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Terminal Position (terminal + position)
Selected AbstractsChemInform Abstract: Highly Stereoselective Intramolecular Diels,Alder Reaction of Decatrienoates Activated by t-Butoxycarbonyl, Chloro, and Sulfonyl Groups at the Terminal Position.CHEMINFORM, Issue 7 2009Seiichi Inoue Abstract ChemInform is a weekly Abstracting Service, delivering concise information at a glance that was extracted from about 200 leading journals. To access a ChemInform Abstract of an article which was published elsewhere, please select a "Full Text" option. The original article is trackable via the "References" option. [source] Highly Regio- and Stereoselective Copper(I) Chloride-Mediated Carbometallation of 2,3-Allenols with Grignard ReagentsADVANCED SYNTHESIS & CATALYSIS (PREVIOUSLY: JOURNAL FUER PRAKTISCHE CHEMIE), Issue 7 2007Zhan Lu Abstract An efficient highly regio- and stereoselective copper(I) chloride-mediated carbometallation of differently substituted 2,3-allenols with primary or secondary alkyl or aromatic Grignard reagents followed by iodination to synthesize fully-substituted allylic alcohols has been developed. This protocol introduces the R4 group from the Grignard reagent to the terminal position of the 2,3-allenols. [source] Reactivity of Tyr,Leu and Leu,Tyr dipeptides: identification of oxidation products by liquid chromatography,tandem mass spectrometryJOURNAL OF MASS SPECTROMETRY (INCORP BIOLOGICAL MASS SPECTROMETRY), Issue 5 2009Conceição Fonseca Abstract The exposure of peptides and proteins to reactive hydroxyl radicals results in covalent modifications of amino acid side-chains and protein backbone. In this study we have investigated the oxidation the isomeric peptides tyrosine,leucine (YL) and leucine,tyrosine (LY), by the hydroxyl radical formed under Fenton reaction (Fe2+/H2O2). Through mass spectrometry (MS), high-performance liquid chromatography (HPLC-MS) and electrospray tandem mass spectrometry (HPLC-MSn) measurements, we have identified and characterized the oxidation products of these two dipeptides. This approach allowed observing and identifying a wide variety of oxidation products, including isomeric forms of the oxidized dipeptides. We detected oxidation products with 1, 2, 3 and 4 oxygen atoms for both peptides; however, oxidation products with 5 oxygen atoms were only present in LY. LY dipeptide oxidation leads to more isomers with 1 and 2 oxygen atoms than YL (3 vs 5 and 4 vs 5, respectively). Formation of the peroxy group occurred preferentially in the C -terminal residue. We have also detected oxidation products with double bonds or keto groups, dimers (YL,YL and LY,LY) and other products as a result of cross-linking. Both amino acids in the dipeptides were oxidized although the peptides showed different oxidation products. Also, amino acid residues have shown different oxidation products depending on the relative position on the dipeptide. Results suggest that amino acids in the C -terminal position are more prone to oxidation. Copyright © 2009 John Wiley & Sons, Ltd. [source] Conformational Study of Short Peptoid Models for Future Applications as Potent Antimicrobial CompoundsMACROMOLECULAR THEORY AND SIMULATIONS, Issue 3 2007Fateh Singh Nandel Abstract Development of peptides as clinically useful drugs is limited by their poor metabolic stability and low bioavailability. Recent progresses in chemical synthesis and design have led to several strategies for producing potent mimetics. This study aims to analyze sequence/structure requirements and composition for antimicrobial peptoid designs, as use of peptoids is one of the most representative approaches to meet the goal of biomimicry. Analysis of the designs showed that for maximum activity and minimum hemolysis, the plane of the aromatic residues should be at an angle between 0 and 90,° with respect to membranes, cationic residues need not be at the terminal position, and central positions should be uniform in NIle, NLys, and NPhe residues. [source] Bioactive fMLF-OMe analogs containing a N -terminal oximic or formylhydrazonic moietyCHEMICAL BIOLOGY & DRUG DESIGN, Issue 2 2000I. Torrini Abstract: In order to obtain chemotactic peptides with selective bioactivity, a new type of structural modification was introduced at the N -terminal position of HCO-Nle-Leu-Phe-OMe. Two groups of analogs have been synthesized both containing a N -terminal residue of the X=C(R)-CO-type replacing the native HCO-NH-CH(R)-CO-. In particular, the A group of pseudopeptides (2a - d) possesses a N -terminal oximic fragment (X=HO-N) and the B group (3a - d) a formylhydrazone fragment (X=HCO-NH-N). These new ligands have been examined for their capacity to induce chemotaxis and other cellular responses such as superoxide anion production and lysozyme release; although significantly active as chemoattractants they have been found to be practically devoid of secretagog activity, thus exhibiting selective behavior. The adopted chemical modification seems extensible in designing a new class of pseudopeptides (hydrazonopeptides) structurally related to both hydrazinopeptides and peptides containing ,,,-unsaturated residues. [source] Density functional study of the relative reactivity in the concerted 1,3-dipolar cycloaddition of nitrile ylide to disubstituted ethylenesINTERNATIONAL JOURNAL OF QUANTUM CHEMISTRY, Issue 6 2001Hsin-Yi Liao Abstract Density functional theory was used to perform a theoretical evaluation of (E)-1,2-disubstituted ethylenes as dipolarophiles for the 1,3-dipolar cycloaddition reaction. The reactivities of electron-withdrawing and -donating substituted ethylenes were examined by estimating their activation energies. The calculated activation energies predicted that the most reactive species is (E)-1,2-C2H2(NO)2, whereas the least reactive is (E)-2-butene. Namely, it was demonstrated that 16-electron 1,3-dipole reactants with more electropositive substituents in terminal positions and ethylenes that possess more strongly electron-withdrawing substituents facilitate 1,3-dipolar cycloaddition reactions. All of the theoretical results can be rationalized using the configuration mixing model. © 2001 John Wiley & Sons, Inc. Int J Quant Chem 83: 318,323, 2001 [source] Ferrocenyl-Ended Thieno,Vinylene Oligomers: Donor,Acceptor Polarization and Mixed-Valence Properties with Emphasis on the Raman Mapping of Localized-to-Delocalized TransitionsCHEMISTRY - A EUROPEAN JOURNAL, Issue 11 2009Juan Casado Dr. Abstract What's your role? New oligothiophene,vinylene compounds have been synthesized to study the role of the conjugated chain in two different cases (see scheme; MV=mixed valence). The electronic and molecular structures were analyzed by means of electronic, X-ray photoelectron, and Raman spectroscopy, together with thermo spectroscopy, electrochemistry, and DFT calculations. New oligothiophene,vinylene compounds have been synthesized in order to study the role of the conjugated chain in two different cases: 1),when push,pull action operates between an electron-donor and an electron-acceptor group at the ends of the thiophene,vinylene conjugated chain, and 2),when mixed-valence action is induced by single oxidation of the same chain functionalized at both terminal positions with ferrocene groups leading to competition between the donor groups. The electronic and molecular structures are analyzed by means of electronic, X-ray photoelectron and Raman spectroscopies, together with thermospectroscopy, electrochemistry and density functional theory calculations. The cyclic voltammetry processes have been followed by spectrochemistry. It is shown that the radical cation of the diferrocenyl derivative is a class,III mixed-valence system (i.e., fully delocalized) according to its Raman spectrum. Moreover, by Raman thermo-spectroscopy the thermal transition of this radical cation from a delocalized (class,III, room temperature) to a localized (class,II, ,160,°C) state is scanned. In all cases the Raman study is paralleled by an electronic absorption spectroscopic analysis. Structure,property relationships are proposed for molecules of two important fields of very active research as that of the non-linear optics (i.e., organic optoelectronic) and that of the mixed-valence systems (i.e., charge-transfer processes). [source] | ||||||||||