- A Stimuli-Responsive Macromolecular Gear: Interlocking Dynamic Helical Polymers with Foldamers
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Herein, macromolecular gears composed of helical poly(phenylacetylenes) (PPAs) bearing short oligopeptides as pendant groups are described, in which the two structural motifs (framework and substituents) are combined. These gears are obtained by polymerization of the acetylene groups introduced at the C-terminus of short oligopeptides formed by achiral (Aib)n units (n=1–3) derivatized at the N-terminus by a single enantiomer (R or S) of α-methoxy-α-trifluoromethylphenylacetic acid (MTPA, Mosher's reagent). The chiral information of the MTPA is transmitted to the achiral Aib fragments and, through either chiral tele-induction and/or chiral harvesting mechanisms, is further transferred to the polyene backbones, which adopt preferentially P or M helical senses. Moreover, these materials also show dynamic behavior and respond to the action of external stimuli by either inverting the P/M sense and/or modifying the elongation in fully reversible processes.
- Freire, Félix,Qui?oá, Emilio,Riguera, Ricardo,Rodríguez, Rafael,Suárez-Picado, Esteban
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supporting information
p. 8616 - 8622
(2020/04/15)
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- Nucleophilic (Radio)Fluorination of Redox-Active Esters via Radical-Polar Crossover Enabled by Photoredox Catalysis
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We report a redox-neutral method for nucleophilic fluorination of N-hydroxyphthalimide esters using an Ir photocatalyst under visible light irradiation. The method provides access to a broad range of aliphatic fluorides, including primary, secondary, and tertiary benzylic fluorides as well as unactivated tertiary fluorides, that are typically inaccessible by nucleophilic fluorination due to competing elimination. In addition, we show that the decarboxylative fluorination conditions are readily adapted to radiofluorination with [18F]KF. We propose that the reactions proceed by two electron transfers between the Ir catalyst and redox-active ester substrate to afford a carbocation intermediate that undergoes subsequent trapping by fluoride. Examples of trapping with O- and C-centered nucleophiles and deoxyfluorination via N-hydroxyphthalimidoyl oxalates are also presented, suggesting that this approach may offer a general blueprint for affecting redox-neutral SN1 substitutions under mild conditions.
- Webb, Eric W.,Park, John B.,Cole, Erin L.,Donnelly, David J.,Bonacorsi, Samuel J.,Ewing, William R.,Doyle, Abigail G.
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supporting information
p. 9493 - 9500
(2020/05/18)
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- Hydrogen-bond-assisted helical folding of propeller-shaped molecules: Effects of extended π-conjugation on chiral selection, conformational stability, and exciton coupling
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Cooperative interaction between multiple chiral centers dictates the absolute handedness of structural folding. We have designed and prepared a series of chiral C3-symmetric tris(N-salicylidenamine) derivatives that adopt three-blade propeller-like conformations. Synthetic access to an expanded family of such constructs was aided by enzymatic resolution and C-C cross-coupling reactions of aryl-substituted chiral propargylic alcohol derivatives. These key structural components were integrated into molecular propellers of predetermined helical screw sense. Through comparative studies on a homologous set of molecules, we found that installation of phenylene-ethynylene-derived π-conjugation profoundly affected the stabilities of the helically folded structures, as evidenced by UV/Vis and circular dichroism (CD) studies. Increasing the number of hydrogen bonds through additional substitution also enhanced the populations of the folded conformations in solution. In addition to introducing steric bias to control structural folding, linearly π-conjugated groups function as spatially well-defined chromophores that give rise to characteristic exciton-coupled circular dichroism. Absolute configurations of chiral centers could thus be further confirmed by comparing the torsional relationships between pairs of chromophores on adjacent subunits, which are fully consistent with the computationally predicted structural models. Twist to fold: Chiral alcohol groups effectively direct the absolute helicities of three-bladed propeller-shaped C3-symmetric molecules through tight O-H...O-H contacts. The stabilities of the conformations of these systems depend upon the numbers of hydrogen bonds and the steric bulk of the π-conjugated substituents on thestereogenic centers. Copyright
- Opsitnick, Elizabeth A.,Jiang, Xuan,Hollenbeck, Andrew N.,Lee, Dongwhan
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supporting information; experimental part
p. 708 - 720
(2012/03/22)
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- Ring strain-promoted allylic transposition of cyclic silyl ethers
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Relief of the ring strain of medium-sized rings promotes a regioselective allylic transposition of a C-O bond when catalyzed by rhenium oxide. Through the allylic transposition, eight-membered cyclic silyl ethers undergo ring contraction to the corresponding six-membered siloxacycles.
- Volchkov, Ivan,Park, Sangho,Lee, Daesung
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supporting information; experimental part
p. 3530 - 3533
(2011/09/15)
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- 1-Benzotriazol-1-yl-3,3,3-trifluoro-2-methoxy-2-phenylpropan-1-ones: Mosher-Bt reagents
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(Chemical Equation Presented) Benzotriazole derivatives of 3,3,3-trifluoro-2-methoxy-2-phenylpropionic acid react with water-soluble amino acids and peptides in an acetonitrile/water (2:1) mixture to give the corresponding amides in quantitative yield.
- Katritzky, Alan R.,Mohapatra, Prabhu P.,Fedoseyenko, Dmytro,Duncton, Matthew,Steel, Peter J.
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p. 4268 - 4271
(2008/02/05)
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- Rhodium Acetate Catalyzes the Addition of Carbenoids α- to Ether Oxygens
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Diazo-carbonyl compounds, when catalyzed by rhodium acetate, insert preferentially adjacent to ether oxygens.This phenomenon was exploited to develop a synthesis of 3(2H)-furanones.
- Adams, Julian,Poupart, Marc-Andre,Grenier, Louis,Schaller, Chris,Ouimet, Nathalie,Frenette, Richard
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p. 1749 - 1752
(2007/10/02)
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- Determination of Enantiomeric Composition of Partly Racemized Carotenols
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Method for determining enantiomeric composition of various racemized carotenols by converting them into diastereomeric esters with subsequent analysis have been studied.Diastereomeric esters of (-)-camphanic acid with carotenols other than α-ketols could not be separated by HPLC.No separation was achieved for diastereomeric esters of methoxytrifluoromethylphenylacetic acid (MTPA). 1H NMR analysis in the presence of Eu(fod)3 of diastereomeric MTPA esters allowed quantitative determination of the enantiomeric composition of carotenols with 2-hydroxy-β- and 3-hydroxy-β-type end groups.
- Aareskjold, Kare,Liaaen-Jensen, Synnove
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p. 499 - 504
(2007/10/02)
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- Magnetic non-equivalence of fluorine atoms of a trifluoromethyl group
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The three fluorine atoms of N,N-dialkyl amide and chloride derivatives of 2-methoxy-2-phenyl-3,3,3-trifluoropropanoic acid are magnetically nonequivalent below -60 deg C and exhibit a clear ABC coupling pattern from which chemical shifts and geminal coupling constants are readily derived.The chemical shifts span a range of about 10 ppm and the geminal coupling constants average about 110 Hz.A five bond coupling of about 5 Hz is observed between the hydrogens of the methoxy group and one of the nonequivalent fluorine atoms of the trifluoromethyl group.Barriers to the hindered rotation about the single bond to the trifluoromethyl group are in the range 36-46 kJ/mol.
- Khan, Mohsin A.,Tavares, Donald F.,Rauk, Arvi
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p. 2451 - 2455
(2007/10/02)
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