bluuebunny/crossref_metadata_2025_split huggingface dataset:
Absolute Configuration of 3‐Substituted 1‐Azabicyclo[2.2.1]heptanes
Authors: Boelsterli Jakob , Eggnauer Ursula , Pombo‐Villar Esteban , Weber Hans‐Peter , Walkinshaw Malcolm , Gould Robert O. 
Published: December 2004

AbstractThe l‐azabicyclo[2.2.1]heptan‐3‐exo‐ol (2) was resolved by fractional crystallisation of its hydrogen tartrate salts. The enantiomers (+)‐ and (−)‐2 were oxidised to the ketones (−)‐4 and (+)‐4, respectively (Scheme). CD spectroscopy suggested that (−)‐4 possesses the (1R,4S)‐configuration. This absolute configuration was confirmed by single‐crystal X‐ray diffraction of the derivative (+)‐(1R,4R)‐3‐(1,3‐dithian‐2‐ylidene)‐1‐azabicyclo [2.2.1]‐heptane ((+)‐5).
Synthesis of Multi‐O4‐phospho‐L‐tyrosine‐Containing Peptides
Authors: Bannwarth Willi , Kitas Eric A. 
Published: December 2004

AbstractMulti‐O4‐phospho‐L‐tyrosine‐containing peptides can be synthesized by the global as well as the buildingblock approach. Thus, we prepared by both strategies the triphosphorylated and the three regioisomeric diphosphorylated insulin‐receptor‐(1142–1153)‐dodecapeptide derivatives 7 and 4–6, respectively, of the parent Thr‐Arg‐Asp‐Ile‐Tyr‐Glu‐Thr‐Asp‐Tyr‐Tyr‐Arg‐Lys (3). These phosphorylated peptides are valuable tools to study the regioselectivity of dephosphorylation by phosphatases.
Stereoselektive Synthese von (2S,6S)‐2,6‐Diaminoheptandisäure und von unsymmetrischen Derivaten der meso‐2,6‐Diaminoheptandisäure
Authors: Bold Guido , Allmendinger Thomas , Herold Peter , Moesch Luzia , Schär Hans‐Peter , Duthaler Rudolf O. 
Published: December 2004

Stereoselective Synthesis of (2S,6S)‐2,6‐Diaminoheptanedioic Acid and of Unsymmetrical Derivatives of meso‐2,6‐Diaminoheptanedioic AcidSpecific inhibition of enzymes of the diaminopimelate pathway (L‐lysine biosynthesis) should, in principle, lead to selective antibacterial agents or herbicides. For this purpose, enantioselective syntheses were devised for (2S, 6S)‐2, 6‐diaminoheptanedioic acid (L, L‐diaminopimelate, 1), (2R,6S)‐2,6‐diamino‐2‐methylhept‐3‐enedioic acid (10), (2R, 6S)‐2, 6‐diaminohept‐3‐enedioic acid (9), (2R, 6S)‐2, 6‐diamino‐4‐fluorohept‐3‐enedioic acid (42), and (2S, 6S)‐2, 6‐diamino‐3‐chloroheptanedioic acid (5). The Schöllkopf bislactim‐ether methodology was applied to control the configuration of C(2) and C(6) of 1, C(2) of 10, as well as C(6) of 9 and 42. Semialdehyde derivatives of L‐glutamate afforded C(6) of 10 and 5, while the (R)‐configurated C(2) of 9 and 42 were derived from L‐serine. For this purpose, the synthesis of the Garner aldehyde 32 has been improved. As chromatographic purifications and the low temperatures for the reduction of the carboxylic acid are eliminated, this valuable intermediate can now be prepared in bulk quantities. An enantio‐ and diastereoselective aldol addition of a glycine titanium‐enolate was applied for the construction of 5 (C(2) and C(3)). As all chiral building blocks and reagents used are available in both enantiomeric forms, these routes should also be suitable for the selective synthesis of the other stereoisomers of these bis(α‐amino acids).
The 6Li,6Li‐INADEQUATE Experiment: A New Tool for 1D‐ and 2D‐NMR Studies of Organolithium Compounds
Authors: Eppers Oswald , Fox Thomas , Günther Harald 
Published: December 2004

AbstractThe 1D‐ and 2D‐6Li, 6Li‐INADEQUATE experiments are described as new tools for the detection of scalar coupled nonequivalent 6Li nuclei in organolithium clusters. Practical applications of these sequences are demonstrated for the 6Li‐NMR spectra of (E)‐1‐lithio‐2‐(2‐lithiophenyl)‐1‐phenylhex‐1‐ene (1) and (E)‐2‐lithio‐1‐(2‐lithiophenyl)‐1‐phenylpent‐1‐ene (2), where signals due to dimers and monomers can be distinguished. The performance of the 2D‐6Li, 6Li‐INADEQUATE and the 6Li, 6Li‐COSY‐45‐LR experiment are compared. The 6Li chemical shifts of 1 and 2 are discussed.
Synthese von 2‐substituierten und 2,3‐disubstituierten Alkyl‐ und Aryl‐thiophenen und 2,3‐anellierten Thiophen‐Derivaten aus Ketonen als C2‐Bausteine und Carbonodithiosäure‐O‐ethyl‐S‐(2‐oxoethyl)ester als C2S‐Baustein
Authors: Aldvogel Erwin 
Published: December 2004

Synthesis of 2‐Substituted and 2,3‐Disubstituted Alkyl‐ und Aryl‐thiophenes and Related 2,3‐Anellated Thiophene Derivatives, Using Ketones and Carbonodithioic Acid O‐Ethyl S‐(2‐Oxoethyl) Ester as the Building BlocksA simple method to synthesize 2‐substituted and 2,3‐disubstituted alkyl‐ and aryl‐thiophenes as well as related 2,3‐anellated thiophenes 6, starting from ketones 1 as C2‐fragment and the S‐protected carbonodithionic‐acid derivative 3 as a C2S‐fragment is described. The aldols 4 are intermediates of the two step process.
Structure and Reactivity of Five‐ and Six‐Ring N, N‐, N, O‐, and O, O‐acetals: A lesson in allylic 1, 3‐strain (A1, 3strain)
Authors: Seebach Dieter , Amatsch Bernd , Amstutz René , Beck Albert K. , Doler Max , Egli Martin , Fitzi Robert , Gautschi Markus , Herradön Bernardo , Hidber Pirmin C. , Irwin John J. , Locher Rita , Maestro Miguel , Maetzke Thomas , Mouriño Antoni , Pfammatter Elmar , Plattner Dietma A. , Schickli Christof , Schweizer W. Bernd , Seiler Paul , Stucky Gerhar , Petter Walter , Escalante Jaime , Juaristi Eusebio , Quintana Delia , Miravitlles Carlos , Molins Elies 
Published: December 2004

AbstractThe X‐ray structures of fifteen 1, 3‐imidazolidine, 1, 3‐oxazolidine, 1, 3‐dioxan‐4‐one, and hydropyrimidine‐4(1H)‐one derivatives are described (Table 2) and compared with known structures of similar compounds (Figs. 1–20). The differences between structures containing exocyclicN‐acyl groups and those lacking this structural element arise from the A1,3effect of the amide moieties. Event‐Bu groups are forced into axial positions of six‐ring half‐chair or into flag‐pole positions of six‐ring twist‐boat conformers by this effect (Figs. 16–20). In theN‐acylated five‐membered heterocycles, a combination of ring strain and A1, 3strain leads to strong pyramidalizations of the amide N‐atoms (Table 1) such that the acyl groups wind up on one side and the other substituents on the opposite side of the rings (Figs. 4–9andScheme 3). Thus, the acyl (protecting!) groups strongly contribute to the steric bias between the two faces of the rings. Observed, at first glance surprizing stereoselectivities of reactions of these heterocycles (Schemes 1and2) are interpreted (Scheme 3) as an indirect consequence of the amide A1, 3strain effect. The conclusions drawn are considered relvant for a better understanding of the ever increasing role which amide groups play in stereoselective syntheses.
Preparation of Campholenal Analogues: Chirons for the lipophilic moiety of sandalwood‐like odorant alcohols
Authors: Chapuis Christian , Brauchli Robert 
Published: December 2004

AbstractIn connection with structure‐activity relationship studies, analogues of campholenal ((+)‐4b), an important building block for sandalwood‐like odorants, were prepared. The five‐membered‐ring analogues 4 were obtained by epoxidation of the corresponding α‐pinene derivatives 2, followed by catalytic ZnBr2 isomerisation (Scheme 2). The six‐membered‐ring skeleton was obtained by ozonolysis of α‐campholenyl acetate ((−)‐14b), followed by intramolecular aldol condensation (Scheme 5). 13C‐NMR assignments are given.
New Heteroaromatic Complexing Agents and Luminescence of Their Europium(III) and Terbium(III) Chelates
Authors: Mukkala Veli‐Matti , Sund Christian , Kwiatkowski Marek , Pasanen Paavo , Högberg Marita , Kankare Jouko , Takalo Harri 
Published: December 2004

AbstractTwelve heteroaromatic complexing agents 9a–I were synthesized with the purpose to develop suitable labels for time‐resolved luminescence‐based bioaffinity assays. The relative luminescence yields, excitation maxima, and emission decay constants of their europium(III) and terbium(III) chelates were determined. According to these results, 2,2′,2″,2‴‐[(2,2′‐bipyridine‐6,6′‐diyl)bis(methylenenitrilo)]tetrakis (acetic acid) (9e) and 2,2′,2″,2‴‐[(2,2′:6′,2″‐terpyridine‐6,6″‐diyl)bis(methylenenitrilo)] tetrakis(acetic acid) (91) are the most promising agents.
Conformational Analysis of 1,2:3,4‐Diepoxides: Ab Initio and semiempirical molecular‐orbital calculations
Authors: Nikles Martin , Bur Daniel , Séquin Urs , Huber Hanspeter 
Published: December 2004

AbstractUsing semiempirical and ab initio procedures, the most stable conformations of meso‐ and rac‐bioxirane and of some substituted 1,2:3,4‐diepoxides were calculated. For threo‐diepoxides (having the same relative configurations as rac‐bioxirane, 3), two stable conformations with CCCC dihedral angles of ca. 90 and ca. 270° were found. For erythro‐diepoxides (derivatives of meso‐bioxirane, 4) the calculations suggest three preferred conformations with corresponding dihedral CCCC angles of ca. 90°, ca. 180°, and ca. 270°. The calculations are in fair agreement with the experimental data available for the unsubstituted compounds 3 and 4.
A Complex Pyrrolo‐oxazinone and Its Iodo Derivative Isolated from a Tunicate
Authors: Yoshida Wesley Y. , Lee Kit K. , Carroll Anthony R. , Scheuer Paul J. 
Published: December 2004

AbstractTwo cytotoxic triphenylpyrrolo‐oxazinones were isolated from a tunicate and their structures elucidated by spectral methods. Lukianol A ( = 3,7,8‐tris(4‐hydroxyphenyl)pyrrolo[2,1‐c][1,4]oxazin‐1 (1H)‐one; 1) had MIC of 1 μg/ml in KB cytotoxicity tests; MIC value for 2 was 100 μg/ml.
Light‐Induced Cycloaddition of Furan and addition of methanol to a 4‐thiacyclohex‐2‐enone ( = 2,3‐dihydro‐4H‐thiin‐4‐one) and a 4‐thiacyclopent‐2‐enone ( = thiophen‐3(2H)‐one)
Authors: Kowalewski Ronald , Margaretha Paul 
Published: December 2004

AbstractIrradiation of newly synthesized 2,2‐dimethyl‐2,3‐dihydro‐4H‐thiin‐4‐one (1) in furan affords the two [4 + 2] cycloadducts 3 and 4 and the [2 + 2] cycloadduct 5 in a 5:4:1 ratio (Scheme 1). Irradiation of 1 in MeOH gives a 3:2 mixture of 5‐ and 6‐methoxy‐2,2‐dimethylthian‐4‐ones 6 and 7. Irradiation in CD3OD affords the same (deuterated) adducts with the CD3O and D groups trans to each other, results compatible with cis‐addition of MeOH to a trans ‐configurated ground‐state enone. Irradiation of the same enone in furan/MeOH 1:1 gives only the furan cycloadducts 3–5 and no MeOH adducts, suggesting that furan interacts with the (excited) triplet enone before the deactivation of this species to a ground‐state (E)‐cyclohexenone, which then reacts with MeOH. On irradiation in furan, the corresponding five‐membered thiaenone, 2,2‐dimethylthiophen‐3(2H)‐one (2) affords only one, cis‐fused, [4 + 2] cycloadduct with ‘exo’‐configuration, i.e. 8, and 2 does not undergo solvent addition but rather cyclodimerization (→9) on irradiation in MeOH (Scheme 1).
Thermal Reaction of Highly Alkylated Azulenes with Dimethyl Acetylenedicarboxylate: HOMO(Azulene) vs. SHOMO(Azulene) Control in the Primary Thermal Addition Step
Authors: Chen Yi , Kunz Roland W. , Uebelhart Peter , Weber Roland H. , Hansen Hans‐JüRgen 
Published: December 2004

AbstractThe reaction of highly alkylated azulenes with dimethyl acetylenedicarboxylate (ADM) in decalin or tetralin at 180–200° yields, beside the expected heptalene‐ and azulene‐1,2‐dicarboxylates, tetracyclic compounds of type ‘anti’‐V and tricyclic compounds of type E (cf. Schemes 2–4 and 8–11). The compounds of type ‘anti’‐V represent Diels‐Alder adducts of the primary tricyclic intermediates A with ADM. In some cases, the tricyclic compounds of type E also underwent a consecutive Diels‐Alder reaction with ADM to yield the tetracyclic compounds of type ‘anti’‐ or ‘syn’‐VI (cf. Schemes 2 and 8–11). The tricyclic compounds of type E, namely 4 and 8, reversibly rearrange via [1,5]‐C shifts to isomeric tricyclic structures (cf. 18 and 19, respectively, in Scheme 6) already at temperatures > 50°. Photochemically 4 rearranges to a corresponding tetracyclic compound 20 via a di‐π‐methane reaction. The observed heptalene‐ and azulene‐1,2‐dicarboxylates as well as the tetracyclic compounds of type ‘anti’'‐V are formed from the primary tricyclic intermediates A via rearrangement (→heptalenedicarboxylates), retro‐Diels‐Alder reaction (→ azulenedicarboxylates), and Diels‐Alder reaction with ADM. The different reaction channels of A are dependent on the substituents. However, the main reaction channel of A is its retro‐Diels‐Alder reaction to the starting materials (azulene and ADM). The highly reversible Diels‐Alder reaction of ADM to the five‐membered ring of the azulenes is HOMO(azulene)/LUMO(ADM)‐controlled, in contrast to the at 200° irreversible ADM addition to the seven‐membered ring of the azulenes to yield the Diels‐Alder products of type E. This competing reaction must occur on grounds of orbital‐symmetry conservation under SHOMO(azulene)/LUMO(ADM) control (cf. Schemes 20–22). Several X‐ray diffraction analyses of the products were performed (cf. Chapt. 4.1).
Thermal Reactions of Guaiazulene and Its 3‐Methyl Derivative with Dimethyl Acetylenedicarboxylate
Authors: Uebelhart Peter , Hansen Hans‐Jürgen 
Published: December 2004

AbstractThe thermal reaction of 7‐isopropyl‐1,3,4‐trimethylazulene (3‐methylguaiazulene; 2) with excess dimethyl acetylenedicarboxylate (ADM) in decalin at 200° leads to the formation of the corresponding heptalene‐ (5a/5b and 6a/6b; cf. Scheme 3) and azulene‐1,2‐dicarboxylates (7 and 8, respectively). Together with small amounts of a corresponding tetracyclic compound (‘anti’‐13) these compounds are obtained via rearrangement (→ 5a/5b and 6a/6b), retro‐Diels‐Alder reaction (→ 7 and 8), and Diels‐Alder reaction with ADM (→ ‘anti’‐13) from the two primary tricyclic intermediates (14 and 15; cf. Scheme 5) which are formed by site‐selective addition of ADM to the five‐membered ring of 2. In a competing Diels‐Alder reaction, ADM is also added to the seven‐membered ring of 2, leading to the formation of the tricyclic compounds 9 and 10 and of the Diels‐Alder adducts ‘anti’‐11 and ‘anti’‐12, respectively of 9 and of a third tricyclic intermediate 16 which is at 200° in thermal equilibrium with 9 and 10 (cf. Scheme 6). The heptalenedicarboxylates 5a and 5b as well as 6a and 6b are interconverting slowly already at ambient temperature (Scheme 4). The thermal reaction of guaiazulene (1) with excess ADM in decalin at 190° leads alongside with the known heptalene‐ (3a) and azulene‐1,2‐dicarboxylates (4; cf. Schemes 2 and 7) to the formation of six tetracyclic compounds ‘anti’‐17 to ‘anti’‐21 as well as ‘syn’‐19 and small amounts of a 4:1 mixture of the tricyclic tetracarboxylates 22 and 23. The structure of the tetracyclic compounds can be traced back by a retro‐Diels‐Alder reaction to the corresponding structures of tricyclic compounds (24‐‐29; cf. Scheme 8) which are thermally interconverting by [1,5]‐C shifts at 190°. The tricyclic tetracarboxylates 22 and 23, which are slowly equilibrating already at ambient temperature, are formed by thermal addition of ADM to the seven‐membered ring of dimethyl 5‐isopropyl‐3,8‐dimethylazulene‐1,2‐dicarboxylate (7; cf. Scheme 10). Azulene 7 which is electronically deactivated by the two MeOCO groups at C(1) and C(2) shows no more thermal reactivity in the presence of ADM at the five‐membered ring (cf. Scheme 11). The tricyclic tetracarboxylates 22 and 23 react with excess ADM at 200° in a slow Diels‐Alder reaction to form the tetracyclic hexacarboxylates 32, ‘anti’‐33, and ‘anti’‐34 (cf. Schemes 10–12 as well as Scheme 13). A structural correlation of the tri‐ and tetracyclic compounds is only feasible if thermal equilibration via [1,5]‐C shifts between all six possible tricyclic tetracarboxylates (22, 23, and 35–38; cf. Scheme 13) is assumed. The tetracyclic hexacarboxylates 32, ‘anti’‐33, and ‘anti’‐34 seem to arise from the most strained tricyclic intermediates (36–38) by the Diels‐Alder reaction with ADM.
Non‐destructive Cleavage of N‐Acylsultams Under Neutral Conditions: Preparation of Enantiomerically, Pure Fmoc‐Protected α‐Amino Acids
Authors: Oppolzer Wolfgang , Lienard Phillippe 
Published: December 2004

AbstractHeating diastereoisomerically pure N‐acylsultams 3 or 4 with allyl alcohol/Ti(OR)4 efficiently yields sultams 1 or 2 and allyl esters 5. Esters 5 are hydrolyzed under nonbasic conditions in the presence of Wilkinson's catalyst to give enantiomerically and diastereoisomerically pure carboxylic acids 7. A series of [(fluoren‐9‐yl)methoxy]‐carbonyl‐(Fmoc)‐protected amino acids 14 were thus prepared from N‐[N'‐(Fmoc)amino]acylsultams 12.
Glycosylidene Carbenes. Part 10. Regioselective Glycosidation of 4,6‐O‐Benzylidene‐D‐altropyranosides
Authors: Bozó Eva , Vasella Andrea 
Published: December 2004

AbstractGlycosidation by the diazirine 1, the trichloroacetimidate 4, and the bromide 5 of the altro‐diol 2, possessing an intramolecular H‐bond (HOC(3) to OC(1)) in solution, but not in the solid state, proceeds with high and complementary regioselectivity. From 2 and 1, one obtains mostly the 1,2‐linked disaccharides 10 and 11 (β‐D > α‐D), together with the 1,3‐linked isomers 12 and 13 (α‐D > β‐D; 1,2‐/1,3‐linked products ca. 9:1), the demethylated 1,3‐linked disaccharides 24–27, the trisaccharides 19–22, the lactone azines 23, and the hydroxyglucal 18, while 2 reacted with 4 or 5 to yield mostly the 1,3‐linked disaccharides (1,2‐/1,3‐linked products ca. 1:9). The disaccharides were additionally characterized as acetates (→ 14–17, 28–31). Yields and stereoselectivity depended upon the donor, stoichiometry, solvent, temperature, and concentration. Glycosidation of the 1,3‐linked disaccharides with 1 yielded the trisaccharides 19–22. Reaction of the β‐D‐altro‐diol 3 with 1 gave the 1,2‐ and 1,3‐linked disaccharides 32/33 and 34/35 in a 1:1 ratio, characterized as the acetates 36–39, while glycosidation with 5 according to Lemieux proceeded regioselectively (1,2‐/1,3‐linked products 91:9). The monotosylates 6 and 7 reacted with 1 to yield the anomeric pairs 40/41, and 42/43 of the tosylated disaccharides; the oxiranes 44 and 45 were not observed.
Interactions between Functional Groups Part IV. The responses of four diazonium groups to adjacent electron‐rich atoms in peri‐substituted naphthalene and quinoline derivatives
Authors: Wallis John D. , Easton Robert J. C. , Dunitz Jack D. 
Published: December 2004

AbstractFour peri‐substituted naphthalene‐1‐diazonium cations all show short attractive interactions between an electron‐rich atom of the peri‐substituent and the α ‐N‐atom of the diazonium group. These are interpreted as models for incipient nucleophilic attack on a NN bond. The diazonium group is a better acceptor of electron density ‘through space’ than the NO2 group, which parallels their relative ‘through‐σ‐bond’ inductive effects.
Enantioselective Synthesis of 3‐ Azabicyclo[4.3.0]nonane Alkaloids
Authors: Cid Maria Magdalena , Pombo‐Villar Esteban 
Published: December 2004

AbstractThe stereospecific synthesis of the monoterpene alkaloids (−)‐α‐skytanthine ((−)‐2), (−)‐N ‐demethyle‐δ‐sky‐tanthine((−)‐7), and (+)‐epidihydrotecomanine (+)‐4 was achieved from a common intermediate 22, which in turn was obtained from (1R,4S,1′S)‐2‐(1′‐phenylethyl)‐2‐azabicyclo[2.2.1]hept‐5‐ene (10),via a ketene aza‐Claisen rearrangement. The piperidine derivative (+)‐31, formally the aza‐analogue of (+)‐isoiridomyrmecin, was also obtained from the same intermediate 22.
Umsetzung von 1,3‐Thiazol‐5(4H)‐thionen mit Diazomethan
Authors: Kägi Martin , Linden Anthony , Heimgartner Heinz , Mlostoń Grzegorz 
Published: December 2004

Reaction of 1,3‐Thaiazole‐5(4H)‐thiones and DiazometaneReaction of 4,4‐dimethyl‐1,3‐thiazol‐5(4H)‐thiones 1 with diazomethane in Et2O at −78° or 0° leads to a mixture of the corresponding 1,4‐dithiane 10,1,3‐dithiole 11, thiirane 12, 4,5‐dihydro‐5methylidene‐1,3‐thiazole 13, and 1,3‐thiazol‐5(4H)‐one 14 (Scheme 3). The structures of 10a, 11a, and 11bhave been established by X‐ray crystallography. The formation of the products can be explained via an intermediate thiocaronyl ylide of type E (Scheme 4), generated by the 1.3‐dipolar cycloaddition of diazomethane with the exocyclic CS bond of 1 and elimination of N2. Head‐to‐head dimerization of E yields 10, 1, 3‐dipolar cycloaddition of E and 1 gives the ‘Schönberg product’ 11, and cyclization of E leads to 12, which undergoes a desulfurization to give 13. The thiazolone 141 is formed by hydolysis of E. The similarity of the ratio of the products at −78° and at 0° shows that, in contrast to the reaction of 1 and 2‐diazopropane, the elimination of N2 in the primary cycloadduct already occurs at −78°.
Identification of The Tocopherol‐Cyclase in the Blue‐Green Algae Anabaena variabilis KÜTZING (Cyanobacteria)
Authors: Stocker Achim , Woggon Wolf‐Dietrich , Rüttimann August 
Published: December 2004

AbstractIncubation of Spheroplats of Anabaena variabilis (Cyanobacteria) with 22 the 2,6‐O‐dimetyl‐ß‐cycoldextrin complex of 2,3‐dimethyl‐5‐phytylhydroquinone (11), revealed the presence of a hitherto unknown enzyme which catalyzes the cyclization of th substrate 11 to give enantiomerically pure γ‐tocopherol (3) in ⩾ 93% yield.
Chiral Acylsilanes in Organic Synthesis. Part 2. The role of the solvent, the organometallic reagent, and the nature of the substrate for the diastereoselectivity of 1,2‐additions to racemic alkoxymethyl‐substituted acylsilanes
Authors: Chapeaurouge Alexander , Bienz Stefan 
Published: December 2004

AbstractThe role of the solvent, the organometallic reagent, and the nature of the substrate for the diastereoselectivity of 1,2‐additions to racemic alkoxymethyl‐substituted acylsilanes was investigated with the acylsilanes 1a–d by variation of the reaction parameters. The results obtained in this study support strongly the previously proposed preferred ‘chelate‐controlled’ reaction path followed under several reaction conditions: highest stereoselectivities were obtained with the best chelating substrates reacting with the most Lewis‐acidic organometallic reagents in the least donating solvents. It is shown that almost complete stereoselectivity can be obtained using optimal reaction conditions.
Magnetic‐Field‐Dependent Electronic Relaxation of Gd3+ in Aqueous Solutions of the Complexes [Gd(H2O)8]3+, [Gd(propane‐1,3‐diamine‐N,N,N′,N′‐tetraacetate)(H2O)2]−, and [Gd(N,N′‐bis[(N‐methylcarbamoyl)methyl]‐3‐azapentane‐1,5‐diamine‐3,N,N′‐triacetate)(H2O)] of interest in magnetic‐resonance imaging
Authors: Powell D. Hugh , Merbach André E. , González Gabriel , Brücher Ernö , Micskei Károly , Ottaviani M. Francesca , Köhler Klaus , Von Zelewsky Alex , Grinberg Oleg Ya. , Lebedev Yakob S. 
Published: December 2004

AbstractEPR Spectra have been measured for aqueous solutions of a series of Gd3+ complexes at variable temperature and a range of magnetic fields; S‐band (0.14 T), X‐band (0.34 T), Q‐band (1.2 T), and 2‐mm‐band (5.0 T). The major contribution to the observed line widths is magnetic‐field‐dependent and is interpreted as being due to the modulation of the zero‐field splitting produced by distortion of the complexes from perfect symmetry. The transverse and longitudinal relaxation matrices for an 8S ion with such an interaction have been calculated using Redfield theory with vector‐coupling methods, and diagonalised numerically to obtain relaxation rates and intensities for the degenerate transitions which contribute to the multiplet. The observed line width, which is inversely proportional to the magnetic field at low temperatures, is best described by the intensity‐weighted mean transverse relaxation time for the four transitions with non‐zero intensity. A least‐squares fit of the data yields the square of the zero‐field splitting tensor, Δ2, and a correlation time, τv, with activation energy, Ev. The physical significance of these parameters and the extent of validity of the theoretical approach are considered. The parameters are used to predict the magnetic‐field dependence of the longitudinal and transverse electronic relaxation times, which are discussed in the context of their relevance to 1H‐NMR relaxivity.