bluuebunny/crossref_metadata_2025_split huggingface dataset:
Reaction of 5,6,7,8‐Tetrahydropterin with Iron(III) Acetylacetonate. Detection of Radical Cations by Electrospray Ionization Mass Spectrometry
Authors: Schäfer Andrea , Hesse Manfred , Viscontini Max , Paul Henning , Fischer Berthold 
Published: December 2004

AbstractThe experimental conditions developed for the detection of rather stable radical cations in solution by electrospray‐ionization mass spectrometry (ESI‐MS) of a FeII complex of 2‐amino‐5,6,7,8‐tetrahydro‐5‐methylpteridin‐4 (3H)‐one (1c) are used to observe the formation of the more unstable radical cations formed from 2‐amino‐5,6,7,8‐tetrahydropteridin‐4(3H)‐one (1a) and tris(pentane‐2,4‐dionato)iron(III) ([FeIII(acac)3]; 4) and to monitor their oxidation to the corresponding p‐quinonoid dihydropterin complexes. These results contribute to the understanding of the important role played by 6β‐5,6,7,8‐tetrahydro‐L‐biopterin (1b; a homologue of 1a) together with iron as constituent of some cofactors. The complexes obtained from 1a and iron may be considered, e.g. as a model of the cofactor of the phenylalanine hydroxylase. Moreover, we describe an improved synthesis of 1c.
Glycosylidene Carbenes. Part 22. α‐D‐Selectivity in the Glycosidation by Carbenes Derived from 2‐Acetamido‐hexoses
Authors: Vasella Andrea , Witzig Christian 
Published: December 2004

AbstractGlycosylidene carbenes derived from the GlcNAc and AllNAc diazirines 1 and 3 were generated by the thermolysis or photolysis of the diazirines. The reaction of 1 with i‐PrOH gave exclusively the isopropyl α‐D‐glycoside of 5 besides some dihydrooxazole 9 (Scheme 2). A similar reaction with (CF3)2CHOH yielded predominantly the α‐D‐anomer of 6, while glycosidation of 4‐nitrophenol (→7) proceeded with markedly lower diastereoselectivity. Similarly, the Allo‐diazirine 3 gave the corresponding glycosides 12–14, but with a lower preference for the α‐D‐anomers (Scheme 3). The reactions of the carbene derived from 1 with Ph3COH (→8) and diisopropylideneglucose 10 (→11) gave selectively the α‐D‐anomers (Scheme 2). The αD‐selectivity increases with increasing basicity (decreasing acidity) of the alcohols. It is rationalized by an intermolecular H‐bond between the acetamido group and the glycosyl acceptor. This H‐bond increases the probability for the formation of a 1,2‐cis‐glycosidic C–O bond. The gluco‐intermediates are more prone to forming a N–H…︁(H)OR bond than the allo‐isomers, since the acetamido group in the N‐acetylallosamine derivatives forms an intramolecular H‐bond to the cis‐oriented benzyloxy group at C(3), as evidenced by δ/T and δ/c experiments.
The Directionality of d‐Orbitals and Molecular‐Mechanics Calculations of Octahedral Transition‐Metal Compounds
Authors: Comba Peter , Ströhle Marc , Hambley Trevor W. 
Published: December 2004

AbstractA novel approach to modeling the angular geometry about the metal centre in transition‐metal complexes, using a variation of classical molecular‐mechanics calculations, is presented. The approach is based on the combination of 1,3‐nonbonded interactions around the metal centre and a harmonic sine function with a ligandfield‐dependent force constant for the L–M–L′ terms. Force‐field parameters for four‐, five‐, and six‐coordinated first‐row transition‐metal coordination centres and a variety of ligands containing N‐, S‐, and O‐donor sets are given. The new ‘electronically doped’ force field is shown to generally lead to computed structures with higher accuracy than those obtained when the coordination geometries are modeled with 1,3‐nonbonded interactions alone.
Cascade Cyclizations of Terpenoid Polyalkenes Triggered by Photoelectron Transfer – Biomimetics with Photons
Authors: Warzecha Klaus‐Dieter , Xing Xuechao , Demuth Martin , Goddard Richard , Kessler Magnus , Krüger Carl 
Published: December 2004

AbstractLight‐induced cyclizations of suitably functionalized polyalkene terpenoids, such as geranyl, all‐trans‐farnesyl, and all‐trans‐geranylgeranyl derivatives, via formation of radical cations are proven to be a powerful method for the single‐step synthesis of mono‐ and mostly all‐trans‐fused polycyclic compounds from readily available precursors. Whereas some of these highly stereo‐ and chemoselective transformations required the use of micel ar media, they can now be conveniently performed in homogeneous solutions upon suitable choice of the electron acceptors and of the functionality pattern of the polyalkene substrates. Moreover, the mode of cyclization, i.e., 6‐ vs. 5‐membered ring formation and termination of the cyclization cascades, are steered efficiently by the substituents of the polyalkenes (polyalkenyl acetate vs. α,β‐unsaturated ethyl polyalkenoate and polyalkene‐1,1‐dicarbonitrile). At the same time, the protic solvents used add highly stereoselectively to the ω‐alkene sites of the polyalkens in anti‐Markovnikov sense which strongly suggests that radical cations are intercepted. Interestingly, the transformations achieved here upon photoelectron transfer parallel the biosynthetic paths of non‐oxidative terpene cyclizations which are thought to occur purely by protonation of the isoprenoid polyalkenes.
Fullerene‐Acetylene Molecular Scaffolding: Chemistry of 2‐functionalized 1‐ethynylated C60, oxidative homocoupling, hexakis‐adduct formation, and attempted synthesis of C
Authors: Timmerman Peter , Witschel Lars E. , Diederich François , Boudon Corinne , Gisselbrecht Jean‐Paul , Gross Maurice 
Published: December 2004

AbstractOn the way to the fullerene‐acetylene hybrid carbon allotropes 2 and 6, the oxidative homocoupling of the 2‐functionalized 1‐ethynylated C60 derivatives 11, 12, 14, and 15 was investigated. Under Glaser‐Hay conditions, the two soluble dumbbell‐shaped bisfullerenes 17 and 18, with two C60 moieties linked by a buta‐1,3‐diynediyl bridge, were formed in 52 and 82% yield, respectively (Scheme 2). Cyclic‐voltammetric measurements revealed that there is no significant electronic communication between the two fullerene spheres via the buta‐1,3‐diynediyl linker. Removal of the 3,4,5,6‐tetrahydro‐2H‐pyran‐2‐yl (Thp) protecting groups in 18 gave in 80% yield the highly insoluble dumbbell 19 with methanol groups in the 2,2′‐positions of the buta‐1,3‐diynediyl‐bridged carbon spheres. Attempted conversion of 19 to the all‐carbon dianion 6 (C) via base‐induced elimination of formaldehyde was not successful presumably due to exo‐dig cyclization of the formed alkoxides. The occurrence of this cyclization under furan formation was proven for 2‐[4‐(trimethylsilyl)buta‐1,3‐diyn‐1‐yl][60]fullerene‐1‐methanol (21), a soluble model compound for 19 (Scheme 3). To compare the properties of ethynylated fullerene mono‐adducts to those of corresponding higher adducts, hexakis‐adducts 26 and 28 with an octahedral functionalization pattern resulting from all‐e (equatorial) additions were prepared by the reversible‐template method of Hirsch (Scheme 4). Reaction of the ethynylated mono‐adducts 25 or 13 with diethyl 2‐bromomalonate/1,8‐diazabicyclo[5.4.0]undec‐7‐ene (DBU) in the presence of 1,9‐dimethylanthracene (DMA) as reversible template led to 26 and 28 in 28 and 22% yield, respectively. Preliminary experiments indicated a significant change in reactivity and NMR spectral properties of the fullerene addends with increasing degree of functionalization.
The Self‐Recognition and Self‐Assembly of Folic Acid Salts in Isotropic Water Solution
Authors: Gottarelli Giovanni , Mezzina Elisabetta , Spada Gian Piero , Carsughi Flavio , Nicola Giovanni Di , Mariani Paolo , Sabatucci Annalaura , Bonazzi Stefania 
Published: December 2004

AbstractThe self‐assembly of alkaline folates in isotropic water solutions, with or without added salts, has been investigated by small‐angle neutron‐scattering, circular‐dichroism, and NMR techniques. The assembled species are chiral, cylindrical aggregates of finite length, composed of stacked tetramers; each tetramer is formed by Hoogsteen‐bonded folate residues. The assembly process is more efficient in the presence of an excess of NaI ions, leading to longer aggregates with stronger tetramer‐tetramer‐tetramer interaction. In pure water, the rods are shorter and the tetramer‐tetramer interaction weaker. Association between folates can be detected by circular‐dichroism spectroscopy starting from a concentration of 6· 10−4 mol l−1, well below the critical concentration for the formation of the cholesteric mesophase (ca. 0.5 mol l−1).
Synthesis of a Building Block for a Nucleic‐Acid Analog with a Chiral, Flexible Peptide Backbone
Authors: Savithri Dhanalekshmi , Leumann Christian , Scheffold Rolf 
Published: December 2004

AbstractWe present here a nine‐step synthesis of the thymine‐containing amino ester 1, starting from commercially available methyl N‐[(tert‐butoxy)carbonyl]‐L‐serinate. Amino ester 1 is considered as a building block for the preparation of a new nucleic‐acid analog with a chiral, flexible polyamide backbone. Key steps in the synthesis are the vitamin‐B12‐catalyzed addition of 3‐bromo‐N‐[(tert‐butoxy)carbonyl]‐L‐alaninate 2 to ethyl acrylate and the homologation of the corresponding N‐protected α‐amino acid 4 into the β‐amino ester 6 by Arndt‐Eistert chemistry. The latter was found to proceed with 10% inversion of configuration at the asymmetric center in 6. Resolution to enantiomerically pure material, however, was easily achieved by simple crystallization of 1.
Chiral Acylsilanes in Organic Synthesis. Part 3. Silicon‐directed stereoselective preparation and Ireland ester‐enolate rearrangement of O‐acyl‐substituted α‐silylated allyl alcohols
Authors: Enev Valentin , Stojanova Diana , Bienz Stefan 
Published: December 2004

AbstractStereocontrolled addition of alk‐1‐enylmetal reagents to the chiral (alkoxymethyl)‐substituted acylsilanes (±)‐6 gave rise to α‐silylated allyl alcohols, which were converted to the corresponding acetates or propionates 11–16 (Scheme 2). Deprotonation and silylation with Me3SiCl afforded – in an Ireland ester‐enolate‐accelerated Claisen rearrangement – stereoselectively αδ‐silylated γδ‐unsaturated carboxylic acids 18–24 (Scheme 4). The Me3Si groups in α‐position to the COOH group of these compounds were removed chemoselectively in presence of the chiral silyl group in δ‐position by treatment with Bu4NF · 3 H2O or Et3N · 3 HF (→ 27–32; Scheme 5). The reaction sequence allows a novel stereocontrolled access to chiral C‐frameworks possessing a vinylsilane moiety with its full reaction potential.
The Tandem Pauson‐Khand Reaction
Authors: Thommen Marc , Veretenov Andrei L. , Guidetti‐Grept Régine , Keese Reinhart 
Published: December 2004

AbstractThe conditions for the novel tandem Pauson‐Khand reaction have been explored. The highly functionalized tetracyclic compounds 11c, 11d, and 16 were prepared from the ene‐diynes 4c, 4d, and 10 by treatment with 2 equiv. of [Co2(CO)8] and 4‐methylmorpholine N‐oxide (NMO) or Me3NO in yields of 24, 22, and 53%, respectively (Table). In the presence of 1–3 equiv. of H2O added to the NMO used for induction of the Pauson‐Khand reaction of 6d, a mixture of cyclopentanones 17/18 and cyclopentenones 12/13 was obtained (Scheme 5). The first example of a [Co2(CO)6]‐induced highly stereoselective ene reaction is described. To account for these results, the formation of intermediates are proposed (Schemes 6 and 7) which hitherto have not been considered in the mechanistic description of the Pauson‐Khand reaction.
Synthesis of 1,6‐Dideoxynojirimycin, 1,6‐Dideoxy‐D‐allo‐nojirimycin, and 1,6‐Dideoxy‐D‐gulo‐nojirimycin via Asymmetric Hetero‐Diels‐Alder Reactions
Authors: Defoin Albert , Sarazin Hervé , Streith Jacques 
Published: December 2004

AbstractAsymmetric Diels‐Alder reaction of sorbaldehyde O‐methyloxime 1d with chiral chloronitroso derivative 2 of D‐mannose, followed by osmylation of the primary cycloadduct, led to diol 6a with excellent enantioselectivity (ee > 99%; Scheme 1). Catalytic hydrogenolysis of 6a gave 1,6‐dideoxy‐D‐allo‐nojirimycin (7a). Nucleophilic ring opening of cyclic sulfate 8 allowed a straightforward synthesis of 1,6‐dideoxynojirimycin (11) and of 1,6‐dideoxy‐D‐gulo‐nojirimycin (12; Scheme 2).
Anion Cryptates: Synthesis, Crystal Structures, and Complexation Constants of Fluoride and Chloride Inclusion Complexes of Polyammonium Macrobicyclic Ligands
Authors: Dietrich Bernard , Dilworth Brid , Lehn Jean‐Marie , Souchez Jean‐Paul , Cesario Michèle , Guilhem Jean , Pascard Claudine 
Published: December 2004

AbstractThree macrobicyclic octamines 1–3 and the macrotricyclic hexadecamine 14 have been synthesized. The octamines 1–3 bind anionic substrates when protonated. The stability constants of the complexes between the protonated forms of the macrobicyclic polyamines and halide anions have been determined by pH‐metric measurements. The stability constants in H2O are very high; 1 in its hexaprotonated form binds F− with high selectivity (selectivity F−/Cl− > 108), while 3 exhibits strong stability constants for both F− and Cl−. Three X‐ray structures have been obtained, one where F− is held inside the cavity of 1 · 6H+, one where Cl− is included in 3 · 6H+, and 3 · 6H+ where the cavity is empty.
Perrhenat‐katalysierte Umlagerung von Ethinyl‐β‐ionol
Authors: Arnold Wolf , Bonrath Werner , Pernin Renaud , Thum Andreas 
Published: December 2004

Perrhenate‐Catalyzed Rearrangement of Ethinyl‐β‐ionolThe rearrangement of 2‐ethinyl‐β‐ionol (1) to α,β‐unsaturated carbonyl compounds using tetraalkylammonium perrhenate catalysts was studied. It was found that the Rupe‐Kambli product, ketone 2, is the main product (ca. 70%) of this rearrangement. The by‐product 4 (ca. 15%) is formed in a ring‐closure reaction.
The Active Centres of Agelastatin A, a Strongly Cytotoxic Alkaloid of the Coral Sea Axinellid Sponge Agelas dendromorpha, as Determined by Comparative Bioassays with Semisynthetic Derivatives
Authors: D'Ambrosio Michele , Guerriero Antonio , Pietra Francesco , Ripamonti Marina , Debitus Cécile , Waikedre Jean 
Published: December 2004

AbstractAgelastatin A (1), an unusual alkaloid of the axinellid sponge Agelas dendromorpha from the Coral Sea, can be selectively acetylated (→7) or methylated at OHC(8a) (→4), peracetylated (→8) or permethylated at OHC(8a), NH(5), and NH(6) (→5), or, finally, subjected to C(9)C(8a) (→14) or C(5b)C(8a) β‐elimination (→11–13), in a regiospecific manner or not, depending on the reaction conditions. Under acidic conditions, compound 12 adds H2O or MeOH, regioselectively though not endo/exo stereoselectively, giving transoid/cisoid mixtures 1/18 or 4/19, respectively. Similarly 11 or 13 add MeOH to give mixtures (−)‐2/20 or 15/16, respectively. Compound 13 also adds AcOH giving mixture 8/17. The intermediate cisoid form obtained on treatment of 21 with H3O+ undergoes N(5)N(6) bridging affording pentacyclic 22 which constitutes a proof for the cisoid configuration. From conformational studies, rules are devised that allow assigning the configuration of these compounds from NMR data. In vitro comparative cytotoxicity assays of these compounds show that for high cytotoxic activity, such as of 1 in vivo, unsubstituted OHC(8a), HN(5), HN(6) moieties are needed in the natural B/D transoid configuration.
Computations of 57Fe‐NMR Chemical Shifts with the SOS‐DFPT Method
Authors: Bühl Michael , Malkina Olga L. , Malkin Vladimir G. 
Published: December 2004

Abstract57Fe Shielding tensors of substituted iron‐carbonyl complexes have been computed employing the density‐functional‐based SOS‐DFPT method (sum‐over‐states density‐functional perturbation theory) with the IGLO (individual gauge for localized orbitals) choice of gauge origins and with large basis sets. The shieldings computed for [Fe(CO)5], [Fe(CO)3(H2CCHCHCH2)], [Fe(CO)3(cyclo‐C4H4)], [Fe(CO)4(H2CCHOMe)], [Fe(CO)4(H2CCHCN)], [Fe(CO)3(H2CCHCHO)], and [Fe(CO)2(C5H5)R] (R = Me, Bu, i‐Pr) correlate with the experimental δ(57Fe) values. However, the slope of the correlation line is 0.55 instead of 1, i.e., only about one half of the substituent effects on σ(Fe) is recovered in the calculations. Nearest‐neighbor effects appear to be described qualitatively (cf. in the [Fe(CO)2(C5H5)R] series, whereas effects of more remote substituents, e.g., for [Fe(CO)4(H2CCHX)] (X = MeO and CN)) are not reproduced. Dissociation energies of these species are discussed because of their relevance to experimental rate constants for substitution processes which are known to correlate with δ(57Fe). Even though the δ(13C) and δ(1H) data of ferrocene (7) are well reproduced theoretically, the computed σ(Fe) shielding of 7 deviates substantially from the σ(calc.)/δ(expt.) correlation, possibly indicating additional shortcomings in the theoretical description of this molecule.
Synthesis, and Solution and Solid‐State Structures of (η3‐Allyl){(4S)‐4‐benzyl‐2‐[2′‐(diphenylphosphino)phenyl]‐ 4,5‐dihydrooxazole‐P,N}palladium(II) Hexafluorophosphates. Comparison with Dichloro{(4S)‐2‐[2′‐(diphenylphosphino)phenyl]‐4,5‐dihydro‐4‐phenyloxazole‐P,N}zinc(II)
Authors: Baltzer Natalie , Macko Ludwig , Schaffner Silvia , Zehnder Margareta 
Published: December 2004

AbstractCrystal and solution structures of the [PdII(η3‐allyl)] and of the [PdII(η3‐1,3‐diphenylallyl)] complexes, 4 and 5, respectively, with (4S)‐4‐benzyl‐2‐[2′‐(diphenylphosphino)phenyl]‐4,5‐dihydrooxazole (3) were determined by X‐ray crystallography and 2D‐NMR spectroscopy. Complex 4 proved to be disordered with both diastereoisomeric complexes in the crystal. The results of X‐ray and NMR experiments demonstrate a good agreement between solution and solid‐state equilibria of the two isomers. A comparison with dichloro{(4S)‐2‐[2′‐(diphenylphosphino)phenyl]‐4,5‐dihydro‐4‐phenyloxazole‐P,N}zinc(II) (2b) shows a surprising conformational stability of the coordinated phosphinooxazole ligand 3.
An Analysis of the Bonding Properties of Benz[a]azulene by X‐Ray, NMR, and Computational Studies
Authors: Bühl Michael , Koźmiński Wiktor , Linden Anthony , Nanz Daniel , Sperandio David , Hansen Hans‐Jürgen 
Published: December 2004

AbstractThe X‐ray crystal structures of 9‐phenylbenz[a]azulene (4) and the corresponding non‐benzannelated form, 4‐phenylazulene (5), have been determined (cf. Fig.2). In contrast to 5, the skeleton of which shows nearly equal C,C bond lengths (cf. Table 1), the seven‐membered ring of 4 exhibits clearly alternating C,C bond lengths (cf. Table 1). This is in agreement with a strong accentuation of the heptafulvene substructure in 4 by the [a] benzannelation. The alternating bond lengths of 4 and of its parent structure 3 are also reflected in the corresponding variations of the 3J(H,H) and 1J(13C,13C) values of these benz[a]azulenes (cf. Tables 4 and 5). Computations on the MP2/6‐31G* level as well as on the BP86/6‐31G* level for azulene (6), benz[a]azulene (3), and heptafulvene (7) are in good agreement with the experimental values (cf. Tables 6–8).
Total Synthesis of (+)‐(8S,13R)‐Cyclocelabenzine
Authors: Schultz Katja , Hesse Manfred 
Published: December 2004

AbstractAn asymmetric synthesis of the spermidine alkaloid (+)‐cyclocelabenzine (1a) and its (−)‐(13S)‐epimer 1b is described using optically active (+)‐(3S)‐3‐amino‐3‐phenylpropionic acid as the chiral building block. The isoquinolin‐1‐one fragment 15 was synthesized by a modified Bischler‐Napieralski reaction. The relative configuration of the (−)‐isomer was determined by an X‐ray crystal‐structure analysis, which enabled us to determine the absolute configuration of natural (+)‐1a as (8S,13R).
Three‐Component Reactions with Sterically Crowded 2,2,4,4‐Tetramethyl‐3‐thioxocyclobutanone, Phenyl Azide, and Electron‐Deficient C,C‐Dipolarophiles
Authors: Mlostoń Grzegorz , Romańnski Jaroslaw , Linden Anthony , Heimgartner Heinz 
Published: December 2004

AbstractIn order to trap ‘thiocarbonyl‐aminides’ A, formed as intermediates in the reaction of thiocarbonyl compounds with phenyl azide, a mixture of 2,2,4,4‐tetramethyl‐3‐thioxocyclobutanone (1), phenyl azide, and fumarodinitrile (8) was heated to 80° until evolution of N2 ceased. Two interception products of the ‘thiocarbonylaminide’ A (ArPh) were formed: the known 1,4,2‐dithiazolidine 3 (cf. Scheme 1) and the new 1,2‐thiazolidine 12 (Scheme 2). The structure of the latter was established by X‐ray crystallography (Fig.1). The analogous ‘three‐component reaction’ with dimethyl fumarate (9) yielded, instead of 8, in addition to the known interception products 3 and 6 (Scheme 1), two unexpected products 15 and 16 (Scheme 3), of which the structures were elucidated by X‐ray crystallography (Fig.2). Their formation is rationalized by a primary [2 + 3] cycloaddition of diazo compound 18 with 1 to give 19, followed by a cascade of further reactions (Scheme 4).
Synthesis, Conformational Properties, and Synthetic Applications of Novel Optically Pure α,α‐Disubstituted (R)‐ and (S)‐Glycines (‘α‐Chimeras’) Combining Side Chains of Asp, Glu, Leu, Phe, Ser, and Val
Authors: Obrecht Daniel , Abrecht Christine , Altorfer Michael , Bohdal Udo , Grieder Alfred , Kleber Martina , Pfyffer Patrick , Müller Klaus 
Published: December 2004

AbstractA series of novel open‐chain and cyclic conformationally constrained α,α‐disubstituted (R)‐ and (S)‐glycine derivatives (‘α‐chimeras’) combining side chains of Asp, Glu, Leu, Phe, Ser, and Val have been efficiently synthesized by using α‐alkylation of racemic 4‐monosubstituted 2‐phenyl‐1,3‐oxazol‐5(4H)‐ones of type 5, resolution after reaction with (S)‐phenylalanine cyclohexylamide (8) as chiral auxiliary, a novel azlactone/dihydrooxazole interconversion reaction to synthesize optically pure α‐substituted (R)‐ and (S)‐serine derivatives coupled with succinimide‐ring formation of aspartic‐acid derivatives. Based on X‐ray structures of (R,S)‐9b, (R,S)‐11c, (R,S)‐18, and (S,S)‐30, the absolute configuration of these novel amino‐acid building blocks could be unambiguously determined and their preferred conformations in the crystalline state be assessed. The high preference of the open‐chain derivatives (R,S)‐1, (S,S)‐3, and (R,S)‐11c for β‐turn type‐I conformations, as well as of the succinimide derivatives (R,S)‐2, (S,S)‐19, (S,S)‐24, (S,S,S)‐26, and (R,S)‐29 for β‐turn type‐II conformations and of (S,S)‐4, (R,S)‐18, (R,S)‐23, and (S,S)‐30 for β‐turn type‐II′ conformations could be confirmed in solution by using CD and NMR spectroscopy. Finally, the spiro derivatives (R,S)‐29 and (S,S)‐30 incorporating the ‘α‐chimera’ of Asp/Glu constitute doubly constrained peptide building blocks combining the properties of α‐substituted prolines and aspartimides.