28 research outputs found
Synthesis of Cyclopropanated [2.2.1] Heterobicycloalkenes: An Improved Procedure
<div><p></p><p>A safer and improved method to our previous report on palladium-catalyzed cyclopropanation of heterobicyclic alkenes has been developed. By using THF as the solvent and a more dilute aqueous NaOH solution for the generation of diazomethane from DiazaldÂź, cyclopropanation could be achieved smoothly with minimal adjustment over the course of reaction. 7-Oxabicyclic substrates with bulky C1 or C2 groups, as well as 2,3-diazabicyclic substrates with various N-substituents effectively underwent cyclopropanation. Using this methodology, yields to previously reported products were markedly increased, and 10 new cyclopropanated [2.2.1] heterobicyclic products were prepared. In addition, this work accounts for the first reported cyclopropanation of 2,3-diazabicyclic alkenes, which all gave excellent yields of >90%.</p></div
Tunable Organophotocatalysts for Polymerization Reactions Under Visible Lights.
New organic photocatalysts derived from pyrene, anthracene,
naphthacene, and pentacene are presented here for the formation of
free radicals through a photoredox catalysis. These OPCs can work
according to <i>an oxidative cycle</i> in a three component
system in combination with diphenyl iodonium salt and a silane or
in <i>a reductive cycle</i> in combination with amine and
alkyl halide. This OPC behavior is highlighted through an investigation
of the associated excited state and redox properties. The free radicals
generated are characterized by ESR or photolysis experiments. Upon
household LED bulb or Xe lamp exposure, the oxidative three-component
system is able to promote the ring-opening polymerization ROP of an
epoxide whereas the reductive three-component system is very efficient
to initiate the free radical photopolymerization FRP of an acrylate.
This ability of OPCs to initiate different polymerization reactions
(ROP and FRP) is clearly an outstanding property
Organic Photocatalyst for Polymerization Reactions: 9,10-Bis[(triisopropylsilyl)ethynyl]anthracene
A new organic photocatalyst (9,10-bisÂ[(triisopropylsilyl)Âethynyl]Âanthracene, <b>An-Si</b>) is proposed here for the formation of free radicals
under very soft irradiation conditions under air through a photoredox
catalysis. It works according to an oxidative cycle that uses the
combination of <b>An-Si</b>, a diphenyl iodonium salt along
with a silane. This behavior is highlighted through an investigation
of its excited state and redox properties. The different chemical
intermediates are characterized by ESR experiments. In addition, the
reversibility of the oxidation reaction of <b>An-Si</b> was
investigated by cyclic voltammetry. This three-component system is
able to promote the ring-opening photopolymerization of an epoxide
as well as the free radical photopolymerization of an acrylate upon
household LED bulb and Xe lamp exposure. Excellent polymerization
profiles (mainly in ROP) are obtained. The specific properties of
this catalyst are outlined
Intramolecular Inverse Electron-Demand [4 + 2] Cycloadditions of Ynamides with Pyrimidines: Scope and Density Functional Theory Insights
4-Aminopyridines
are valuable scaffolds for the chemical industry
in general, from life sciences to catalysis. We report herein a collection
of structurally diverse polycyclic fused and spiro-4-aminopyridines
that are prepared in only three steps from commercially available
pyrimidines. The key step of this short sequence is a [4 + 2]/<i>retro</i>-[4 + 2] cycloaddition between a pyrimidine and an
ynamide, which constitutes the first examples of ynamides behaving
as electron-rich dienophiles in [4 + 2] cycloaddition reactions. In
addition, running the <i>ih</i>DA/<i>r</i>DA reaction
in continuous mode in superheated toluene, to overcome the limited
scalability of MW reactions, results in a notable production increase
compared to batch mode. Finally, density functional theory investigations
shed light on the energetic and geometric requirements of the different
steps of the <i>ih</i>DA/<i>r</i>DA sequence
Iridium Photocatalysts in Free Radical Photopolymerization under Visible Lights
A series of iridiumÂ(III) complexes was designed and investigated
as new potential photocatalysts for radical polymerization reactions.
The photocatalytic reduction cycle involves the combination of an
iridium complex with an amine (e.g., <i>N</i>-methyldiethanolamine)
and an alkyl halide (e.g., phenacylbromide). The different three-component
systems herein investigated are very attractive for acrylate polymerization
upon visible light irradiation. They are much more reactive than those
based on RuÂ(bpy)<sub>3</sub><sup>2+</sup>. Free radicals generated
during the reaction were investigated by ESR spectroscopy and the
chemical mechanisms are discussed. The crucial role played by the
photocatalyst (reduction ability and excited state lifetime) is also
demonstrated
Photopolymerization of Cationic Monomers and Acrylate/Divinylether Blends under Visible Light Using Pyrromethene Dyes
New photoinitiating systems based on boron-dipyrromethene
dye (bodipy)/iodonium
salt and optionally trisÂ(trimethylsilyl)Âsilane are proposed for the
polymerization of divinylether and epoxy monomers upon visible-light
exposure. The presence of the silane increases the epoxide rate of
polymerization and conversion. Using acrylate/vinyl ether blends,
the synthesis of cross-linked polymer networks (possessing two <i>T</i><sub>g</sub> values: â11 and 111 °C) is also
successfully achieved through concomitant cationic and radical polymerization
pathways. The chemical mechanisms associated with these initiating
systems are investigated by steady-state photolysis and ESR experiments
Synthetic Variants of Mycolactone Bind and Activate WiskottâAldrich Syndrome Proteins
Mycolactone is a complex macrolide
toxin produced by <i>Mycobacterium
ulcerans</i>, the causative agent of skin lesions called Buruli
ulcers. Mycolactone-mediated activation of neural (N) WiskottâAldrich
syndrome proteins (WASP) induces defects in cell adhesion underpinning
cytotoxicity and disease pathogenesis. We describe the chemical synthesis
of 23 novel mycolactone analogues that differ in structure and modular
assembly of the lactone core with its northern and southern polyketide
side chains. The lactone core linked to southern chain was the minimal
structure binding N-WASP and hematopoietic homolog WASP, where the
number and configuration of hydroxyl groups on the acyl side chain
impacted the degree of binding. A fluorescent derivative of this compound
showed time-dependent accumulation in target cells. Furthermore, a
simplified version of mycolactone mimicked the natural toxin for activation
of WASP in vitro and induced comparable
alterations of epithelial cell adhesion. Therefore, it constitutes
a structural and functional surrogate of mycolactone for WASP/N-WASP-dependent
effects
Synthetic Variants of Mycolactone Bind and Activate WiskottâAldrich Syndrome Proteins
Mycolactone is a complex macrolide
toxin produced by <i>Mycobacterium
ulcerans</i>, the causative agent of skin lesions called Buruli
ulcers. Mycolactone-mediated activation of neural (N) WiskottâAldrich
syndrome proteins (WASP) induces defects in cell adhesion underpinning
cytotoxicity and disease pathogenesis. We describe the chemical synthesis
of 23 novel mycolactone analogues that differ in structure and modular
assembly of the lactone core with its northern and southern polyketide
side chains. The lactone core linked to southern chain was the minimal
structure binding N-WASP and hematopoietic homolog WASP, where the
number and configuration of hydroxyl groups on the acyl side chain
impacted the degree of binding. A fluorescent derivative of this compound
showed time-dependent accumulation in target cells. Furthermore, a
simplified version of mycolactone mimicked the natural toxin for activation
of WASP in vitro and induced comparable
alterations of epithelial cell adhesion. Therefore, it constitutes
a structural and functional surrogate of mycolactone for WASP/N-WASP-dependent
effects
Soft Photopolymerizations Initiated by Dye-Sensitized Formation of NHC-Boryl Radicals under Visible Light
A procedure for the production of N-heterocyclic carbeneâboryl
radicals (NHC-BH<sub>2</sub><sup>âą</sup>) upon visible light
irradiation under soft conditions is presented. New acridine orange
(dye)/diphenyl disulfide/NHCâBH<sub>3</sub> and dye/sulfonium
salt/NHCâBH<sub>3</sub> three-component initiating systems
are introduced for the efficient visible light photopolymerization
of trimethylolpropane triacrylate. The new systems could be extendend
to polymerization reactions in water (hydroxyethyl acrylate and hydroxyethyl
methyl acrylate), which proceeded with strongly improved polydispersity.
The chemical mechanisms are investigated through EPR and photolysis
experiments
Grafting subdominant SM9 at GRA6<sub>II</sub> C-terminus enhances its presentation, overturns the dominance hierarchy and provides efficient cyst control.
<p>(A) Schematic description of the GRA6<sub>II</sub>-SM9 chimeric constructs. For GRA6<sub>II</sub>-SM9<sub>Cter</sub>, SM9 was introduced at the C-terminus, downstream of HF10. For GRA6<sub>II</sub>-SM9<sub>internal</sub>, SM9 was introduced internally within GRA6<sub>II</sub> after residue 153, before the putative transmembrane domain. In both cases, SM9 was preceded by a leucine to mimic the endogenous flanking sequence of HF10. (B) Western blot analysis of GRA6 (upper panels) and SAG1 (loading control, lower panels) of the indicated parasite clones. (C,D) SM9 and HF10 presentation by BMDMs infected for 24 h with the indicated parasites, assessed with the BDSM9Z (C) and CTgEZ.4 (D) hybridomas, respectively. (E,F,G) <i>Ex vivo</i> evaluation of the CD8 responses and the parasite load 3 weeks post-infection with the indicated parasites. (E) IFN-Îł intracellular staining of spleen cells restimulated with the SM9 peptide (left panel) or the HF10 peptide (right panel). (F) Tetramer staining of brain cells with SM9:L<sup>d</sup> (left panel) and HF10:L<sup>d</sup> (right panel). Bars represent the mean +/â SEM. Data pooled from 2 independent experiments. (G) Parasite burden in the brains of mice infected with control CEP (white bar), CEP+GRA6<sub>II</sub>-SM9<sub>internal</sub> (black bar) or CEP+GRA6<sub>II</sub>-SM9<sub>Cter</sub> (hatched bar), evaluated microscopically by enumerating the cysts. Histograms represent the mean +/â SEM of 14 mice per group, pooled from 3 independent experiments. *: <i>P</i><0.05 ; **: <i>P</i><0.005.</p