Man-Kin Wong

Gold-Catalyzed Highly Enantioselective Synthesis of Axially Chiral Allenes

Axially chiral allenes are synthesized from chiral propargylamines catalyzed by KAuCl4 in high yields (up to 93% yield) and excellent enantioselectivities (up to 97% ee) in CH3CN at 40 degrees C. The reaction has been applied to the synthesis of novel allene-modified artemisinin derivatives with the delicate endoperoxide moieties remaining intact. A tentative mechanism regarding gold(I)-catalyzed intramolecular hydride transfer was proposed on the basis of deuterium-labeling experiments and ESI-MS analysis of the reaction mixture.

Gold(I)‐Catalyzed Intermolecular Hydroarylation of Alkenes with Indoles under Thermal and Microwave‐Assisted Conditions

An efficient method for intermolecular hydroarylation of aryl and aliphatic alkenes with indoles using a combination of [(PR(3))AuCl]/AgOTf as catalyst under thermal and microwave-assisted conditions has been developed. The gold(I)-catalyzed reactions of indoles with aryl alkenes were achieved in toluene at 85 degrees C over a reaction time of 1-3 h with 2 mol% of [(PR(3))AuCl]/AgOTf as catalyst. This method works for a variety of styrenes bearing electron-deficient, electron-rich, and sterically bulky substituents to give the corresponding products in good to high yields (60-95%). Under microwave irradiation, coupling of unactivated aliphatic alkenes with indoles gave the corresponding adducts in up to 90% yield. Selective hydroarylation of terminal C=C bond of conjugated dienes with indoles gave good product yields (62-81%). On the basis of deuterium-labeling experiments, a reaction mechanism involving nucleophilic attack of Au(I)-coordinated alkenes by indoles is proposed.

Ruthenium-Catalyzed Alkylation of Indoles with Tertiary Amines by Oxidation of a sp3 CH Bond and Lewis Acid Catalysis

Ruthenium porphyrins (particularly [Ru(2,6-Cl(2)tpp)CO]; tpp=tetraphenylporphinato) and RuCl(3) can act as oxidation and/or Lewis acid catalysts for direct C-3 alkylation of indoles, giving the desired products in high yields (up to 82% based on 60-95% substrate conversions). These ruthenium compounds catalyze oxidative coupling reactions of a wide variety of anilines and indoles bearing electron-withdrawing or electron-donating substituents with high regioselectivity when using tBuOOH as an oxidant, resulting in the alkylation of N-arylindoles to 3-{[(N-aryl-N-alkyl)amino]methyl}indoles (yield: up to 82%, conversion: up to 95%) and the alkylation of N-alkyl or N-H indoles to 3-[p-(dialkylamino)benzyl]indoles (yield: up to 73%, conversion: up to 92%). A tentative reaction mechanism involving two pathways is proposed: an iminium ion intermediate may be generated by oxidation of an sp(3) C-H bond of the alkylated aniline by an oxoruthenium species; this iminium ion could then either be trapped by an N-arylindole (pathway A) or converted to formaldehyde, allowing a subsequent three-component coupling reaction of the in situ generated formaldehyde with an N-alkylindole and an aniline in the presence of a Lewis acid catalyst (pathway B). The results of deuterium-labeling experiments are consistent with the alkylation of N-alkylindoles via pathway B. The relative reaction rates of [Ru(2,6-Cl(2)tpp)CO]-catalyzed oxidative coupling reactions of 4-X-substituted N,N-dimethylanilines with N-phenylindole (using tBuOOH as oxidant), determined through competition experiments, correlate linearly with the substituent constants sigma (R(2)=0.989), giving a rho value of -1.09. This rho value and the magnitudes of the intra- and intermolecular deuterium isotope effects (k(H)/k(D)) suggest that electron transfer most likely occurs during the initial stage of the oxidation of 4-X-substituted N,N-dimethylanilines. Ruthenium-catalyzed three-component reaction of N-alkyl/N-H indoles, paraformaldehyde, and anilines gave 3-[p-(dialkylamino)benzyl]indoles in up to 82% yield (conversion: up to 95%).

Modification of N-Terminal α-Amino Groups of Peptides and Proteins Using Ketenes

A method of highly selective N-terminal modification of proteins as well as peptides by an isolated ketene was developed. Modification of a library of unprotected peptides XSKFR (X varies over 20 natural amino acids) by an alkyne-functionalized ketene (1) at room temperature at pH 6.3 resulted in excellent N-terminal selectivity (modified α-amino group/modified ε-amino group = >99:1) for 13 out of the 20 peptides and moderate-to-high N-terminal selectivity (4:1 to 48:1) for 6 of the 7 remaining peptides. Using an alkyne-functionalized N-hydroxysuccinimide (NHS) ester (2) instead of 1, the modification of peptides XSKFR gave internal lysine-modified peptides for 5 out of the 20 peptides and moderate-to-low N-terminal selectivity (5:1 to 1:4) for 13 out of the 20 peptides. Proteins including insulin, lysozyme, RNaseA, and a therapeutic protein BCArg were selectively N-terminally modified at room temperature using ketene 1, in contrast to the formation of significant or major amounts of di-, tri-, or tetra-modified proteins in the modification by NHS ester 2. The 1-modified proteins were further functionalized by a dansyl azide compound through click chemistry without the need for prior treatment.

Surface decoration of selenium nanoparticles by mushroom polysaccharides–protein complexes to achieve enhanced cellular uptake and antiproliferative activity

By using mushroom polysaccharides–protein complexes (PSP) as the capping agent, size controllable and highly stable selenium nanoparticles (SeNPs) have been successfully created in a simple redox system of sodium selenite and ascorbic acid. SeNPs were capped with PSP through strong physical adsorption of hydroxyl groups of polysaccharides and imino groups of proteins on the surface of SeNPs. PSP surface decoration significantly enhanced the cellular uptake of SeNPs through endocytosis. Treatment with PSP–SeNPs significantly inhibited the growth of MCF-7 human breast cacinoma cells through induction of apoptosis with the involvement of PARP cleavage and caspase activation. Moreover, PSP–SeNPs not only significantly induced dose-dependent disruption of mitochondrial membrane potential in MCF-7 cells after 24 h treatment, but it also enhanced reactive oxygen species (ROS) generation as early as 15 min, indicating that ROS-mediated mitochondrial dysfunction may play an important role in PSP–SeNPs-induced apoptosis. Our results suggest that PSP–SeNPs may be a candidate for further evaluation as a chemopreventive agent for human cancers, and the strategy to use PSP as a surface decorator could be a highly efficient way to enhance the cellular uptake and anticancer efficacy of nanomaterials.

Gold(I)-Catalyzed Enantioselective Intermolecular Hydroarylation of Allenes with Indoles and Reaction Mechanism by Density Functional Theory Calculations

Chiral binuclear gold(I) phosphine complexes catalyze enantioselective intermolecular hydroarylation of allenes with indoles in high product yields (up to 90%) and with moderate enantioselectivities (up to 63% ee). Among the gold(I) complexes examined, better ee values were obtained with binuclear gold(I) complexes, which displayed intramolecular Au(I)-Au(I) interactions. The binuclear gold(I) complex 4c [(AuCl)(2)(L3)] with chiral biaryl phosphine ligand (S)-(-)-MeO-biphep (L3) is the most efficient catalyst and gives the best ee value of up to 63%. Substituents on the allene reactants have a slight effect on the enantioselectivity of the reaction. Electron-withdrawing groups on the indole substrates decrease the enantioselectivity of the reaction. The relative reaction rates of the hydroarylation of 4-X-substituted 1,3-diarylallenes with N-methylindole in the presence of catalyst 4c [(AuCl)(2)(L3)]/AgOTf [L3 = (S)-(-)-MeO-biphep], determined through competition experiments, correlate (r(2) = 0.996) with the substituent constants σ. The slope value is -2.30, revealing both the build-up of positive charge at the allene and electrophilic nature of the reactive Au(I) species. Two plausible reaction pathways were investigated by density functional theory calculations, one pathway involving intermolecular nucleophilic addition of free indole to aurated allene intermediate and another pathway involving intramolecular nucleophilic addition of aurated indole to allene via diaurated intermediate E2. Calculated results revealed that the reaction likely proceeds via the first pathway with a lower activation energy. The role of Au(I)-Au(I) interactions in affecting the enantioselectivity is discussed.

Electron‐Deficient Alkynes as Cleavable Reagents for the Modification of Cysteine‐Containing Peptides in Aqueous Medium

An efficient method has been developed for the chemoselective cysteine modification of unprotected peptides and proteins in aqueous media through the formation of a vinyl sulfide linkage by using electron-deficient alkynes, including alkynoic amides, esters and alkynones. The terminal alkynone-modified peptides could be converted back into the unmodified peptides (81% isolated yield) by adding thiols under mild conditions. The usefulness of this thiol-assisted cleavage of the vinyl sulfide linkage in peptides has been exemplified by the enrichment of a cysteine-containing peptide (71% recovery) from a mixture of cysteine-containing and non-cysteine-containing peptides.

Subcellular localization of a fluorescent artemisinin derivative to endoplasmic reticulum.

A cytotoxic artemisinin derivative conjugated with a fluorescent dansyl moiety was synthesized and its subcellular localization in Hep3B cells was examined. Comparison of the localization signals of the fluorescent artemisinin derivative with organelle specific dyes revealed that endoplasmic reticulum (ER) is the main site of its accumulation.

Gold-mediated selective cysteine modification of peptides using allenes

A new approach for selective modification of cysteine-containing peptides through gold-mediated oxidative allene-thiol coupling reaction in aqueous medium is developed.

Conjugation of latent membrane protein (LMP)-2 epitope to gold nanoparticles as highly immunogenic multiple antigenic peptides for induction of Epstein-Barr virus specific cytotoxic T-lymphocyte responses in vitro?

Nasopharyngeal carcinoma is a neoplasm with a high incidence in Southeast Asia, and it is strongly associated with Epstein-Barr virus (EBV) activation involving the expression of a weakly immunogenic protein, namely, latent membrane protein (LMP)-2. Previous immunological studies already identified the human leukocyte antigen (HLA)-A11 restricted peptide epitope (SSCSSCPLSK) in the LMP-2 antigen. In this work, we prepared gold nanoparticle (AuNP)-peptide conjugate 1 by treating the nanoparticles with the N-cysteinated LMP-2 epitope. The AuNP-peptide conjugates have been characterized by TEM (15-24 nm in diameter) and UV-vis spectroscopy (surface plasmon resonance absorption band at lambda(max) = 520 nm). In the presence of a CALNN capping peptide, the AuNP-peptide conjugates are stable in solution without aggregation at room temperature for at least 48 h. By ELIspot studies, AuNP-peptide conjugate 1 was found to elicit a significantly stronger INF-gamma response [number of spot forming cells (SPC) = 727 +/- 198] from peripheral blood mononuclear cells of healthy HLA-A11 donors when compared to that induced by the unconjugated LMP-2 peptides (SFC = 73 +/- 28). Further studies showed that dendritic cells treated with conjugate 1 can effect CD8+ T-cell activation leading to epitope-specific cytotoxic T lymphocyte killing responses in vitro.