bluuebunny/crossref_metadata_2025 huggingface dataset:
Structure‐Lipophilicity and Structure‐Polarity relationships of amino acids and peptides
Authors: Vallat Philippe , Gaillard Patrick , Carrupt Pierre‐Alain , Tsai Ruey‐Shiuan , Testa Bernard
Published: December 2004

The objectives of this study were to gain insights into the structure‐lipophilicity relationships of peptides and to propose an improved model for estimating their lipophilicity. First, existing databases were extended to obtain the distribution coefficients of a total of 208 free or protected peptides (di‐ to pentapeptides). The polarity parameters (Λ) of 23 free amino acids and 19 protected amino acids (AcNHCHRCONH2) and of their side chains were calculated from experimental distribution coefficients and computed molecular volumes. An analysis of the polarity parameters revealed that the hydrophobicity of the amino‐acid side chains is largely reduced due to the polar field of the backbone. The polarity parameters of the peptides were then obtained in a similar manner and shown to be highly correlated with the sum of the polarity parameters of their side chains, i.e., the lipophilicity of peptides can be calculated from their molecular volume and the sum of their side‐chain polarities using the regression established for each individual series of peptides (Fig. 1). This last restriction is essential since the polarity and lipophilic increment of a NHC*HCO unit were shown to decrease with increasing length of backbone.
Oligosaccharides related to tumor‐associate antigens. Part 3. Synthesis of the propyl glycosides of the trisaccharide β‐D‐Galp‐(1→3)‐β‐D‐GalpNAc‐(1→3)‐α‐D‐Galp and of the Tetrasaccharide α‐L‐Fucp‐(1→2)‐β‐D‐Galp‐(1→3)‐β‐D‐GalpNAc‐(1→3)‐α‐D‐Galp, components of a tumor antigen recognized by the antibody MBr1
Authors: Lay Luigi , Panza Luigi , Russo Giovanni , Colombo Diego , Ronchetti Fiamma , Adobati Elena , Canevari Silvana
Published: December 2004

The synthesis of the trisaccharide β‐D‐Galp‐(1→3)‐β‐D‐GalpNAc‐(1→3)‐α‐D‐Galp‐1‐OPr (2) and of the tetrasaccharide α‐L‐Fucp‐(1→2)‐β‐D‐GalpNAc‐(1→3)‐β‐D‐Galp‐1‐OPr (3), starting from the disaccharidic dihydrooxazole donor 5, is described. Glycosylation of 5 with 6 in the presence of Me3SiOTf gave the trisaccharide 7 which was deprotected with standard methods to give, via 8, compound 2 (Scheme 1). Alternatively, protection of 8 as the 4′,6′‐O‐benzylidene derivative 9 followed by glycosylation with 10 and by standard deprotection afforded the tetrasaccharide 3 (Scheme 2). Biological testing showed that trisaccharide 2 is unable to inhibit the binding of the monoclonal antibody MBr1 to the target tumor cells MCF7, while tetrasaccharide 3 inhibits the binding in ca. 7‐fold extent with respect to the previously tested trisaccharide α‐L‐Fucp‐(1→2)‐β‐D‐Galp‐(1→3)‐β‐D‐GalpNAc‐1‐OPr. These results indicate that the galactose corresponding to the unit D of compound 1 plays an important role in defining the MBr1‐recognized epitope and confirm the essential role of fucose for MAb recognition.
Oligosaccharides Related to Tumor‐Associate Antigens. Part 4. Modeling of the terminal tetrasaccharide of an antigen recognized by the MBr1 antibody and of overlapping Di‐ and trisaccharide sequences
Authors: Toma Lucio , Colombo Diego , Ronchetti Fiamma , Panza Luigi , Russo Giovanni
Published: December 2004

The conformational space of the tetrasaccharide α‐L‐Fuc‐(1→2)‐β‐D‐Gal‐(1→3)‐β‐D‐GalNAc‐(1→3)‐α‐D‐GallPr (3) and of some overlapping di‐ and trisaccharide sequences was investigated with the aid of molecular‐mechanics energy minimizations, molecular‐dynamics simulations, and 1H‐NMR analysis. These investigations suggested that in compound 3 a certain rigidity of the first two glycosidic linkages (Fuc‐Gal and Gal‐GalNAc) is combined with the flexibility of the third one (GalNAc‐Gal).
Synthesis, Structure, and Antimalarial Activity of Some Enantiomerically Pure, cis‐fused cyclopenteno‐1,2,4‐trioxanes
Authors: Jefford Charles W. , Kohmoto Shigeo , Jaggi Danielle , Timári Géza , Rossier Jean‐Claude , Rudaz Manyck , Barbuzzi Olivier , Gérard David , Burger Ulrich , Kamalaprija Philippe , Mareda Jiri , Bernardinelli Gérald , Manzanares Ignacio , Canfield Craig J. , Feck Suzanne L. , Robinson Brain L. , Peters Wallace
Published: December 2004

Two pairs of enantiomerically pure cis‐fused cyclopenteno‐1,2,4‐trioxanes (7, ent‐7 and 8, ent‐8) are prepared (Schemes 1–3). Their identities are established by dye‐sensitized photo‐oxygenation of ent‐7 and 8, ent‐8 to the allylichydroperxides, reduction to the corresponding alcohols, and conversion to the (1S)‐camphanates (Scheme 4), the structures of which are determined by X‐ray analysis. The dynamic properties of ent‐7 are investigated by NMR spectroscopy and PM3 calculations. Evidence for an easily accessible twist‐boat conformation is obtained. The in vitro and in vivo antimalarial activities of 7, ent‐7,8, and ent‐8 as well as those of the racemic mixtures are evaluated against Plasmodium falciparum, P. berghei, and P. yoelii. No correlation is observed between configuration and activity. Racemates and pure enantiomers have commensurate activities. The mode of action on the intraerythrocytic parasite is rationalized in terms of close docking by the twist‐boat conformer of the trioxane on the surface of a molecule of heme, single‐electron transfer to the OO σ* orbital, and scission to the acetal radical which then irreversibly isomerizes to a C‐centered radical, the ultimate lethal agent (Scheme 5).
Synthesis of (±)‐Pyrenolide B
Authors: Moricz Attila , Gassmann Elisabeth , Bienz Stefan , Hesse Manfred
Published: December 2004

In the synthesis of the title compound 12, the important intermediate 7 was obtained in good yield from the easily available ethyl 5, 5‐ethylenedioxy‐2‐oxocyclohexane‐1‐carboxylate (1) via ring enlargement of the bicyclic enol ether 5 (Scheme). Its reduction (NaBH4 in EtOH) and subsequent protection with (t‐Bu)Me2Si resulted in the highly functionalized ten‐membered lactone 9. Introduction of the (Z)‐configurated double bond, followed by deprotection and elimination of H2O, gave (±)‐pyrenolide B (12) in 16% overall yield.
O‐(1‐Phenyl‐1H‐tetrazol‐5‐yl) Glycosides: Alternative synthesis and transformation into glycosyl fluorides
Authors: Palme Monica , Vasella Andrea
Published: December 2004

A number of new glycosyl donors, O‐(1‐phenyl‐1H‐tetrazol‐5‐yl) glycosides, are prepared from the corresponding hemiacetals, commercially available 5‐chloro‐1‐phenyl‐1H‐tetrazole (2), and tetrabutylammonium fluoride (Bu4NF) in either THF or DMF. The mild reaction conditions are compatible with a variety of protecting groups. The glycosyl donors are treated with hydrogen fluoride‐pyridine complex (HF·py) to rapidly provide glycosyl fluorides in good‐to‐excellent yields, apparently by a (single or double) SN2 mechanism as studied by both 1H‐ and 19F‐NMR spectroscopy. Under acidic conditions, glycosyl fluorides equilibrate partially or completely, equilibration requiring a large excess of HF · py.
Calculating the Thermodynamics of Weakly Hydrogen‐Bonded Complexes from Heteronuclear NMR Data: Base‐pairing stabilities of a 5‐methyl(15N2)[O2, O4‐17O2]uridine (= (15N2)[O2,O4‐17O2]ribosylthymine) derivative, and structural implications
Authors: Strazewski Peter
Published: December 2004

2′,3′‐O‐Isopropylidene‐5‐methyl(15N2)[O2,O4‐17O2]uridine (= 2′,3′‐O‐isopropylidene (15N2)[O2,O4‐17O2]‐ribosylthymine; 1) was analyzed by 15N‐and 17O‐NMR spectroscopy. The 15N and 17O chemical shifts revealed, in the absence and presence of unlabelled 2′,3′‐O‐isopropylideneadenosine (2), the formation of thymine‐thymine and thymine‐adenine base pairs in CHCl3. As expected, cyclic complexes stabilized by two H‐bonds occurred at low temperatures, but at elevated temperatures, the data suggest that open complexes involving only one H‐bond prevailed. The 17O‐NMR data showed the cyclic thymine‐adenine pair in a reverse base pair geometry. The open base pair involved contacts to the urea‐derived carbonyl O‐atom of thymine. The thermodynamics of complex formation of the cyclic and open forms in both homo and hetero pairs were calculated from the temperature and concentration dependence of the 15N‐NMR data using a new method. It involves a fitting procedure onto the experimental isotherms using a theoretically derived function with the standard Gibbs free energy as a parameter to be optimized. ΔH° and ΔS° were derived from a linear regression of ΔG°(T) vs. T. The fitting procedure circumvents the baseline problem and could be automated and used to calculate correct thermodynamics from UV‐monitored melting curves of oligonucleotides. Since titrations are not involved, this dilution method should also be a useful alternative for stability studies of supramolecular complexes in H2O and in organic solvents.
Crystal Smectic‐B Phase from a Mesomorphic Ferrocene Derivative
Authors: Deschenaux Robert , Marendaz Jean‐Luc , Santiago Julio , Goodby John W.
Published: December 2004

The thermal properties of two ferrocene derivatives, substituted by either one or two cholesteryloxycarbonyl units, were investigated. While the monosubstituted ferrocene derivative 1 was found to be non‐mesomorphic, the disubstituted ferrocene derivative 2 exhibited a crystal smectic‐B phase. This result shows that ferrocene‐containing thermotropic liquid crystals, despite the bulkiness of the metallocene core, are not limited to disordered calamitic phases.
Synthesis, Structure, and Reactivity of Secosteroids Containing a Medium‐Sized Ring. Part 36. Transannular photocyclization of some unsaturated 5,10‐secosteroidal ketones: A reevaluation
Authors: Heckendorn Roland , Fuhrer Hermann , Kalvoda Jaroslav , Lorenc Ljubinka , Pavlović Vladimir , Mihailović Mihailo Lj.
Published: December 2004

The structures 9 and 8 are proposed for the single isolated irradiation product of 5‐oxo‐5,10‐secocholest‐1(10)‐en‐3α‐yl acetate (6) [2] and for the minor product of irradiation of 5‐oxo‐5,10‐secocholest‐1(10)‐en‐3β‐yl acetate (1) [3], respectively. These compounds are formed in an alternative reaction with respect to the originally observed intramolecular Paterro‐Büchi photoprocess (transformation of 1 to oxetane 2). The formerly postulated ‘active’ conformations for 1 and 6 still allow explanation of their generation.
Short, Enantiospecific Syntheses of Indolizidines 209B and 209D, and Piclavine A from Diethyl‐L‐Glutamate
Authors: Jefford Charles W. , Sienkiewicz Krzysztof , Thornton Steven R.
Published: December 2004

The 1H‐pyrrole derivative obtained from diethyl L‐glutamate hydrochloride and tetrahydro‐2,5‐dimethoxyfuran was cyclized with BBr3 to ethyl (5S)‐5,6,7,8‐tetrahydro‐8‐oxoindolizine‐5‐carboxylate (18). Catalytic hydrogenation of 18 over Pd/C in AcOH gave ethyl (5S,8aR)‐octahydroindolizine‐5‐carboxylate (21), whereas hydrogenation over Rh/Al2O3 in EtOH/AcOH 99:1 afforded mainly ethyl (5S,8S,8aS)‐octahydro‐8‐hydroxyindolizine‐5‐carboxylate (22). By functional‐group interconversions, 21 was transformed into piclavine A (1) and indolizidine 209D (2). Similarly, (5R,8R,8aS)‐octahydro‐5‐pentylindolizine‐8‐methanol (37), the final relay for indolizidine 209B (3), was obtained from 22.
Reassignment of the Configuration of Several Keto‐cyclolignans Prepared from Podophyllotoxin
Authors: Miguel del Corral José María , Gordaliza Marina , López José‐Luis , Olmo Esther Del , Angeles Castro M. , Luisa López M.
Published: December 2004

The configuration of several keto‐cyclolignans related to podophyllotoxin has been reviewed. Under basic catalysis, the configuration at the C‐atom in α‐position to the lactone carbonyl group in podophyllotoxone is inverted instead of the C‐atom in α‐position to the ketone group, as it has been reported.
Syntheses and PE‐Spectroscopic Investigations of 2,6‐ and 3,5‐Bridged 1,4‐Dimethylidenecyclohexanes
Authors: Chou Teh‐Chang , Lange Holger , Gleiter Rolf , Gögh Tibor , Kríž Miroslav , Pfenninger Max H. , Valentíny Marian , Ganter Camille
Published: December 2004

Three 1,4‐dimethylidenecyclohexanes, bridged in the 2,6‐ and 3,5‐positions by two ethano (4), one ethano and one propano (5), and two propano bridges (6) have been synthesized. The interaction of the two exocyclic methylidene groups has been investigated by He(I) photoelectron (PE) spectroscopy. It revealed a slightly larger energy difference (0.8 eV) for 4 and 5 as compared to the parent 1,4‐dimethylidenecyclohexane (7) (0.7 eV). The interpretation of the PE spectra was based on the comparison with PE data of related systems and with the results of semiempirical calculations on 4–6.
Oligosaccharide Analogues of Polysaccharides. Part 5. Studies on the cross‐coupling of alkynes and haloalkynes
Authors: Cai Chengzhi , Vasella Andrea
Published: December 2004

The cross‐coupling of the homopropargylic ether 1 and the halopropargylic ethers 2a and 2b was optimized, and aspects of the coupling mechanism were studied. Coupling promoted by Pd° and Cu1 in the presence of an amine yielded a mixture of the heterodimer 3 and the homodimers 4 and 5 (Scheme 1). Optimizations were first directed at suppressing homo‐coupling. Homo‐coupling is partially due to a H/I exchange (1 + 2a ⇌ 6 + 7) promoted by CuI and an amine. The exchange, but not the formation of homodimers, was largely suppressed in DMSO. The influence of phosphine ligands was also evaluated. Weaker σ‐donors (with the exception of PPh3) lead to a faster coupling and to a higher ratio of hetero‐ to homodimers, with P(fur)3 leading to the cleanest reaction. Homodimers are also formed (together with I2 · (i‐Pr)2NH) by reductive dimerization of the iodoalkyne 2a in the presence of [Pd2(dba)3], CuI, and (i‐Pr)2NH. Bulky and acceptor‐substituted amines reduced the extent of the dimerization of 2a, but the bulkiest amines did not promote coupling. Better results were obtained by using the bromoalkyne 2b. Neither dimerization of 2b, nor H/Br exchange between 1 and 2b were observed. Coupling of 1 and 2b was slower than the one of 1 and 2a, but gave higher yields of the heterodimer 3. The yield of 3 and the ratio of hetero‐to homodimers was greatly improved by addition of LiI; no phosphine ligand is then required. While the oxidative addition of the iodoalkyne 2a to [Pd(PPh3)4] (2a → 8a) was rapid, the one of the bromoalkyne 2b was much slower and proceeded via the η2‐complex 9 as evidenced by 1H‐NMR spectroscopy. The rearrangement of 9 to the bromopalladium σ‐complex 8b follows first‐order kinetics (k = 0.014 min−1). CuBr greatly increased the rate of this rearrangement. LiI caused rapid substitution of Br by I in the Pd σ‐complex (8b → 8a), but not in 9, nor in 2b. The σ‐complex 8a did not react with the alkyne 1 in the presence of (i‐Pr)2NH, unless CuI was added. The alkynes 10 or 1 did not react with CuI and either TMEDA or (i‐Pr)2NH to yield detectable amounts of the Cu‐acetylides 11 or 12. These observations are rationalized by the mechanism shown in Scheme 3, postulating the intermediacy of the binuclear alkyne‐Pd‐Cu complexes C and J, and some or all of E–H, and highlighting the role of CuI in this coupling.
A 1H‐NMR Spectroscopic Investigation of the Conformation of the Acetamido Group in Some Derivatives of N‐Acetyl‐D‐allosamine and ‐D‐glucosamine
Authors: Fowler Paul , Bernet Bruno , Vasella Andrea
Published: December 2004

The population of the conformations obtained by rotation around the C(2)N and the NC(O) bonds of AllNAc, GlcNAc, and GlcNMeAc derivatives was investigated by 1H‐NMR spectroscopy. The AllNAc‐derived α‐D‐and β‐D‐pyranosides 4–7, the AllNAc diazirine 16, and the GlcNAc‐derived axial anomers α‐D‐8–10 prefer the (Z)‐anti‐conformation. A significant population of the (Z)‐syn‐conformer in the (Z)‐syn/(Z)‐anti‐equilibrium for the equatorial anomers β‐D‐8–10 and the GlcNAc diazirine 17 was evidenced by an upfield shift of HC(2), downfield shifts of HC(1) and HC(3), and by NOE measurements. The population of the (Z)‐syn‐conformation depends on the substituent at C(1) and is highest for the hexafluoroisopropyl glycoside. The population of the (Z)‐syn‐conformation of β‐D‐14 decreases with increasing polarity of the solvent, but a substantial population is still observed for solutions in D2O. Whereas the α‐D‐anomers of the hemiacetal 22 and the methyl glycoside 21 prefer the (Z)‐anti‐conformation in D2O solution, the corresponding β‐D‐anomers are mixtures of the (Z)‐anti‐and (Z)‐syn‐conformers. The diazirine 17 self‐associates in CD2Cl2 solution at concentrations above 0.005M at low temperatures. The axial anomers of the GlcNMeAc derivatives α‐D‐26–28 are 2:1 to 3:1 mixtures of (Z)‐anti‐and (E)‐anti‐conformers, whereas the corresponding β‐D‐glycosides are ca. 1:3:6 mixtures of (Z)‐syn‐, (Z)‐anti‐, and (E)‐anti‐conformers.
Novel Thiazole‐Containing Complexing Agents and Luminescence of Their Europum(III) and Terbium(III) Chelates
Authors: Mukkala Veli‐Matti , Liitti Päivi , Hemmilä Ilkka , Takalo Harri , Matachescu Cristina , Kankare Jouko
Published: December 2004

The synthesis of novel thiazole‐containing complexing agents and their luminescence properties with EuIII and TbIII ions are reported. One of these terpyridine analogues was also tested as an EuIII labelling reagent, and its luminescence properties as an antibody conjugate were studied.
Kinetics and Mechanisms of Racemization: 5‐Substituted Hydantoins (= Imidazolidine‐2,4‐diones) as Models of Chiral Drugs
Authors: Reist Marianne , Carrupt Pierre‐Alain , Testa Bernard , Lehmann Søren , Hansen Jan Jørn
Published: December 2004

A chiral center in a drug molecule increases the complexity of synthetic, metabolic, pharmacological, and clinical studies, an additional problem being a possible lack of configurational stability. Here, we report detailed kinetic and mechanistic studies on the deuteration and racemization of seven 5‐monosubstituted hydantions (= imidazolidine‐2,4‐diones) used as model compounds. Using 1H‐NMR and chiral RP‐HPLC, rates of reaction and thermodynamic parameters of activation were determined for the reactions of deuteration and racemization. Energies of deprotonation were obtained by molecular‐orbital calculations performed at the AM1 level. It is demonstrated that the deuteration and racemization of 5‐monosubstituted hydantoins follow general‐base catalysis. The identical (within experimental errors) activation energies of deuteration and racemization indicate that the two reactions share a common reaction mechanism. The fact that the pseudo‐first‐order rate constants of deuteration are about half of those of racemization suggests that deuteration occurs with inversion of configuration. Very large differences in reaction rates were observed between the seven compounds, indicating the marked influence of substituents on chiral stability. These results, together with the small isotope effects observed, and the comparison between experimental activation energies and calculated energies of deprotonation, suggest a SE2 push‐pull mechanism for the racemization of 5‐monosubstituted hydantoins.
Variation and Selection
Authors: Quinkert Gerhard , Bang Holger , Reichert Dietmar
Published: December 2004

Ninety years ago, chemical methods were used in solving biological problems. In our days, methods of evolutionary biology are going to be used in solving problems of synthetic chemistry.
Trinuclear Double Helicates of Iron(II) and Nickel(II): Self‐assembly and resolution into helical enantiomers
Authors: Hasenknopf Bernold , Lehn Jean‐Marie
Published: December 2004

The synthesis of the linear tris[terpyridine] 1 with three tridentate binding sites is described. The reaction with metal ions of octahedral coordination geometry, such as FeII or NiII, leads to the self‐assembly of trinuclear complexes [M3(1)2]6+, which display properties in agreement with a double helical structure. The trinuclear iron (II) helicate has been resolved into its enantiomers.
Synthesis, Separation, and Characterization of Optically Pure C76 Mono‐Adducts
Authors: Herrmann Andreas , Diederich François
Published: December 2004

Nucleophilic 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 Step

A 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

An 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

Rarisetenolide (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 Systems

The [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

The 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) Octanitrile

A 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

Substituted 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

1‐(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

The 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

The 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

β‐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

New Bi(cyclopropylidenes) by CuCl2‐Induced ‘Carbene Dimerization’ of 1‐Bromo‐1‐lithiocycIopropanes

A 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

It 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

Chiral 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

Various 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

(−)‐(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

The 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.
New Types of Potentially Antimalarial Agents: Epidioxy‐substituted norditerpene and norsesterpenes from the marine sponge Diacarnus levii
Authors: D'Ambrosio Michele , Guerriero Antonio , Deharo Eric , Debitus Cécile , Munoz Victoria , Pietra Francesco
Published: December 2004

Natural free carboxylic acids from the hadromerid sponge Diacarnus levii (Kelly‐Borges and Vacelet) were esterified to yield the new cyclic norditerpene peroxides ent‐muqubilin benzyl ester (= (αR,3S,6R)‐α,6‐dimethyl‐6‐[(E)‐4‐methyl‐6‐(2,6,6‐trimethyl‐cyclohex‐1‐en‐1‐yl)hex‐3‐enyl]‐1,2‐dioxan‐3‐acetic acid benzyl ester; 6, diacarnoate B methyl ester(= (αS,3R,6R)‐α,6‐dimethyl‐6‐{2‐(4aS,8aS)‐3,4,4a,5,6,7,8,8a‐octahydro‐3‐oxo‐2,5,5,8a‐tetramethylnaphthalen‐1‐yl)ethyl}‐1,2‐dioxan‐3‐acetic acid methyl ester; 9), and deoxydiacarnoate B benzyl ester (= (αS,3R,6R)‐α,6‐dimethyl‐6‐{2‐[(4aS,8aS)‐3,4,4a,5,6,7,8,8a‐octahydro‐2,5,5,8a‐tetramethyl‐1‐naphthalenyl]ethyl}‐1,2‐dioxan‐3‐acetic acid benzyl ester; 10), which were isolated following extensive chromatography. The relative configuration of the peroxide/α‐methylacetate moiety of 6, 9, and 10, was directly determined from their NMR spectra. The absolute configurations of the peroxide/α‐methylacetate moiety was deduced from comparative 1H‐NMR data of the (S)‐ and (R)‐phenylglycine methyl ester derivatives 7 and 8 as well as 11/13 and 12/14, all obtained from a mixture of the precursors of 3, 6, and 10. The absolute configuration at the carbobicyclic moiety of enone 9 and of 10, is identical, as established by chemical interconversion, 9 and 10 belong to the normal labdane series according to empirical CD rules, applied either directly to 9 or to the parent (+)‐sclareolide‐derived enone 20. In contrast, molar rotation additivity rules suggest the ent‐labdane configuration for 9 and 10. The epidioxides 1–3, 6, and 10 proved active in vitro against the malaria parasite Plasmodium falciparum; especially the previously isolated methyl 3‐epinuapapuanoate (2) was active against a chloroquine‐resistant strain, and this with a good security index.
Minor Coumarins from Calophyllum teysmannii var. inophylloide and Synthesis of Cytotoxic Calanone Derivatives
Authors: Cao Shu‐Geng , Wu Xiao‐Hua , Sim Keng‐Yeow , Tan Benny H. K. , Vittal Jagadese J. , Pereira Joan T. , Goh Swee‐Hock
Published: December 2004

A Chemotaxonomic survey for biologically active compounds from Malaysian Calophyllum species led to the findings of the four new benzoylcoumarins 1a, 2, 3, and 4a (including the unusual prenylated 6‐benzoylcoumann 1a), two uncommon coumarins 5 and 6a with a pyran‐4‐one moiety fused at C(6) and C(7), and compounds 7a, 9, and 10, all isolated from the bark of C. teysmannii var. inophylloide. Their structures were determined by spectroscopic analysis and chemical transformations. X‐Ray crystal‐structure determination of 2 provided information on the conformational preferences of substituents in this class of coumarins. The syntheses of the cytotoxic calanone (7a) and of some related coumarins are described.
Nucleosides. Part LXIII. Acetals as new 2′‐O‐protecting functions for the synthesis of oligoribonucleotides: Synthesis of uridine building blocks and evaluation of their relative acid stability
Authors: Matysiak Stefan , Fitznar Hans‐Peter , Schnell Ralf , Pfleiderer Wolfgang
Published: December 2004

A broad variety of new acyclic vinyl ethers (see6–41) have been synthesizedviathe vinyl‐interchange reaction of ethyl vinyl ether at room temperature using mercury(II) trifluoroacetate as a highly efficient catalyst. The appropriate vinyl ethers were reacted under acidic conditions with 3′,5′‐O‐silyl‐protected uridine42to the corresponding 2′‐O‐(1‐alkoxyethyl) derivatives43–83which gave, on desilylation of F−ions, in high yields the uridine‐2′‐O‐acetal derivatives84–124. The relative stabilities of the newly synthesized compounds under acidic and basic conditions were determined using TLC and HPLC techniques. Protected protecting groups offer the best properties for oligoribonucleotide syntheses. Interestingly, the very acid‐stable acetals of the β‐substituted ethyl‐type118–121and123can be cleaved by a β‐elimination process providing a series of base‐labile acetals of potential synthetic value.
Bonding mechanism evidence in a ceramic–nonprecious alloy system
Authors: Anusavice K. J. , Ringle R. D. , Fairhurst C. W.
Published: December 2004

Bonding between porcelain and dental ceramic alloys is thought to be dependent upon the establishment of a continous electron structure across the ceramicmetal interface (Pask, Proc. Procelain Enamel Inst., 33, 1, 1971). Such a structure most likely results from the compatibility of metal ions at the metal surface saturated in an oxide form with the complex oxide structure of the ceramic matrix. Reaction zone compounds are expected to play a prominent role in the strength of the ceramic–metal bonds but thus far none have been detected or identified.

The present study was centered on the determination of possible adherence zone compounds in four composite ceramic–metal couples examined by x‐ray energy analysis. Elemental analysis of four couples revealed the presence of a predominant AlCr interaction resulting from the formation of an AlCrO compound or mixed oxide complex. The Cr ions were supplied by the Cr2O3 oxide layer at the metal surface and the Al was provided by the initial bonding‐agent coating.