Fourier transform NMR investigations of 13C labeled silyl and stannyl acetylenes

The ¹³C NMR spectra of ¹³C labeled mono- and di-substituted silyl- and stannyl-acetylenes have been studied. It was found that the values of ¹J(CC) coupling constants between acetylenic carbons decrease very sharply in the series Alk₃SiCCH, Alk₃SnCCH, Alk₃SiCCSiAlk₃ and Alk₃SnCCSnAlk₃. These results and the observed changes in the geminal hetero-atom β-acetylenic carbon couplings suggest a very strong p_πd_π interaction between the π-electrons of the triple bond and the vacant d orbitals of silicon and tin.     

Phenolic Compounds in the Flowers of Lavatera trimestris L. (Malvaceae)

JPC – Journal of Planar Chromatography – Modern TLC - Plants of the Malvaceae family are very rich in phenolic compounds. TLC, 2D TLC, and RP HPLC have been used for qualitative...     

Diastereoselective Diels-Alder Reaction of 2-Thienyl and 2-Furyl Substituted 3-Propenethioamides with Electron Deficient Dienophiles

Summary. The hetero-Diels-Alder reaction of N-aryl-3-(2-thienyl)-2-propenethioamides with N-phenylmaleimide and maleimide yielded a mixture of endo- and exo-2-arylimino-4-(2-thienyl)tetrahydrothiopyran[2,3-c]pyrroles. Cycloaddition to diethyl fumarate required acylation and furnished a mixture of diastereoisomeric 5-(N-acetylphenylamino)-2,3-bis-(ethoxycarbonyl)-4-(2-thienyl)-3,4-dihydro-2H-thiopyrans. Reactions of 3-(2-furyl)-2-propenethioamides with N-arylmaleimides furnished the correspondent 2-arylimino-4-(2-furyl)tetrahydrothiopyran[2,3-c]pyrroles. In the cycloadditions of the heterodienes with N-arylmaleimides the endo-cycloadducts were formed as the major products.     

Polychromatic selective population inversion for TROSY experiments with large proteins

This paper presents polychromatic selective polarization inversion (PC-SPI) as an alternative to the polarization transfer methods recently developed for the application of NMR to large biological molecules. Theoretical and numerical considerations indicate that PC-SPI has the potential for more efficient polarization transfer under conditions of rapid transverse relaxation compared to J coupling- and cross-correlated relaxation-based transfers. The main advantage offered by the method presented here is the maintenance of near-optimal trajectories of inversion of the individual components of the spin magnetization while using broadband optimized pulses. A 2D experiment was implemented combining PC-SPI with TROSY-based chemical shift correlation. The experiment was applied to detect (15)N-(1)H chemical shift correlation spectra of a 200 kDa complex consisting of an 80% (2)H- and uniformly (15)N,(13)C-labeled 22 kDa portion of complement receptor type 1 and unlabeled C3b of complement (180 kDa).     

Capillary electrophoretic determination of inorganic selenium species

The performance of pyromellitic electrolyte for capillary zone electrophoresis of inorganic selenium species in the presence of selected common anions with indirect UV detection was investigated. The separation was achieved with pyromellitic electrolyte at pH 8.8 and hexamethonium hydroxide as the electroosmotic flow modifier. Obtained detection limits of 0.17 microg ml(-1) for Se(VI) and 0.29 microg ml(-1) for Se(IV) were improved by a factor of 5-7 in comparison with chromate electrolyte, which has been mainly employed for selenium analysis. Good resolution for nitrate-Se(VI) peaks were obtained.     

Application of Organolithium and Related Reagents in Synthesis,Part 29 [1]. A Concise Regiospecific Conversion of Benzoic Acids into 5-(2-Carboxyphenyl)-5-phenylpent-2-enoic Acids

Summary. A convenient two-step protocol preparation of ortho-alkylated (substituent with the carbomethoxy group at the end of five carbon atoms alkyl chain) aromatic carboxylic acids from benzoic acids anilides is described. Ortho-lithiation of benzanilides and subsequent reaction of the generated bis(N- and C-ortho-)-lithiated anilides with aromatic aldehydes provided 3-arylphthalides. In the next step, these phthalides were converted into 5-(2-carboxyphenyl)-5-phenylpent-2-enoic acids by treatment with 1-methoxy-1-trimethylsilyloxybuta-1,3-diene.     

The Photophysics of β-Tyrosine and Its Simple Derivatives

Synthesis and photophysical studies of (O-methyl)--tyrosine (-tyrosine; an analogue of tyrosine, in which the amino group is moved from the - to the -carbon, closer to the phenol ring) and its derivatives with a blocked amino and/or carboxyl group were performed to explain the nature of the fluorescence of tyrosine derived analogues. All -tyrosine derivatives, except Ac-Tyr(Me), displayed the monoexponential fluorescence decay. The biexponential fluorescence decay observed for Ac-Tyr(Me) is assumed to be the result of the presence of two low-energy conformations (extended and with an intramolecular hydrogen bond). Higher quenching of the fluorescence of -tyrosine derivatives by the N-acetyl group than by the N-methylamide group moved farther was found, contrary to the data found for the respective derivatives of natural tyrosine. The obtained photophysical data are discussed with theoretical calculations (AMBER, AM1) on the basis of the rotamer model.     

Transition metal complexes of tetradentate and bidentate Schiff bases as catalysts for ethylene polymerization: Effect of transition metal and cocatalyst

This article compares catalytic performance of ethylene polymerization in similar polymerization conditions of transition metal complexes having two ligands [O,N] (phenoxy‐imine) and having one tetradentate ligand [O,N,N,O] (salphen or salen). It is shown that the activity of both complex types as well as the product properties depend in the same way on the type of central metal in the complex and on the cocatalyst used. Although the type of ligand has some effect on the catalyst activity, yet it does not control the properties of the obtained products. The vanadium and zirconium complexes, irrespective of the cocatalyst used, yield linear polyethylene with high molecular weight (a few hundred thousand g/mol). Similar products are formed when titanium complexes activated with MAO are employed. On the other hand, the same titanium complexes in conjunction with Et₂AlCl, yield low molecular weight polyethylene (of a few thousand) and additionally a mixture of oligomers. © 2008 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem 47: 565–575, 2009