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Excellent Enantiomeric Excess (excellent + enantiomeric_excess)
Selected AbstractsCutting Long Syntheses Short: Access to Non-Natural Tyrosine Derivatives Employing an Engineered Tyrosine Phenol LyaseADVANCED SYNTHESIS & CATALYSIS (PREVIOUSLY: JOURNAL FUER PRAKTISCHE CHEMIE), Issue 4 2010Birgit Seisser Abstract The chemical synthesis of 3-substituted tyrosine derivatives requires a minimum of four steps to access optically enriched material starting from commercial precursors. Attempting to short-cut the cumbersome chemical synthesis of 3-substituted tyrosine derivatives, a single step biocatalytic approach was identified employing the tyrosine phenol lyase from Citrobacter freundii. The enzyme catalyses the hydrolysis of tyrosine to phenol, pyruvate and ammonium as well as the reverse reaction, thus the formation of tyrosine from phenol, pyruvate and ammonium. Since the wild-type enzyme possessed a very narrow substrate spectrum, structure-guided, site-directed mutagenesis was required to change the substrate specificity of this CC bond forming enzyme. The best variant M379V transformed, for example, o -cresol, o -methoxyphenol and o -chlorophenol efficiently to the corresponding tyrosine derivatives without any detectable side-product. In contrast, all three phenol compounds were non-substrates for the wild-type enzyme. Employing the mutant, various L -tyrosine derivatives (3-Me, 3-OMe, 3-F, 3-Cl) were obtained with complete conversion and excellent enantiomeric excess (>97%) in just a single ,green' step starting from pyruvate and commercially available phenol derivatives. [source] Highly Efficient Threonine-Derived Organocatalysts for Direct Asymmetric Aldol Reactions in WaterADVANCED SYNTHESIS & CATALYSIS (PREVIOUSLY: JOURNAL FUER PRAKTISCHE CHEMIE), Issue 6 2007Xiaoyu Wu Abstract The introduction of siloxy groups at the hydroxy function of natural threonine resulted in efficient hydrophobic organocatalysts, which could efficiently catalyze the direct aldol reactions of both cyclic and acyclic ketones with aromatic aldehydes in water with excellent enantiomeric excess. [source] Enzymatic Synthesis of Enantiopure ,- and ,-Amino Acids by Phenylalanine Aminomutase-Catalysed Amination of Cinnamic Acid DerivativesCHEMBIOCHEM, Issue 2 2009Bian Wu Abstract The phenylalanine aminomutase (PAM) from Taxus chinensis catalyses the conversion of ,-phenylalanine to ,-phenylalanine, an important step in the biosynthesis of the N -benzoyl phenylisoserinoyl side-chain of the anticancer drug taxol. Mechanistic studies on PAM have suggested that (E)-cinnamic acid is an intermediate in the mutase reaction and that it can be released from the enzyme's active site. Here we describe a novel synthetic strategy that is based on the finding that ring-substituted (E)-cinnamic acids can serve as a substrate in PAM-catalysed ammonia addition reactions for the biocatalytic production of several important ,-amino acids. The enzyme has a broad substrate range and a high enantioselectivity with cinnamic acid derivatives; this allows the synthesis of several non-natural aromatic ,- and ,-amino acids in excellent enantiomeric excess (ee >99,%). The internal 5-methylene-3,5-dihydroimidazol-4-one (MIO) cofactor is essential for the PAM-catalysed amination reactions. The regioselectivity of amination reactions was influenced by the nature of the ring substituent. [source] Development of a New Spiro-BOX Ligand and Its Application in Highly Enantioselective Palladium-Catalyzed Cyclization of 2-Iodoanilines with AllenesADVANCED SYNTHESIS & CATALYSIS (PREVIOUSLY: JOURNAL FUER PRAKTISCHE CHEMIE), Issue 17 2009Wei Shu Abstract In this communication, we report the synthesis of a new chiral spiro-bisoxazoline ligand, i.e., ,-naphthylmethyl-substituted spiro-BOX [(Ra,S,S)- L7] and have successfully applied it to the palladium-catalyzed enantioselective cyclization reaction of simple allenes with o -aminoiodobenzenes, affording highly optically active 3-alkylideneindolines in good yields with excellent enantiomeric excesses. [source] Rational Design of Sterically and Electronically Easily Tunable Chiral Bisimidazolines and Their Applications in Dual Lewis Acid/Brønsted Base Catalysis for Highly Enantioselective Nitroaldol (Henry) ReactionsCHEMISTRY - A EUROPEAN JOURNAL, Issue 6 2007Kuoyan Ma Abstract A new addition to the rational design of sterically and electrically easily tunable chiral bis(imidazoline) ligands from chiral amino alcohols has been developed. Vast structural variation of chiral bis(imidazoline) ligands can be simply achieved by the choice of both the 1,2-amino alcohol and its N-1 R1 substituent. A small library of chiral bisimidazolines (1,a,h) has been constructed. The method has provided an easy and simplified route to a diverse set of air-stable and water-tolerant chiral bis(imidazoline) ligands on 10,g scales. The dual Lewis Acid/Brønsted base catalytic system generated from the (S)- 1,a/Cu(OTf)2 complex and Et3N was able to catalyze Henry reactions between aldehydes and nitromethane effectively at room temperature, and also to tolerate a wide scope of aldehydes with excellent enantiomeric excesses. Not only aromatic aldehydes but also aliphatic aldehydes afforded the nitroalcohol products, with enantiomeric excesses in the 93,98,% range. This dual catalytic system is among the most effective systems so far reported for the asymmetric parent Henry reactions. This work also represents the first members of the class of chiral bisimidazolines to have been demonstrated to achieve excellent enantioselectivities. [source] | ||||||||||