Selective redox reaction chiral synthesis

Many stereo- and regioselective redox reactions have been carried out on a preparative scale by the catalysis of enzymes or microorganisms. The majority of these reactions involves the reduction of keto groups to chiral secondary alcohols or derivatives thereof Not only the reductions of CX-double bonds, but also selective dehydrogenations are of interest. A well-known example of a regioselective biocatalytic dehydrogenation is the formation of L-sorbose from D-sorbitol which is actually the key step in the synthesis of vitamin C. 

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. 

The synthesis is initiated by the organocatalyzed cascade that activates a,p-unsaturated aldehyde 8 with the formation of an iminium ion (Scheme 14.2). In this way, the imidazolidinone catalyst allows hydride transfer from the Hantzsch dihydropyridine 9 onto the highly activated conjugated alkene 11, which creates the nucleophilic enamine intermediate 12. Because of the chirality of the organocatalyst, stereoselective Michael addition (97% ee) to the adjacent enone occurs, with minor preference for the cis diastereomer (2 1 dr). Fortunately, this undesired diastereomer slowly epimerizes to the required trans isomer, which produces (-l-)-ricciocarpin A when treated with samarium triisopropoxide. Besides the Cannizzaro-like redox disproportionation, which allows the lactone producing Evans-Tihchenko reaction to occur, samarium(III) also enhances the epimerization to the trans isomer and therefore produces the desired isomer in high selectivity. 

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