retropinacolcross-pinacol-coupling-reactions-catalytic-access-to-12

Retropinacol/Cross-pinacol Coupling Reactions - A Catalytic Access to 1,2-Unsymmetrical Diols

Unsymmetrical 1,2-diols are hardly accessible by reductive pinacol coupling processes. A successful execution of such a transformation is bound to a clear ...

Humboldt University of Berlin

A histidine-catalyzed asymmetric direct cross-aldol reaction of enolizable aldehydes is described. In contrast to proline, histidine is able to clearly differentiate the reactivity of various aldehydes. In addition, this approach provides access to syn-configured beta-hydroxyaldehydes. Thus, by application of this new methodology, defined-configuration quaternary stereocenters can be constructed with ease. The utility of this method is demonstrated in several total syntheses of branched-chain carbohydrates.

Asymmetric histidine-catalyzed cross-aldol reactions of enolizable aldehydes: access to defined configured quaternary stereogenic centers.

A highly, stereo- and regioselective Meerwein-Ponndorf-Verley-Aldol etherification process of enolizable aldehydes is described. This new transformation is catalyzed by trifluoroacetic acid. The method also allows cross-aldol reactions with alpha-branched enolizable aldehydes and thus provides access to defined configured quaternary stereogenic centers.

Asymmetric acid-catalyzed Meerwein-Ponndorf-Verley-Aldol reactions of enolizable aldehydes.

Extensive studies of asymmetric cross-aldol addition between enolizable aldehydes are described and provide a deeper insight into histidine-catalyzed aldol additions. In particular, aspects of enantio- as well as diastereoselectivity of these reactions are discussed. Rules and predictions of configurative outcome are explained by using different transition-state models. These discussions are confirmed by extensive computations.

Histidine-catalyzed asymmetric aldol addition of enolizable aldehydes: insights into its mechanism.

Quantum mechanical calculations reveal the origin of diastereo- and enantioselectivities of aldol reactions between aldehydes catalyzed by histidine, and differences between related reactions catalyzed by proline. A stereochemical model that explains both the sense and the high levels of the experimentally observed stereoselectivity is proposed. The computations suggest that both the imidazolium and the carboxylic acid functionalities of histidine are viable hydrogen-bond donors that can stabilize the cyclic aldolization transition state. The stereoselectivity is proposed to arise from minimization of gauche interactions around the forming C-C bond.

Stereoselectivities of histidine-catalyzed asymmetric aldol additions and contrasts with proline catalysis: a quantum mechanical analysis.

Aldol additions of unprotected carbohydrates to 1.3-dicarbonyl compounds have been described. This transformation is based on a dual activation by tertiary amines and 2-hydroxypyridine.

Stereoselective amine-catalyzed carbohydrate chain elongation.

Beyond a doubt organocatalysis belongs to the most exciting and innovative chapters of organic chemistry today. Organocatalysis has emerged not only as a complement to metal-catalyzed reactions and to biocatalysis over the last decade, but also provides new asymmetric organocatalyzed reactions that cannot be accomplished by metal- or biocatalyzed reactions so far. A large number of organocatalytic processes are already well established in organic synthesis. Nevertheless, the number of publications in this field is still on the increase; new important results are produced constantly. This review gives a detailed overview of the latest developments and main streams in organocatalyzed asymmetric CC bond formation processes of the last three years. It is intended to outline the most important current findings focused on especially new synthetic methodologies.

Ulf Scheffler

Department of Chemistry

Humboldt University of Berlin

Retropinacol/Cross-pinacol Coupling Reactions - A Catalytic Access to 1,2-Unsymmetrical Diols