Organic synthesis, enantioselective strategy

Recent developments have impressively enlarged the scope of Pauson-Khand reactions. Besides the elaboration of strategies for the enantioselective synthesis of cyclopentenones, it is often possible to perform PKR efficiently with a catalytic amount of a late transition metal complex. In general, different transition metal sources, e.g., Co, Rh, Ir, and Ti, can be applied in these reactions. Actual achievements demonstrate the possibility of replacing external carbon monoxide by transfer carbonylations. This procedure will surely encourage synthetic chemists to use the potential of the PKR more often in organic synthesis. However, apart from academic research, industrial applications of this methodology are still awaited. 

Finally, another possibility is to design enantioselective syntheses by using external chiral auxiliaries either in catalytic or in stoichiometric quantities [21], Since these strategies are nowadays of great interest in organic synthesis, we will consider here some of the most recent results achieved in enantioselective aldol condensations, as well as in the asymmetric epoxidation and hydroxylation of olefmic double bonds. 

Electroenzymatic reactions are not only important in the development of ampero-metric biosensors. They can also be very valuable for organic synthesis. The enantio- and diasteroselectivity of the redox enzymes can be used effectively for the synthesis of enantiomerically pure compounds, as, for example, in the enantioselective reduction of prochiral carbonyl compounds, or in the enantio-selective, distereoselective, or enantiomer differentiating oxidation of chiral, achiral, or mes< -polyols. The introduction of hydroxy groups into aliphatic and aromatic compounds can be just as interesting. In addition, the regioselectivity of the oxidation of a certain hydroxy function in a polyol by an enzymatic oxidation can be extremely valuable, thus avoiding a sometimes complicated protection-deprotection strategy. 

Selected recent developments in the area of asymmetric organocatalysis in our laboratory have been briefly summarized. Enamine catalysis, Brpnsted acid catalysis, and iminium catalysis turn out to be powerful new strategies for organic synthesis. Using Hantzsch ester as the hydride source, highly enantioselective transfer hydrogenantion reactions have been developed. We have also developed an additional new con-... 

Taber DF, Neubert TD, Schlecht MF (2004) The enantioselective synthesis of morphine. In Harmata M (ed) Strategies and tactics in organic synthesis, vol 5. Elsevier, London, p 353... 

The catalytic, enantioselective, vinylogous Mannich reaction has recently emerged as a very powerful tool in organic synthesis for the assembly of highly functionalized and optically enriched 6 amino carbonyl compounds. Two distinctly different strategies have been developed. The first approach calls for the reaction of preformed silyl dienolates as latent metal dienolates that react in a chiral Lewis acid or Bronsted acid catalyzed Mukaiyama type reaction with imines. Alternatively, unmodified CH acidic substrates such as a,a dicyanoalkenes or 7 butenolides were used in vinylo gous Mannich reactions that upon deprotonation with a basic residue in the catalytic system generate chiral dienolates in situ. 

The recent explosion in the development of asymmetric strategies for organic synthesis has fostered investigations into the discovery of methods for enantioselective and diastereoselective Diels-Alder reactions. Some early forays into this field focused on the use of chiral auxiliaries covalently attached to one of the reaction partners however, nearly all recent investigations have centered on developing chiral catalysts. The multitude of new catalysts spans the range of Lewis acids and Bronsted acids and bases as well as metal-based and organic molecules. 

The development of new strategies in organic synthesis with a minimum of chemical steps has become more and more necessary for the efficient creation of complex molecular structures. The ability of palladium(O) catalysts to exercise control in bond forming has made it an excellent candidate for the synthesis of biologically active molecules. Allylic alkylations catalyzed by palladium have widely been studied and have proved unusually productive because of the extraordinary chemo-, regio-, and diastereoselectivity and the continuing possibility for the development of enantioselectivity. 

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