Palladium-Mediated Ring-Opening Reactions of Strained Cyclotetrasilanes

Room-temperature ring-opening polymerization of the strained monomer 1,2,3,4-tetramethyl-1,2,3,4-tetraphenylcyclotetrasilane mediated by palladium complexes was attempted. Examination of the reaction products between the cyclotetrasilane and the metal complex at high concentrations indicated that insertion of one cyclotetrasilane molecule between the ligand–palladium bond was followed by reductive elimination of the tetrasilane. Among the palladium complexes examined were (³-allyl)PdCp [Cp=⁵-cyclopentadienyl], [(³-allyl) PdCl]₂, (³-allyl)PdCl(PPh₃), (³-allyl)PdCl(PPh₃)₂, (³-allyl)Pd(PPh₃)₃ and PdCl₂(PPh₃)₂. Reaction of 1,1,2,3,4-pentamethyl-2,3,4-triphenylcyclotetrasilane with (³-allyl)PdCp provides similar products.     

Asymmetric Ring-Opening Reactions of Donor-Acceptor Cyclopropanes and Cyclobutanes

Donor-acceptor (DA) cyclopropanes are particularly useful synthetic building blocks, which have been widely applied in the total synthesis of natural products and important chiral molecules in organic synthesis. The asymmetric ring-opening reactions of racemic DA cyclopropanes and cyclobutanes, for example, aryl-substituted 1,1-cyclopropane diesters and aryl-substituted 1,1-cyclobutane diesters, with nucleophiles provides versatile access to optically active γ- and δ-functionalized carbon skeletons, as well as the kinetic resolution of racemic DA cyclopropanes, which are useful chiral skeletons in organic synthesis. Recently, we have developed a series of highly enantioselective ring-opening and annulation reactions of DA cyclopropanes and cyclobutanes with various nucleophiles, such as amines, alcohols, nitrones, azomethine imines, enol silyl ethers, and indoles, by employing nickel and copper catalysts with TOX and SaBOX as ligands. The reactions worked smoothly with excellent diastereoselectivities and enantioselectivities (up to >99/1 dr and up to 99 % ee) over broad substrate scopes.     

Ring-Opening Metathesis Polymerization and Related Olefin Metathesis Reactions in Benzotrifluoride as an Environmentally Advantageous Medium

A tremendous number of solvents, either as liquids or vapors, contaminate the environment on a daily basis worldwide. Olefin metathesis, which has been widely used as high-yielding protocols for ring-opening metathesis polymerization (ROMP), ring-closing metathesis (RCM), and isomerization reactions, is typically performed in toxic and volatile solvents such as dichloromethane. In this study, the results of our systematic experiments with the Grubbs G1, G2, and Hoveyda-Grubbs HG2 catalysts proved that benzotrifluoride (BTF) can replace dichloromethane (DCM) in these reactions, providing high yields and similar or even higher reaction rates in certain cases. The ROMP of norbornene resulted not only in high yields but also in polynorbornenes with a high molecular weight at low catalyst loadings. Ring-closing metathesis (RCM) experiments proved that, with the exception of the G1 catalyst, RCM occurs with similar high efficiencies in BTF as in DCM. It was found that isomerization of (Z)-but-2-ene-1,4-diyl diacetate with the G2 and HG2 catalysts proceeds at significantly higher initial rates in BTF than in DCM, leading to rapid isomerization with high yields in a short time. Overall, BTF is a suitable solvent for olefin metathesis, such as polymer syntheses by ROMP and the ring-closing and isomerization reactions.     

Ring-Opening Reactions of Propylene Oxide (Methyloxirane) over Au/MgO and Unsupported Au Catalysts

The ring-opening reactions of propylene oxide (methyloxirane) were studied over an Au/MgO catalyst and Au powder at various temperatures in a pulse microreactor. The supported catalyst was significantly more active, and its activity was dramatically influenced by the method of pretreatment. When it was finished with hydrogen, conversion values were always above 70%, but often it was in the 90%+ range. The major reaction pathways were deoxygenation and isomerization. The comparison in the behavior of Au/MgO and Au powder pointed to the crucial importance of the support–metal interface in oxygen traffic and as a landing place for hydrogen-rich residues being the hydrogen source for water and alcohol formation (hydrogenation).     

Expanding the Genetic Code for a Dinitrophenyl Hapten

Haptens, such as dinitrophenyl (DNP) are small molecules that induce strong immune responses when attached to proteins or peptides and, as such, have been exploited for diverse applications. We engineered a Methanosarcina barkeri pyrrolysyl‐tRNA synthetase (mbPylRS) to genetically encode a DNP‐containing unnatural amino acid, N⁶‐(2‐(2,4‐dinitrophenyl)acetyl)lysine (DnpK). Although this moiety was unstable in Escherichia coli, we found that its stability was enhanced in mammalian HEK 293T cells and was able to induce selective interactions with anti‐DNP antibodies. The capability of genetically introducing DNP into proteins is expected to find broad applications in biosensing, immunology, and therapeutics.     

Cyclodimerisation and Cyclotrimerisation During the Ring-Opening Reactions of Ethylene Sulfide (Thiirane) over Acidic Molecular Sieves and Alumina

Liquid phase benzylation (by benzyl chloride) and acylation (by benzoyl chloride) of benzene and toluene over a Ga–Mg hydrotalcite (which is a basic anionic clay) with or without HCl gas pretreatment and also over the hydrotalcite used earlier in the toluene benzylation (at 110°C) have been investigated. The fresh, used and HCl-gas-pretreated hydrotalcite catalysts were characterized for their surface area, basicity, crystalline structure and phases and also for their surface composition. The fresh hydrotalcite showed no catalytic activity for both the benzylation and benzoylation reactions for a long period. However, after a long induction period (2.7 h), the catalyst showed a very high activity in the toluene benzylation (at 110°C). Whereas, the catalyst after its use in the toluene benzylation or after its HCl gas pretreatment showed very high activity for both the reactions with a much shorter induction period. The hydrotalcite-derived catalyst consists essentially highly dispersed gallium and magnesium chlorides on MgO. The presence of moisture in the reaction mixture has beneficial effects on the acylation reactions over the used catalyst.     

拟除虫菊酯类农药半抗原合成及酶联免疫分析研究进展

酶联免疫技术是近年来发展的农药残留检测方法,具有方便快速、特异灵敏等优点。主要介绍了拟除虫菊酯类农药半抗原的合成方法以及酶联免疫分析技术的研究进展和展望。半抗原的合成入手点主要为菊酯分子的三碳环、中间和芳环部分,连接一个连接臂作为半抗原。目前制得的抗体多数是多克隆抗体,最低检测限最好的能达到0.2μg/L。酶联免疫分析技术在拟除虫菊酯类农药的残留检测中会发挥越来越大的作用。     

农药小分子半抗原合成的研究与应用

在农药残留免疫分析中,农药小分子半抗原的分子设计及合成是基础和关键。本文对近年来国内外关于农药小分子半抗原的设计及合成、合成的影响因素、半抗原与载体蛋白偶联方法的研究现状及应用作一综述。     

几种有机可逆反应实验装置的设计

反应的可逆性是有机化学反应的一大特点。文章从实验操作上对如何提高可逆反应的产率进行了分析,对基础有机化学实验中常见的可逆反应实验装置进行了归纳和讨论。     

Integrated Design for Solid Catalysts in Multiphase Reactions

The integrated design for solid catalysts in multiphase reactions requires consideration of all the different levels of scale involved in the reaction which should be simultaneously addressed in an integrated view with the reactor design. Emphasis is given here to three main levels (nano-, micro- and macro-scale) at which catalyst design should be considered. The main concepts discussed are (i) control of the local effective concentration of reagents and possible deactivating molecules around the active sites by incorporating them into nanoporous cavities and by tuning the hydrophilic character of the support, (ii) possibilities offered by microstructuring the catalyst composition (multilayer design and microassembled catalyst bodies) and (iii) opportunities offered by the use of macrostructured catalysts (monoliths, cloths and membranes). The various levels of design are not independent. They must be considered in an integrated view and in close relationship with reactor design.     

Single-Site Calcium Initiators for the Controlled Ring-Opening Polymerization of Lactides and Lactones

Summary Single-site calcium initiators containing chelating tmhd (H-tmhd = 2,2,6,6-tetramethylheptane-33-dione) ligands [(THF)Ca(tmhd)_]2[-N(SiMe₃)₂](-tmhd) (2) and [(THF)Ca(tmhd)]₂[-OCH(Me)Ph](-tmhd) (3) have been synthesized and applied for the ring-opening polymerization of L-lactide and -caprolactone. Both 2 and 3 were highly reactive and promoted a fast polymerization of L-lactide and -caprolactone to high monomer conversions under mild conditions (THF as a solvent, room temperature). More importantly, results showed that the ring-opening polymerizations of lactides and lactones initiated by either 3 or 2 in the presence of equivalent 2-propanol are living, to provide polymers and block copolymers of controlled molecular weights and tailored end-groups. The polymerizations were first-order in monomer up to high conversions, in which the in situ initiating system 2/2-propanol revealed no induction period and much faster polymerization kinetics as compared to 3.     

脱羧反应途径及其机制的研究进展

脱羧反应广泛地应用于化工、生物、医药、食品等领域.通过脱羧反应可使废弃化合物得到高值化利用,有利于新能源开发和环境保护.目前,对很多羧酸类化合物的脱羧反应提出了不同的途径,如热化学脱羧、电化学脱羧、光化学脱羧、光一电化学脱羧、催化脱羧等.综述了脱羧反应的途径、机制及其研究领域的主要进展.     

Approaches to Antibody-Catalyzed Cationic Cyclizations: Chemical Studies of Leaving Groups and Cyclization Modes

The solvolysis of a range of esters of 2-methylbenzyl alcohol 10 has been studied. It was found that sulphonate esters are solvolyzed very rapidly in aqueous solution (t_1/2 ≈ 35 s), whereas sulphinates react slowly (t_1/2 ≈ 35 h) and a phosphonate diester is stable. The fluorescent 5-dimethylaminonaphthalene-1-sulphinate (dansinate) esters (e.g. 22) held the most promise as substrates for antibody-catalyzed cationic cyclizations. It was shown by ¹⁸O-labeling studies that ester 13 is hydrolyzed predominantly (but not exclusively) by S_N1 reaction at the benzylic position. A number of conditions were tried to effect cationic cyclizations of an unsaturated benzylic alcohol 26, whose ester is intended as the substrate for the antibody-catalyzed reaction. The products obtained were the result of attack of the non-benzylic alcohol on either the benzylic cation, to give 31 and 32, or on the protonated alkene, to give 29. No cyclization of the alkene onto the benzylic cation, to give 4, was observed, suggesting that this is a disfavored reaction in solution.     

Ring-Opening Polymerization of Epoxidized Soybean Oil

Ring-opening polymerization of epoxidized soybean oil (ESO) catalyzed by boron trifluoride diethyl etherate (BF₃·OEt₂) in methylene chloride was conducted in an effort to develop useful biodegradable polymers. The resulting polymerized ESO (PESO) were characterized using infrared (IR) spectroscopy, differential scanning calorimetry (DSC), thermogravimetric analysis (TGA), ¹H NMR, ¹³C NMR, solid state ¹³C NMR and gel permeation chromatography (GPC). The results indicated that PESO materials were highly crosslinked polymers. They had glass transition temperatures ranging from −16 to −48 °C. TGA results showed the PESO polymers were thermally stable at temperatures up to 220 °C. Decomposition of the polymers was found to occur at temperature greater than 340 °C. GPC results indicated the extracted soluble substances from PESO polymers were ESO dimers, trimers and polymers with low molecular weights. The resulting crosslinked polymers can be converted into hydrogels by chemical modification, such as hydrolysis. These soy based hydrogels will find applications in personal care and health care areas.     

Silyl Triflate-Initiated Ring-Opening Polymerizations of Cyclosiloxanes

Polymer yields between 70 and 85% form in ring-opening polymerizations of trimeric cyclosiloxanes initiated with trimethylsilyl triflate in the presence of a selective scavenger for protic acids, 2,6-di-t-butylpyridine. Examples include the polymerization of 1,3,5-trimethyl-1,3,5-tris(3,3,3-trifluoropropyl)cyclotrisiloxane (F₃) and hexamethylcyclotrisiloxane (D₃). Siloxane bond redistribution is essentially absent and data are presented establishing a nonterminating polymerization giving an average of roughly one trimethylsilyl group per chain. The present observations contrast with other work that suggests that the presence of free triflic acid is required for initiation with trimethylsilyl triflate.     

苄基化甘露糖原酸酯的开环研究

以甘露糖为原料合成了苄基化甘露糖原酸酯,并对其在酸催化下的开环反应进行了研究,采用1HNMR对各阶段产物进行了表征.结果表明,冰乙酸的催化效果最好,产率可迭90%,同时对冰乙酸催化苄基化甘露糖原酸酯的开环机理进行了探讨,合理地解释了用80%或50%乙酸代替冰乙酸时产率下降的原因,主要是水与亲核试剂乙酸根形成了竞争,生成...     

Effects of Hapten Density on the Induced Antibody Repertoire

Small peptides and oligosaccharides are important antigens for the development of vaccines and the production of monoclonal antibodies. Because of their small size, peptides and oligosaccharides are non‐immunogenic on their own and typically must be conjugated to a larger carrier protein to elicit an immune response. Selection of a suitable carrier protein, conjugation method, and hapten density are critical for generating an optimal immune response. We used a glycan array to compare the repertoire of antibodies induced after immunizing with either low or high‐density conjugates of the tumor‐associated Tn antigen. At high hapten density, a broader range of antibodies was induced, and reactivity to the clustered Tn antigen was observed. In contrast, antibodies induced by the low‐density conjugate had narrower reactivity and did not bind the clustered Tn antigen.     

Dual Activities of Oxidation and Oxidative Decarboxylation by Flavoenzymes

Specific flavoenzyme oxidases catalyze oxidative decarboxylation in addition to their classical oxidation reactions in the same active sites. The mechanisms underlying oxidative decarboxylation by these enzymes and how they control their two activities are not clearly known. This article reviews the current state of knowledge of four enzymes from the l -amino acid oxidase and l -hydroxy acid oxidase families, including l -tryptophan 2-monooxygenase, l -phenylalanine 2-oxidase and l -lysine oxidase/monooxygenase and lactate monooxygenase which catalyze substrate oxidation and oxidative decarboxylation. Apart from specific interactions to allow substrate oxidation by the flavin cofactor, specific binding of oxidized product in the active sites appears to be important for enabling subsequent decarboxylation by these enzymes. Based on recent findings of l -lysine oxidase/monooxygenase, we propose that nucleophilic attack of H₂O₂ on the imino acid product is the mechanism enabling oxidative decarboxylation.