| Abstract
| - Whole cells of an Escherichia coli strain that overexpresses Acinetobacter sp. NCIB 9871cyclohexanone monooxygenase have been used for the Baeyer−Villiger oxidations of a variety of4-mono- and 4,4-disubstituted cyclohexanones. In cases where comparisons were possible, this newbiocatalytic reagent provided lactones with chemical yields and optical purities that were comparableto those obtained from the purified enzyme or a strain of bakers' yeast that expresses the sameenzyme. The efficient production of cyclohexanone monooxygenase in the E. coli expression system(ca. 30% of total soluble protein) allowed these oxidations to reach completion in approximatelyhalf the time required for the engineered bakers' yeast strain. Surprisingly, 4,4-disubstitutedcyclohexanones were also accepted by the enzyme, and the enantioselectivities of these oxidationscould be rationalized by considering the conformational energies of bound substrates along withthe enzyme's intrinsic enantioselectivity. The enzyme expressed in E. coli cells also oxidized several4-substituted cyclohexanones bearing polar substituents, often with high enantioselectivities. Inthe case of 4-iodocyclohexanone, the lactone was obtained in >98% ee and its absolute configurationwas assigned by X-ray crystallography. The crystal belongs to the monoclinic P21 space group witha = 5.7400(10), b = 6.1650(10), c = 11.377(2) Å, b = 99.98(2)°, and Z = 2. Taken together, theseresults demonstrate the utility of an engineered bacterial strain in delivering useful chiral buildingblocks in an experimentally simple manner.
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