Science (Vol. 311, No. 5760, January 2006)

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A simple fragment deformation approach also has been applied and appears to work better for the larger rings. Barriers to interconversion of enantiomers in cyclic allenes are reduced with increasing strain. Estimated levels of strain parallel the known reactivity of these substances.

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A strategy utilizing N-heterocyclic carbenes NHCs derived from thiazolium salts has been developed for the generation of carbonyl anions from acylsilanes. These organocatalytic reactions are air- and water-tolerant methods to execute robust carbonyl anion addition reactions. Additionally, polysubstituted aromatic furans and pyrroles have been efficiently synthesized in a one-pot process using this carbonyl anion methodology.

The addition of alcohols to the reaction renders the process catalytic in thiazolium salt. In an effort to synthesize a potential intermediate along the proposed reaction pathway, silylated thiazolium carbinols have been identified to provide good yields of carbonyl anion addition products when subjected to the standard reaction conditions in the presence of suitable electrophiles. The palladium acetate-catalyzed coupling reaction of aryl boronic acid with carboxylic anhydride or acyl chloride was carried out smoothly in water in the presence of poly ethylene glycol PEG or 1-butylmethylimidazolium hexafluorophosphate [bmim][PF6] to give high yields of ketones without the use of phosphine ligands.

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A new multicomponent bifunctional catalytic system based on a titanium complex was used for the efficient enantioselective cyanation of aldehydes. It was revealed that the combination of 1e, 2a, 3b, and Ti IV was essential in this cyanation. A catalytic cycle based on experimental phenomena was proposed to explain the origin of the asymmetric induction.

The copper-catalyzed asymmetric conjugate addition of dialkyl zinc leads to homochiral zinc enolates. These intermediates were trapped in situ with activated allylic electrophiles, without the need of additional palladium catalysis. The functionalized nature of the electrophiles makes the new synthons potential candidates for further elaboration. When stereodefined alkenyl bromides are employed, the resulting styrene derivatives are accessed stereospecifically.


A variety of functional groups are tolerated in both coupling partners. When HAT derivative 7 with six perylenediimide moieties is incorporated into the one-dimensional aggregates, an efficient energy transfer takes place from the self-assembled HAT moiety as a light-harvesting antenna to the perylenediimide moiety as an energy acceptor. Further, when HAT derivative 8 with six triphenylamino moieties is newly added to the light-harvesting system, an intermolecular electron transfer occurs subsequently between the electron-accepting perylenediimide molecule and the electron-donating triphenylamino molecule.

The total synthesis of the proposed structure of feigrisolide A is reported. Ethyl S hydroxybutyrate was the chiral starting material. A Brown asymmetric allylation and an Evans aldol reaction were key steps of the synthesis. The NMR data of the synthetic product are different from those of the natural product.

The published structure of feigrisolide A is therefore erroneous. Phosphonium salts e. We recently reported the use of covalently attached DNA as a structural constraint for rational control of macromolecular conformation. These procedures should be generally applicable for attaching DNA to biomolecules. Selective excitation of a variety of anthracene derivatives in the presence of anthracene led to the formation of only the cross-cyclomer products, while irradiating these mixtures at wavelengths where anthracene also absorbs led to both dianthracene and the mixed cyclomers being formed.

This method was shown to be general, so long as the chromophore was inert toward forming its homodimer and could be irradiated at wavelengths at which anthracene does not absorb. Magnesium bistrifluoromethanesulfonimide catalyzed the acetylation of phenols, alcohols, and thiols under solvent-free conditions at room temperature and in short times. Electron-deficient and sterically hindered phenols provided excellent yields. The catalyst was found to be general for acylation with other anhydrides, such as propionic, isobutyric, pivalic, chloroacetic, and benzoic anhydrides.

The rate of acylation was influenced by the electronic and steric factors associated with the anhydride. The reaction with less electrophilic anhydrides e. A series of diazo carbonyl compounds bearing different substituents have been prepared in order to investigate the steric effect in 1,2-migration reaction of rhodium II carbene. Through the investigation on the diazo decomposition of these compounds with Rh2 OAc 4, it was found that the steric effect could dramatically influence the migratory aptitude. In many cases, the steric effect could override the inherent electronic effect of the substituent.

Symmetrically and unsymmetrically 2,5-difunctionalized phospholes bearing ester groups were prepared in a one-pot procedure from the corresponding diynes and dichloro phenyl phosphine via titanacyclopentadienes. Bis-dioxirane 3B is formed when two oxygens are incorporated into biacetyl, while mono-dioxirane 3A incorporated only one. The Kumada and Negishi cross-coupling reactions failed to provide any of the desired product. However, the Stille and Suzuki reactions furnished hippadine in low yields starting from the electron-deficient methyl 6-iodo- and 6-bromopiperonate, respectively.

Regioselective halogenation of the terminal isopropylidene unit of different acyclic polyolefinic polyprenoids farnesyl acetate, geranylgeranyl acetate, squalene, etc. Three-fold methyllithium addition to 2,4,6-trimethoxybenzene-1,3,5-tricarbaldehyde gives the anti,syn triol exclusively by 1H NMR ; addition of HMPA to the reaction or replacement of the substrate's methoxy groups with ethyl groups affords a statistical anti,syn:syn,syn diastereomeric product ratio.

Analogous asymmetric induction is found upon hydride reduction using LiAlH4 or NaBH4 of the complementary triketone, 2,4,6-trimethoxybenzene-1,3,5-triethanone. Chelation and steric gearing effects about the crowded aromatic core contribute to the observed stereoselectivity. This protocol tolerates a wide variety of functional groups or substrates and does not require the use of either acid or base catalysts.

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Aromatic azides are inert toward triethylsilane under thermal conditions in the presence of a radical initiator, but in the presence of additional catalytic amounts of tert-dodecanethiol, they afford anilinosilanes and thence the corresponding anilines in virtually quantitative yields. Hindered protected and unprotected epoxy alcohols were regioselectively cleaved using copper-catalyzed cis- and transpropenylmagnesium bromide. The reaction exhibited good yield and excellent regioselectivity in systems where organocuprates and organoalanes failed.

The cis Grignard reagent displayed no double-bond isomerization, whereas the trans isomer showed partial trans-to-cis equilibration, which was minimized by controlling the reagent formation conditions.

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Electronic effect of the alkyne component has shown to play an important role on the rate of the cycloaddition, and the reactivity of the alkyne component increases dramatically as the alkyne becomes more electron deficient. Increase in the steric bulk of the alkyne component decreases the reactivity of the alkyne component. A convenient method for preparing pyridines from air-stable, commercially available catalyst precursors is described. The in situ method also converts diynes and carbon dioxide to the corresponding pyrones.

The solid-phase reagent was reacted with unprotected nucleosides e. Pair your accounts. Your Mendeley pairing has expired. Please reconnect. July 1, Volume 71, Issue 15 Pages About the Cover: In natural double-stranded nucleic acids, the bases form Watson-Crick pairs in the core of the duplex. Incorporation of unnatural "bisheaded" nucleosides provides opportunities for additional interactions in the grooves of double-stranded synthetic DNA.

The new base-base interactions are symbolically represented by the different hands trying to communicate with each other.

See Herdewijn and co-workers, p View the article. In this issue: Share. Sort By: Page Date Articles. Crawford Christopher S. Straub Susan N. Pieniazek K. Kucera Katheryn E. Harrison Sogole Bahmanyar Jill M. Abstract Full text PDF ABSTRACT It is often found in mass spectrometry that when a molecule is protonated at the thermodynamically most favorable site, no fragmentation occurs, but a major reaction is observed when the proton migrates to a different position. Candeias Pedro M. Gois Carlos A.

Orendt Scott W. Roberts Jon D. France Huw M.

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Pearlman Sterling R. Putt Jeffrey A. Interconversion of Bicyclo[2. Abstract Full text PDF ABSTRACT This work provides unambiguous results for the full equilibration of cycloalkyne 2 with its isomeric ring-contracted vinylidene 3 prior to their reaction with 2,3-dihydropyran 4 , as evidenced by the products formed when 2 and 3 are produced directly from their precursors having bicyclo[2.

Butler Aoife M. Fahy Anthony Fox John C. Stephens P. McArdle D.

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Cunningham A. Doering Ting-hu Zhang Eckhart K. Kireev Oleg N. Nadein Vincent J. Agustin Nancy E. Ogasawara Shiva K. Rastogi Snezna Rogelj Scott T. Abstract Full text PDF ABSTRACT Pancratistatin is a potent anticancer natural product, whose clinical evaluation is hampered by the limited natural abundance and the stereochemically complex structure undermining practical chemical preparation.

Daoust Susanna M. Hernandez Kaleen M.

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