Lee-Chiang Lo

Fluorous oxime palladacycle: a precatalyst for carbon-carbon coupling reactions in aqueous and organic medium.

To facilitate precatalyst recovery and reuse, we have developed a fluorous, oxime-based palladacycle 1 and demonstrated that it is a very efficient and versatile precatalyst for a wide range of carbon-carbon bond formation reactions (Suzuki-Miyaura, Sonogashira, Stille, Heck, Glaser-type, and Kumada) in either aqueous or organic medium under microwave irradiation. Palladacycle 1 could be recovered through F-SPE in various coupling reactions with recovery ranging from 84 to 95% for the first cycle. Inductively coupled plasma optical emission spectrometry (ICP-OES) analyses of the Pd content in the crude product from each class of transformation indicated extremely low levels of leaching and the palladacycle could be reused four to five times without significant loss of activity.

Development of a fluorous, oxime-based palladacycle for microwave-promoted carbon–carbon coupling reactions in aqueous media

A thermally stable, fluorous oxime-based palladacycle has been developed and was shown to efficiently promote various carbon–carbon bond formation reactions (Suzuki–Miyaura, Sonogashira and Stille) in aqueous media under microwave irradiation. The palladacycle gave extremely low levels of Pd leaching and could be reused five times with no significant loss of activity.

Ferric chloride, an anomerization catalyst for the preparation of alkyl α-glycopyranosides

Anhydrous FeCl3 in CH2Cl2 has been found to readily anomerize β-glycopyranosides to their corresponding α-anomers in good yields and selectivities at room temperatures. Acetyl- and benzoyl-protected oxygen sugars yielded the best results.

Nitrophenylboronic acids as highly chemoselective probes to detect hydrogen peroxide in foods and agricultural products.

Hydrogen peroxide is commonly used in the food processing industry as a chlorine-free bleaching and sterilizing agent, but excessive amounts of residual hydrogen peroxide have led to cases of food poisoning. Here we describe the development of a novel nonenzymatic colorimetric method for the determination of residual hydrogen peroxide in foods and agricultural products. Nitrophenylboronic acids chemoselectively react with hydrogen peroxide under alkaline conditions to produce yellow nitrophenolates. Of the three nitrophenylboronic acid isomers tested, the p-isomer displayed the highest sensitivity for hydrogen peroxide and the fastest reaction kinetics. The reaction product, p-nitrophenolate, has an absorption maximum at 405 nm and a good linear correlation between the hydrogen peroxide concentration and the A(405) values was obtained. We successfully applied this convenient and rapid method for hydrogen peroxide determination to samples of dried bean curds and disposable chopsticks, thereby demonstrating its potential in foods and agricultural industries.

Development of an Activity-Based Probe for Steroid Sulfatases

the enzyme family responsiblefor the removal of the sulfate moieties from various sulfatedbiomolecules has gained considerable attention. For example,human steroid sulfatase (STS, EC 3.1.6.2) has become a targetofdrug development over the last decade due to its involve-ment in estrogen-dependent tumors.

A VERSATILE MECHANISM BASED REACTION PROBE FOR THE DIRECT SELECTION OF BIOCATALYSTS

Abstract A mechanism based reaction probe was synthesized and shown to modify a bacterial phosphotriesterase; this strategy for generating a probe is general and should allow the isolation of a host of unique catalysts.

Mutagenesis and mechanistic study of a glycoside hydrolase family 54 α-L-arabinofuranosidase from Trichoderma koningii

A GH (glycoside hydrolase) family 54 alpha-L-arabinofuranosidase from Trichoderma koningii G-39 (termed Abf) was successfully expressed in Pichia pastoris and purified to near homogeneity by cation-exchange chromatography. To determine the amino acid residues essential for the catalytic activity of Abf, extensive mutagenesis of 24 conserved glutamate and aspartate residues was performed. Among the mutants, D221N, E223Q and D299N were found to decrease catalytic activity significantly. The kcat values of the D221N and D299N mutants were 7000- and 1300-fold lower respectively, than that of the wild-type Abf. E223Q was nearly inactive. These results are consistent with observations obtained from the Aspergillus kawachii alpha-L-arabinofuranosidase three-dimensional structure. This structure indicates that Asp221 of T. koningii Abf is significant for substrate binding and that Glu223 as well as Asp299 function as a nucleophile and a general acid/base catalyst for the enzymatic reaction respectively. The catalytic mechanism of wild-type Abf was further investigated by NMR spectroscopy and kinetic analysis. The results showed that Abf is a retaining enzyme. It catalyses the hydrolysis of various substrates via the formation of a common intermediate that is probably an arabinosyl-enzyme intermediate. A two-step, double-displacement mechanism involving first the formation, and then the breakdown, of an arabinosyl-enzyme intermediate was proposed. Based on the kcat values of a series of aryl-alpha-L-arabinofuranosides catalytically hydrolysed by wild-type Abf, a relatively small Brønsted constant, beta(lg)=-0.18, was obtained, suggesting that the rate-limiting step of the enzymatic reaction is the dearabinosylation step. Further kinetic studies with the D299G mutant revealed that the catalytic activity of this mutant depended largely on the pK(a) values (>6) of leaving phenols, with beta(lg)=-1.3, indicating that the rate-limiting step of the reaction becomes the arabinosylation step. This kinetic outcome supports the idea that Asp299 is the general acid/base residue. The pH activity profile of D299N provided further evidence strengthening this suggestion.

Diterpenoids with Anti-Inflammatory Activity from the Wood of Cunninghamia konishii

Two new diterpenoids, konishone (1) and 3β-hydroxy-5,6-dehydrosugiol (2), along with three known diterpenoids—hinokiol (3), sugiol (4), and 12-hydroxy-6,7-secoabieta-8,11,13-triene-6,7-dial (5)—were isolated from the wood of Cunninghamia konishii. Compound 1 is a novel skeleton of the 7,20-dinorabietane-type diterpene. In addition, when RAW264.7 macrophages were treated with different concentrations of compounds 1, 3, and 5 together with LPS, a significant concentration-dependent inhibition of NO production was detected. The IC50 values for inhibition of nitrite production of compounds 1, 3, and 5 were about 9.8 ± 0.7, 7.9 ± 0.9, and 9.3 ± 1.3 μg/mL, respectively. This study presents the potential utilization of compounds 1, 3, and 5, as lead compounds for the development of anti-inflammatory drugs.

Palladium-catalyzed annulation of internal alkynes in aqueous medium

To facilitate precatalyst recovery and reuse, we have developed a fluorous, oxime-based palladacycle 1 and demonstrated that it is a very efficient and versatile precatalyst for carbo- and heteroannulation of internal alkynes with functionalized aryl halides in aqueous medium. A uniform reaction condition for these annulation reactions was also developed.

Circular dichroic studies of 2-amino-2-deoxy-galactopyranosides - conformations of the 2-(N-acetyl-p-bromobenzamido) group

Abstract The CD spectra of methyl 2-amino-2-deoxy-D-galactopyranoside mono- N -acylates (acetyl or p -bromobenzoyl) resemble those of the corresponding O -acylates and can be accounted for by the additivity rule. However, the CD of pyranosides containing the N -acetyl- p -bromobenzamido imide group (NAcBz) are far more complex than those of mono- N -acylates and their O -counterparts, and furthermore, the solvent- and temperature-dependent changes differ for the α- and β-anomers. These differences can be accounted for by the different conformations of the 2-NAcBz group.