132 research outputs found
Mediterranean-type diet and brain structural change from 73 to 76 years in a Scottish cohort
STUDY FUNDING The data were collected by a Research into Ageing programme grant; research continues as part of the Age UKâfunded Disconnected Mind project. The work was undertaken by The University of Edinburgh Centre for Cognitive Ageing and Cognitive Epidemiology, part of the cross-council Lifelong Health and Wellbeing Initiative (MR/K026992/1), with funding from the BBSRC and Medical Research Council. Imaging and image analysis was performed at the Brain Research Imaging Centre (sbirc.ed.ac.uk/), Edinburgh, supported by the Scottish Funding Council SINAPSE Collaboration. Derivation of mean cortical thickness measures was funded by the Scottish Funding Councilâs Postdoctoral and Early Career Researchers Exchange Fund awarded by SINAPSE to David Alexander Dickie. L.C.A.C. acknowledges funding from the Scottish Government's Rural and Environment Science and Analytical Services (RESAS) division.Peer reviewedPublisher PD
Palladium-Catalyzed Direct Arylation of 5âHalouracils and 5âHalouracil Nucleosides with Arenes and Heteroarenes Promoted by TBAF
The
1-<i>N</i>-benzyl-5-iodoÂ(or bromo)Âuracil undergoes
Pd-catalyzed [Pd<sub>2</sub>(dba)<sub>3</sub>] direct arylation with
benzene and other simple arenes in the presence of TBAF in DMF without
the necessity of adding any ligands or additives to give 5-arylated
uracil analogues. The TBAF-promoted coupling also occurs efficiently
with electron rich heteroarenes at 100 °C (1 h) even with only
small excess of heteroarenes. The protocol avoids usage of the arylboronic
acid or stannane precursors for the synthesis of 5-(2-furyl, or 2-thienyl,
or 2-pyrrolyl)Âuracil nucleosides, which are used as important RNA
and DNA fluorescent probes. The fact that 1-<i>N</i>-benzyl-3-<i>N</i>-methyl-5-iodouracil did not undergo the TBAF-promoted
couplings with arenes or heteroarenes suggests that the C4-alkoxide
(enol form of uracil) facilitates coupling by participation in the
intramolecular processes of hydrogen abstraction from arenes. TBAF-promoted
arylation was extended into the other enolizable heterocyclic systems
such as 3-bromo-2-pyridone. The Ï-excessive heteroarenes also
coupled with 5-halouracils in the presence of PdÂ(OAc)<sub>2</sub>/Cs<sub>2</sub>CO<sub>3</sub>/PivOH combination in DMF (100 °C, 2 h)
to yield 5-arylated uracils
Mechanisms and Origins of Switchable Chemoselectivity of Ni-Catalyzed C(aryl)âO and C(acyl)âO Activation of Aryl Esters with Phosphine Ligands
Many experiments have shown that
nickel with monodentate phosphine
ligands favors the CÂ(aryl)âO activation over the CÂ(acyl)âO
activation for aryl esters. However, Itami and co-workers recently
discovered that nickel with bidentate phosphine ligands can selectively
activate the CÂ(acyl)âO bond of aryl esters of aromatic carboxylic
acids. The chemoselectivity with bidentate phosphine ligands can be
switched back to CÂ(aryl)âO activation when aryl pivalates are
employed. To understand the mechanisms and origins of this switchable
chemoselectivity, density functional theory (DFT) calculations have
been conducted. For aryl esters, nickel with bidentate phosphine ligands
cleaves CÂ(acyl)âO and CÂ(aryl)âO bonds via three-centered
transition states. The CÂ(acyl)âO activation is more favorable
due to the lower bond dissociation energy (BDE) of CÂ(acyl)âO
bond, which translates into a lower transition-state distortion energy.
However, when monodentate phosphine ligands are used, a vacant coordination
site on nickel creates an extra NiâO bond in the five-centered
CÂ(aryl)âO cleavage transition state. The additional interaction
energy between the catalyst and substrate makes CÂ(aryl)âO activation
favorable. In the case of aryl pivalates, nickel with bidentate phosphine
ligands still favors the CÂ(acyl)âO activation over the CÂ(aryl)âO
activation at the cleavage step. However, the subsequent decarbonylation
generates a very unstable <i>t</i>Bu-NiÂ(II) intermediate,
and this unfavorable step greatly increases the overall barrier for
generating the CÂ(acyl)âO activation products. Instead, the
subsequent CâH activation of azoles and CâC coupling
in the CÂ(aryl)âO activation pathway are much easier, leading
to the observed CÂ(aryl)âO activation products
Theoretical Elucidation of the Origins of Substituent and Strain Effects on the Rates of DielsâAlder Reactions of 1,2,4,5-Tetrazines
The DielsâAlder
reactions of seven 1,2,4,5-tetrazines with
unstrained and strained alkenes and alkynes were studied with quantum
mechanical calculations (M06-2X density functional theory) and analyzed
with the distortion/interaction model. The higher reactivities of
alkenes compared to alkynes in the DielsâAlder reactions with
tetrazines arise from the differences in both interaction and distortion
energies. Alkenes have HOMO energies higher than those of alkynes
and therefore stronger interaction energies in inverse-electron-demand
DielsâAlder reactions with tetrazines. We have also found that
the energies to distort alkenes into the DielsâAlder transition-state
geometries are smaller than for alkynes in these reactions. The strained
dienophiles, <i>trans</i>-cyclooctene and cyclooctyne, are
much more reactive than unstrained <i>trans</i>-2-butene
and 2-butyne, because they are predistorted toward the DielsâAlder
transition structures. The reactivities of substituted tetrazines
correlate with the electron-withdrawing abilities of the substituents.
Electron-withdrawing groups lower the LUMO+1 of tetrazines, resulting
in stronger interactions with the HOMO of dienophiles. Moreover, electron-withdrawing
substituents destabilize the tetrazines, and this leads to smaller
distortion energies in the DielsâAlder transition states
Contour plots (two-dimensional) for the regularization methods.
<p>The regularization parameters are <i>λ</i> = 1 and <i>α</i> = 0.2 for the HLR method.</p
The most frequently selected 10 genes found by the five sparse logistic regression methods from the lung cancer dataset.
<p>The most frequently selected 10 genes found by the five sparse logistic regression methods from the lung cancer dataset.</p
The performance of the AUC from ROC analyzes of each method on prostate, lymphoma and lung cancer datasets.
<p>The performance of the AUC from ROC analyzes of each method on prostate, lymphoma and lung cancer datasets.</p
Hydrogermylation of 5âEthynyluracil Nucleosides: Formation of 5â(2-Germylvinyl)uracil and 5â(2-Germylacetyl)uracil Nucleosides
A stereoselective
radical-mediated hydrogermylation of the protected
5-ethynyluracil nucleosides with trialkyl-, triaryl,- or trisÂ(trimethylsilyl)Âgermanes
gave (<i>Z</i>)<i>-</i>5-(2-germylvinyl)Âuridine,
2âČ-deoxyuridine, or <i>ara</i>-uridine as major products.
Reaction of the ÎČ-triphenylgermyl vinyl radical intermediate
with oxygen and fragmentation of the resulting peroxyradical provided
also 5-[2-(triphenylgermyl)Âacetyl]Âpyrimidine nucleosides in low to
moderate yields. Thermal isomerization of the latter in MeOH occurred
via a four-centered activated complex, and subsequent hydrolysis of
the resulting <i>O-</i>germyl substituted enol yielded 5-acetyluracil
nucleosides in quantitative yield
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