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194 8 Racemizable Acyl Donors for Enzymatic Dynamic Kinetic Resolution

class of compounds, also considering that usually biocatalysts achieve their best
performance under near-physiological conditions.
In the case of carboxylic esters, which are widely employed as acyl donors,
in principle, it is possible by reaction engineering to find a suitable window of
conditions that permit their employment in DKR, and a number of successful
cases is indeed documented in the literature; in particular, α-amino acid esters are
particularly fruitful in this context because they are efficiently racemized under
mild conditions in the presence of a suitable aldehyde.
A particularly promising opportunity, in the context of substrate engineering, is
the use of thioester substrates as substitutes for their oxygenated analogs in DKR
reactions. First of all, thioesters are far more prone to racemization in mild con-
ditions. Secondly, their structure is not very different from the oxoesters, allowing
them to be accepted by many enzymes as acyl donors. Last but not least, they
possess a good kinetic stability in the presence of water despite being thermody-
namically activated. These characteristics render thioesters highly attractive from
the synthetic point of view, especially when aiming at an efficient production of
enantiopure acids, esters, or amides.

Acknowledgments

Support from COST Action CM0701-CASCAT (Cascade Chemo-Enzymatic Pro-
cesses: New Synergies between Chemistry and Biochemistry) is gratefully acknowl-
edged.

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