Yeast-mediated stereoselective reduction

Bakers s yeast is the most widely employed biocatalyst for asymmetric ketone reductions [43]. A group at Roussel Uclaf described an industrial application of bakers yeast mediated ketone reduction for the synthesis of trimegestone [44], a progestomimetic compound for the treatment of postmenopausal diseases. The key step of the nine-step synthesis is a chemo-, regio- and stereoselective bakers yeast reduction of the triketone 5 (Scheme 4.9). This transformation could not be performed efficiently using nonenzymatic methods. 

Figure 1 Strategy for tailoring the stereoselectivity of whole-cell baker s yeast-mediated reductions. The level of a yeast reductase can be increased by placing the corresponding gene under control of a strong yeast promoter on an extrachromosomal plasmid. Alternatively, the catalytic activity of a reductase can be eliminated by replacing the corresponding gene on the chromosome with a defective copy prepared in vitro. (Reprinted with permission from Organic Letters 1999, 1. 1999 American Chemical Society.)...

Using rational metabolic engineering to tailor the stereoselectivity of yeast-mediated reductions is feasible only when all of the relevant genes are known and the relationships between enzyme and acceptable substrates are defined. We are far from this ideal. For a few reductase proteins, amino acid sequence data has... 

Asymmetric oxidoreductions performed in isopropyl ether allow syntheses of optically active alcohols with ee >95% on a 1-10 mmol scale. Nakamura et al. investigated the effect of organic solvents on the reduction of ot-keto esters mediated by bakers yeast [140]. The reduction of ethyl 2-oxoheptanoate was tested in various solvents. Best results were achieved with benzene so further experiments were performed with benzene. Conversion only takes place in the presence of small amounts of water. The reduction of six ot-keto esters was examined regarding the stereoselectivity of the corresponding ot-hydroxy esters. The reactions were performed in aqueous systems as well as in benzene. In aqueous systems, the formed hydroxy esters show (S)-stereoselectivity while the stereochemistry of the reaction shifts markedly towards the production of (/ )-ot-hydroxy esters in benzene. 

A familiar example of a stereoselective reaction would be the formation of a higher yield of f rans-2-butene than cis-2-butene in an E2 reaction, no matter whether the starting material is (R)- or (S)-2-bromobutane. The addition of bromine to trans-2-butene to produce mcso-2,3-dibromobutane or the addition of bromine to the cis isomer to produce an equimolar mixture of the two enantiomers of 2,3-dibromobutane is a stereospecific reaction. Note that a reaction that gives only one of a pair of eirantiomers is not necessarily stereospecific. A yeast-mediated reduction of 3-chloropropiophenone gives (S)-3-chloro-l henylpropan-l-ol, with no evidence for formation of the R enantiomer. Because the reactant cannot exist as stereoisomers, it is not possible for stereoisomerically different reactants to give stereoisomerically different products, and the reaction can only be considered stereoselective, not stereospecific. 

It seems that the enantioselectivity of the reduction of keto esters with the keto group being part of a five-membered ring proceeds better than that of open-chain keto esters substituted at carbon C-2 [225, 226]. The reduction of 50 (Figure 21.16) gave 80% of (1R,2S)-51 showing both an ee and a de of 100%. In another experiment, however, 51 was obtained with a de of only 60% [217, 227]. Several mold strains have been shown to perform the same reduction with high enanho- and dias-tereoselection, whereas from the baker s yeast-mediated reactions quite often mixtures of the stereoisomeric products were obtained [227]. More complex substrates have also been studied [228-232], but yields and /or stereoselectivity were often only moderate. 

III. MULTIPLE BAKER S YEAST REDUCTASES AND STEREOSELECTIVITY IN WHOLE CELL-MEDIATED REDUCTIONS... 

Perrone, M.G., Santandrea, E., Sdlimati, A., and Syldatk, C. (2007) Stereoselective prostereogenic 3-oxo ester reduction mediated by a novel yeast alcohol dehydrogenase derived from Kluyveromyces marxianus CBS 6556. Adv. Synth. Catal., 349 (7), 1111-1118. 

---