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ABSTRACT: N-Toluensulfonyl-l-prolin amide was tested as catalyst in the enantioselective Michael addition of carbonyl compounds to (E)-β-nitrostyrene in nine ionic liquids under different reaction conditions. The reaction rates and enantioselectivities were
strongly dependent on the ionic liquids. Change of enantioselectivity was observed too and it is attributed to both the cation
and the anion of ionic liquid. The best yields (up to 98%) and enantioselectivity (70% ee) of product were obtained in a basic ionic liquid [bmim]BF4 at room temperature.
Monatshefte fuer Chemie/Chemical Monthly 10/2007; 138(11):1181-1186. · 1.53 Impact Factor
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ABSTRACT: N-Toluenesulfonyl-L-proline amide was tested as catalyst in the enantioselective aldol addition of acetone to aromatic aldehydes in different ionic liquids. The aldolisation products were isolated in high yields (up to 98%) with enantioselectivities ranging from 50 to 90% Immobilisation of the catalyst in ionic liquids allows simple product isolation and repeated use. The chemical yields and enantioselectivities achieved were comparable to those obtained when using this catalyst in DMSO. Overall, the trends regarding enantioselectivity and yields are comparable to those using L-proline as catalyst in ionic liquids.
Letters in Organic Chemistry 05/2006; 3(6):437-441. · 0.82 Impact Factor
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ABSTRACT: In the presence of short solid-phase bound peptide catalysts, the Juliá-Colonna epoxidation of enones (such as chalcone) with hydrogen peroxide can be performed with high enantiomeric excess (> or = 95% ee). It was proposed earlier (A. Berkessel, N. Gasch, K. Glaubitz, C. Koch, Organic Letters, 2001, Vol. 3, pp. 3839-3842) that this remarkable catalysis is governed by the N-terminus of individual and helical peptide strands. This mechanistic proposal was scrutinized further. (i) Nonaggregation of the peptide catalysts: five solid-phase bound statistic mixtures (0/100; 30/70; 50/50; 70/30; 100/0) of D-Leu and L-Leu heptamers were generated and assayed as catalysts. A linear dependence of the epoxide ee on the enantiomeric composition of the catalysts resulted. (ii) Catalyst helicity [introduction of the helix-stabilizing C(alpha)-methyl-L-leucine, L-(alphaMe)Leu]: solid-phase bound Leu/(alphaMe)Leu-pentamers of composition TentaGel-NH-[(alphaMe)-L-Leu]n-(L-Leu)m-H (n = 0-4; m = 5-n) were prepared and assayed as catalysts. The introduction of up to two (alphaMe)-L-Leu residues (n = 1, 2) significantly enhanced the catalyst activity relative to the L-Leu homopentamer (n = 0). Higher (alphaMe)-L-Leu contents (n = 3, 4) led to a decrease in both catalyst activity and enantiopurity of the product epoxide. In summary, both the individual catalytic action of the peptide strands and the helical conformation as the catalytically competent state of the peptide catalysts were further supported.
Biopolymers 01/2006; 84(1):90-6. · 2.87 Impact Factor
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ABSTRACT: Proline catalysis of the asymmetric direct aldol reaction involves both the secondary amine function and the carboxyl group of the amino acid. N-Sulfonylcarboxamides are known to be of similar acidity as carboxylic acids, and three N-arylsulfonyl derivatives of L-proline amide were synthesized as functionalized and versatile derivatives of L-Pro. Their catalytic performance was evaluated in the direct aldol addition of acetone to 4-nitrobenzaldehyde. Significantly improved reactivities and enantioselectivities were achieved in various solvents at low catalyst loadings (5–10 mol %) and at room temperature, with ees ranging up to 98%, whereas L-proline itself afforded a maximum ee of 80% (in DMSO). Thus, N-arylsulfonyl derivatives of proline amide represent a novel class of highly enantioselective catalysts for direct aldol reactions. Furthermore, the N-arylsulfonyl substituent suggests possibilities for incorporation into larger catalyst assemblies (including immobilization) without affecting the catalytically active functional groups.
Advanced Synthesis & Catalysis 09/2004; 346(9‐10):1141 - 1146. · 6.05 Impact Factor
