posted on 1998-03-31, 00:00authored byJon D. Stewart, Kieth W. Reed, Carlos A. Martinez, Jun Zhu, Gang Chen, Margaret M. Kayser
Cyclohexanone monooxygenase (E.C. 1.14.13.22) 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.