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The microbial reduction of carbonyl compounds is a classical and well-documented method to prepare chiral alcohols. Diversely functionalized monoalcohols as well as alpha-and beta-diols can be produced with high enantiomeric purity by different microorganisms: yeasts, bacteria, and fungi (1). However, this kind of laboratory experiment is rarely proposed to students, essentially for practical reasons.
The main difficulty is the time required; the whole process usually takes several days. The microbial reduction time itself is generally 24 or 48 h. The reaction is carried out in dilute solution, and due to the hydrophilic nature of the product (often an alcohol or a hydroxyester of low molecular weight), an overnight continuous extraction procedure is needed. Accumulation of various metabolites produced by the microorganism, especially in fermenting conditions, makes the purification more difficult and more time-consuming. The preparation of the biocatalyst requires some knowledge of microbiology and some specific material. This constitutes another possible hindrance to the realization of microbial reduction by students, but in a pure chemical environment, readily available bakers' yeast can be used. Generally, the use of fresh, pressed bakers' yeast can be disappointing because optical purity can depend upon the nature, age, and storage conditions of the yeast.
In the case of our studies (2) on the microbial reduction of carbonyl compounds, we found ideal conditions for the reduction of commercial 1-phenyl-1,2-propanedione 1, so the experiment can be completed in 4 h. 2,3-Pentanedione and 2,3-octanedione are reduced in a similar way, but we preferred using 1 because 2,3-pentanediols are more volatile and more difficult to extract than 1-phenyl-1,2-propanediols. Also 2,3-octanedione is not a commercial product.
The reaction was carried out on 500 mg of dione 1 with 40 g of freeze-dried bakers' yeast in water. After 1 h, the medium was filtered, and the filtrate was extracted with ether, giving 90% pure (1R,2S)-1-phenyl-1,2-propanediol 3. Purity can be detected by TLC or GC. No more than 10% of 1-phenyl-1,2-propanedione and 1-phenyl-2-hydroxy-1-propanone are present in the crude mixture. sup 1 H NMR spectrum of the crude product is practically identical to that of the purified diol. However, to measure the optical rotation, a purification is necessary. It is realized by column chromatography. A CH sub 2 Cl sub 2 fraction draws away the less polar impurities,...