Page 232 - Cascade_Biocatalysis_Integrating_Stereoselective_and_Environmentally_Friendly_Reactions

208 9 Stereoselective Hydrolase-Catalyzed Processes in Continuous-Flow Mode

A porous vessel filled with the biocatalyst and immersed in recirculating toluene
was applied for the preparation of methyl trans-(2R,3S)-(4-methoxyphenyl)glycidate
(an intermediate of Diltiazem) from its racemic methyl ester [88]. In this system,
hydrolysis of the ester with Lecitase ® Ultra (a commercially available phospholi-
pase) immobilized in macroporous gelatin organo-gel (gelozyme) gave the desired
product in 47% yield and >99% ee.
A recirculated PBR filled with lactonase from Fusarium proliferatum adsorbed
onto cotton cloth and cross-linked with glutaraldehyde was applied for enantiomer
selective hydrolysis of racemic 2-hydroxy-γ-butyrolactone (HBL) [89]. The system
was operated for 60 cycles with an average productivity of 2.48 g l −1 h −1 and
90.0–96.4% ee.
Hydrolytic reactions in real flow-through systems are summarized in Figure 9.3
and Table 9.4.
Hydrolysis in a PBR filled with CrL adsorbed on silica gel was evaluated for the
influence of flow rate, height to diameter ratio (H/D) and substrate concentration to
yield (S)-ketoprofen [(S)-13] from its racemic 2-chloroethyl ester (Figure 9.4, entry 1)
[90]. By using a semicontinuously operated PBR, enantiopure (S)-13 (>99% ee) was
−1
produced with a conversion of 30% and productivity of 1.5 mg g −1 biocatalyst h .
The hollow fiber MBR (Figure 9.4) proved to be a versatile tool to perform
continuous-flow KRs by lipase-catalyzed hydrolysis of racemic esters. The system
consists of an organic phase containing the racemic ester, a lipase-loaded
membrane, and an aqueous phase that extracts the product. A full-scale plant
was built for the chiral separation of the above mentioned Diltiazem intermediate
2
containing 1440 m of hollow fiber in a MBR and producing 75 tons/year of
resolved product [96].
(S)-Ibuprofen [(S)-14] was produced from its racemic 2-ethoxyethyl ester by using
lipase-catalyzed hydrolysis in hollow fiber MBR (Table 9.4, entries 2 and 3) [91,
92]. The operating temperatures, organic phase and aqueous phase flow rates, and
enzyme loadings were varied leading to a KR with enantiomeric ratio (E)of13and
85–90% ee of (S)-14 [91, 92]. On the other hand, hydrolysis of racemic ibuprofen
heptyl ester under the same conditions gave only an E value of 1–4 [92].

MeO
O O O
OH OH OH
O
(S)-Ketoprofen: (S)-13 (S)-Ibuprofen: (S)-14 (S)-Naproxen: (S)-15
Alcohol in parent ester: Alcohol in parent ester: Alcohol in parent ester:
HOCH CH CI [90], HOCH CH OEt [91, 92], HOMe [93, 94]
2
2
2
2
H
HOCH 2 CH OEt [87], HOC 7 15 [92]
2
HOEt [95]
Figure 9.3 Products of kinetic resolutions of the esters indicated by enzyme-catalyzed
hydrolysis in continuous-flow mode [87, 90–95].