Asymmetric Baeyer−Villiger Oxidations of 4-Mono- and 4,4-Disubstituted Cyclohexanones by Whole Cells of EngineeredEscherichiacoli

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Abstract
Whole cells of an Escherichia coli strain that overexpresses Acinetobacter sp. NCIB 9871 cyclohexanone monooxygenase have been used for the Baeyer−Villiger oxidations of a variety of 4-mono- and 4,4-disubstituted cyclohexanones. In cases where comparisons were possible, this new biocatalytic reagent provided lactones with chemical yields and optical purities that were comparable to those obtained from the purified enzyme or a strain of bakers' yeast that expresses the same enzyme. 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 approximately half the time required for the engineered bakers' yeast strain. Surprisingly, 4,4-disubstituted cyclohexanones were also accepted by the enzyme, and the enantioselectivities of these oxidations could be rationalized by considering the conformational energies of bound substrates along with the enzyme's intrinsic enantioselectivity. The enzyme expressed in E. coli cells also oxidized several 4-substituted cyclohexanones bearing polar substituents, often with high enantioselectivities. In the case of 4-iodocyclohexanone, the lactone was obtained in >98% ee and its absolute configuration was assigned by X-ray crystallography. The crystal belongs to the monoclinic P21 space group with a = 5.7400(10), b = 6.1650(10), c = 11.377(2) Å, b = 99.98(2)°, and Z = 2. Taken together, these results demonstrate the utility of an engineered bacterial strain in delivering useful chiral building blocks in an experimentally simple manner.

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