@article{MachumiYenesewMidiwoetal.2012,
  author    = {Machumi, Francis and Yenesew, Abiy and Midiwo, Jacob O. and Heydenreich, Matthias and Kleinpeter, Erich and Tekwani, Babu L. and Khan, Shabana I. and Walker, Larry A. and Muhammad, Ilias},
  title     = {Antiparasitic and anticancer carvotacetone derivatives of Sphaeranthus bullatus},
  series = {Natural product communications : an international journal for communications and reviews},
  volume    = {7},
  journal   = {Natural product communications : an international journal for communications and reviews},
  number    = {9},
  publisher = {NPC},
  address   = {Westerville},
  issn      = {1934-578X},
  pages     = {1123 -- 1126},
  year      = {2012},
  abstract  = {The CH2Cl2-MeOH (1:1) extract of the aerial parts of Sphaeranthus bullatus, an annual herb native to tropical East Africa, showed activity against chloroquine sensitive D6 (IC50 9.7 mu g/mL) and chloroquine resistant W2 (IC50 15.0 mu g/mL) strains of Plasmodium falciparum. Seventeen secondary metabolites were isolated from the extract through conventional chromatographic techniques and identified using various spectroscopic methods. The compounds were evaluated for their in vitro antiplasmodial, antileishmanial and anticancer activities revealing activity of four carvotacetone derivatives, namely 3-acetoxy-7-hydroxy-5-tigloyloxycarvotacetone (1) 3,7-dihydroxy-5-tigloyloxycarvotacetone (2), 3-acetoxy-5,7-dihydroxycarvotacetone (3) and 3,5,7-trihydroxycarvotacetone (4); with antiplasmodial IC50 values of 1.40, 0.79, 0.60 and 3.40 mu g/mL, respectively, against chloroquine sensitive D6 strains of P. falciparum; antiplasmodial activity of IC50 2.00, 0.90, 0.68 and 2.80 mu g/mL respectively, against chloroquine resistant W2 strains of P. falciparum, antileishmanial IC50, values of 0.70, 3.00, 0.70 and 17.00 mu g/mL, respectively, against the parasite L. donovanii promastigotes, and anticancer activity against human SK-MEL, KB, BT-549 and SK-OV-3 tumor cells, with IC50 values between <1.1 - 5.3 mu g/mL, for 1-3. In addition, cytotoxic effects of the active compounds were evaluated against monkey kidney fibroblasts (VERO) and pig kidney epithelial cells (LLC-PK11). The structures of carvotacetone derivatives were determined by ID and 2D NMR spectroscopy; the absolute stereochemical configuration of 3-acetoxy-7-hydroxy-5-tigloyloxycarvotacetone (I) was determined as 3R, 4R, 5S by circular dichroism, specific rotation, H-1 NMR and 2D NMR ROESY and NOESY experiments.},
  language  = {en}
}
@article{KleinpeterShainyanKirpichenkoetal.2012,
  author    = {Kleinpeter, Erich and Shainyan, Bagrat A. and Kirpichenko, Svetlana V. and Shlykov, Sergei A.},
  title     = {Structure and Conformational Properties of 1,3,3-Trimethyl-1,3-Azasilinane : Gas Electron Diffraction, Dynamic NMR, and Theoretical Study.},
  issn      = {1089-5639},
  year      = {2012},
  language  = {en}
}
@article{ShainyanSuslovaKleinpeter2012,
  author    = {Shainyan, Bagrat A. and Suslova, Elena N. and Kleinpeter, Erich},
  title     = {Conformational analysis of 4,4-dimethyl-1-(trifluoromethylsulfonyl)-1,4-azasilinane and 2,2,6,6-tetramethyl-4- (trifluoromethylsulfonyl)-1,4,2,6-oxazadisilinane},
  year      = {2012},
  abstract  = {4,4-Dimethyl-1-(trifluoromethylsulfonyl)-1,4-azasilinane 1 and 2,2,6,6-tetramethyl-4-(trifluoromethylsulfonyl)- 1,4,2,6-oxazadisilinane 2 were studied by variable temperature dynamic 1H, 13C, 19F NMR spectroscopy and theoretical calculations at the DFT (density functional theory) and MP2 (Moller-Plesset 2) levels of theory. Both kinetic (barriers to ring inversion) and thermodynamic data (frozen conformational equilibria) could be obtained for the two compounds. The computations revealed two minima on the potential energy surface for molecules 1 and 2 corresponding to the rotamers with the CF3SO2 group directed inward and outward the ring, the latter being 0.20.4 kcal/mol (for 1) and 1.1 kcal/mol (for 2) more stable than the former. The vibrational calculations at the DFT and MP2 levels of theory give the values of the free energy difference Delta G degrees for the 'inward' reversible arrow 'outward' equilibrium consistent with those determined from the experimentally measured ratio of the rotamers. The structure of crystalline compound 2 was ascertained by X-ray diffraction analysis.},
  language  = {en}
}
@article{LazarevaAlbanovShainyanetal.2012,
  author    = {Lazareva, Nataliya F. and Albanov, Alexander I. and Shainyan, Bagrat A. and Kleinpeter, Erich},
  title     = {Synthesis and conformational properties of substituted 1,4,2-oxazasilinanes: low temperature NMR study and quantum chemical calculations},
  doi       = {10.1016/j.tet.2011.11.077},
  year      = {2012},
  language  = {en}
}
@article{KleinpeterStojanovicMarkovicetal.2012,
  author    = {Kleinpeter, Erich and Stojanovic, Milovan and Markovic, Rade and Baranac-Stojanovic, Marija},
  title     = {Synthesis of thiazolidine-fused heterocycles via exo-mode cyclizations of vinylogous N-acyliminium ions},
  issn      = {1477-0520},
  year      = {2012},
  abstract  = {Syntheses of thiazolidine-fused heterocycles via exo-mode cyclizations of vinylogous N-acyliminium ions incorporating heteroatom-based nucleophiles have been examined and discussed. The formation of (5,6)-membered systems was feasible with all nucleophiles tried (O, S and N), while the closing of the five-membered ring was restricted to O- and S-nucleophiles. The closure of a four-membered ring failed. Instead, the bicyclic (5,6)-membered acetal derivative and the tricyclic system with an eight-membered central ring were obtained from the substrates containing O and S nucleophilic moieties, respectively. The reaction outcome and stereochemistry are rationalized using quantum chemical calculations at B3LYP/6-31G(d) level. The exclusive cis-stereoselectivity in the formation of (5,6)- and (5,5)-membered systems results from thermodynamic control, whereas the formation of the eight-membered ring was kinetically controlled.},
  language  = {en}
}
@article{ShainyanKleinpeter2012,
  author    = {Shainyan, Bagrat A. and Kleinpeter, Erich},
  title     = {Conformational preferences of Si-Ph,H and Si-Ph,Me silacyclohexanes and 1,3-thiasilacyclohexanes : Additivity of conformational energies in 1,1-disubstituted heterocyclohexanes},
  issn      = {0040-4020},
  year      = {2012},
  abstract  = {The conformational equilibria of 1-phenyl-1-silacyclohexane 1, 3-phenyl-1,3-thiasilacyclohexane 2, 1-methyl-1- phenyl-1-silacyclohexane 3, and 3-methyl-3-phenyl-1,3-thiasilacyclohexane 4 have been studied for the first time by low temperature C-13 NMR spectroscopy at 103 K. Predominance of the equatorial conformer of compound 1 (Ph-eq/Ph-ax=78\%:22\%) is much less than in its carbon analog, phenylcyclohexane (nearly 100\% of Ph-eq). And in contrast to 1-methyl-1- phenylcyclohexane, the conformers with the equatorial Ph group are predominant for compounds 3 and 4: at 103 K, Ph-eq/Ph- ax ratios are 63\%:37\% (3) and 68\%:32\% (4). As the Si-C bonds are elongated with respect to C-C bonds, the barriers to ring inversion are only between 5.2-6.0 (ax -> eq) and 5.4-6.0 (eq -> ax) kcal mol(-1). Parallel calculations at the DFT and MP2 level of theory (as well as the G2 calculations for compound 1) show qualitative agreement with the experiment. The additivity/nonadditivity of conformational energies of substituents on cyclohexane and silacyclohexane derivatives is analyzed. The geminally disubstituted cyclohexanes containing a phenyl group show large deviations from additivity, whereas in 1-methyl-1-phenyl-1-silacyclohexane and 3-methyl-3-phenyl-1,3-thiasilacyclohexane the effects of the methyl and phenyl groups are almost additive. The reasons for the different conformational preferences in carbocyclic and heterocyclic compounds are analyzed using the homodesmotic reactions approach.},
  language  = {en}
}
@article{KleinpeterHeydenreichKochetal.2012,
  author    = {Kleinpeter, Erich and Heydenreich, Matthias and Koch, Andreas and Linker, Torsten},
  title     = {Synthesis and NMR spectroscopic conformational analysis of esters of 4-hydroxy-cyclohexanone-the more polar the molecule the more stable the axial conformer},
  issn      = {0040-4020},
  year      = {2012},
  abstract  = {The esters of 4-hydroxy-cyclohexanone and a series of carboxylic acids R-COOH with R of different electronic and steric influence (R=Me, Et, n-Pr, i-Pr, n-Bu, i-Bu, sec-Bu, t-Bu, CF3, CH2Cl, CHCl2, CCl3, CH2Br, CHBr2, and CBr3) were synthesized and the conformational equilibria studied by 1H and 13C NMR spectroscopy at 103 K and at 295 K, respectively. The geometry of optimized structures of the axial/equatorial chair conformers was computed at the ab initio MO and DFT levels of theory. Only one preferred conformation was obtained for the axial and the equatorial conformer as well. When comparing the conformational equilibria of the cyclohexanone esters with those of the corresponding cyclohexyl esters a certain polarity contribution of the cyclohexanone framework was revealed, which is independent of the substituent effects and increases the stability of the axial conformers by a constant amount.},
  language  = {en}
}
@article{KleinpeterNeuvonenNeuvonenetal.2012,
  author    = {Kleinpeter, Erich and Neuvonen, Kari and Neuvonen, Helmi and Koch, Andreas},
  title     = {Taft equation in the light of NBO computations : Introduction of a novel polar computational substituent constant scale for alkyl groups},
  issn      = {2210-271X},
  year      = {2012},
  language  = {en}
}
@article{KleinpeterLazarevaShainyanetal.2012,
  author    = {Kleinpeter, Erich and Lazareva, Nataliya F. and Shainyan, Bagrat A. and Schilde, Uwe and Chipania, Nina N.},
  title     = {Synthesis, Molecular Structure, Conformational Analysis, and Chemical Properties of Silicon-Containing Derivatives of Quinolizidine},
  issn      = {0022-3263},
  year      = {2012},
  language  = {en}
}
@article{KleinpeterKoch2012,
  author    = {Kleinpeter, Erich and Koch, Andreas},
  title     = {Spatial magnetic properties subject to lone pair and pi electron delocalization in benzenoid and quinoid structures : are quinoid tautomers really nonaromatic?},
  issn      = {1551-7004},
  year      = {2012},
  abstract  = {The spatial magnetic properties, through-space NMR shieldings (TSNMRS), of benzenoid and quinoid tautomeric structures such as benzodifurantrione and phenazine-type molecules have been calculated using the GIAO perturbation method employing the nucleus independent chemical shift (NICS) concept of Paul von Rague Schleyer and visualized as iso- chemical-shielding surfaces (ICSS) of various size and direction. The TSNMRS values were employed to quantify and visualize the partial aromaticity of the studied compounds. In the case of the surprisingly more stable quinoid tautomers, the aromaticity-synonymous with stability due to the conjugation of p electrons and lone pairs-was not found to be particularly reduced.},
  language  = {en}
}