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ABSTRACT: A pseudo-enantiomeric pair of helicenes derived from 3,7-gallamide substituted (10R,11R)-dimethoxymethyl-dibenzosuberane and 8-phenyl-α-azo-tetralin was synthesized. Irradiation of pure (10R,11R,M)-helicene at 280 nm in CH2Cl2 gel state led to exclusive formation of (10R,11R,P)-helicene (M/P', <1/>99). Its preferential return to the original helicene was achieved with high selectivity (M/P', 90/10) at 343 nm. Irradiations of either helicenes at 318 nm led to a photostationary state of nearly 50/50 composition by which a micelle- or vesicle-containing solution state is resulted. Bundled fibers with hollow helical channel (ca 105 nm in diameter) of complementary helicity were observed from the xerogels of (M) or (P') helicene-containing CH2Cl2 gels, respectively, by TEM and AFM images. The decisive intermolecular N-H bonding, Π-π stacking, and paired van der Waals interactions were identified by IR, concentration dependent 1H NMR spectra and molecular simulation to explain the observed helical assembly. For the first time, the helical organic superstructure with a helical tunnel that can be reversibly tuned by light thru the (-)-gel-sol-(+)-gel interconversion augurs well for its potential application as optically switchable helical fibers of ternary logics.
Journal of the American Chemical Society 03/2013; · 9.91 Impact Factor
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ABSTRACT: A highly efficient and mild method for azido glycosylation of glycosyl β-peracetates to 1,2-trans glycosyl azides was developed by using inexpensive FeCl(3) as the catalyst. In addition, we demonstrated, for the first time, that FeCl(3) in combination with copper powder can promote 1,3-dipolar cycloaddition (click chemistry) of azido glycosides with terminal alkynes. Good to excellent yields were obtained with exclusive formation of a single isomer in both glycosyl azidation and subsequent cycloaddition processes.
Chemical Communications 08/2011; 47(37):10440-2. · 6.17 Impact Factor
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Advanced Synthesis & Catalysis 05/2011; 353(8):1234 - 1240. · 6.05 Impact Factor
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ABSTRACT: Chiral oxidovanadium(V) methoxides prepared from 3,5-disubstituted-N-salicylidene-l-tert-butylglycines and vanadyl sulfate in air-saturated MeOH serve as highly enantioselective catalysts for asymmetric aerobic oxidations and kinetic resolution of alkyl, aryl, and heteroaryl α-hydroxy-ketones with differed α-substituents at ambient temperature in toluene or TBME (tert-butyl methyl ether). The best scenarios involve the use of complexes which bear the tridendate templates derived from 3,5-diphenyl- or 3-o-biphenyl-5-nitro-salicyaldehyde. The kinetic resolution selectivities of the aerobic oxidation process are in the range of 12 to >1000 based on the selectivity factors (k(rel)).
Organic Letters 01/2011; 13(1):26-9. · 5.86 Impact Factor
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ABSTRACT: A series of C(2)-symmetric, 10,11-disubstituted dibenzosuberane (DBS)-based helicenes 6 a-c with a common 7-bromo-α-tetralin-based bottom fragment were synthesized. Their absolute stereochemistry was determined to be 10R,11R,P after reductive desulfurization of the corresponding (10R,11R,1'S)-episulfides with complete stereospecificity. Photoisomerization of the diastereomerically pure (P)-6 c in hexane led to virtually exclusive formation of the opposite M-form diastereomer (P/M', <1:>99) at 290 nm. The preferential return of (M')-6 c to (P)-6 c was also achieved with high selectivity (P/M', 90:10) at 330 nm. Molecular simulations of (P)-6 c and (M')-6 c with both DBS conformations suggest that the selectivities of photoswitching are controlled by the conformation of the top DBS template as evidenced by their (1)H NMR spectra. Doping 6 c into a nematic liquid crystal (E7) led to a cholesteric mesophase with modulated pitches, reversible helical senses, and with a switch memory of ternary logic.
Chemistry 10/2010; 16(43):12822-30. · 5.93 Impact Factor
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Advanced Functional Materials 09/2010; 20(22):3948 - 3958. · 10.18 Impact Factor
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ABSTRACT: An unprecedented dual-mode optical switch by combining helicene and bis-azobenzene moieties in proximity allows for cooperative, highly diastereoselective chirochromic (92/8 to 3/97) and photochromic (>99/<1 to 19/81) switchings both in solution and in nematic LC with tunable pitches and handedness by permutation of four different metastable states.
Organic Letters 03/2010; 12(7):1472-5. · 5.86 Impact Factor
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Advanced Synthesis & Catalysis 12/2009; 352(1):188 - 194. · 6.05 Impact Factor
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ABSTRACT: A series of bipolar OLED materials were subjected to pulsed radiolysis experiments to determine their transient absorption and lifetime profiles of the independently in situ generated radical cations and anions in solutions. Moreover, their emission behaviors from the charge recombination of their radical ions were also determined by the pulse radiolysis method. It was found the absorption bands in doubly ortho-linked quinoxaline/diphenylfluorene hybrids 1a-e are red-shifted progressively with increasing electron-donating nature at the C5 and C8 positions of the quinoxaline template. The incipient radical anions in 1a-e are mainly localized on the quinoxaline heterocyclic moiety, whereas the incipient radical cations are mainly distributed onto the attached electron-donating groups at the C5 and C8 positions of the quinoxaline template. For other doubly ortho-linked cis-stilbene derivatives 3d, 3f, and 4f, the radical anions are mainly localized on the cis-stilbene central moiety and the radical cation is mainly distributed onto both para substituents of the cis-stilbene templates. It was also shown that there is a correlation between their optoelectronic emission efficiencies and the radiolysis induced emission intensities. In addition, the charge transporting behaviors within an OLED device were found to show the relationship with transient absorption half-lives (tau(1/2)) of the radical ions. Charge recombination mechanisms in both the OLED and pulsed radiolysis experiments were proposed to rationalize these observations, allowing us to establish some guidelines for an ultimate molecular design of ideal bipolar optoelectronic materials with a judicious choice of local charge appendages in the optoelectronic templates.
Journal of the American Chemical Society 05/2009; 131(19):6698-707. · 9.91 Impact Factor
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ABSTRACT: A directed assembly process to form C4-symmetric, vanadate-centered quadruplexes, for the first time, from a given chiral oxidovanadium(V) complex allows for highly efficient K+- and Ag+-specific transport from aqueous phase containing three other alkali metal cations into organic solvents, reminiscent of the K+ specific transport exerted by four homochiral glycine residues of the opening site in KcsA membrane protein.
Journal of the American Chemical Society 10/2008; 130(39):12842-3. · 9.91 Impact Factor
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ABSTRACT: Compared to monovalent carbohydrates, multivalent carbohydrate ligands exhibit significantly enhanced binding affinities to their interacting proteins. Here, we report globotriose (P(k) ligand)-functionalized gold nanoparticle (AuNP) probes for the investigation of multivalent interactions with the B(5) subunit of Shiga-like toxin I (B-Slt). Six P(k)-ligand-encapsulated AuNPs (P(k)-AuNPs) of varying particle size and linker length were synthesized and evaluated for their potential as multivalent affinity probes by using a surface plasmon resonance competition assay. The affinity of these probes for the interacting proteins was greatly affected by nanoparticle size, linker length, and ligand density on nanoparticle surface. For example, the 20-nm 20-P(k)-l-AuNP, which had a relatively long linker showed a >10(8)-fold increase in affinity compared with the mono P(k) ligand. This intrinsic high-affinity AuNP probe specifically captured the recombinant B-Slt from Escherichia coli lysate, and the resulting purity of the B-Slt was >95 %. We also developed a robust P(k)-AuNP-based detection method for Slt-I by combining the technique with silver enhancement.
ChemBioChem 05/2008; 9(7):1100-9. · 3.94 Impact Factor
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ABSTRACT: A series of carbamates, amides, N-tosyl amides, (hetero)arenes, and hydrogen phosphines/phosphites has been examined as nucleophiles for (hetero)Michael-type additions to enones and enamides catalyzed by amphoteric vanadyl triflate under mild and neutral conditions. The newly developed C-N, C-P, and C-C bond-formation protocols were carried out smoothly in good to high yields without intervention of any 1,2-additions.
Organic Letters 01/2008; 9(25):5195-8. · 5.86 Impact Factor
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ABSTRACT: A series of chiral vanadyl(V) methoxides bearing 3-t-butyl-5-substituted N-salicylene-L-valinate and L-t-leucinate as chiral auxiliaries has been prepared. In all cases except the 3,5-di-t-butyl analogue, they exist as monomers both in solution and in the single crystal state. In the case of the 3,5-di-t-butyl analogue, the architectural nature of the vanadyl(V) complex highly depends on the base used during the complex formation event. A pentanuclear C4-symmetric complex was formed when potassium salts were employed instead of the corresponding sodium salts. A central vanadate(V) unit serves to grip four identical chiral monomeric vanadyl(V) units together, by which a potassium ion sits on top of the four flanking units through carbonyl coordinations and serves to hold the whole cluster by cooperation with the central vanadate(V) unit. In comparison with the corresponding monomeric vanadyl(V) methoxide complex, the cluster complex was utilized to facilitate the asymmetric aerobic oxidations of various racemic alpha-hydroxyesters, -amides, and -thioesters with excellent selectivity factors (krel 40 to >500).
The Journal of Organic Chemistry 10/2007; 72(22):8175-85. · 4.45 Impact Factor
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Journal of the American Chemical Society 07/2007; 129(24):7478-9. · 9.91 Impact Factor
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ABSTRACT: Acetal, trityl, and TBDMS protecting groups on saccharides were subjected to alcoholysis using a catalytic amount of vanadyl triflate in an MeOH-CH2Cl2 solvent system. The configuration at the anomeric positions of saccharides was retained, and no glycosidic bond cleavage and oxidation of sulfides were observed. The presented method was easily implemented, compatible with diverse functional groups, and regioselective in some cases.
The Journal of Organic Chemistry 02/2007; 72(1):299-302. · 4.45 Impact Factor
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ABSTRACT: Among 18 oxometallic species, MoO2Cl2 was found to be the most reactive in catalytic thioglycosylation of O-acetylated glycosides with functionalized thiols in CH2Cl2, leading cleanly to 1,2-trans-thioglycosides with exclusive diastereocontrol. The new catalytic protocol is applicable to a monoglycoside building block and beta-(1-->6)-S-linked-thiodisaccharide synthesis. [reaction: see text].
Organic Letters 11/2006; 8(24):5633-6. · 5.86 Impact Factor
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ABSTRACT: Vanadyl triflate has been identified as a mild and efficient catalyst for the chemoselective O-isopropylidenation of functionalized carbohydrates with acetone and acetone equivalents. The current protocol is compatible with a diverse array of protecting groups and the products can be readily isolated by simple aqueous wash.
Carbohydrate Research 09/2006; 341(11):1948-53. · 2.33 Impact Factor
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ABSTRACT: The titled dipolar hybrids bearing a central quinoxaline-fused dibenzosuberene optoelectronic unit with functional C5 and C8 appendages and spiro-fluorene junction act as fluorescent bipolar OLED chameleons. The emission colors can be tuned from blue, green, yellow, to red with operation brightness of 205, 1268, 1542, and 1102 cd/m2, respectively, at 20 mA/cm2.
Journal of the American Chemical Society 09/2006; 128(34):10992-3. · 9.91 Impact Factor
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ABSTRACT: An unprecedented vanadyl(V) methoxide complex 4 derived from 3,5-dibromo-N-salicylidene-l-tert-leucinate enables highly enantioselective aerobic oxidations of alpha-hydroxyphosphonates at ambient temperature with selectivity factors ranging from 3 to >99.
Journal of the American Chemical Society 06/2006; 128(19):6308-9. · 9.91 Impact Factor
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ABSTRACT: A series of chiral vanadyl carboxylates derived from N-salicylidene-L-alpha-amino acids and vanadyl sulfate has been developed. These configurationally well defined complexes were examined for the kinetic resolution of double- and mono-activated 2 degrees alcohols. The best chiral templates involve the combination of L-tert-leucine and 3,5-di-t-butyl-, 3,5-diphenyl-, or 3,4-dibromo-salicylaldehyde. The resulting vanadyl(V)-methoxide complexes after recrystallization from air-saturated methanol serve as highly enantioselective catalysts for asymmetric aerobic oxidation of alpha-hydroxyl-esters and amides with a diverse array of alpha-, O-, and N-substituents at ambient temperature in toluene. The asymmetric inductions of the oxidation process are in the range of 10 to >100 in terms of selectivity factors (k(rel)) in most instances. The previously undescribed aerobic oxidation protocol is also applicable to the kinetic resolution of C-13 taxol side chain with high selectivity factor (k(rel) = 35). X-ray crystallographic analysis of an adduct between a given vanadyl complex and N-benzyl-mandelamide allows for probing the stereochemical origin of the nearly exclusive asymmetric control in the oxidation process.
Proceedings of the National Academy of Sciences 04/2006; 103(10):3522-7. · 9.68 Impact Factor
