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Low Selectivity (low + selectivity)
Selected Abstracts2,5-Disubstituted Pyrrolidines as Chiral Auxiliaries in Radical Reactions: A Theoretical ApproachEUROPEAN JOURNAL OF ORGANIC CHEMISTRY, Issue 25 2010Miglena K. Georgieva Abstract The radical addition to amides derived from chiral 2,5-disubstituted pyrrolidines has been theoretically studied by the use of density functional methods and the results compared with known experimental data. The results agree quite well with those obtained experimentally and allow the full rationalization of the factors influencing the diastereoselectivity. Steric effects are the main factors determining the selectivity, but electronic interactions can also be very important when the attacking alkene is ,,,-conjugated, as in acrylic esters or acrylamides. Additions at the ,- and ,-positions of the amide chain in the auxiliary are subject to different rules, with the former usually yielding high diastereoselectivities both experimentally and theoretically, whereas the latter is theoretically predicted to occur with low selectivity. We fully rationalize these two opposite behaviours and suggest several ways to circumvent this limitation, thus strongly increasing the interest of this type of structure as chiral auxiliaries in radical reactions. [source] Divalent Samarium Triflate Mediated Stereoselective Pinacol Coupling of Planar Chiral Phosphanyl and Phosphoryl FerrocenecarbaldehydeEUROPEAN JOURNAL OF ORGANIC CHEMISTRY, Issue 5 2009Shin-ichi Fukuzawa Abstract The pinacol coupling reaction of (Rp)-2-diphenylphosphanyl ferrocenecarbaldehyde (1) was smoothly mediated by divalent samarium triflate to give (R,R)-diol 2a predominantly, whereas the use of samarium(II) iodide resulted in low selectivity as described in the previous literature. In contrast, the coupling reaction of (Rp)-2-diphenylphosphoryl ferrocenecarbaldehyde (3) with Sm(OTf)2 gave the (S,S)-diol as the major isomer, which was the opposite stereochemistry of that obtained in the reaction with 1. The rhodium complexes of diphosphanes 2a were good catalysts for the asymmetric hydrogenation of ,-acetamidocinnamic acid, and the product was obtained quantitatively with up to 92,%,ee. (© Wiley-VCH Verlag GmbH & Co. KGaA, 69451 Weinheim, Germany, 2009) [source] On the Enantioselectivity of Transition Metal-Catalyzed 1,3-Cycloadditions of 2-Diazocyclohexane-1,3-dionesHELVETICA CHIMICA ACTA, Issue 9 2003Paul Müller The formal 1,3-cycloaddition of 2-diazocyclohexane-1,3-diones 1a,1d to acyclic and cyclic enol ethers in the presence of RhII -catalysts to afford dihydrofurans has been investigated. Reaction with a cis/trans mixture of 1-ethoxyprop-1-ene (13a) yielded the dihydrofuran 14a with a cis/trans ratio of 85,:,15, while that with (Z)-1-ethoxy-3,3,3-trifluoroprop-1-ene (13b) gave the cis -product 14b exclusively. The stereochemical outcome of the reaction is consistent with a concerted rather than stepwise mechanism for cycloaddition. The asymmetric cycloaddition of 2-diazocyclohexane-1,3-dione (1a) or 2-diazodimedone (=2-diazo-5,5-dimethylcyclohexane-1,3-dione; 1b) to furan and dihydrofuran was investigated with a representative selection of chiral, nonracemic RhII catalysts, but no significant enantioselectivity was observed, and the reported enantioselective cycloadditions of these diazo compounds could not be reproduced. The absence of enantioselectivity in the cycloadditions of 2-diazocyclohexane-1,3-diones is tentatively explained in terms of the Hammond postulate. The transition state for the cycloaddition occurs early on the reaction coordinate owing to the high reactivity of the intermediate metallocarbene. An early transition state is associated with low selectivity. In contrast, the transition state for transfer of stabilized metallocarbenes occurs later, and the reactions exhibit higher selectivity. [source] New chiral phosphinite ligands with C2 -symmetric axis and their possible applications in Ru-catalyzed asymmetric transfer hydrogenationAPPLIED ORGANOMETALLIC CHEMISTRY, Issue 3 2010Murat Aydemir Abstract The new chiral ligands N,N,-bis-[(1R)-1-ethyl-2- O -(diphenylphosphinite)ethyl]ethanediamide, 1, and N,N,-bis-[(1S)-1-isobutyl-2- O -(diphenylphosphinite)ethyl]ethanediamide, 2, and the corresponding ruthenium complexes 3 and 4 were prepared and their structures were elucidated by a combination of multinuclear NMR spectroscopy, IR spectroscopy and elemental analysis. Following activation by NaOH, these chiral ruthenium complexes serve as catalyst precursors for the asymmetric transfer hydrogenation of acetophenone derivatives in iPrOH. The complexes 3 and 4 showed high catalytic activity but low selectivity in asymmetric transfer hydrogenation reactions. Copyright © 2009 John Wiley & Sons, Ltd. [source] Proline Analogue of Nitrosourea as a New Cytotoxic ProdrugARCHIV DER PHARMAZIE, Issue 11 2009Anna Stankiewicz-Kranc Abstract Carmustine is frequently used as anticancer drug. High toxicity and low selectivity reduces the application of this drug. Though, there is a necessity to find new compounds characterized by similar therapeutic effects but a higher selectivity and safety. As a result, the proline analogue of nitrosourea, N -[N,-(2-bromophenyl)- N,-nitrosocarbamoyl]proline (AC), has been synthesized. The aim of this study was to compare the influence of carmustine and the proline analogue of nitrosourea on the antioxidant abilities of fibroblasts and leukemia cells, MOLT4. It was shown that carmustine as well as AC cause an increase in hydrogen peroxide concentration in normal and neoplastic cells. Incubation with both compounds led to a diminution of the activity of antioxidant enzymes such as superoxide dismutase, glutathione peroxidase, and reductase. Changes in activity / level of antioxidant parameters were accompanied by augmentation of lipid and oxidative protein modifications. In conclusion, carmustine and AC cause changes in the antioxidative system of normal and MOLT4 cells and are a reason of oxidative stress formation. [source] Hydrogenolysis of 1,2-Propanediol for the Production of Biopropanols from GlycerolCHEMSUSCHEM CHEMISTRY AND SUSTAINABILITY, ENERGY & MATERIALS, Issue 6 2010Yasushi Amada Abstract Production of propanols from glycerol, which are known as biopropanols, requires catalysts for the hydrogenolysis of 1,2-propanediol, which has been easily derived from glycerol. It is found that the Rh/SiO2 catalysts modified with ReOxspecies exhibited high activity and selectivity in the hydrogenolysis of 1,2-propanediol to propanols with low selectivity to degradation products and high stability. The optimized RhReOx/SiO2 (Re/Rh=0.5) catalyst gave high yields of 1-propanol (66,%) and propanols (1-propanol +2-propanol) (85,%) in the hydrogenolysis of 1,2-propanediol. In addition, the catalyst was applicable to the one-pot conversion of glycerol to propanols. The structure of Rh metal particles with attached ReOxclusters is suggested from the catalyst characterization. It is proposed that 1,2-propanediol hydrogenolysis proceeds by the hydrogenolysis of the alkoxide species on Re with hydrogen species on the Rh metal surface. [source] | ||||||||||