Asymmetric Baeyer-Villiger oxidations of 4-mono- and 4,4-disubstituted cyclohexanones by whole cells of engineered Escherichia coli

Whole cells of an Escherichia coli strain that overexpresses Acinetobacter sp. NCIB 9871 cyclohexanone monooxygenase have been used for the Baeyer-Vil liger oxidations of a variety of 4-mono- and 4,4-disubstituted cyclohexanon es. In cases where comparisons were possible, this new biocatalytic reagent provided lactones with chemical yields and optical purities that were comp arable to those obtained from the purified enzyme or a strain of bakers' ye ast that expresses the same enzyme. The efficient production of cyclohexano ne monooxygenase in the E. coli expression system (ca. 30% of total soluble protein) allowed these oxidations to reach completion in approximately hal f 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 consideri ng the conformational energies of bound substrates along with the enzyme's intrinsic enantioselectivity. The enzyme expressed in E. coli cells also ox idized several 4-substituted cyclohexanones bearing polar substituents, oft en with high enantioselectivities. In the case of 4-iodocyclohexanone the l actone was obtained in > 98% ee and its absolute configuration was assigned by X-ray crystallography. The crystal belongs to the monoclinic P2(1) spac e group with a = 5.7400(10), b = 6.1650(10), c = 11.377(2) Angstrom, b = 99 .98(2)degrees, and Z = 2. Taken together, these results demonstrate the uti lity of an engineered bacterial strain in delivering useful chiral building blocks in an experimentally simple manner.