Showing papers on "Intramolecular force published in 2002"
TL;DR: The glutathione peroxidase-like enzyme Gpx3 is identified as a second component of the pathway, serving the role of sensor and transducer of the hydroperoxide signal to Yap1, revealing a redox-signaling function for a GPx- like enzyme and elucidate a eukaryotic Hydroperoxide-sensing mechanism.
838 citations
TL;DR: Characterization of the gelation process, from the molecular level up through the macroscopic properties of the material, suggests that by linking the intramolecular folding of small designed peptides to their ability to self-assemble, responsive materials can be prepared.
Abstract: A general peptide design is presented that links the pH-dependent intramolecular folding of β-hairpin peptides to their propensity to self-assemble, affording hydrogels rich in β-sheet. Chemical responsiveness has been specifically engineered into the material by linking intramolecular folding to changes in solution pH, and mechanical responsiveness, by linking hydrogelation to self-assembly. Circular dichroic and infrared spectroscopies show that at low pH individual peptides are unstructured, affording a low-viscosity aqueous solution. Under basic conditions, intramolecular folding takes place, affording amphiphilic β-hairpins that intermolecularly self-assemble. Rheology shows that the resulting hydrogel is rigid but is shear-thinning. However, quick mechanical strength recovery after cessation of shear is observed due to the inherent self-assembled nature of the scaffold. Characterization of the gelation process, from the molecular level up through the macroscopic properties of the material, suggests ...
812 citations
TL;DR: A protocol is described for the treatment of molecular polarization in force field calculations and a group‐based scheme for including direct intramolecular polarization is shown to be most successful in accounting for the conformational dependence of electrostatic potentials.
Abstract: A protocol is described for the treatment of molecular polarization in force field calculations. The resulting model is consistent in that both inter- and intramolecular polarization are handled within a single scheme. An analytical formula for removing intramolecular polarization from a set of atomic multipoles for an arbitrary static structure or conformation is given. With the help of the intramolecular polarization, these permanent atomic multipoles can then be applied in modeling alternative conformations of a molecule. Equipped with this simple technique, one can derive transferable electrostatic parameters for peptides and proteins using flexible model compounds such as dipeptides. The proposed procedure is tested for its ability to describe the electrostatic potential around various configurations of the N-methylacetamide dimer. The effect of different intramolecular polarization schemes on the accuracy of a force field model of the electrostatic potential of alanine dipeptide is investigated. A group-based scheme for including direct intramolecular polarization is shown to be most successful in accounting for the conformational dependence of electrostatic potentials.
535 citations
TL;DR: A family of chiral triazolium salts has been developed for inducing the asymmetric intramolecular Stetter reaction, with optimal results, with the product keto esters formed in 82-97% ee and very good chemical yield.
Abstract: A family of chiral triazolium salts has been developed for inducing the asymmetric intramolecular Stetter reaction. The use of an aminoindanol-derived catalyst affords optimal results, with the product keto esters formed in 82-97% ee and very good chemical yield. Aromatic and aliphatic aldehydes are equally competent substrates for this reaction. The reaction conditions are reasonably mild and allow the isolation of the newly formed stereocenter without epimerization, although the presumed carbenic intermediates are strong bases.
391 citations
TL;DR: In this paper, a force field for liquid-state simulations on ionic liquids containing 1-ethyl-3-methylimidazolium and 1-n-butyl-3methyloroaluminate and tetrafluoroborate anions is presented.
Abstract: We present a complete force field for liquid-state simulations on ionic liquids containing 1-ethyl-3-methylimidazolium and 1-n-butyl-3-methylimidazolium cations and the tetrachloroaluminate and tetrafluoroborate anions. The force field is compatible with the AMBER methodology and is easily extendable to other dialkylimidazolium salts. On the basis of the general AMBER procedures to develop lacking intramolecular parameters and the RESP approach to calculate the atomic point charges, we obtained an all-atom force field which was validated against the experimental density, diffusion coefficient, vibrational frequencies, as well as X-ray (crystal state) and neutron (liquid state) diffraction structural data. Moreover, molecular mechanics calculations for the developed force field produce the cation's structures and dipole moments in very good agreement with quantum mechanical ab initio calculations. In addition, a basic study concerning the simulated liquid structure in terms of the radial distribution funct...
311 citations
237 citations
235 citations
TL;DR: In this paper, the excited-state intramolecular proton transfer (ESIPT) of a series of water-soluble 2-(2‘-hydroxyphenyl)benzazole derivatives has been studied under physiological conditions using absorbance and steady-state emission spectroscopy.
Abstract: The excited-state intramolecular proton transfer (ESIPT) of a series of water-soluble 2-(2‘-hydroxyphenyl)benzazole derivatives has been studied under physiological conditions using absorbance and steady-state emission spectroscopy. At neutral pH in the presence of 0.1 M ionic background, the fluorescence properties of these derivatives differ substantially compared to previously reported data in nonaqueous solvents. The ESIPT process is disrupted, presumably due to intermolecular hydrogen bonding with surrounding water molecules combined with increased stabilization of the trans-rotamer, which cannot undergo the ESIPT process. The emission spectrum of the benzimidazole derivative depends significantly on the solvent polarity, as revealed by titrations with Zn(II) in methanol, ethanol, and under physiological conditions. Inhibition of ESIPT via metal coordination shows a significant wavelength shift together with a substantial ratio increase by a factor of 13.7. Titration of the benzoxazole derivative wit...
209 citations
TL;DR: A catalytic Michael cycloisomerization of bis(enones) under Morita-Baylis-Hillman conditions is reported, finding that upon exposure to 10 mol % tributylphosphine, bis( enone) substrates afford both five- and six-membered ring products.
Abstract: The utilization of enones as latent enolates enables regioselective enolate formation from chemically robust presursors. In this communication, we report a catalytic Michael cycloisomerization of bis(enones) under Morita−Baylis−Hillman conditions. Upon exposure to 10 mol % tributylphosphine, bis(enone) substrates afford both five- and six-membered ring products. Notably, unsymmetrical bis(enones) possessing sufficient steric or electronic bias yield single isomeric products.
200 citations
TL;DR: The inter- and intramolecular Diels-Alder reactions of in situ generated masked o-benzoquinones produced cycloadducts in excellent selectivities and the efficacy of the methodology is demonstrated by several examples of the total synthesis of natural products.
Abstract: An account of the synthetic utility of masked o-benzoquinones is provided. The inter- and intramolecular Diels-Alder reactions of in situ generated masked o-benzoquinones produced cycloadducts in excellent selectivities. New synthetic methodologies have been developed for the synthesis of highly substituted ring systems including bicyclo[2.2.2]octenones, oxatricycles, triquinanes, polysubstituted cyclohexanes, and bicyclo[4.2.2]decenones with complete stereocontrol from easily accessible 2-methoxyphenols via the Diels-Alder reaction of masked o-benzoquinones. Other reactions of adducts derived from masked o-benzoquinones are also described. The efficacy of our methodology is demonstrated by several examples of the total synthesis of natural products.
197 citations
TL;DR: In this article, the enthalpy difference between the hydrogen-bonded (HB) form and the lowest energy conformer in which the OH is rotated into the “away” position was defined.
Abstract: For a series of 61 2-substituted phenols, 2-X−PhOH, forming a total of 73 intramolecular hydrogen bonds, the intramolecular hydrogen bond enthalpy, ΔHintra-HB, has been determined by density functional theory (DFT) calculations on the B3LYP/6-31G(d,p)//B3LYP/6-31G(d,p) level. The ΔHintra-HB was defined as the enthalpy difference between the hydrogen-bonded (HB) form and the lowest-energy conformer in which the OH is rotated into the “away” position. The correlation of ΔHintra-HB with geometrical factors such as r(O−H), or r(OH···A), with A as the hydrogen bond accepting atom, was generally very poor, showing that none of these parameters can be used as an universal descriptor for the hydrogen bond strength. The relation between ΔHintra-HB and ν(O−H) intra-HB is also insignificant, in contrast with previous estimates. The data clearly demonstrate that the genuine ΔHintra-HB of a phenolic compound cannot be unequivocally derived by simple rotation of the OH group into the “away” orientation, because additio...
TL;DR: Acyloxyl radicals RC(O)O* (with R = alkyl, aryl) could be trapped through addition to cyclic and open-chain alkynes, where they were found to act as a donor of oxygen atoms.
Abstract: Acyloxyl radicals RC(O)O• (with R = alkyl, aryl) could be trapped through addition to cyclic and open-chain alkynes, where they were found to act as a donor of oxygen atoms. Mechanistically, this radical oxygenation proceeded through a transannular or intramolecular, respectively, radical cyclization cascade, which was finally terminated by release of an acyl radical RC•(O). The reaction led to stereoselective formation of cyclized products, which contained a carbonyl group at the former site of the alkyne triple bond.
TL;DR: Theoretical ab initio calculations were carried out to simulate the hydrolysis of the siloxane bond under neutral and acidic conditions in this article, where the most important factors reducing the energy barrier of the reaction are the protonation of the nucleophile (acid catalysis) and the basic assistance provided by hydrogen bond complexes, which help to transfer a proton from a nucleophile to the leaving group.
TL;DR: Intramolecular insertion of a C-C double bond into aC-C single bond was achieved by treatment of cyclobutanone bearing an o-styryl group at the 3-position with a catalytic amount of a cationic rhodium(I)-dppp complex.
Abstract: Intramolecular insertion of a C−C double bond into a C−C single bond was achieved by treatment of cyclobutanone bearing an o-styryl group at the 3-position with a catalytic amount of a cationic rhodium(I)-dppp complex. Initially, rhodium is inserted between the carbonyl carbon and the α-carbon of the cyclobutanone. Intramolecular coordination of the vinyl group results in its migratory insertion into the C−Rh linkage. Reductive elimination affords benzobicyclo[3.2.1]octan-3-one. Notably, a ring-opened α,β-unsaturated ketone was obtained when dppe was used instead of dppp. In this reaction, rhodium cleaved the bond between the α sp3 carbon and the β sp3 carbon of the cyclobutanone. The coordinating vinyl group directs this new regioselectivity of cleavage observed with the dppe ligand.
TL;DR: This work presents for the first time the single-molecule spectroscopy of a new molecular probe which uses an intramolecular electron transfer mechanism to detect binding, local structure, and interfacial processes.
Abstract: Detecting structure, dynamics, and chemical reactions at the single-molecule level represents the ultimate degree of sensitivity for sensing and imaging. There is a tremendous need to develop new molecular systems and methodology for single-molecule-based sensing. This work presents for the first time the single-molecule spectroscopy of a new molecular probe which uses an intramolecular electron transfer mechanism to detect binding, local structure, and interfacial processes. Moreover, we show how information about the interaction of these probes with their environment is obtained from an analysis of the intensity, duration and time-varying behavior of the single-molecule fluorescence.
TL;DR: The solvent-dependent ground-state conformational equilibrium and excited-state dynamics of 2-(2'-hydroxyphenyl)benzoxazole have been characterized in several solvents on the femtosecond to nanosecond time scales.
Abstract: The solvent-dependent ground-state conformational equilibrium and excited-state dynamics of 2-(2‘-hydroxyphenyl)benzoxazole have been characterized in several solvents on the femtosecond to nanosecond time scales. The only observable ground-state tautomer is the enol, which exists in equilibrium between the syn- and anti-rotational isomers. In the anti-enol isomer, the phenyl hydroxyl group appears to not interact strongly with solvent but rather forms a strong intramolecular hydrogen bond with the benzoxazole oxygen atom. In the syn-enol isomer, the phenyl hydroxyl proton may interact with solvent or form an internal hydrogen bond with the benzoxazole nitrogen atom. Upon excitation, the proton is transferred from the oxygen atom to the nitrogen atom of the internally hydrogen bonded syn-enol isomer in 170 fs, regardless of the solvent. The lifetime of the resulting excited keto tautomer is solvent dependent and on the order of picoseconds. In addition to these dynamics, several additional dynamic process...
TL;DR: An N-heterocyclic carbene complex was found to be the active catalyst in the Rh(I)-catalyzed intramolecular coupling of an alkenyl group to a C-H bond of a substituted benzimidazole.
Abstract: An N-heterocyclic carbene complex was found to be the active catalyst in the Rh(I)-catalyzed intramolecular coupling of an alkenyl group to a C−H bond of a substituted benzimidazole. Kinetic studie...
TL;DR: The calculated values fit the observed data within reasonable limits and prove that two-point hydrogen bonding can be sufficient to achieve a preparatively useful face differentiation in solution phase photochemistry.
Abstract: The [2 + 2] photocycloaddition of 4-alkoxy-2-quinolones was conducted in the presence of the chiral lactams 5 or ent-5. At −60 °C in toluene as the solvent the intramolecular reaction of quinolones 6 and 8 as well as the intermolecular photocycloaddition of various alkenes 13 to quinolone 12 proceeded with excellent enantioselectivity (81−98% ee) and in high yields (61−89%). Styrene (13d) reacted sluggishly in the intermolecular reaction (29% yield, 83% ee). The absolute configuration of the intramolecular photocycloaddition products 7 and 9 was elucidated by single-crystal X-ray crystallography of the corresponding diastereomeric N-menthyloxycarbonyl derivatives. The relative configuration of the intermolecular photocycloaddition products 14 and 15 was assigned on the basis of NOESY experiments and on crystallographic evidence. The differentiation of the enantiotopic faces in the prochiral quinolones 6, 8, and 12 can be explained by assuming a coordination of these substrates to the lactams 5 or ent-5 vi...
TL;DR: These new iron porphyrin carbene complexes were characterized by NMR and UV-vis spectroscopy, mass spectrometry, and elemental analyses and produced allylic or benzylic C-H insertion products in up to 83% yield.
Abstract: Reactions of [Fe(TPFPP)] (TPFPP = meso-tetrakis(pentafluorophenyl)porphyrinato dianion) with diazo compounds N2C(Ph)R (R = Ph, CO2Et, CO2CH2CHCH2) afforded [Fe(TPFPP)(C(Ph)R)] (R = Ph (1), CO2Et (2), CO2CH2CHCH2 (3)) in 65−70% yields. Treatment of 1 with N-methylimidazole (MeIm) gave the adduct [Fe(TPFPP)(CPh2)(MeIm)] (4) in 65% yield. These new iron porphyrin carbene complexes were characterized by NMR and UV−vis spectroscopy, mass spectrometry, and elemental analyses. X-ray crystal structure determinations of 1·0.5C6H6·0.5CH2Cl2 and 4 reveal FeCPh2 bond lengths of 1.767(3) (1) and 1.827(5) A (4), together with large ruffling distortions of the TPFPP macrocycle. Complexes 2 and 4 are reactive toward styrene, affording the corresponding cyclopropanes in 82 and 53% yields, respectively. Complex 1 is an active catalyst for both intermolecular cyclopropanation of styrenes with ethyl diazoacetate and intramolecular cyclopropanation of allylic diazoacetates. Reactions of 2 and 4 with cyclohexene or cumene prod...
TL;DR: The core of teleocidin B4, a complex fragment of a natural product containing two quaternary stereocenters and a penta-substituted benzene ring, was synthesized in four C-C bond-forming steps starting from tert-butyl derivative 1.
Abstract: The core of teleocidin B4, a complex fragment of a natural product containing two quaternary stereocenters and a penta-substituted benzene ring, was synthesized in four C-C bond-forming steps starting from tert-butyl derivative 1. The first step involved alkenylation of the tert-butyl group with a vinyl boronic acid, followed by the successful annulation of the cyclohexane ring to the benzene nucleus via an intramolecular Friedel-Crafts reaction. The third step required a diastereoselective oxidative carbonylation of the geminal dimethyl group, followed at last by indole assembly via the alkenylation of the phenol nucleus, to afford the teleocidin B4 core. Noteworthy is the fact that steps 1 and 3 critically depended on the directing role of the aniline nitrogen (directed C-H bond functionalization).
TL;DR: These investigations probe the conformational order of bulk octadecane and low molecular weight polyethylene as they undergo solid/liquid phase transitions and demonstrate that Raman spectroscopy is sensitive to very subtle changes in alkane chain structure and conformation.
Abstract: The correlation of Raman spectral indicators for the determination of alkyl chain interactions and conformational order is presented. These investigations probe the conformational order of bulk octadecane and low molecular weight polyethylene as they undergo solid/liquid phase transitions. Spectral indicators are quantitatively correlated to the I[νa(CH2)]/I[νs(CH2)], as this is the primary indicator of rotational and conformational order obtained empirically from Raman spectra. These indicators are interpreted in terms of alkane intramolecular motion, intermolecular interactions between alkyl chains, crystal structure of these solid materials, and the presence of methylene conformers. Results demonstrate that Raman spectroscopy is sensitive to very subtle changes in alkane chain structure and conformation. These results can be used to understand molecular interactions and structure−function relationships in alkane-based materials.
TL;DR: In this paper, a novel synthesis of benzimidazoles by a palladium-catalysed intramolecular N-arylation reaction from (o-bromophenyl)amidine precursors is described.
TL;DR: 1,8-Naphthalenediol, 5, and its 4-methoxy derivative, 6, were found to be potent H-atom transfer (HAT) compounds on the basis of their rate constants for H- atom transfer to the 2,2-di(4-t-octylphenyl)-1-picrylhydrazyl radical (DOPPH)*, or as antioxidants during inhibited styrene autoxidation, k(ArOH/ROO
Abstract: 1,8-Naphthalenediol, 5, and its 4-methoxy derivative, 6, were found to be potent H-atom transfer (HAT) compounds on the basis of their rate constants for H-atom transfer to the 2,2-di(4-t-octylphenyl)-1-picrylhydrazyl radical (DOPPH•), kArOH/DOPPH•, or as antioxidants during inhibited styrene autoxidation, kArOH/ROO•, initiated with AIBN. The rate constants showed that 5 and 6 are more active HAT compounds than the ortho-diols, catechol, 1, 2,3-naphthalenediol, 2, and 3,5-di-tert-butylcatechol, 3. Compound 6 has almost twice the antioxidant activity, kArOH/ROO• = 6.0 × 106 M-1 s-1, of that of the vitamin E model compound, 2,2,5,7,8-pentamethyl-6-chromanol, 4. Calculations of the O−H bond dissociation enthalpies compared to those of phenols, (ΔBDEs), of 1−6 predict a HAT order of reactivity of 2 < 1 < 3 ≈ 4 < 5 < 6 in general agreement with kinetic results. Calculations on the diols show that intramolecular H-bonding stabilizes the radicals formed on H-atom transfer more than it does the parent diols, and ...
TL;DR: In this article, a force field for the room temperature molten salt 1-ethyl-3methylimidazolium tetrachloroaluminate was developed and successfully tested against experimental data (neutron diffraction, diffusion constants) by molecular dynamics computer simulation.
Abstract: A classical force field for the room temperature molten salt 1-ethyl-3-methylimidazolium tetrachloroaluminate has been developed and successfully tested against experimental data (neutron diffraction, diffusion constants) by molecular dynamics computer simulation corresponding to a temperature of 298 K. The force field parameters for the cation have been derived from the AMBER description for the protonated amino acid histidine, whereas the AlCl4- parameters have been achieved by parametrization of intramolecular terms with van der Waals parameters taken from the literature. All atomic partial charges have been obtained from ab initio calculations using the RESP methodology.
TL;DR: This Account describes the development and mechanistic study of the cycloisomerization and cyclization/hydrosilylation of functionalized dienes catalyzed by cationic palladium(II) complexes, characterized by good functional group compatibility, high regio- and stereoselectivity, and low air- and moisture-sensitivity.
Abstract: This Account describes the development and mechanistic study of the cycloisomerization and cyclization/hydrosilylation of functionalized dienes catalyzed by cationic palladium(II) complexes. These transformations are characterized by good functional group compatibility, high regio- and stereoselectivity, and low air- and moisture-sensitivity. Mechanistic studies of palladium phenanthroline-catalyzed diene cycloisomerization and cyclization/hydrosilylation established mechanisms involving C-C bond formation via intramolecular carbometalation of an alkyl olefin chelate complex, which was directly observed in the context of cyclization/hydrosilylation.
TL;DR: A localized orbital model is proposed in which low-lying acceptor orbitals on the center portion of the ligands effectively quench the Ru(II)-based MLCT emission via a mechanism that can be viewed as intramolecular electron transfer to specific subunits of the bridges.
Abstract: Two new ruthenium(II) polypyridyl dimers containing the large planar aromatic bridging ligands 9,11,20,22-tetraazatetrapyrido[3,2-a:2‘3‘-c:3‘ ‘,2‘ ‘-l:2‘ ‘‘,3‘ ‘‘-n]pentacene (tatpp) and 9,11,20,22-tetraazatetrapyrido[3,2-a:2‘3‘-c:3‘ ‘,2‘ ‘-l:2‘ ‘‘,3‘ ‘‘-n]pentacene-10,21-quinone (tatpq) have been synthesized and characterized by 1H and 13C NMR, MALDI mass spectrometry, and elemental analyses. The electronic properties (UV−vis, redox, photophysical) of these dimers are analyzed in the context of orbital calculations (PM3 level) on the bridging ligands. A localized orbital model is proposed in which low-lying acceptor orbitals on the center portion of the ligands effectively quench the Ru(II)-based MLCT emission via a mechanism that can be viewed as intramolecular electron transfer to specific subunits of the bridges.
TL;DR: Excellent enantioselectivities up to 96% ee are reached for the first time in a Heck reaction with monodentate ligands.
Abstract: An efficient enantioselective intramolecular Heck reaction of cyclohexadienones, using readily available and modular TADDOL-based mono- and bidentate phosphoramidites as chiral ligands and not requiring any additives, has been developed. Excellent enantioselectivities up to 96% ee are reached for the first time in a Heck reaction with monodentate ligands.
TL;DR: In this paper, a unique combination of copper iodide and cesium acetate was found to mediate intramolecular amination of aryl halides under mild conditions.
Abstract: A unique combination of copper iodide and cesium acetate was found to mediate intramolecular amination of aryl halides under mild conditions. The reaction proceeds at room temperature with primary or N-benzyl amines and at moderately elevated temperatures with other amine derivatives. The reaction has been applied to the formation of 5-, 6-, and 7-membered rings. Remarkably, halogens at the meta-position were retained, providing a definitive advantage over palladium-catalyzed systems.