Synthesis and Characterization of Privileged Monodentate Phosphoramidite Ligands and Chiral Brønsted Acids Derived from d-Mannitol

The synthesis of several novel chiral phosphoramidite ligands (L1–L8) with C₂ symmetric, pseudo C₂ symmetric secondary amines and chiral Brønsted acids 1a,b has been achieved. These chiral auxiliaries were obtained from commercially available d-mannitol, and secondary amines in moderate to excellent yields. Excellent diastereoselectivites of ten chiral auxiliaries were obtained. The chiral phosphoramidite ligands and chiral Brønsted acids were fully characterized by spectroscopic methods.     

8-oxoguanine DNA glycosylases: one lesion, three subfamilies

Amongst the four bases that form DNA, guanine is the most susceptible to oxidation, and its oxidation product, 7,8-dihydro-8-oxoguanine (8-oxoG) is the most prevalent base lesion found in DNA. Fortunately, throughout evolution cells have developed repair mechanisms, such as the 8-oxoguanine DNA glycosylases (OGG), which recognize and excise 8-oxoG from DNA thereby preventing the accumulation of deleterious mutations. OGG are divided into three subfamilies, OGG1, OGG2 and AGOG, which are all involved in the base excision repair (BER) pathway. The published structures of OGG1 and AGOG, as well as the recent availability of OGG2 structures in both apo- and liganded forms, provide an excellent opportunity to compare the structural and functional properties of the three OGG subfamilies. Among the observed differences, the three-dimensional fold varies considerably between OGG1 and OGG2 members, as the latter lack the A-domain involved in 8-oxoG binding. In addition, all three OGG subfamilies bind 8-oxoG in a different manner even though the crucial interaction between the enzyme and the protonated N7 of 8-oxoG is conserved. Finally, the three OGG subfamilies differ with respect to DNA binding properties, helix-hairpin-helix motifs, and specificity for the opposite base.     

Sulfonic Acid‐Catalyzed Autoxidative Carbon‐Carbon Coupling Reaction under Elevated Partial Pressure of Oxygen

An aerobic organocatalytic oxidative C C bond formation reaction of benzylic C H bonds with various C‐nucleophiles is described. The coupling reaction proceeds by simply stirring the substrates under elevated partial pressure of oxygen in the presence of a sulfonic acid catalyst at room temperature. Elevation of the pressure enables the reaction of a broad scope of nucleophile substrates otherwise showing poor reactivity at ambient pressure. The benzylic C H bonds of xanthene, acridanes, isochromane and related heterocycles could be functionalized with nucleophiles including ketones, 1,3‐dicarbonyl compounds and aldehydes. Electron‐rich arenes could be utilized as nucleophiles at elevated temperatures. The reactions are believed to proceed via autoxidation of the benzylic C H bonds to the hydroperoxides and subsequent nucleophilic substitution catalyzed by sulfonic acids.     

Catalytic Electrophilic Halogenation of Silyl‐Protected and Terminal Alkynes: Trapping Gold(I) Acetylides vs. a Brønsted Acid‐Promoted Reaction

In the presence of a cationic gold(I) catalyst and N‐halosuccinimide, both trimethylsilyl‐protected and terminal alkynes are converted into alkynyl halides. Further experiments showed that silyl‐protected alkynes undergo electrophilic iodination and bromination under Brønsted acid catalysis, whilst terminal alkynes require a cationic gold catalyst. The former reactions probably proceed via activation of the electrophile, whilst the latter reactions proceed via a gold(I) acetylide intermediate. Gold‐catalysed halogenation was further combined with gold‐catalysed hydration and subsequent annulation to provide convenient routes to iodomethyl ketones and five‐membered aromatic heterocycles.     

Brønsted Acid‐Promoted Divergent Reactions of Enaminones: Efficient Synthesis of Fused Pyrroles with Different Substitution Patterns

Brønsted acid‐promoted divergent approaches for the synthesis of multi‐functionalized fused pyrroles with different substituted patterns (44 examples) have been established. The divergent pathways were controlled by varying the charging sequence to give a series of new fused pyrroles selectively. The direct C(sp³) S bond formation was achieved for the two‐step, one‐pot reaction.     

催化环氧乙烷氢甲酯化的催化体系研究

以Co2(CO)8为催化剂,甲醇为溶剂,在一定CO压力下,催化环氧乙烷氢甲酯化合成3-羟基丙酸甲酯。考察了不同配体、催化剂/配体的摩尔比、温度、CO压力、甲醇用量对环氧乙烷氢甲酯化反应的影响。得到较佳合成工艺条件为:咪唑为配体、n(催化剂)∶n(配体)=1∶1.5、反应温度75℃、反应压力6.0 MPa、V(甲醇)∶V(环氧乙烷)=5∶1。在该优化条件下,环氧乙烷转化率为92.24%,3-羟基丙酸甲酯收率达84.35%。     

不同温度下U_3O_8结构的XRD研究

在大气环境及25～850℃条件下，用X射线衍射(XRD)法研究了八氧化三铀(U3O8)的结构变化情况。在实验温度范围内发现，U3O8存在两种相结构，相变温度约为300℃。低于300℃时，U3O8为底心正交结构(Amm2)，在300℃附近转变为对称性更高的简单六方结构(P-62m)。在两种相结构稳定的温度范围内，U3O8的点阵参数随温度的增加呈现出规律性变化。     

A phenolic antioxidant from the Pacific oyster (Crassostrea gigas) inhibits oxidation of cultured human hepatocytes mediated by diphenyl-1-pyrenylphosphine

3,5-Dihydroxy-4-methoxybenzyl alcohol (DHMBA), an antioxidant isolated from the Pacific oyster (Crassostrea gigas), was studied in a cell-based fluorometric antioxidant assay using human hepatocyte-derived cells (C3A) and diphenyl-1-pyrenylphosphine (DPPP) as a fluorescent probe. In comparison with two hydrophilic antioxidants, DHMBA showed the stronger inhibition of DPPP-mediated fluorescence than chlorogenic acid and l-ascorbic acid: at a concentration of 320 μM of DPPP, the inhibition was 26.4 ± 2.6%, 11.1 ± 1.2%, and 0 ± 2.0% for DHMBA, chlorogenic acid, and l-ascorbic acid, respectively (mean ± SD, n = 4). Their relative oxygen radical absorbance capacities (ORAC) were dissociated with their cell-based antioxidant activities: 1.47 ± 0.40, 4.57 ± 0.30, and 0.53 ± 0.13 μmol TE/μmol for DHMBA, chlorogenic acid, and l-ascorbic acid, respectively (mean ± SD, n = 4). The amphiphilicity of DHMBA was better than chlorogenic acid and l-ascorbic acid might underlie this dissociation. Since the C3A cells are human hepatoma-derived cells, DHMBA might be useful in the prevention and treatment of liver diseases by involving an oxidation process.