bluuebunny/crossref_metadata_2025_split huggingface dataset:
Hourglass‐Shaped Dendrimers on Surfaces: A Comparison of Different Scanning‐Tunneling‐Microscopy Approaches
Authors: Widmer I. , Hubler U. , Stöhr M. , Merz L. , Güntherodt H.‐J. , Hermann B. A. , Samorí P. , Rabe J. P. , Rheiner P. B. , Greiveldinger G. , Murer P. 
Published: January 2003

AbstractLarge molecules adsorbed on surfaces can be analyzed by scanning tunneling microscopy (STM) under various environmental conditions: on a dry surface in air or vacuum, and at the solid‐liquid interface. However, can measurements under dissimilar conditions be compared, e. g., when sample A was studied at the solid‐liquid interface and sample B in a dry environment? Only rarely can the same substance be examined with more than one approach, since completely different set‐up and preparations are necessary. Furthermore, few substances are suitable for several methods of sample preparation and characterization. We have chosen a large, flexible, nonplanar molecule, namely an alkoxy‐substituted second‐generation dendritic compound with a chiral core unit, which is peculiar for its ‘hourglass' conformation. The assembly properties have been explored by STM both in solution‐cast self‐organized monolayers (SOMs) and multilayer films, as well as at the solid‐liquid interface. The complexity and limits of the three approaches applied to our hourglass‐shaped dendritic compound are discussed. Depending on the approach and environmental conditions, several quality levels of image resolution could be achieved; measurements carried out at low temperatures led to highest resolution on the aromatic parts of the molecule. A comparison of equally sized images obtained under these varying conditions reveals not only different packing arrangements, but also spots of unlike shape. Therefore, when the approach, preparation, and/or environmental conditions are not the same, STM measurements of different compounds have to be compared with greatest care.
Theoretical Studies on the Diastereoselectivity in the Lewis Acid Catalyzed CarbonylEne Reaction: A Fundamental Role of Electrostatic Interaction
Authors: Yamanaka Masahiro , Mikami Koichi 
Published: January 2003

AbstractLewis acids affect reactivity, selectivity, and mechanism in the carbonyl‐ene reaction. The diastereoselectivity in the glyoxylate‐ene reaction depends on Lewis acids. While the SnCl4‐promoted reaction can be achieved with a high level of anti‐selectivity, the use of Al reagents leads to a high syn‐selectivity. The origin of the Lewis acid dependency of the diastereoselectivity in the carbonylene reaction of (E)‐but‐2‐ene with glyoxylate was theoretically studied (HF/6‐31G*) from the point of view of differences and similarities between the ene and the Diels–Alder reactions. Though it has been widely accepted that the endo‐preference would be less obvious in the ene reaction than in the Diels–Alder reaction, our ab initio molecular studies showed that the electrostatic interaction between carbonyl O‐atom lone pair and cationic allylic central C‐atom of ene component exists in the Lewis acid‐promoted carbonyl–ene reaction to affect the transition‐state conformation. It is illustrated that such an electrostatic interaction is essential to control the exo/endo‐selectivity, which provides the diastereoselectivity of the product in the transition state of the Lewis acid promoted carbonylene reaction.
Highly Diastereo‐ and Enantioselective Aziridination of α,β‐Unsaturated Amides with Diaziridine and Mechanistic Consideration on Its Stereochemistry
Authors: Ishihara Hiroyuki , Hori Kiyoto , Sugihara Hiroyasu , Ito Yoshio N. , Katsuki Tsutomu 
Published: January 2003

AbstractDuring studies of aziridination of α,β‐unsaturated amides with diaziridine, we found that we could prepare both the cis‐ and trans‐aziridinecarboxamides by choosing an appropriately substituted diaziridine. While 3‐monosubstituted diaziridine 2 was suitable for the trans‐selective aziridination, employment of 3,3‐dialkyldiaziridine 1 resulted in the formation of cis‐aziridine carboxamides, irrespective of the geometry of the substrate (Scheme 1 and Tables 1 and 2). To elucidate the unique nonstereospecificity and to expand these aziridinations to asymmetric ones, several optically active diaziridines were newly prepared. Aziridination with an optically active 3‐monosubstituted diaziridine, 3‐cyclohexyl‐1‐[(1R)‐1‐phenylethyl]diaziridine 16, proceeded smoothly with high trans‐selectivity as well as excellent enantioselectivity (up to 98% ee; see Table 3). On the other hand, highly enantioselective cis‐aziridination was achieved (>99% ee) with optically active 3,3‐dimethyl‐1‐[(1R)‐1‐phenylethyl]diaziridine 15, though the yield was low (4%). This aziridination was considered to proceed stepwise by way of the enolate intermediate (Scheme 2). Careful inspection of the stereochemistry and its solvent‐dependence suggested that the diastereoselection of the reaction was kinetically controlled: the 1,4‐addition of N‐lithiated diaziridine was a crucial step for determination of the stereochemical course of the aziridination (Figs. 2–4).
Synthesis and Reactivity of π‐Electron‐Deficient (Arylsulfonyl)acetates
Authors: Alonso Diego A. , Nájera Carmen , Varea Montserrat 
Published: January 2003

AbstractDifferent π‐electron‐deficient (arylsulfonyl)acetates 9 were synthesized (Scheme 1, Table 1), and their behavior as soft nucleophiles in the dialkylation reaction under phase‐transfer catalysis conditions was studied (Schemes 2 and 3, Tables 2 and 3). The [3,5‐bis(trifluoromethyl)phenyl]sulfonyl group was shown to be the best substituent for the stereoselective synthesis of (E)‐aconitates 18 via an alkylation hydro‐sulfonyl‐elimination integrated process under very mild phase‐transfer‐catalysis conditions (Scheme 5, Table 4). Sulfonylacetates 9h,i also underwent smooth Diels‐Alder reactions with acyclic and cyclic dienes via in situ formation of the appropriate dienophile through a Knoevenagel condensation with paraformaldehyde (Scheme 6). Reductive desulfonylation with Zn and NH4Cl in THF was shown to be an efficient method for removal of the synthetically useful sulfonyl moiety (Scheme 7).
Catalyzed Asymmetric DielsAlder Reactions of Benzoquinone. Total Synthesis of (−)‐Ibogamine
Authors: White James D. , Choi Younggi 
Published: January 2003

AbstractThe DielsAlder reaction of 1,4‐benzoquinone with 1,3‐dienes catalyzed by Mikami's [Ti{(S)‐binol(2−)}Cl2] complex (binol=[1,1′‐binaphthalene]‐2,2′‐diol) gives cycloadducts in good yield and in high enantiomer excess. A model is proposed to explain the observed absolute configuration of cycloadducts, and the reaction is used as the key step in an asymmetric synthesis leading to the alkaloid (−)‐ibogamine.
Steric and Electronic Effects on an Antibody‐Catalyzed Diels–Alder Reaction
Authors: Gozin Yael , Hilvert Donald 
Published: January 2003

AbstractA series of substituted thiophene dioxides was tested as diene substrates for the antibody 1E9, which was elicited with a hexachloronorbornene derivative and normally catalyzes the inverse electron‐demand Diels–Alder reaction between 2,3,4,5‐tetrachlorothiophene dioxide (TCTD) and N‐ethylmaleimide (NEM). Previous structural and computational studies had suggested that the catalytic efficiency of this system derives in part from a very snug fit between the apolar active site and the transition state of this reaction. Nevertheless, replacing all the Cl‐atoms in the hapten with Br‐atoms leads to no loss in affinity (Kd=0.1 nM), indicating substantial conformational flexibility in the residues that line the binding pocket. Consistent with this observation, the 2,3,4,5‐tetrabromothiophene dioxide is a good substrate for the antibody (kcat=1.8 min−1, KNEM=14 μM), despite being considerably larger than TCTD. In contrast, normal electron‐demand Diels–Alder reactions between NEM and unsubstituted thiophene dioxide or 2,3,4,5‐tetramethylthiophene dioxide, which are much smaller or nearly isosteric with TCTD, respectively, are not detectably accelerated. These results show that the electronic properties of the 1E9 active site are optimized to a remarkable degree for the inverse electron‐demand Diels–Alder reaction for which it was designed. Indeed, they appear to play a more important role in catalysis than simple proximity effects.
Facile Preparation and Activation of High‐Productivity Single‐Site Nickel Catalysts for Highly Linear Polyethylene
Authors: Pickel Marco , Casper Thomas , Rahm Annette , Dambouwy Christian , Chen Peter 
Published: January 2003

AbstractA easy‐to‐prepare series of nickel complexes are reported, which, upon activation with near‐stoichiometric alkylating agents, polymerize ethylene with turnover number (TON) of over 105, which brings the catalyst into the range of commercial single‐site catalysts based on early transition metals. However, in contrast to the usual metallocenes, the catalyst shows the same activity in coordinating solvents such as THF and dioxane as it shows in toluene. Moreover, the polymer produced is highly linear, which, together with the long catalyst lifetime, suggests that the combination of the nickel catalyst and a Lewis acid suppresses formation of nickel hydrides.
Cyclic RGD Peptides Containing Azabicycloalkane Reverse‐Turn Mimics
Authors: Belvisi Laura , Caporale Andrea , Colombo Matteo , Manzoni Leonardo , Potenza Donatella , Scolastico Carlo , Castorina Massimo , Cati Matilde , Giannini Giuseppe , Pisano Claudio 
Published: January 2003

AbstractThe Fmoc‐protected lactams 3 and 4 were used to prepare cyclo(Arg‐Gly‐Asp‐lactam) 1 and cyclo(Arg‐Gly‐Asp‐Phe‐lactam) 2, which contain the Arg‐Gly‐Asp (RGD) recognition motif. Their solid‐phase synthesis, conformational analysis, and binding to purified αVβ3 and αVβ5 integrins are reported. Compound 1 was found to act as an active and selective inhibitor of the αVβ5 integrin.
Regioselective Benzoylation of 6‐O‐Protected and 4,6‐O‐Diprotected Hexopyranosides as Promoted by Chiral and Achiral Ditertiary 1,2‐Diamines
Authors: Hu Guixian , Vasella Andrea 
Published: January 2003

AbstractMonobenzoylation of triols (6‐O‐silylated glycopyranosides) or diols (4,6‐O‐benzylidenated glycopyranosides) with benzoyl chloride and triethylamine at −60° to 23° is promoted by catalytic amounts of ditertiary 1,2‐diamines. The regioselectivity depends mostly on the structure of the alcohols; it is modulated by the configuration and constitution of the diamines, as shown by comparing the effect of Oriyama's catalyst ((S)‐1 and (R)‐1), N,N,N′,N′‐tetramethylethylenediamine (TMEDA), N,N,N′,N′‐tetraethylethylenediamine (TEEDA), Et3N, and EtNMe2. The effect of the catalysts on the reactivity is impaired by their steric hindrance. In agreement with the modest enantioselectivity of the mono‐ and dibenzoylation of rac‐cyclohexane‐1,2‐diol in the presence of Oriyama's catalyst, the influence of these diamines on the regioselectivity is rather limited. While associated with procedural simplicity, these catalysts lead, in a few cases, to higher yields of a single benzoate than established methods, viz. in the preparation of the 3‐O‐benzoyl β‐D‐glucopyranoside 4, the 2‐O‐benzoyl α‐D‐galactopyranoside 22, the 3‐O‐benzoyl α‐D‐galactopyranoside 23, and the benzylidenated 2‐O‐benzoyl α‐D‐galactopyranoside 44. The regioselective benzoylation of the benzylidenated β‐D‐mannopyranoside 47, leading to 48, appears to be new.
Construction and Analysis of LifsonRoig Models for the Helical Conformations of α‐Peptides
Authors: Kemp Daniel S. 
Published: January 2003

AbstractNew forms of LifsonRoig algorithms are introduced for modeling stabilities of helices formed by polypeptides derived from α‐amino acids. The principles of constructing and generalizing these algorithms are developed, and their application to modeling of circular dichroism ellipticities and protection factors derived from H/D exchange of α‐peptide backbone amide residues are critically reviewed. With the aim of comparing the properties of structured conformations formed by α‐ and β‐peptides, the intrinsic limitations of LifsonRoig algorithms and their underlying assumptions are analyzed. LifsonRoig state sums that provide easy structural analysis are generated by new algorithms based on products of 8×8 and 16×16 matrices, and a simple protocol is introduced for generating new state sums that are tailored for specific purposes. The N‐ and C‐capping of highly helical α‐peptides by means of tailored helix‐stabilizing templates is shown to result in helical conformational manifolds that approach those of helical β‐peptides in conformational homogeneity.
Synthesis of a Conformationally Flexible β‐Hairpin Mimetic
Authors: Hoffmann Reinhard W. , Schopfer Ulrich , Müller Gerhard , Brandl Trixi 
Published: January 2003

AbstractRational conformation design led us to a synthesis of the ω‐amido‐undecenamide 4, which was shown by theoretical means (simulated annealing techniques) and by NMR and IR spectroscopy to have a high tendency to populate a conformation corresponding to a natural β‐II′‐type hairpin, despite possessing a conformationally fully flexible open‐chain backbone.
Designed Beta‐Turn Mimic Based on the Allylic‐Strain Concept: Evaluation of Structural and Biological Features by Incorporation into a Cyclic RGD Peptide (Cyclo(‐L‐arginylglycyl‐L‐α‐aspartyl‐))
Authors: Sukopp Martin , Marinelli Luciana , Heller Markus , Brandl Trixi , Goodman Simon L. , Hoffmann Reinhard W. , Kessler Horst 
Published: January 2003

AbstractThe (3R,5S,6E,8S,10R)‐11‐amino‐3,5,8,10‐tetramethylundec‐6‐enoic acid (ATUA; 1), which was designed as a βII′‐turn mimic according to the concepts of allylic strain and 2,4‐dimethylpentane units, was incorporated into a cyclic RGD peptide. The three‐dimensional structure of cyclo(‐RGD‐ATUA‐) (=cyclo(‐Arg‐Gly‐Asp‐ATUA‐)) 4 in H2O was determined by NMR techniques, distance geometry calculations and molecular‐dynamics simulations. The RGD sequence of 4 shows high conformational flexibility but some preference for an extended conformation. The structural features of the RGD sequence of 4 were compared with the RGD moiety of cyclo(‐RGDfV‐) (=cyclo(‐Arg‐Gly‐Asp‐D‐Phe‐Val‐)). In contrast to cyclo(‐RGDfV‐), which is a highly active αvβ3 antagonist and selective against αIIbβ3, cyclo(‐RGD‐ATUA‐) shows a lower activity and selectivity. The structure of the ATUA residue in the cyclic peptide resembles a βII′‐turn‐like conformation. Its middle part, adjacent to the CC bond, strongly prefers the designed and desired structure.
Synthesis and Reactions of New Dithiolopyrroles
Authors: Stachel Hans‐Dietrich , Eckl Eduard , Immerz‐Winkler Elisabeth , Kreiner Christine , Weigand Wolfgang , Robl Christian , Wünsch Ralf , Dick Stefan , Drescher Norbert 
Published: January 2003

AbstractThe title compounds were prepared starting from pyrrolinone 4. Nucleophilic‐displacement and ring‐closure reactions yielded the dithiolopyrrole 5a, which formed salts with electrophiles (7, 8) as well as with bases. The crystal structure of 5a was determined. Oxidation of the dithioles 5a and 6a led to S(2)‐oxides (10a, 11a) and the corresponding S(2)‐dioxides (10b, 11b) depending on reaction conditions. The thiosulfinate 10a was converted by a ring‐opening/ring‐closure reaction sequence to the bicyclic sulfinamide 12. The oxidative addition reactions of [Pt(η2‐C2H4) (PPh3)2] (14) with the disulfides 5a and 13 led to the dithiolatoplatinum(II) complexes 15 and 16, respectively. Complex 16 was characterized structurally. The sulfenato‐thiolato complex 17 was synthesized via reaction of 14 with the thiosulfinate 10a. The thiosulfonato PtII complex 18 was prepared by an oxidative insertion of Pt0 into the CS bond of the corresponding thiosulfonate 10b. Furthermore, complex 18 was characterized by single‐crystal X‐ray‐diffraction studies.
Synthesis of α‐Nitro‐α‐diazocarbonyl Derivatives and Their Applications in the Cyclopropanation of Alkenes and in OH Insertion Reactions
Authors: Charette André B. , Wurz Ryan P. , Ollevier Thierry 
Published: January 2003

AbstractA facile and highly efficient method for the preparation of α‐nitro‐α‐diazocarbonyl derivatives by a diazo‐transfer reaction involving (trifluoromethyl)sulfonyl azide has been developed. These substrates undergo a rhodium‐catalyzed cyclopropanation reaction with a variety of alkenes. A systematic study of the reaction indicated that the diastereoselectivity of the cyclopropanation could be effectively controlled through the modification of the steric bulk of the diazo reagent. A novel OH insertion reaction of the metalcarbene complex derived from the α‐nitro‐α‐diazocarbonyl reagent afforded the corresponding novel α‐nitro‐α‐alkoxy carbonyl derivatives.
Toward an Understanding of the Chemical Etiology for DNA Minor‐Groove Recognition by Polyamides
Authors: Marques Michael A. , Doss Raymond M. , Urbach Adam R. , Dervan Peter B. 
Published: January 2003

AbstractCrescent‐shaped polyamides composed of aromatic amino acids, i.e., 1‐methyl‐1H‐imidazole Im, 1‐methyl‐1H‐pyrrole Py, and 3‐hydroxy‐1H‐pyrrole Hp, bind in the minor groove of DNA as 2 : 1 and 1 : 1 ligand/DNA complexes. DNA‐Sequence specificity can be attributed to shape‐selective recognition and the unique corners or pairs of corners presented by each heterocycle(s) to the edges of the base pairs on the floor of the minor groove. Here we examine the relationship between heterocycle structure and DNA‐sequence specificity for a family of five‐membered aromatic amino acids. By means of quantitative DNase‐I footprinting, the recognition behavior of polyamides containing eight different aromatic amino acids, i.e., 1‐methyl‐1H‐pyrazole Pz, 1H‐pyrrole Nh, 5‐methylthiazole Nt, 4‐methylthiazole Th, 3‐methylthiophene Tn, thiophene Tp, 3‐hydroxythiophene Ht, and furan Fr, were compared with the polyamides containing the parent‐ring amino acids Py, Im, and Hp for their ability to discriminate between the four WatsonCrick base pairs in the DNA minor groove. Analysis of the data and molecular modeling showed that the geometry inherent to each heterocycle plays a significant role in the ability of polyamides to differentiate between DNA sequences. Binding appears sensitive to changes in curvature complementarity between the polyamide and DNA. The Tn/Py pair affords a modest 3‐fold discrimination of T⋅A vs. A⋅T and suggests that an S‐atom in the thiophene ring prefers to lie opposite T not A.
Lewis Base Catalyzed Aldol Reaction of Trimethylsilyl Enolates with Aldehydes
Authors: Mukaiyama Teruaki , Fujisawa Hidehiko , Nakagawa Takashi 
Published: January 2003

AbstractA new Lewis base catalyzed aldol reaction of trimethylsilyl enolates with aldehydes is established in DMF or pyridine solvent by using a Lewis base such as lithium diphenylamide (Tables 4 and 5) or lithium 2‐pyrrolidone (Tables 6–8). The effect of solvent suggests that this reaction proceeds via the pentacoordinated hypervalent silicate generated by the coordination of the above Lewis base to a trimethylsilyl enolate. Successive coordination of the solvent to the thus‐formed pentacoordinated silicate leads to an active enolate intermediate having hexacoordinated silicate, which, in turn, attacks carbonyl compounds to form the desired aldols (Scheme 5).
The Development of a Convergent and Efficient Enantioselective Synthesis of the Bengamides via a Common Polyol Intermediate
Authors: Boeckman, Jr. Robert K. , Clark Tammy J. , Shook Brian C. 
Published: January 2003

AbstractAn efficient, general synthetic route to the bengamide family of antitumor agents from a common polyol thioester is described. Consecutive aldol condensations afford the protected polyol thioester side chain suitable for coupling to the bengamides. A novel chiral‐phase‐transfer‐catalyzed enantioselective alkylation affords the properly functionalized caprolactams required for the synthesis of more‐complex members of the bengamide family. Use of the methyl 2‐naphthyl ether protecting group, compatible with the boron Lewis acids required for enantioselective aldol condensation, allows direct access to all the bengamides.
Synthesis of Cyclic Peptides by Photochemical Decarboxylation of N‐Phthaloyl Peptides in Aqueous Solution
Authors: Griesbeck Axel G. , Heinrich Thomas , Oelgemöller Michael , Molis Arne , Heidtmann Axel 
Published: January 2003

AbstractThe synthesis of a variety of cyclic peptides from N‐phthaloyl‐protected di‐, tri‐, tetra‐, and pentapeptides with different aminocarboxylic acid tethers by photodecarboxylation – initiated by intramolecular electron transfer – has been explored in aqueous media. The progress and the chemoselectivity of the follow‐up processes after CO2 extrusion were traced by the respective pH/time‐profiles, as well as by the overall change in pH after completion of the reaction. The competition between cyclization and simple oxidative decarboxylation depends on spacer length and geometry, H‐bonding interaction between the electron accepting phthalimide CO groups and amide H‐atoms, as well as the geometric reorganization coupled with the radical combination step and the formation of the lactam rings. With progressing reaction, hydrolysis of the phthalimide chromophore becomes an increasingly important side reaction due to the constant increase in pH. The use of phosphate‐buffered aqueous media consequently improved the cyclization yields. The ground‐state interactions between amide groups and the terminal COO− group with the imide CO groups were studied for the model system [N‐(phthaloyl)glycyl]sarcosine (1) by NMR spectroscopy where the amide (E/Z)‐equilibrium depends on the presence of carboxylate vs. free carboxylic acid, demonstrating the role of H‐bonding and metal coordination.
Rationale for active vitamin D and analogs in the treatment of osteoporosis
Authors: Nishii Yasuho 
Published: January 2003

AbstractIn 1981, Chugai Pharmaceutical succeeded in marketing alfacalcidol, a prodrug of calcitriol, as a therapeutic agent for renal osteodystrophy. In 1983, Chugai succeeded in extending the application of alfacalcidol to the treatment of osteoporosis as well. Clinicians in Japan have accepted alfacalcidol as a remedy for osteoporosis. However, the use of calcitriol and its analogs for the treatment of osteoporosis is still controversial. Some misunderstandings exist internationally about the efficacy of the active form of vitamin D for the treatment of osteoporosis. It is important to emphasize that patients with osteoporosis have intestinal calcium malabsorption and dysfunction in renal activation of vitamin D. When massive doses of parent vitamin D were administered to OVX rats, bone mass increased, but surprisingly, many porotic area were observed in the cortical bone. On the other hand, administration of alfacalcidol increased physiological bone without porotic observation. It is necessary to give the active form of vitamin D, D‐hormone, with an RDA‐equivalent supply of calcium. Alfacalcidol forms physiological strong bones that are hardly fractured by regulating calcium and bone metabolism. We proposed a new vitamin D analog, 2β (3‐hydroxypropoxy)calcitriol [ED‐71] as a therapeutic drug for osteoporosis, which is more potent than calcitriol. ED‐71 is now being investigated in phase 2 clinical studies in Japan. ED‐71 will appear as more improved drugs for osteoporosis until 2010. J. Cell. Biochem. 88: 381–386, 2003. © 2002 Wiley‐Liss, Inc.