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212 9 Stereoselective Hydrolase-Catalyzed Processes in Continuous-Flow Mode

R
HO OAcyl
R R
Acyl-X (R)-17a,b
HO OH Immobilized lipase + AcylO OAcyl
Solvent
16a (R = OH) Acyl-X = (PhCO) O R 18a,b
2
AcylO OH
16b (R = CH 2 Ph) Acyl-X = AcOC(CH 3 ) = CH 2
(S)-17a,b
Figure 9.6 Enantioselective acylation of prochiral diols 16a,b in continuous-flow lipase-filled
PBR.
Some of the stereoselective enzymic acylations were performed not in true flow-
through systems but in systems comprising continuously operated recirculated
reactor elements.
For example, lipase-catalyzed KR of 1-phenylethanol was performed in a recir-
culated system with lipase-filled PBR [103]. The problem that different parameters
are required for acylation and racemization in the enzyme-catalyzed DKR transfor-
mation of 1-phenylethanol was solved in a way by operating in two separate vessels
[104]. The biocatalyst performing acylation was retained in a lower temperature
vessel with a microfiltration membrane while the racemization was performed in
a higher temperature vessel.
(S)-Flurbiprofen and (R)-flurbiprofen butyl ester were prepared with ≥90% ee by
a sequential catch/release KR protocol by a serial system of a lipase-filled PBR and
an adsorbent-filled catch/release column [105].
Although with lipase-catalyzed acylations all types of selectivities including
enantiotopic selectivity can be realized [29, 30, 60, 79, 81, 83], there are only a
few examples of hydrolase-catalyzed enantioselective biotransformations in real
flow-through mode (Figure 9.6).
Optically active (R)-monobenzoyl glycerol (R)-16a was synthesized by Candida
antarctica lipase B (CaLB)-catalyzed enantiotopic selective esterification of glycerol
with benzoic anhydride in organic solvents [106]. Although the enantiotopic
selectivity of the first acylation was low, CaLB preferred the conversion of the
minor enantiomer (S)-17a into 1,3-dibenzoyl glycerol 18a resulting in significant
improvement in the enantiopurity of unreacted (R)-17a (up to 70% ee). (R)-17a
was produced in the PBR over a period of 1 week, with an average productivity of
0.79 g (R)-17a day −1 g −1 biocatalyst.
Continuous-flow mode asymmetric acetylation of the prochiral diol 16b with
isopropenyl acetate was performed in a small stainless steel PBR filled with
sol–gel/Celite entrapped Pseudomonas fluorescens lipase (PfL) (Lipase AK) [107].
Optimization of the process resulted in (R)-17b with up to 91% ee.
The most typical type of hydrolase-catalyzed stereoselective reactions under
continuous-flow conditions is KR. Continuous-flow KRs were performed on
racemic acids by direct esterification with alcohols [100–102, 108, 109] or on
racemic amines by acylation with esters (Figure 9.7 and Table 9.5) [110–114].
However, the most frequent so far is the continuous-flow KR of racemic primary