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56 3 Monooxygenase-Catalyzed Redox Cascade Biotransformations

OH O O O
Biocatalytic oxidation Biocatalytic reduction Biocatalytic oxidation
E1 E2 E3
O + O
ADH ERED BVMO

Cofactor reduction

Self-sustaining RedOx system NADPH NADP +
Provided by the host
E. coli cell factory

Scheme 3.16 Concept of a biocatalytic redox toolbox based on simple functional group
transformations.
regeneration. This approach was applied completely in an in vivo environment,
concomitantly providing access to diverse chemical entities. Scheme 3.17 displays
the model cascade applied by the authors to prove the feasibility of their concept.
Combination of three different enzymes, an ADH from L. kefir (LK-ADH) for
the oxidation of the allylic alcohol moiety, an enoate reductase from P. putida
sp. (XenB), and a BVMO from Acinetobacter sp., were assembled in an artificial
redox pathway. After expression of all enzymes in one E. coli host, the subsequent
biotransformation of cyclohexenol yielded in >99% the desired lactone after 8 h.
The versatility of this approach was demonstrated by applying it to seven dif-
ferent substrates (Scheme 3.17). The corresponding chiral products were obtained
in very good to excellent conversions (63–99%) and absolute stereoselectivities
(>99%), particularly when regiodivergent oxygenations were in principle possible.
Thereby, the major classes of reactions in asymmetric synthesis or racemate reso-
lution were investigated (desymmetrization, kinetic resolution, and regiodivergent
transformations).

3.1.7
Chemo-Enzymatic Cascade Reactions
Synthetic chemistry and biochemistry are somehow complementary because of
the reactions they can catalyze. The enormous efforts in organic and metal-organic
chemistry in the last century, which allowed the development of new types of
reactions and investigation of new catalytic entities, led to a variety of bioorthog-
onal transformations (e.g., metathesis, click chemistry, 2 + 3 cycloadditions).
Specifically, enzymatic and homogeneous catalysis offer complementary means
to address synthetic challenges in chemistry and biology. The implementation
of such cascade reactions that combine an enzyme and an organometallic or
metallic catalyst proved to be challenging because of the mutual inactivation
of both catalysts. Two distinct approaches will be outlined in the following, as