittig reaction. The synthetic compound 5 was found to have a negative value for the specific rotation. This is in contrast to that of the natural product reported in the literature. Therefore, as an outcome of this synthesis, the absolute configuration of the natural alkaloid should be inverted to be as shown in structure V.
Rarisetenolide, Epoxyrarisetenolide, and Epirarisetenolide, New‐Skeleton Sesquiterpene Lactones as Taxonomic Markers and Defensive Agents of the Marine Ciliated Morphospecies Euplotes rariseta
Authors: Guella Graziano , Pietra Francesco , Dini Fernando 
Published: December 2004

AbstractRarisetenolide (3) and epoxyrarisetenolide (4), new‐skeleton α,β‐conjugated sesquiterpene lactones, were isolated from various strains of the unicellular ciliated protist Euplotes rariseta collected from marine coasts widely far apart from each other: northern and southern Australia, southern Brazil, and Canary Islands. A strain of E. rariseta from New Zealand gave epirarisetenolide (5) instead, revealing a subtle variability in secondary metabolism for this ciliated morphospecies. Nonetheless, these metabolites – which are the first non‐aldehydic terpenoids so far isolated from ciliates – represent a unique whole that constitutes a reliable taxonomic tool at the morphospecies level. Epirarisetenolide (5) and rarisetenolide (3), in this order, showed higher toxicity towards nonproductive ciliates than the chemically more reactive natural epoxide 4 and the semisynthetic aldehyde/protected‐aldehyde froms 8/7a/7b. This inverse trend of biological vs. chemical effectiveness suggests that these cytotoxic agents interact noncovalently with membrane receptors.
Konfigurations‐ und konformationsisomere antiaromatische [28]Tetraoxaporphyrinoide(4.2.4.2) und aromatische [26]Tetraoxaporphyrin(4.2.4.2)‐dikationen. Eine neue Form molekularer Dynamik in makrocyclischen Systemen
Authors: Märkl Gottfried , Stiegler Jürgen , Kreitmeier Peter , Burgemeister Thomas , Kastner Fritz , Dove Stefan 
Published: December 2004

Configurational and Conformational Isomeric Antiaromatic [28]Tetraoxaporphyrinoids(4.2.4.2) and Aromatic [26]Tetraoxaporphyrin(4.2.4.2) Dications. A New Type of Molecular Dynamics in Macrocyclic SystemsThe [28]tetraoxaporphyrinoids(4.2.4.2) 6 are synthesized by cyclizing Wittig reaction of (E, E)‐5, 5′‐(buta‐1, 3‐diene‐1, 4‐diyl)bis[furan‐2‐carbaldehyde] (8) with (E, E)‐{(buta‐1, 3‐diene‐diyl)bis[(furan‐5, 2‐diyl)methylene]}bis‐[triphenylphosphonium] dibromide (9) and 3, 3′‐{[(E)‐ethene‐1, 2‐diyl]bis(furan‐5, 2‐diyl)}bis[(E)‐prop‐2‐enal] (22) with (E)‐{(ethene‐1, 2‐diyl)bis[(furan‐5.2‐ diy)methylene]}bis[triphenylphosphonium] dibromide (23). An alternative path to get 6 is the McMurry condensation of 8. Four different configurational isomers of 6 could be isolated and characterized by 1H‐NMR spectroscopy. The (Z, EE, Z, EE)‐isomer 6a is the first macrocyclic system where the inner and outer protons of the (E, E)‐dienediyl bridges exchange by rotation around the adjacent single bonds. In the (Z, EE, E, EE)‐isomer 6b, the (E)‐ethenediyl bridge is rotationally active, while in the (E, ZE, E, EZ)‐isomer 6c and in the (E, EZ, E, EZ)‐isomer 6e, the rotation of both (E)‐ethenediyl bridges is observed. When in the dynamic systems the rotation of the active (E)‐double bonds at temperatures T < −90° is frozen, all configurational isomers of 6 appear to be antiaromatic and paratropic. The oxidation of the [28]tetraoxaporphyrinoids 6c and 6e with DDQ yields the aromatic, diatropic [26]tetraoxaporphyrin(4.2.4.2) dications 21e/21e′ both with (E, EZ, E, EZ)‐configuration but different fixed conformations. (Z, EE, Z, EE)‐Isomer 6a is oxidized to give the (Z, EE, Z, EE)‐dication 21a, while the oxidation of 6b yields a mixture of 21a and 21e/21e′. The standard formation enthalpies of the obtained and expected [28]tetraoxaporphyrinoids 6 and [26]tetraoxaporphyrin dications 21 have been calculated with the AM1 method, showing good accordance with the experimental results.
Cyclo‐β‐peptides: Structure and tubular stacking of cyclic tetramers of 3‐aminobutanoic acid as determined from powder diffraction data
Authors: Seebach Dieter , Matthews Jennifer L. , Meden Anton , Wessels Thomas , Baerlocher Christian , McCusker Lynne B. 
Published: December 2004

AbstractThe solid‐state structures of three stereoisomer,1–3, of the cyclic tetramer of 3‐aminobutanoic acid are presented. These cyclo‐β‐peptides were found to be highly insoluble materials, and it proved to be impossible to grow crystals of sufficient quality for X‐ray single‐crystal analysis. The samples of 1–3 were, however, suitable candidates for structure determination from powder diffraction data (Fig. 1), and the application of this method is described. All three isomers have been found to adopt tubular structures (Figs. 2–4) in a fashion similar to those already observed for certain cyclo‐α‐peptides. The stacks of 16‐membered rings are held together by four nonlinear CO…︁HN H‐bonds between pairs of molecules (Fig.5).
Chemie von α‐Aminonitrilen 22. Mitteilung Regioselektive Synthese und Kristallstruktur von Uroporphyrinogen‐(Typ I)‐octanitril
Authors: Lehmann Christian , Schweizer Bernd , Leumann Christian , Eschenmoser Albert 
Published: December 2004

Chemistry of α‐Aminonitriles. Regioselective Synthesis and Crystal Structure of Uroporphyrinogen (Type I) OctanitrileA regioselective synthesis of uroporphyrinogen‐octanitrile (type I) based on the strategy of multiple use of (dimethylmethylidene)ammonium iodide for stepwise regioselective functionalization of the pyrrole nucleus is described. This uroporphyrinogen derivative is remarkably stable and beautifully crystallizes in space group P1 with one molecule per unit cell. The crystal structure of the compound shows interesting conformational characteristics which are interpreted to be caused by subtle stereoelectronic effects. The English Footnotes to Schemes 1‐3 and Figs. 1‐12 provide an extension of this summary.
1, 3‐Cycloadditions to Highly Substituted, Strained Double Bonds: Spiro β‐lactams from α‐methylidene‐β‐lactams by reactions with diphenylnitrilimine, acetonitrile oxide, nitrones, and diazomethane
Authors: Strauss Arthur , Otto Hans‐Hartwig 
Published: December 2004

AbstractSubstituted dihydropyrazole‐spiro‐β‐lactams and isoxazolidine‐spiro‐β‐lactam derivatives are regio‐ and stereoselectively prepared by 1, 3‐cydoadditions between substituted α‐methylidene‐β‐lactams and diazomethane, nitrones, or the in‐situ‐prepared dipoles ‘diphenylnitrilimine’ and acetonitrile oxide. These reactions represent examples for 1, 3‐cycloadditions to the highly substituted, strained double bonds of α‐methylidene‐β‐lactams, and they need special experimental conditions as all reaction products are relatively unstable. Especially in solution, the reverse reaction is highly favoured. Regioselectivity and stereoselectivity of the reactions are elucidated mainly by NMR techniques such as 2D‐INEPT, ATP, and NOE experiments.
Structural Comparison of Oligoribonucleotides and Their 2′‐Deoxy‐2′‐fluoro Analogs by heteronuclear NMR spectroscopy
Authors: Reif Bernd , Wittmann Valentin , Schwalbe Harald , Griesinger Christian , Wörner Karlheinz , Jahn‐Hofmann Kerstin , Engels Joachim W. , Bermel Wolfgang 
Published: December 2004

Abstract1‐(2′‐Deoxy‐2′‐fluororibofuranosyl)pyrimidines were synthesized and incorporated into an RNA oligonucleotide to give 5′‐r[CfGCf(UfUfCfG)GCfG]‐3′ (Cf: short form of C= 2′‐deoxy‐2′‐fluorocytidine; Uf: short form of U= 2′‐deoxy‐2′‐fluorouridine). The oligomer was investigated by means of UV, CD, and NMR spectroscopy to address the question of how F‐labels can substitute13C‐labels in the ribose ring. Through‐space (NOE) and through‐bond (scalar couplings) experiments were performed that make use of the ameliorated chemical‐shift dispersion induced by19F as an alternative heteronucleus. A comparison of the structures of fluorinatedvs.unmodified oligomer is given. It turns out that the fluorinated oligonucleotide exists in a 14:3 equilibrium between a hairpin and a duplex conformation, in contrast to the unmodified oligonucleotide which predominantly adopts the hairpin conformation. Furthermore, the fluorinated hairpin structure adopts two distinct conformations that differ in the sugar conformation of the Uand Cnucleoside units, as detected by the19F‐NMR chemical shifts. The role of the 2′‐OH group as stabilizing element in RNA secondary structure is discussed.
Reduction Potentials of Tetraaminecopper(II/I) Couples
Authors: Comba Peter , Jakob Holger 
Published: December 2004

AbstractThe difference in steric strain between the oxidized and the reduced forms of tetraaminecopper complexes is correlated with the corresponding reduction potentials. The experimentally determined data considered range from −0.54 to −0.04 V (vs. NHE) in aqueous solution and from −0.35 to −0.08 V (vs. NHE) in MeCN. The observed and/or computed geometries of the tetraaminecopper(II) complexes are distorted octahedral or square‐pyramidal (4 + 2 or 4+1) with (distorted) square‐planar CuN4 chromophores (CuIIN = 1.99–2.06 Å; CuO ≈ 2.5 Å; CuO ≈ 2.3 Å), those of the tetraaminecopper(I) complexes are (distorted) tetrahedral (four‐coordinate; CuIN = 2.12–2.26 Å; tetrahedral twist angle ∅︁ = 30–90°). The reduction potentials of CuII/I couples with primary‐amine ligands and those with macrocyclic secondary‐amine ligands were correlated separately with the corresponding strain energies, leading to slopes of 70 and 61 kJ mol−1 V−1, with correlation coefficients of 0.89 and 0.91, respectively. The approximations of the model (entropy, solvation, electronic factors) and the limits of applicability are discussed in detail and in relation to other approaches to compute reduction potentials of transition‐metal compounds.
Triple‐Helix Formation by Pyrimidine Oligonucleotides Containing Nonnatural Nucleosides with Extended Aromatic Nucleobases: Intercalation from the major groove as a method for recognizing C·G and T · A base pairs
Authors: Lehmann Thomas E. , Greenberg William A. , Liberles David A. , Wada Carol K. , Dervan Peter B. 
Published: December 2004

AbstractThe sequence‐specific recognition of double‐helical DNA by oligonucleotide‐directed triple helix formation is limited primarily to purine tracts. To identify potential lead compounds which are able to extend the sequence repertoire of triple helical complexes, we designed two carbocyclic nucleosides with nucleobases attached via amide bonds. N5‐[(1R, 2S, 3R, 4R)‐3‐hydroxy‐4‐(hydroxymethyl)‐2‐methoxycyclopentyl]‐2‐{[(1H‐pyrrol‐2‐yl)carbonyl]‐amino}thiazole‐5‐carboxamide (L1) and 2‐benzamido‐N5‐[(1R, 2S, 3R, 4R)‐3‐hydroxy‐4‐(hydroxymethyl)‐2‐methoxycyclopentyl]thiazole‐5‐carboxamide (L2) were synthesized and incorporated into pyrimidine oligonucleotides. The 2‐(trimethylsilyl)ethoxymethyl (SEM) protecting group for the 1H‐pyrrole NH was found to be compatible with DNA solid‐phase synthesis of pyrimidine Oligonucleotides. By quantitative DNase I footprinting analysis, both nonnatural nucleosides L1 and L2 showed preferential binding of pyrimidine over purine bases: L1/2·(C·G) ≈ L1/2·(T · A) > L1/2·(G·C) ≈ L1/2·(A · T). Comparison with the previously reported nonnatural nucleosides with extended aromatic nucleobases 1‐(2‐deoxy‐β‐D‐ribofuranosyl)‐4‐(3‐benzamidophenyl)‐imidazole (D3) and N4‐[6‐(benzamido)pyridin‐2‐yl]‐2′‐deoxycytidine (bzM) suggests that the observed binding selectivity C · G ≈ T · A > G · C ≈ A · T for the nucleoside analogs L1, L2, D3, and bzM is derived from sequence‐specific intercalation with preferential stacking of their nucleobases over pyrimidine · purine Watson‐Crick base pairs.
‘Mixed’ β‐peptides: A unique helical secondary structure in solution. Preliminary communication
Authors: Seebach Dieter , Gademann Karl , Schreiber JüRg V. , Matthews Jennifer L. , Hintermann Tobias , Jaun Bernhard , Oberer Lukas , Hommel Ulrich , Widmer Hans 
Published: December 2004

Abstractβ‐Hexapeptides 1–5 and a β‐dodecapeptide 6 with sequences containing two different types of β‐amino acids (aliphatic proteinageous side chains in the 2‐ or in the 3‐position) have been prepared. CD (Fig. 1) and NMR measurements indicate that, with one exception, the secondary structures formed by these new β‐peptides differ from those of isomers studied previously. Detailed NMR analysis of the β‐hexapeptide 5 (with alternating β2,β3‐building blocks) and molecular‐dynamics simulations have produced a minimum energy conformation (Fig. 2,b)which might be described as a novel irregular helix containing ten‐ and twelve‐membered H‐bonded rings. This demonstrates the great structural variability of β‐peptides, since three different helical secondary structures have been discovered to date.
Neue Bi(cyclopropylidene) durch CuCl2‐katalysierte ‘Carben‐Dimerisieiung’ von 1‐Bromo‐1‐lithiocyclopropanen
Authors: Borer Markus , Neuenschwander Markus 
Published: December 2004

AbstractNew Bi(cyclopropylidenes) by CuCl2‐Induced ‘Carbene Dimerization’ of 1‐Bromo‐1‐lithiocycIopropanesA series of so far unknown bi(cyclopropylidenes) 5 are prepared in a simple one‐pot reaction by halogenolithio exchange between 1,1‐dibromocyclopropanes 1a–c as well as 1e–i and BuLi at −95°, to give 1‐bromo‐1‐lithiocyclopropanes 2a–c as well as 2e–i, followed by treatment with CuCl2 at low temperature and a simple workup at room temperature (Table 1). The influence of reaction parameters on yields of 5 (Tables 2, 4, and 5) and diastereoselectivity of the reaction 2 → 5 (Table 3) are discussed. In view of an elucidation of the reaction mechanism, first kinetic experiments of the quantitative reaction 1c → 2c → 5c are reported.
A ‘One‐Pot’ Anellation Method for the Transformation of Heptalene‐4,5‐dicarboxylates into Benzo[a]heptalenes
Authors: Abou‐Hadeed Khaled , Hansen Hans‐Jürgen 
Published: December 2004

AbstractIt has been found that dimethyl heptalene‐4,5‐dicarboxylates, when treated with 4 mol‐equiv. of lithiated N,N‐dialkylamino methyl sulfones or methyl phenyl sulfone, followed by 4 mol‐equiv. of BuLi in THF in the temperature range of −78 to 20°, give rise to the formation of 3‐[(N,N‐dialkylamino)sulfonyl]‐ or 3‐(phenylsul‐fonyl)benzo[a]heptalene‐2,4‐diols of. (cf. Scheme 4, and Tables 2 and 3). Accompanying products are 2,4‐bis{[(N,N‐dialkylamino)sulfonyl]methyl}‐ or 2,4‐bis[(phenylsulfonyl)methyl]‐4,10a‐dihydro‐3H‐heptaleno[1,10‐bc]furan‐3‐carboxylates as mixtures of diastereoisomers of. cf. Scheme 4, and (Tables 2 and 3) which are the result of a Michael addition reaction of the lithiated methyl sulfones at C(3) of the heptalene‐4,5‐dicarboxylates, followed by (sulfonyl)methylation of the methoxycarbonyl group at C(5) and cyclization of. (cf. Scheme 5). It is assumed that the benzo[a]heptalene formation is due to (sulfonyl)methylation of both methoxycarbonyl groups of the heptalene‐4,5‐dicarboxylates of. (cf. Schemes 6 and 8). The resulting bis‐enolates 35 are deprotonated further. The thus formed tris‐anions 36 can then cyclize to corresponding tris‐anions 37 of cyclopenta[d]heptalenes which, after loss of N,N‐dialkylamido sulfite or phenyl sulfinate, undergo a ring‐enlargement reaction by 1,2‐C migration finally leading to the observed benzo[a]heptalenes of. (cf. Schemes 8 and 9). The structures of the new product types have been finally established by X‐ray crystal‐structure analyses (cf. Figs. 1 and 2 as well as Exper. Part).
Supramolecular Chirality: Chiral hydrogen‐bonded supermolecules from achiral molecular components
Authors: Suárez Mauricio , Branda Neil , Lehn Jean‐Marie , Decian André , Fischer Jean 
Published: December 2004

AbstractChiral supermolecules may be obtained from suitable achiral molecular constituents associated through a dissymmetrizing interaction mode. This is the case for the supermolecules I–IV formed by hydrogen‐bonding association between the achiral complementary components 1a,b and 2a,b,c. The crystal structures of the supermolecular pairs I–III and of the homochiral aggregate of two ternary supermolecules IV have been determined. The structural data are discussed.
Nucleotides. Part LV. Synthesis and application of a novel linker for solid‐phase synthesis of modified oligonucleotides
Authors: Waldvogel Siegfried R. , Pfleiderer Wolfgang 
Published: December 2004

AbstractVarious bifunctional amino‐protecting groups such as the phthaloyl, succinyl, and glutaryl group were investigated as potential linker molecules for attachment to solid‐support materials. Pentane‐1,3,5‐tricarboxylic acid 1,3‐anhydride (16) offered the best properties and reacted with the amino groups of differently sugar‐protected adenosine (see 20 and 22), cytidine (see 29), and guanosine derivatives (see 32) to the corresponding 2‐(2‐carboxyethyl)glutaryl derivatives 23, 24, 30, and 33. The usefulness of the new linker‐type molecules was demonstrated by the solid‐support synthesis of the potentially antivirally active 3′‐deoxyadenylyl‐(2′–5′)‐2′‐adenylic acid 2′‐{2‐[(adenin‐9‐yl)methoxy]ethyl} ester (38) starting from the 2′‐end with N6,N6‐[2‐(2‐carboxyethyl)glutaryl]‐9‐{{2‐[(4,4′‐dimethoxytrityl)ethoxy]methyl}adenine (12).
New Synthesis and Chirality of (−)‐4,4,4,4′,4′,4′‐Hexafluorovaline
Authors: Eberle Marcel K. , Keese Reinhart , Stoeckli‐Evans Helen 
Published: December 2004

Abstract(−)‐(R)‐4,4,4,4′,4′,4′‐Hexafluorovaline hydrochloride ((R)‐5) of 98% ee is prepared from β,β‐bis(trifluoromethyl)acrylic acid (= benzyl 4,4,4‐trifluoro‐3‐(trifluoromethyl)but‐2‐enoate; 1) in 4 steps with an overall yield of 9.6%. Key step is the separation of the TsOH salts of the diastereoisomers obtained by anti‐Michael addition of (+)‐(R)‐1‐phenylethylamine (2) to 1 (→ (R,R)‐3). In contrast to the published (S)‐chirality, the X‐ray structure analysis of (R,S)‐6 reveals, that (R)‐chirality has to be assigned to the levorotatory (−)‐4,4,4,4′,4′,4′‐hexafluorovaline hydrochloride.
Aldehyde‐Promoted Addition of 2‐(Trimethylsilyl)thiazole to α,α′‐Dialkoxy Ketones: A new way to branched‐chain monosaccharides
Authors: Carcano Michela , Vasella Andrea 
Published: December 2004

AbstractThe reaction of 2‐(trimethylsilyl)thiazole (2‐TST) with several ketones was tested in the presence or absence of aldehydes. The keto aldehyde5(Scheme 2) was prepared from1viathe hydroxy aldehyde4in 3 steps. It reacted with 2‐TST to give, after desilylation and acetylation, the bis‐thiazole6. The ketone11, obtained from4in 3 steps, reacted with 2‐TST to give, after desilylation,12. The ketofuranose17(Scheme 3) reacted with 2‐TST to yield exclusively the more stableD‐glucoepimer18. The reaction of the ketone11(Scheme 2) with 2‐TST was faster in the presence of 1 equiv. of the keto aldehyde5, suggesting that an aldehyde promotes the indirect and intermolecular addition of 2‐TST to a ketone. We have studied the effect of several aldehydes on the rate of the reaction of the ketones11and17with 2‐TST at different temperatures and at different concentrations of the ketones and of the aldehydes. Electrophilic aldehydes, and particularly 2‐fluorobenzaldehyde (0.1 equiv.), promote the addition of 2‐TST to electrophilic ketones.