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428 19 Chemoenzymatic Multistep One-Pot Processes

approach is based on the isomerization of a substrate which is catalyzed by means
of a chemocatalyst (Scheme 19.1, concept (a)). This isomerization is coupled in situ
with an enzymatic transformation of one of the isomers. As isomers, mostly racemic
mixtures of enantiomers are used, which then leads to a so-called dynamic kinetic
resolution (DKR) in which permanent racemization of the favored enantiomer
is coupled with a selective transformation of one of the two enantiomers (in a
resolution process). However, the isomers need not necessarily be enantiomers.
Any types of isomers are conceivable as long as they can be transformed into
each other in a (reversible) manner under reaction conditions capable of being
combined with enzyme catalysis. The details will be discussed in the subsequent
section 19.3.

(a) Concept of combined isomerizations and subsequent derivatization of one isomer
based on chemo- and biocatalysis (exemplified for dynamic kinetic resolutions)
Resolution
via
Biocatalysis
(R)-Substrate (R)-Product
Racemization
via
Chemocatalysis

(S)-Substrate
(b) Concept of combined substrate synthesis with subsequent product formation
based on chemo- and biocatalysis
Chemocatalysis
Biocatalysis or ‘‘classic’’
chemical rection
(Chiral)
Intermediate (Sequence type I)
product
In situ-formed,
(Prochiral) not isolated
substrate Chemocatalysis
or ‘‘classic’’ Biocatalysis
chemical rection
(Chiral)
Intermediate (Sequence type II)
product
In situ-formed,
not isolated
Scheme 19.1 Concepts of one-pot processes with combined chemo- and biocatalytic
reactions.

A second option consists in combining a substrate synthesis (not formed by an
isomerization process) in an initial step, followed by a subsequent transformation
of the substrate (Scheme 19.1, concept (b)). Often, one of the two processes (if not
both) are asymmetric reactions. In such a concept of a chemoenzymatic one-pot