bluuebunny/crossref_metadata_2025_split huggingface dataset:
Iodonium ion‐promoted glycosidation of sugar 1,2‐thio‐orthoesters
Authors: Zuurmond H. M. , van der Marel G. A. , van Boom J. H. 
Published: September 2010

AbstractSugar 1,2‐thio‐orthoesters could be condensed with terminal glycosidic acceptors using halonium ion sources (IDCP, NIS/TfOH, NCS/TfOH) as activators. IDCP or NIS/TfOH‐assisted glycosidation of sugar 1,2‐thio‐orthoesters with disarmed alkyl 1‐thioglycosides gave chemoselectively the 1,2‐disarmed orthoester or 1,2‐trans linked disaccharides, respectively.
Chiral catalysts for enantioselective carbenoid cyclopropanation reactions
Authors: Doyle Michael P. 
Published: September 2010

AbstractChiral copper(I) and rhodium(II) catalysts designed to achieve enantioselection in cyclopropanation reactions have recently been reported. These catalysts react with diazo compounds to form intermediate metal carbenes that subsequently react with alkenes, in both intermolecular and intramolecular transformations, to form cyclopropane derivatives. They are constructed with chiral ligands whose function is to orient carbene substituents and control olefin approach to the carbenoid center. Enantioselectivities that are greater than 95% have been achieved.In this review, the major advances in chiral catalyst design and development are described, and a uniform mechanistic interpretation is advanced to explain the results accumulated in reported investigations. Focus is placed on chiral salicylaldimine and semicorrin/bis‐oxazoline copper catalysts and on the newest class of chiral catalysts for enantioselective carbenoid reactions, rhodium(II) carboxamides. Although early in their development, sufficient advances have been made to justify placement of this catalytic methodology among the most promising for enantioselective cyclopropanation reactions.
Solid‐phase synthesis of RNA via a silyl‐protecting‐group strategy
Authors: Dreef‐Tromp C. M. , van Dam E. M. A. , van den Elst H. , van den Boogaart J. E. , van der Marel G. A. , van Boom J. H. 
Published: September 2010

AbstractRibonucleoside phosphoramidites have been prepared of which the exocyclic amino functions and 2′‐hydroxyl functions are protected with 2‐[(tert‐butyldiphenylsiloxy)methyl]benzoyl groups and tert‐butyldimethylsilyl groups, respectively. The N‐SiOMB/O‐TBDMS‐blocked nucleosides proved to be suitable building units in a solid‐phase synthesis of the RNA‐fragment 5′‐AGAGUACCU‐3′ via a phosphite triester approach.
Synthesis of 17‐(isocyanotosylmethylene) steroids: Precursors to pregnane derivatives
Authors: van Leusen Daan , van Leusen Albert M. 
Published: September 2010

AbstractThe synthesis is described of a series of twenty 17‐(isocyanotosylmethylene) steroids 5 from 17‐oxo steroids and 4‐tolylsulfonylmethyl isocyanide (TosMIC). Furthermore, nine analogous 17‐(isocyanosulfonylmethylene) steroids (15–21a) have been prepared using sulfonylmethyl isocyanides other than TosMIC. The synthesis of four new TosMIC analogues is described.
Reductive cleavage of the carbonphosphorus bond with alkali metals. II. Cleavage of mixed functionalized triarylphosphines; Birch reduction of arylphosphines
Authors: van Doom Johannes A. , Frijns John H. G. , Meijboom Nico 
Published: September 2010

AbstractThe reductive cleavage of mixed ortho‐ and para‐functionalized triarylphosphines with Na/NH3 and Li/THF depends strongly on the nature and positions of the substituents. Reduction occurs readily with phosphines PhAr2P (4, 6 and 9) and Ph2ArP 19 when the corresponding phosphine Ar3P is not reduced. Cleavage of para‐substituted compounds 7 and 9 leads to mixtures of secondary phosphines. By contrast, cleavage of mixed ortho‐substituted triphenylphosphines is very selective. The functionalized phenyl group is split off in high yield when it carries CH3, (CH3)2N and CH3O substituents (2, 3, 5, 6, 8, 10, 11, 13, 14, 17, 19, 24, 25). Reaction of 4 is not selective due to loss of methoxy groups. By contrast, the phenyl group is split off when the mixed phosphine contains phenoxide groups (1, 15, 16). The reactions of bis‐ and tris(diphenylphosphino)benzenes with Li/THF lead predominantly to cleavage of the diphenylphosphino group from the respective substrates.In a number of cases, the product of a Birch reduction with an isolated diene system is formed in NH3 (1, 9, 12, 21, 23) via a phosphino‐stabilized cyclohexadienyl anion. This reduction does not occur in the aprotic solvent THF. Base‐catalyzed isomerization leads to a conjugated double‐bond system with a vinylphosphine moiety.We also report interesting large aJ(PP) couplings in 1,3‐diphosphinobenzenes and complicated 13C resonances of para‐substituted phosphines.
Conformational analysis of longicyclene: A 600 MHz 1H‐NMR and molecular mechanics study
Authors: Lankhorst Peter P. , van Beek Teris A. , Haasnoot Cornells A. G. 
Published: September 2010

AbstractThe complete assignment of the 1H‐NMR spectrum of the fully saturated tetracyclic sesquiterpene hydrocarbon longicyclene is presented. The assignment was achieved using phase‐sensitive double‐quantum filtered COSY at 600 MHz, heteronuclear correlation spectroscopy and COLOC spectroscopy. Accurate vicinal 1H‐1H coupling constants involving the seven‐membered ring protons were obtained using extensive simulation of the ID spectrum.Molecular mechanics calculations indicate that the title compound may adopt two conformations which differ mainly in the geometry of the seven‐membered ring. These geometries are best described as twist‐chair‐like conformations denoted TC11 and TC8, respectively. The energy of the TC11 conformer is predicted to be slightly lower (4.6 kJ/mol) than the energy of the TC8 conformer. The vicinal 1H‐1H coupling constants of these two conformers were predicted from a generalized Karplus equation. The predicted coupling constants agree well with the experimental coupling constants if it is assumed that, in solution, longicyclene is engaged in a conformational equilibrium between TC11 (86%) and TC8 (14%).
Polymer networks: Formation and structure
Authors: Du Šek Karel 
Published: September 2010

AbstractThe features of network‐formation processes, network‐formation theories and their applications to simple and more complicated systems are briefly reviewed. Network‐formation theories describe changes in structure as a function of an evolution parameter of the system. The existing theories can be grouped into two major categories: (a) off‐space graph models of structure growth (statistical simulation of the growth from units and kinetic simulation) and (b) simulation of the growth in space (percolation). However, models of group (a) can be modified to treat spatial correlations approximately. The applications of the network formation theories to various model systems important for testing network formation and rubber elasticity theories are briefly discussed.
Reactions of styrene and derivatives with sulfur trioxide; characterization and chemistry of the initial products
Authors: Schonk Ruud M. , Bakker Bert H. , Cerfontain Hans 
Published: September 2010

AbstractThe reactions of styrene derivatives 1a‐14a with sulfur trioxide were studied in the temperature range −60 to 25°C using dichloromethane as solvent and 1.5 mol equiv. of dioxane as reactivity moderator. Reaction of styrene (1a) at low temperature yields the carbyl sulfate 1c as the only observable intermediate in the ultimate formation of (E)‐2‐phenylethene‐1‐sulfonic acid (1d). The (phenyl) substituted styrenes 3a‐5a clearly demonstrate the presence of the β‐sultones 3b‐5b as the sole initial products leading to the corresponding carbyl sulfates 3c‐5c. The initially formed β‐sultone b in the reaction of styrene derivatives 3a‐5a, 9a and 10a with 1.0 mol equiv. of SO3 disproportionates to yield equal amounts of the corresponding carbyl sulfate and the starting styrene. This is thought to indicate that there is an equilibrium between the β‐sultone and the starting styrene. The group of the 1‐methyl‐substituted styrenes 11a‐13a are an exception to the generally observed eventual conversion of the initial intermediates into the 2‐sulfonic acids d, since they give the corresponding 3‐sulfonic acids f.Mechanisms for the formation of the various products are proposed.
Synthesis and biological evaluation of A‐ring analogs of the natural germination stimulant strigol
Authors: Mangnus E. M. , Zwanenburg B. 
Published: September 2010

AbstractAn A‐ring‐derived analog of the natural germination stimulant, Strigol (1), has been prepared from citral in an unambiguous manner. This analog, 2, a γ‐hydroxy aldehyde for which a high stimulant activity was claimed in the literature, has been re‐evaluated as a germination stimulant for seeds of parasitic weeds and was found to be inactive. It was also shown that analog 2 is rather stable in aqueous solution, in contrast to other reports.In addition, some related A‐ and AB‐ring‐derived analogs have been prepared and biologically evaluated. The γ‐hydroxy acid 12 was obtained in one step from a mixture of α‐ and β‐cyclocitral in 58% yield. The dihydroxy analog 14 was prepared by reduction of the γ‐hydroxy ester 13. The AB‐ring analog 15 has been synthesized in five steps from ester 13 following published procedures. All analogs have been evaluated using Striga and Orobanche seeds; none of the compounds induced germination. It was concluded that the actiphore of strigol does not reside in the AB part of the molecule.
Competing concepts in electronic control of face selection
Authors: Li Haifang , Le Noble William J. 
Published: September 2010

AbstractThe addition of reagents to trigonal carbon to give tetrahedral centers and its reverse involve the stereogenetic aspect known as face selection, which has ramifications throughout organic chemistry. Some of the root factors of face selection (e.g., steric and conformational) have long been understood in principle, but the electronic contribution of substituents and skeletal changes is controversial. The principal models for the electronic basis of face selection are those of Felkin, Anh and Cieplak. It is argued that none of the three are either right or wrong, but that the contribution each can make, varies from case to case. Each of them may become dominant depending on the reaction at issue.
Computer tools for reaction retrieval and synthesis planning in organic chemistry. A brief review of their history, methods, and programs
Authors: Ott Martin A. , Noordik Jan H. 
Published: September 2010

AbstractAn overview is presented of computer programs designed to assist the synthetic organic chemist. Reaction retrieval, reaction prediction, and synthesis design programs are discussed, the development of these systems is outlined, and their aims and possibilities are described. Brief accounts of some representative systems are given.
Computer‐assisted synthetic analysis. Quantitative assessment of transform utilities
Authors: Corey E. J. , Long Alan K. , Lotto Gerald I. , Rubenstein Stewart D. 
Published: September 2010

AbstractA novel scheme for assigning initial, or substrate‐independent, ratings for transforms (retroreactions) in the chemical knowledge base for the LHASA program for computer‐assisted synthetic analysis is described. The new rating scheme is based on keyword scales defined for eight rating criteria. Initial ratings are calculated as weighted averages of the ratings in the eight categories. Various applications of initial ratings in LHASA are outlined.
Calix[4]arenes as building blocks for molecular receptors
Authors: van Loon Jan‐Dirk , Heida Jan F. , Verboom Willem , Reinhoudt David N. 
Published: September 2010

AbstractMolecular receptors for organic molecules are described in which calix[4]arene is used as a molecular platform rather than for its cavity. This is illustrated by the synthesis of a receptor for barbiturates, a calix[4]arene in a cone conformation containing two diaminotriazine moieties at the upper rim. This receptor (9) forms a molecular cleft in which phenobarbital complexes by hydrogen bonding both triazine rings. The association constant K is 520 M 1, when the self‐association of the barbiturate is taken into account.
Molecular recognition of barbiturates by a metalloreceptor
Authors: van Doorn Arie R. , Rushton David J. , van Straaten‐Nijenhuis Wilma F. , Verboom Willem , Reinhoudt David N. 
Published: September 2010

AbstractMetallomacrocycle 2, possessing an immobilized Lewis‐acidic uranyl cation, was synthesized by reaction of aldehyde 9 with 1,2‐cis‐cyclohexanediamine in the presence of Ba2+ as a template cation and subsequent transmetallation with UO22+. Docking experiments with barbituric acid revealed that a ‐(CH2)6– spacer between the salen‐uranyl and 2,6‐diamidopyridine moieties of 2 is optimal. According to 1H NMR titration experiments, the barbituric acid complex of 2 is up to 2.7 kcal · mol−1 more stable than the complexes with five disubstituted derivatives. The flux of barbituric acid through a supported liquid membrane is enhanced 3.7 times by carrier 2 mediated transport.
Spectroscopy and photochemistry of a series of alkyl‐substituted o‐ and p‐benzoquinones and their mono‐oxime O‐methyl ethers
Authors: Stunnenberg Frank , Cerfontain Hans , Rexwinkel Roel B. 
Published: September 2010

AbstractFollowing our previous reports on the spectroscopy and photochemistry of methyl ethers of cyclic α‐oxo oximes, we have now studied a number of o‐ and p‐benzoquinone mono‐oxime O‐methyl ethers and, for comparison, some corresponding o‐ and p‐benzoquinones. The ground‐state electronic structure of o‐benzoquinones is described satisfactorily by the sum of the two propenal fragments. The structure of the corresponding (E)‐ and (Z)‐mono‐oxime ethers can be described similarly as the sum of the propenal and (E)‐ or (Z)‐(methoxyimino)propene fragments. This method fails when used for the construction of the ground‐state electronic structure of the isomeric p‐benzoquinones and their mono‐oxime ethers.The UV spectra of o‐benzoquinone mono‐oxime ethers are quite different from those of the parent o‐benzoquinones in that the free lying n+ π* absorption of the latter is, for the former, shifted to lower wavelength and apparently hidden under the long‐wavelength ππ* absorptions. For the p‐benzoquinone mono‐oxime O‐methyl ether 12, however, the long‐wavelength nπ* absorption remains visible.Direct irradiation of the tert‐butyl‐substituted o‐benzoquinone mono‐oxime O‐methyl ethers (E)‐5–(E)‐7 dissolved in acetonitrile at λ 350 nm leads (in addition to E‐Z isomerization) to the formation of the (E)‐azobenzenes 16–19 by dimerization of the respective phenylnitrenes. However, in addition, the 2,4‐di‐tert‐butyl derivative (E)‐7 undergoes mainly γ‐hydrogen abstraction, probably from the S1(nπ*) state, leading to subsequent cyclization to the relatively stable dienol (E)‐14, which eventually isomerizes thermally to the enone (E)‐15. Evidence is presented that (Z)‐7 exhibits only Z‐E photoisomerization and no photocyclization, as has been observed previously with the saturated six‐membered ring analogue. Direct irradiation of the corresponding 3,5‐di‐tert‐butyl‐o‐benzoquinone (3) at 350 nm leads to decarbonylation with formation of cyclopentadienone 13. Sensitized irradiation of mono‐oxime O‐methyl ether (E)‐7, using 1‐acetylnaphthalene and 1‐fluorenone as triplet sensitizers and acetonitrile as solvent, leads to E‐Z isomerization only with eventual photo‐stationary state [E]/[Z] ratios of 0.78 and 0.88, respectively.The p‐benzoquinone mono‐oxime O‐methyl ethers 11 and 12 are photostable against 254, 300 and 350 nm radiation; however, any occurring E‐Z isomerization escapes detection for symmetry arguments.
Reactions of alkenoic acids with sulfur trioxide; sulfonation and sulfo‐lactonization
Authors: Schonk Ruud M. , Bakker B. H. , Cerfontain H. 
Published: September 2010

AbstractThe reactions of the alkenoic acid derivatives 1a–19a with sulfur trioxide were studied in the temperature range −60 to 25°C using dichloromethane as solvent and 1.5 equiv. of dioxane as reactivity moderator. The alkenoic acids 13a and 14a, having a ‐(CH2)4‐ and ‐(CH2)7‐ linkage between the C = C and CO2H moieties, respectively, react as simple alkenes; at low temperatures, they yield the β‐sultones 13d, 14c and 14d, which at 25°C are slowly converted into the corresponding carbyl sulfates 13h, 14g and 14h. Reactions of the alkenoic acids 4a–12a with 1.2 equiv. of SO3 at −60°C leads to the quantitative formation of the γ‐lactones 4e–9e and 11e, and in some cases to the γ‐lactones 10f and 12f. The stereochemistry of this sulfo‐lactonization is dependent on the SO3 concentration, which is clearly demonstrated for the reactions of (E)‐ (5a) and (Z)‐3‐hexenoic acid (6a) with 1.0 equiv. and 10.0 equiv. of SO3. For reactions of 5a and 6a with ≤ 1.0 equiv. of SO3, the β‐sultones 5c and 6c are the presumed sole initial products undergoing subsequent lactonization. Upon using an excess of SO3, in addition, the corresponding carbyl sulfates 5g and 6g will undergo sulfocyclization to give the cis‐ (6e) and trans‐γ‐lactones (5e), respectively.Reaction of 5‐norbornene‐endo‐2,endo‐3‐dicarboxylic anhydride (15a) with 1.2 equiv. of SO3 at 25°C yields slowly but quantitatively the β‐sultone 15c. This β‐sultone is surprisingly stable and it is very slowly converted into the γ‐lactone 15e only in the presence of H2O. Reactions of the norbornenecarboxylic acids and derivatives 16a–19a with SO3 yield the γ‐lactones 16e, 18e and 19e. Mechanisms for the formation of the various products are suggested.
Diastereo‐ and enantio‐selective synthesis of trans‐3‐phenyl‐ and trans‐3‐styryl‐2‐azetidinones via zinc ester enolates and imines
Authors: Kleijn Henk , van Maanen Hendrik L. , Jastrzebski Johann T. B. H. , van Koten Gerard 
Published: September 2010

AbstractZinc enolates of phenyl‐ and trans‐styryl‐acetic acid methyl ester react with excellent diastereoselectivety and good enantioselectivety (e.e. 64 to 91.5%) with imines in high yields to trans‐2‐azetidinones. The corresponding lithium enolates reacted with much lower selectivity and yields.
Enantioselective cleavage of esters by histidine‐containing tripeptides in micellar solutions of various hexadecyltrialkylammonium bromide surfactants
Authors: Cleij Marco C. , Drenth Wiendelt , Nolte Roeland J. M. 
Published: September 2010

AbstractCleavage of chiral p‐nitrophenyl esters derived from the amino acid phenylalanine by histidine‐containing tripeptides has been studied in micellar solutions of four quaternary ammonium surfactants. Enzyme‐like enantioselectivities up to kI/kD = 131 (at 0°C) are observed. The enantioselectivity can be rationalized by assuming a hydrophobically driven stabilizing hydrogen bond between the I. enantiomer of the ester and the tripeptide in the transition state of the reaction. This hydrogen bond is absent in the reaction with the D enantiomer of the ester. The transition state has an amphipolar character and is stabilized by the micellar environment. The hydrophilic‐hydrophobic balance of the reactants, which affects the transition state, was optimized by varying the composition of the tripeptide and the length of the N‐protecting groups in the tripeptide and the substrate. The activities and enantioselectivities depend on the structure of the quaternary ammonium surfactant headgroup. Increasing the size of this headgroup leads to an increase in rate of hydrolysis of the L ester and hence to an increase in enantioselectivity. This effect is attributed to a change in the degree of ion‐pair formation with a carboxylate group that is present in the peptides. Compared to previous studies the results indicate that a chiral surfactant is not required for obtaining high enantioselectivities.
Formation and [4 + 2] cycloaddition reactions of 2,3‐dimethylene‐2,3‐dihydrothiophene
Authors: van den Berg Keimpe J. , van Leusen Albert M. 
Published: September 2010

Abstract2,3‐Dimethylene‐2,3‐dihydrothiophene (1), the thiophene analog of o‐xylylene (o‐quinodimethane), was generatedin situfrom the (trialkylammoniomethyl)‐(trimethylsilylmethyl)thiophene iodides 4 or 5 by fluoride‐induced 1,4 climination, and was trapped by [4 + 2] cycloadditions with a series of dienophiles. The reaction of 1 with dimethyl fumarate was considerably faster than with dimethyl maleate. Unsymmetrical dienophiles gave mixtures of regio‐isomers. In the absence of dienophiles, 1 formed [4 + 2] spiro dimers, of which15ais the major component, as determined by1H‐NMR comparison with a tetradeutero analog of15a. The syntheses of the new precursors of1and dideutero‐1: compounds4, 5a,b, as well as the 3.4‐isomer of4, are described.
Molecular‐mechanics calculation on oligomers of tert‐butyl isocyanide
Authors: Huige Cornelis J. M. , Hezemans Alfons M. F. , Nolte Roeland J. M. , Drenth Wiendelt 
Published: September 2010

AbstractThe consistent force‐field method developed by Niketic and Rasmussen was applied in molecular‐mechanics calculations on the hexamer and hexadecamer of tert‐butyl isocyanide, [(CH3)3C‐N = C < ]n, n = 6 and 16. Due to the limitations of the computer programme, calculations on the latter oligomer required a united atom approximation for the methyl groups. From the results, it is concluded that a compact helical conformation with a backbone torsional angle of 78.6° and 3.76 units per turn is the energetically most favourable state of these oligomers.
A new convenient route for the synthesis of 4(5)‐(ω‐aminoalkyl)‐1 H‐imidazoles
Authors: Vollinga Roeland C. , Menge Wiro M. P. B. , Timmerman Hendrik 
Published: September 2010

AbstractA new route for the synthesis of 4(5)‐(ω‐aminoalkyl)‐1 H‐imidazoles 1 on a preparative scale through C5 lithiation of a 1,2‐diprotected imidazole is described. When the described 1,2‐diprotected 5‐lithio‐imidazole is treated with a 1‐chloro‐ω‐iodoalkane 5, selective substitution of the iodo group takes place. The chloro group of the resultant 1.2‐diprotected 5‐(ω chloroalkyl)imidazole can be easily converted into an amino group, but obviously other functional groups can be introduced as well. Subsequent deprotection of the 5‐substituted imidazole affords 4(5)‐(ω‐aminoalkyl)‐1 H‐imidazoles 1, as precursors for a large range of histaminergic compounds, in good overall yields.
Structural analysis of κ‐opioid receptor agonists
Authors: Ijzerman Adriaan P. 
Published: September 2010

AbstractIn the last decade, much interest has been shown in the development of agonists selective for the κ‐opioid receptor, which are potentially useful as analgesics with fewer side effects than morphine. The prototype drug is the benzeneacetamide U 50488, which was used in the present study as a starting point for molecular modelling of newer congeners. The heat of formation corresponding to the recently reported crystal structure of U 50488 was minimized with the semi‐empirical molecular orbital program MOPAC. Similar minimization and subsequent superimposition of all compounds used in the study yielded a template that was used for further discussion and manipulation. From the template, it appeared that compounds conformationally less restrained than U 50488, such as ICI 199441, may better accommodate the receptor environment. In addition, the template was used to generate ideas for the design of novel compounds. Finally, the little resemblance of the template to older, only marginally selective compounds, such as ethylketocyclazocine and the endogenous peptide dynorphin, supports the biochemical/pharmacological hypothesis of the existence of different subtypes of κ‐opioid receptors.