Conjugated, with baker yeast

Huyng-Ba, T., Matthey-Doret, W, Fay, L. B., Bel-Rhlid, R. (2003). Generation of thiols by biotransformation of cystein-aldehyde conjugates with baker yeast. J. Agric. Food Chem., 51, 3629-3635. 

A conjugated 3-oxo group is not easily reduced by yeast however. Clostridium paraputrificum selectively reduces these steroids to the 3a-hydroxy compounds. Reduction of the 17-oxo group to 17/ -hydroxysteroids can be achieved with baker s yeast, while reduction of the 20-oxo group can be performed in most cases with S treptomyces strains. 

Michael-type reactions have been achieved with baker s yeast using trifluoroethanol as the co-substrate (Scheme 5.17). These transformations probably are related to the acyloin condensation in that it has been proposed that the trifluoroethanol is initially oxidized enzymically to the aldehyde, which is then converted to a reactive acyl anion that can add to the electrophilic enone in a conjugate manner. The a, -unsaturated ketones are good Michael acceptors, leading to the corresponding y-hydroxyketones (via the diketone intermediate) in moderate yields (26-40%) and good optical purities (e.e. = 79%). The absolute configurations of the products (16) were not determined. 

Baker s yeast reduces conjugated nitro compounds to nitroalkanes and also the C=C unit of conjugated ketones. Other enzymatic reductions are possible. A reductase from Nicotiana tabacum reduced a conjugated ketone to the saturated ketone, with excellent enantioselectivity. Enzyme YNAR-I and NADP-H reduces conjugated nitro compounds to nitroalkanes. ... 

---