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Albert S C Chan

Publications and source records attributed to Albert S C Chan.

At least 19 recordsLinked to original sources
PubMed2006

High-performance thin-layer chromatographic fingerprints of isoflavonoids for distinguishing between Radix Puerariae Lobate and Radix Puerariae Thomsonii.

The roots of Pueraria lobata (Wild.) Ohwi and Pueraria thomsonii Benth have been officially recorded in all editions of Chinese Pharmacopoeia under the same monograph 'Gegen' (Radix Puerariae, RP). However, in its 2005 edition, the two species were separated into both individual monographs, namely 'Gegen' (Radix Puerariae Lobatae, RPL) and 'Fenge' (Radix Puerariae Thomsonii, RPT), respectively, due to their obvious content discrepancy of puerarin, the major active constituent. In present paper, the fingerprint of high-performance thin-layer chromatography (HPTLC) combining digital scanning profiling was developed to identify and distinguish the both species in detail. The unique properties of the HPTLC fingerprints were validated by analyzing ten batches of Pueraria lobata and P. thomsonii samples, respectively. The common pattern of the HPTLC images of the roots of Pueraria spp. and the respective different ratios of the chemical distribution can directly discern the two species. The corresponding digital scanning profiles provided an easy way for quantifiable comparison among the samples. Obvious difference in ingredient content and HPTLC patterns of the two species questioned their bio-equivalence and explained that recording both species separately in the current edition of Chinese Pharmacopoeia (2005 edition) is reasonable due to not only the content of major constituent, puerarin, but also the peak-to-peak distribution in the fingerprint and integration value of the total components. Furthermore, the HPTLC fingerprint is also suitable for rapid and simple authentication and comparison of the subtle difference among samples with identical plant resource but different geographic locations.

Chromatography, Thin Layer↗
PubMed2006

Highly enantioselective addition of in situ prepared arylzinc to aldehydes catalyzed by a series of atropisomeric binaphthyl-derived amino alcohols.

The direct addition of in situ prepared arylzinc to aldehydes with chiral binaphthyl-derived amino alcohols as catalysts can afford optically active diarylmethanols in high yields and with excellent enantioselectivities (up to 99 % ee, ee = enantiomeric excess). By using a single catalyst, both enantiomers of many pharmaceutically interesting diarylmethanols can be obtained by the proper combination of various arylzinc reagents with different aldehydes; this catalytic system also works well for the phenylation of aliphatic aldehydes to give up to 96 % ee.

Aldehydes↗
PubMed2006

A new class of versatile chiral-bridged atropisomeric diphosphine ligands: remarkably efficient ligand syntheses and their applications in highly enantioselective hydrogenation reactions.

A series of chiral diphosphine ligands denoted as PQ-Phos was prepared by atropdiastereoselective Ullmann coupling and ring-closure reactions. The Ullmann coupling reaction of the biaryl diphosphine dioxides is featured by highly efficient central-to-axial chirality transfer with diastereomeric excess >99%. This substrate-directed diastereomeric biaryl coupling reaction is unprecedented for the preparation of chiral diphosphine dioxides, and our method precludes the tedious resolution procedures usually required for preparing enantiomerically pure diphosphine ligands. The effect of chiral recognition was also revealed in a relevant asymmetric ring-closure reaction. The chiral tether bridging the two aryl units creates a conformationally rigid scaffold essential for enantiofacial differentiation; fine-tuning of the ligand scaffold (e.g., dihedral angles) can be achieved by varying the chain length of the chiral tether. The enantiomerically pure Ru- and Ir-PQ-Phos complexes have been prepared and applied to the catalytic enantioselective hydrogenations of alpha- and beta-ketoesters (C=O bond reduction), 2-(6'-methoxy-2'-naphthyl)propenoic acid, alkyl-substituted beta-dehydroamino acids (C=C bond reduction), and N-heteroaromatic compounds (C=N bond reduction). An excellent level of enantioselection (up to 99.9% ee) has been attained for the catalytic reactions. In addition, the significant ligand dihedral angle effects on the Ir-catalyzed asymmetric hydrogenation of N-heteroaromatic compounds were also revealed.

Hydrogen↗
PubMed2006

Alpha-glucosidase inhibition of natural curcuminoids and curcumin analogs.

Natural curcumin (1), demethoxycurcumin (2) and bisdemethoxycurcumin (3) isolated from Curcuma longa (turmeric), and synthetic curcumin analogs (A(1-7), B(1-7), C(1-6) and D(1-7)) were evaluated in vitro for the alpha-glucosidase inhibitory activity via UV and circular dichroism (CD) spectroscopy. The results indicated that natural curcuminoid compound 3 showed a remarkable inhibitory effect with IC(50) of 23.0 microM, and the synthetic compounds A(2), B(2), C(2) and D(2) showed potent inhibitory effects with IC(50) of 2.8, 2.6, 1.6 and 8.2 microM, respectively. Kinetic study exhibited that the mechanism of alpha-glucosidase inhibition of both 3 and C(2) was non-competitive. The structure activity relationship revealed that the ortho dihydroxyl groups could form a more tight interaction with alpha-glucosidase to exert more potential inhibitory activities.

Animals↗
PubMed2006

Curcumin analogs as potent aldose reductase inhibitors.

In the present study, curcuminoids isolated from curcuma longa were demonstrated to possess inhibitory activities on bovine lens aldose reductase. In order to find more potent aldose reductase inhibitor, curcumin analogs were synthesized and evaluated for their ability to inhibit bovine lens aldose reductase enzyme. The results indicated that the compounds with tetrahydroxyl groups, 2,6-bis(3,4-dihydroxybenzylidene)cyclohexanone (A(2)), 2,5-bis(3,4-dihydroxybenzylidene)cyclopentanone (B(2)), 1,5-bis(3,4-dihydroxyphenyl)-1,4-pentadiene-3-one (C(2)), and 3,5-bis(3,4-dihydroxybenzylidene)-4-piperidone (D(2)) showed remarkably potent inhibitory effects on aldose reductase with IC(50) of 2.9 microM, 2.6 microM, 3.4 microM, and 4.9 microM, respectively. The structure-activity relationship revealed that the curcumin analogs with ortho-dihydroxyl groups could form a more tight affinity with aldose reductase to exert more potential inhibitory activities.

Aldehyde Reductase↗
PubMed2006

P-Phos: a family of versatile and effective atropisomeric dipyridylphosphine ligands in asymmetric catalysis.

This Account outlines our efforts in the design and synthesis of a family of highly effective atropisomeric dipyridylphosphine ligands (P-Phos and its variants) and in the development of their widespread applications in transition-metal-catalyzed asymmetric reactions including hydrogenation, hydrosilylation, and C-C bond formation. Desirable attributes, such as air stability, broad substrate scope, fast rates of reaction, excellent enantioselectivities, low catalyst loading, and mild conditions, make the catalyst systems highly attractive and thus may provide excellent opportunities for practical applications.

Boronic Acids↗
PubMed2006

Transforming capacity of two novel genes JS-1 and JS-2 located in chromosome 5p and their overexpression in human esophageal squamous cell carcinoma.

Esophageal squamous cell carcinoma (ESCC) has a high mortality rate and geographic differences in incidence. Previous studies of comparative genomic hybridization (CGH) showed that chromosomal 5p is frequently amplified in cell lines and primary ESCC of Hong Kong Chinese origin. In this report, attempt was made to study two novel genes, named as JS-1 and JS-2, which are located in chromosome 5p15.2 and are 5' upstream to delta catenin for their roles in molecular pathogenesis of ESCC. Eleven cell lines, 27 primary ESCC cases and multiple human tissue cDNA panels (MTC) of digestive system were studied for the expression level of JS-1 and JS-2 by RT-PCR. The full-length cDNA sequences of JS-1 and JS-2 were determined from a non-tumor esophageal epithelial cell line by 3' and 5' rapid amplification of cDNA ends (RACE). The transforming capacity of JS-1 and JS-2 was also investigated by transfecting NIH 3T3 cells with the expression vector pcDNA3.1(-) cloned with the full coding sequences and it was followed by the study of foci formation of the transfected cells under confluence growth and the anchorage-independent growth in soft agar. Forty-five percent (5/11) and 18% (2/11) of the ESCC cell lines showed overexpression of JS-1 and JS-2 respectively, while 55% (15/27) and 14% (3/22) primary ESCC cases showed overexpression of JS-1 and JS-2 respectively. JS-1 overexpression was most common in patients with stage II ESCC (6/27; 22%) whereas JS-2 was only overexpressed in a dysplastic lesion (1/22; 4%) and stage III tumors (2/22; 9%). The expression levels of JS-1 and JS-2 are both low in normal esophageal tissues. Overexpression of JS-1 in NIH 3T3 cells caused foci formation in confluence growth and colony formation in soft agar but not for JS-2. A high grade sarcoma was formed in the athymic nude mice when NIH 3T3 cells overexpressing JS-1 were injected subcutaneously. Our results thus indicate that the frequent overexpression of JS-1 in ESCC and its transforming capacity in normal cells may play a critical role in the molecular pathogenesis of ESCC. The present study also forms the ground work for further identification of novel mechanisms of molecular carcinogenesis in ESCC and other cancers.

Aged↗
PubMed2005

Synthesis and biological evaluation of functionalized coumarins as acetylcholinesterase inhibitors.

Three series of functionalized coumarin compounds were designed and prepared as cholinesterase (AChE and BuChE) inhibitors. The biological profile against AChE and BuChE of the prepared compounds was determined. Compound 7b exhibited a mixed-type of AChE inhibitor with IC50 value for the AChE inhibition of 0.19+/-0.01 microM and a high selectivity for AChE/BuChE, and compound 6b acted as non-competitive AChE inhibitor with IC50 value of 0.43+/-0.02 microM. Structure-activity relationships (SARs) of prepared compounds were discussed.

Acetylcholinesterase↗
PubMed2005

The effect of triptolide on CD4+ and CD8+ cells in Peyer's patch of SD rats with collagen induced arthritis.

Triptolide is a purified component from a traditional Chinese herb Tripterygium wilfordii Hook F. It has been shown to have anti-inflammatory and immunosuppressive activities by its inhibitory effect on T cells. But the effect of triptolide on Peyer's patch cells is unknown. Enteric mucosal immune system, including Peyer's patch, is regarded as one of the sites for inducing immunity tolerance, and this intolerance effect has been used to induce oral tolerance which can considerably reduce arthritis severity in several models of experimental polyarthritis and RA patients. In this study, we investigated the effect of triptolide on the Peyer's patch cells and peripheral lymphocytes in collagen induced arthritis (CIA) in rats. CIA in rat is a widely studied animal model of inflammatory polyarthritis with similarities to rheumatoid arthritis (RA). Our data show that triptolide could lower the arthritic scores and delay the onset of CIA. There are more Peyer's patches in triptolide treated rats than in control rats, while there is no difference in Peyer's patch numbers between CIA rats and triptolide treated rats. In the Peyer's patch, more CD4+ cells are observed in CIA rats, and the numbers of CD4+ cells in triptolide treated rats and control rats are similar. While more CD8+ cells are observed in triptolide treated rats, and the numbers of CD8+ cells in CIA rats and control rats are similar. In periphery, more CD4+ cells and less CD4+ cells in CIA rats and triptolide treated rats are respectively observed. Therefore, the regulation on Peyer's patch might explain some of the immunosuppressive activities of triptolide, and enteric immune response might be actively involved in CIA pathogenesis. It is suggested that the Peyer's patch is one of the primary targets of the immunosuppressive activity of triptolide.

Animals↗
PubMed2005

Catalytic asymmetric alkynylation of alpha-imino ester: a versatile approach to optically active unnatural alpha-amino acid derivatives.

The catalytic asymmetric introduction of alkynyl functionality to alpha-amino acid derivatives was realized by the direct addition of terminal alkynes to alpha-imino ester in the presence of chiral Cu(I) complex under mild reaction conditions. Owing to the rich chemistry to which alkyne can be subjected, the present system provides a remarkably versatile tool for the construction of optically active alpha-amino acid derivatives. Good yields and enantiomeric excess values were achieved with an array of terminal alkynes and challenging, biologically active, unnatural alpha-amino acid derivatives could be conveniently obtained.

Alkynes↗
PubMed2005

The molecular mechanisms involved in the cytotoxicity of alkannin derivatives.

In order to better understand the molecular aspects of the cytotoxic action mechanisms, the cytotoxicity of alkannin derivatives, 1-10, on five human tumor cell lines were examined and their standard redox potentials in aprotic medium were tested by means of cyclic voltammetry. It was suggested that the oxidative potential is closely related to the cytotoxicity. The more negative the oxidative potential of the hydroquinones, the higher cytotoxicity of these derivatives. The results of the compounds 5, 7, 9 and 10 with bad leaving groups, have higher cytotoxic action is not agreed with the bioreductive alkylation mechanism of quinones. It indicates that the molecular mechanism involving cytotoxicity of alkannin derivatives may favor the mechanism of production of reactive oxygen.

Cell Line, Tumor↗
PubMed2005

Rhodium-catalyzed asymmetric aqueous Pauson-Khand-type reaction.

An interesting rhodium-catalyzed asymmetric aqueous Pauson-Khand-type reaction was developed. A chiral atropisomeric dipyridyldiphosphane ligand was found to be highly effective in this system. This operationally simple protocol allows both catalyst and reactants to be handled under air without precautions. Various enynes were transformed to the corresponding bicyclic cyclopentenones in good yield and enantiomeric excess (up to 95 % ee). A study of the electronic effects of the enyne substrates revealed a correlation between the electronic properties of the substrates and the ee value obtained in the product of the Pauson-Khand-type reaction. A linear free-energy relationship was observed from a Hammett study.

Alkynes↗
PubMed2005

A remarkably effective copper(II)-dipyridylphosphine catalyst system for the asymmetric hydrosilylation of ketones in air.

The combination of catalytic amounts of optically active dipyridylphosphine and CuF(2) along with hydride donor PhSiH(3) generated in situ a remarkably reactive catalyst system (substrate-to-ligand molar ratio up to 100,000) for the highly enantioselective hydrosilylation of a broad spectrum of aryl alkyl ketones (up to 97% enantiomeric excess) in normal atmosphere and at mild conditions (ambient temperature to -20 degrees C, compatible with traces of moisture) in the absence of base additives. Furthermore, a highly effective catalytic asymmetric hydrosilylation of unsymmetrical diarylketones using this catalyst system was also realized (up to 98% enantiomeric excess). The introduction of the dipyridylphosphine ligands in the air-accelerated and inexpensive metal-mediated asymmetric hydrosilylation of ketones makes the present system highly attractive and thus provides an excellent opportunity for practical applications.

Journal Article↗
PubMed2005

Highly enantioselective phenyl transfer to aryl aldehydes catalyzed by easily accessible chiral tertiary aminonaphthol.

A new chiral tertiary aminonaphthol ligand 3b served as a highly efficient ligand for the asymmetric catalytic phenyl transfer to aromatic aldehydes and a variety of chiral diarylmethanols was prepared in high ee values (ee up to 99%) and chemical yields. The straightforward syntheses of both 3b and its enantiomer provide an excellent opportunity for large-scale applications.

Journal Article↗
PubMed2004

A potential prodrug for a green tea polyphenol proteasome inhibitor: evaluation of the peracetate ester of (-)-epigallocatechin gallate [(-)-EGCG].

Green tea has been shown to have many biological effects, including effects on metabolism, angiogenesis, oxidation, and cell proliferation. Unfortunately, the most abundant green tea polyphenol (-)-epigallocatechin gallate or (-)-EGCG is very unstable in neutral or alkaline medium. This instability leads to a low bioavailability. In an attempt to enhance the stability of (-)-EGCG, we introduced peracetate protection groups on the reactive hydroxyls of (-)-EGCG (noted in text as 1). HPLC analysis shows that the protected (-)-EGCG analog is six times more stable than natural (-)-EGCG under slightly alkaline conditions. A series of bioassays show that 1 has no inhibitory activity against a purified 20S proteasome in vitro, but exhibits increased proteasome-inhibitory activity in intact leukemic cells over natural (-)-EGCG, indicating an intercellular conversion. Inhibition of cellular proteasome activity by 1 is associated with induction of cell death. Therefore, our results indicate that the protected analog 1 may function as a prodrug of the green tea polyphenol proteasome inhibitor (-)-EGCG.

Catechin↗