American Chemical Society
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Recombinant Baker's Yeast as a Whole-Cell Catalyst for Asymmetric Baeyer−Villiger Oxidations

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journal contribution
posted on 1998-03-31, 00:00 authored by Jon D. Stewart, Kieth W. Reed, Carlos A. Martinez, Jun Zhu, Gang Chen, Margaret M. Kayser
Cyclohexanone monooxygenase (E.C. from Acinetobacter sp. NCIB 9871 has been expressed in baker's yeast (Saccharomyces cerevisiae) to create a general reagent for asymmetric Baeyer−Villiger oxidations. This “designer yeast” approach combines the advantages of using purified enzymes (single catalytic species, no overmetabolism, etc.) with the benefits of whole-cell reactions (experimentally simple, no cofactor regeneration necessary, etc.). The yeast reagent was used to systematically examine a series of 2-, 3-, and 4-substituted cyclohexanones (R = Me, Et, n-Pr, i-Pr, allyl, n-Bu), almost all of which were oxidized to the corresponding ε-caprolactones in good yields and high enantioselectivities (typically ≥ 95%). Mesomeric 4-substituted cyclohexanones were oxidized to ε-caprolactones in ≥ 92% ee. The engineered yeast strain also effected kinetic resolutions of 2-substituted cyclohexanones with enantioselectivity values ≥ 200 for substituents larger than methyl. The behavior of 3-substituted cyclohexanones depended upon the size of the substituent. The engineered yeast strain cleanly converted the antipodes of 3-methyl- and 3-ethylcyclohexanone to divergent regioisomers. On the other hand, for cyclohexanones with larger substituents (n-Pr, allyl, n-Bu), both antipodes were oxidized by the enzyme to a single regioisomer. In these cases, the observed enantioselectivities were due to a combination of a modest preference for one enantiomer by the enzyme and an unfavorable conformational preequilibrium required prior to binding of the less-favored antipode, a phenomenon we refer to as substrate-assisted enantioselectivity.