Search results for "Activity"
showing 10 items of 7178 documents
Conjugate addition of organolithium reagents to α,β-unsaturated carboxylic acids
1999
Abstract Conjugate addition of primary, secondary, tertiary alkyl and phenyl lithium reagents to 2-alkenoic acids affords good yields of branched saturated carboxylic acids. Methyl groups at the α- and β-carbon of the 2-alkenoic acid decrease reactivity as acceptors, and foster deprotonation, respectively. The lithium enediolate resulting from the conjugate addition can react with electrophiles. PM3 calculations are in agreement with the substituent effects.
Synthesis and Characterization of theO-Alkylation Products of Resorcinarene
2013
O-Substitution reactions of tetramethoxyresorcinarene with alkyl halides produced a variety of partially O-alkylated resorcinarene derivatives with terminal alkyne functionality. The degree of alkylation was affected by the reactivity of the alkylating halide used. NMR spectroscopy proved to be an ideal tool for analyzing the complex reaction mixtures and the isolated products based on the symmetry and degree of alkylation of the resorcinarene derivatives. Single-crystal X-ray diffraction studies furthermore showed diversity in the self-assembly of the various O-alkylation products that was greatly affected by the degree of alkylation, as well as the nature of the alkyne moiety.
Group 4 and Group 8 unbridged metallocene derivatives with a pendant fluorenyl group. X-ray structure of 1,1â²-bis[2-(2-fluorenyl)propyl]ferrocene
1999
Abstract The action of one equivalent of BuLi on 2-cyclopentadienyl-2-fluorenylpropane (C5H5CMe2C13H9) led to the monoanionic salt LiC5H4CMe2C13H9. This anion was reacted with Fe2Cl4(THF)3, TiCl3(THF)3, ZrCl4, HfCl4 or CpZrCl3, DME and, in a mixture with CpLi, with Fe2Cl4(THF)3 affording the corresponding metallocenes and metallocene dichlorides. The X-ray structure of Fe(η5-C5H4CMe2C13H9)2 is described. The first results about the reactivity of the fluorenyl group are reported together with the synthesis of Zr(η5-C5H5)[η5-C5H4CMe2-η5-C13H8Rh(cod)]Cl2, which has been tested using hydroformylation and cyclotrimerisation catalysis.
Anti-/Pro-Oxidant Behavior of Naturally Occurring Molecules in Polymers and Biopolymers: A Brief Review
2019
Polymers and biopolymers are continuously subjected to the action of different stress factors, such as oxygen, heat, UV light, mechanical stress, humidity, etc., during their processing and service life, undergoing overall oxidative degradation, that causes performance and property deterioration. To improve the resistance at high temperatures and long-term weatherability of the polymers and biopolymers, usually, during their manufacturing, synthetic antioxidants and UV-light stabilizers are added. In the past decade, several concerns related to the impact of the synthetic stabilizers have emerged, and to reduce their negative effect on human health and the environment, their replacement wit…
Understanding the Electronic Reorganization along the Nonpolar [3+2] Cycloaddition Reactions of Carbonyl Ylides
2011
[EN] The nonpolar [3+2] cycloaddition (32CA) reaction of the carbonyl ylide (CY) 23 with tetramethylethylene (TME) 24 has been studied with DFT methods at the B3LYP/6-31G* level. This cycloaddition reaction, which has a very low activation energy of 4.7 kcal/mol, takes place through a synchronous transition structure. A topological analysis of the ELF along the 32CA reaction provides a new scope of the electronic structure of CY 23 as a pseudodiradical species offering a sound explanation of the high reactivity of this CY in nonpolar reactions. In addition, this analysis points to the nonparticipation of the oxygen lone pairs in the 32CA reaction. This cycloaddition can be seen as a pseudod…
Photoelectrocatalytic selective oxidation of 4-methoxybenzyl alcohol in water by TiO2 supported on titanium anodes
2013
The photoelectrocatalytic partial oxidation of 4-methoxybenzyl alcohol in aqueous solution irradiated by near-UV light was carried out in a three-electrode batch reactor. TiO2 films were either deposited by dip-coating of a TiO2 sol onto a Ti foil and subsequent calcination or generated on Ti plates by thermal oxidation in air at 400-700 degrees C. The effects of the anode preparation method and bias potential values on conversion and selectivity to the corresponding aldehyde were investigated. The photoelectrocatalytic results were compared with the photocatalytic and electrocatalytic ones. The results indicated that no reaction occurred during the electrocatalytic experiments, whereas the…
Localization and Activity of a Carboxypeptidase in Germinating Seeds of Scots Pine, Pinus sylvestris
1976
Extracts prepared from the endosperm of germinating seeds of Scots pine, Pinus sylvestris L., hydrolysed two typical carboxypeptidase substrates, Z-Phe-Ala and Z-Phe-Phe, with pH optima at 4.2 and 5.0. The activities were completely destroyed by diisopropylfluorophosphate. Identical heat inactivation curves and elution patterns in gel chromatography on Sephadex G-200 suggest that the two activities are due to a single enzyme. In resting seeds very low carboxypeptidase activity was present in both the endosperm and the embryo. During germination on agar gel at 20°C in the dark the activities, expressed as enzyme units per seed, increased in the seedling and particularly in the endosperm up t…
Poly(S-ethylsulfonyl-l-homocysteine): An α-Helical Polypeptide for Chemoselective Disulfide Formation
2018
Homocysteine and cysteine are the only natural occurring amino acids that are capable of disulfide bond formations in peptides and proteins. The chemoselective formation of asymmetric disulfide bonds, however, is chemically challenging and requires an activating group combining stability against hard nucleophiles, e.g., amines, with reactivity toward thiols and soft nucleophiles. In light of these considerations, we introduced the S-alkylsulfonyl cysteines in our previous work. Here, we present the synthesis and ring-opening polymerization of S-ethylsulfonyl-l-homocysteine N-carboxyanhydrides. We demonstrate that the polymerization leads to narrowly distributed polypeptides (Đ = 1.1–1.3) wi…
Orthogonally reactive amino acids and end groups in NCA polymerization
2017
Functional amino acids whose reactivity is compatible with the polymerization of α-amino acid-N-carboxyanhydrides (NCAs) have received a lot of attention in recent years. The appeal of these reactive monomers lies in the fact that the resulting polymers can be easily modified in one controlled post-polymerization step, leading to a variety of polypeptidic materials like helical non-natural polycations or glycopeptides. This review highlights recent developments in the field and focuses on the different reactive groups like alkynes, alkenes, azides, chlorides and S-alkylsulfonyls. Furthermore, the modifications after polymerization are discussed, pointing out advantages and challenges. Besid…
Poly(S-ethylsulfonyl-l-cysteines) for Chemoselective Disulfide Formation
2016
The amino acid cysteine possesses a unique role in nature due to its ability to reversibly cross-link proteins. To transfer this feature to polypeptides and control the process of disulfide formation, a protective group needs to provide stability against amines during synthesis, combined with chemoselective reactivity toward thiols. A protective group providing these unique balance of stability and reactivity toward different nucleophiles is the S-alkylsulfonyl group. In this work we report the polymerization of S-ethylsulfonyl-l-cysteine N-carboxyanhydride and kinetic evaluations with respect to temperature (−10, 0, and +10 °C) and monomer concentration. The polymerization degree of poly(S…