Aliphatic ketone reduction yeast

Simple aliphatic ketones are reduced by baker s yeast predominantly to the (S)-configurated alcohols, often with high enantioselectivity. As in other cases, a large difference in the size of the two residues in the substrate is advantageous. Compounds with two larger residues (e.g., propyl butyl ketone) are not accepted, but if one residue is methyl then compounds with many different groups as the second are accepted for reduction. For example, 6-heptyn-2-one is reduced to (S)-6-heptyn-2-ol in >99% ee235. 

In addition to baker s yeast, several systems that selectively reduce aliphatic ketones are now known. Lactic acid bacteria, e.g., Lactobacillus fermentum, Lactobacillus brevis or Leuconostoc paramesenteroides, reduce 2-pentanone or acetophenone in high yield (50- 100%) and high enantioselectivity (94-100% ee) to the (S )-configurated alcohols247. (5)-Alcohols are also obtained with high enantiomeric excess from 2-pentanone, 2-heptanone, 2-octanone and the substituted ketones 3-methyl-2-butanone and 4-methylpentane-2,3-dione by reduction with resting cells of the thermophilic archaebacterium Sulfolobus so/fataricus24s. 

Capillary gas chromatographic determination of optical purities, investigation of the conversion of potential precursors, and characterization of enzymes catalyzing these reactions were applied to study the biogenesis of chiral volatiles in plants and microorganisms. Major pineapple constituents are present as mixtures of enantiomers. Reductions, chain elongation, and hydration were shown to be involved in the biosynthesis of hydroxy acid esters and lactones. Reduction of methyl ketones and subsequent enantioselective metabolization by Penicillium citrinum were studied as model reactions to rationalize ratios of enantiomers of secondary alcohols in natural systems. The formation of optically pure enantiomers of aliphatic secondary alcohols and hydroxy acid esters using oxidoreductases from baker s yeast was demonstrated. 

Aliphatic a-hydroxy ketones are easily reduced by baker s yeast to yield the (7 )-configurated 1,2-diols. The reduction of l-hydroxy-2-propanone to (/ )- ,2-propanediol is still of preparative interest227,232. The corresponding a-hydroxy ketones with higher alkyl chain length are also reduced in high enantioselectivity with moderate to high yield. 

Scheme 2.122 Reduction of aliphatic and aromatic ketones using baker s yeast...

In the laboratoiy, baker s yeast is a commonly used microorganism for the enantioselective reduction of ketones. It is readily available and can be handled using standard, non-sterile laboratory equipment. Furthermore, it displays large substrate scope that includes /l-ketoesters and aliphatic, aromatic, cyclic, and acyclic ketones [150, 151, 154, 155). Its applicability and usefulness were recognized very early this is underscored by the fact that the first comprehensive review covering yeast reductions dates back to 1949 [156] A few examples of reductions that provide alcohols with high selectivity are shown in Scheme 2.31 methyl isopropyl ketone (246) [157], hy-... 

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