Efficient chemoenzymatic synthesis of (2S,3R)-3-hydroxy-3-methylproline,a key fragment in polyoxypeptin A and FR225659

We report an efficient synthesis of protected (2S,3R)-3-hydroxy-3-methylproline that proceeds in three steps with complete stereoselectivity. This route represents a significant improvement over previous approaches to this noncanonical amino acid. Key to this success is the development of a one-pot chemoenzymatic procedure for the preparation of (2S,3S)-3-methylproline from l-isoleucine. This work lays the foundation for future chemoenzymatic syntheses of polyoxypeptin A and FR225659.     

First principles investigation on the stabilization mechanisms of the polar copper terminated Cu2O(1 1 1) surface

The stabilization of the unstable, polar copper terminated Cu₂O(1 1 1) surface by reconstruction and hydroxylation was studied theoretically with static and molecular dynamics calculations at ab initio density functional theory (DFT) level. Surface reconstruction was investigated using extensive finite temperature molecular dynamics (MD) combined with a simulated annealing technique. Both the global minimum energy structure obtained during annealing the system at higher temperature (300 K) and the final ‘quenched’ structure which was obtained after cooling the system to 0 K show the expected reconstruction of the adsorbate-free surface. The copper atoms in the first layer and oxygen atoms in the second and third layers are markedly displaced, and the atomic planes merge together to form a uniform mixed layer, thereby minimizing the polarity of the surface. Surface hydroxylation by adsorption of OH⁻ or dissociated water was investigated using static optimization at 0 K. The results show that adsorption is exothermic and that the reconstruction characterizing the annealed OH-free surface does not occur in the presence of adsorbed OH. A surface coverage of 50% results in the surface structure that is the closest to the unrelaxed bulk terminated surface.     

Oxidative Degradation of 2,4‐Dihydroxybenzoic Acid by the Fenton and Photo‐Fenton Processes: Kinetics,Mechanisms, and Evidence for the Substitution of H2O2 by O2

The kinetics and mechanisms of the oxidative degradation of 2,4‐dihydroxybenzoic acid (2,4‐DHBA) by the Fenton and photo‐Fenton processes were investigated in detail by a combination of HPLC, IC, and TOC analyses. The formation of 2,3,4‐trihydroxybenzoic acid (2,3,4‐THBA) at an early oxidation stage shows that hydroxylation of the aromatic ring is the first step of the process. This intermediate was able to reduce Fe^III and to contribute to the recycling of Fe^II. Complete mineralization could only be achieved under irradiation (photo‐Fenton). A detailed study of the dependence of the rate of mineralization on the concentration of H₂O₂ and dissolved O₂ was carried out. It was found that, even at a low initial concentration of H₂O₂, mineralization by the photo‐Fenton process was complete in a relatively short time, provided that the O₂ concentration was high enough, indicating that O₂ may, at least in part, substitute H₂O₂. Channeling reaction pathways toward O₂ rather than H₂O₂ consumption is of particular interest for the technical development of the photo‐Fenton process.     

A facile and efficient method for hydroxylation of azabenzanthrone compounds

A novel and facile method of introducing 4-hydroxyl group into the aromatic ring of azabenzanthrone compounds was carried out by reacting azabenzanthrone compounds with hydrazine hydrate or hydroxylamine hydrochloride and sodium hydroxide in diethylene glycol(DEG) solvent.The mechanism of reaction may involve an amino intermediate and follow a hydroxyl substitution process.     

Concise Stereoselective Total Synthesis of Leiocarpin C

A simple and highly concise strategy has been developed for the stereoselective total synthesis of leiocarpin C starting from commercially available mandelic ester. The strategy utilizes the OsO₄‐catalyzed cis‐hydroxylation and selective reduction with K‐Selectride as key steps.     

Metalloporphyrin-catalyzed hydroxylation of the N,N-dimethylamide function of the drug molecule SSR180575 to a stable N-methyl-N-carbinolamide

The metalloporphyrin-catalyzed oxidation of SSR180575, a ligand of the peripheral benzodiazepine receptor, that contains both N-methylindole and N,N-dimethylamide functions, produces a high yield of a stable carbinolamide derivative.     

Synthesis of water-soluble porphyrin and the corresponding highly planar benzoporphyrin without meso-substituents

Water-soluble bicycloporphyrin and benzoporphyrin having octacarboxyl groups are synthesized from the corresponding octaester porphyrin by simple hydrolyzing and heating procedures. These compounds are characterized by UV-vis, NMR and mass spectroscopies. The benzoporphyrin octacarboxylic acid exhibits extremely high planarity in spite of its poly-substitution.     

Cu-incorporated mesoporous materials: Synthesis, characterization and catalytic activity in phenol hydroxylation

A series of novel Cu-incorporated mesoporous materials (CMMs) with molar ratios of Cu/Si ranging from 1/200 to 1/20 were synthesized by sol–gel method using glutaric acid as template. The materials were characterized by powder X-ray diffraction (XRD), N₂ adsorption, diffuse reflectance UV–vis spectroscopy and transmission electron microscopy (TEM). The results indicate that the CMMs have a three-dimensional (3D) worm-like mesoporous structure with a surface area between 600 and 800 m² g⁻¹ and the copper ions are incorporated into the matrix with octahedral environment. The catalytic activity of these CMMs in the phenol hydroxylation using H₂O₂ as oxidant is comparable to that of TS-1. Effects of Cu/Si molar ratio in the catalyst and the influence of various reaction parameters on the catalytic activity were investigated in detail and the optimized reaction condition was acquired.     

META 4. Prediction of the metabolism of polycyclic aromatic hydrocarbons

Polycyclic aromatic hydrocarbons (PAHs) are widely distributed in the environment and are often implicated as potential carcinogens. It is generally believed that the carcinogenic potential of polycyclic aromatic hydrocarbons is linked to the formation of ultimate carcinogens generated by metabolic biotransformations. In this paper we propose a methodology that consists of using both quantum chemical properties and structural features of the reaction sites to predict PAH metabolism. Two essential questions have been answered: at which sites will the reaction take place and does the transformation consist of epoxidation or hydroxylation? This methodology has been successfully implemented into an expert system, META, for the evaluation of metabolic transformations of new chemicals. Received: 3 April 1998 / Accepted: 6 July 1998 / Published online: 7 October 1998     

Surface modification of silica-coated zirconia by chemical treatments

Zirconia surface modification by various chemical treatments after silica coating by sandblasting was investigated in this study. The surface of silica-coated dental zirconia was hydroxylated by treatment with different acids at room temperature for 4 h, rinsed with deionized water and air-dried. The modified surfaces were characterized by X-ray photoelectron spectroscopy (XPS) and atomic force microscopy (AFM). Shifts in binding energies for Zr 3d_5/2 and Si 2p peaks were observed after treatment with acids, thereby showing a change in the chemical states of zirconium and silicon on the surface layer of silica-coated zirconia. The XPS analysis revealed that the silica-coated zirconia (SiO₂-ZrO₂) surfaces had changed to hydrous silica-coated zirconia (SiO₂-ZrO₂·nH₂O). One-way ANOVA analysis revealed there was significant difference in both surface roughness parameters of silica-coated zirconia after chemical treatments and the surface topography varied depending on the acid treatment.