Page 244 - Cascade_Biocatalysis_Integrating_Stereoselective_and_Environmentally_Friendly_Reactions

220 9 Stereoselective Hydrolase-Catalyzed Processes in Continuous-Flow Mode

were maintained over 15 days. KR of racemic alcohols (e.g., rac-23i)without theuse
of organic solvents was developed by the combination of KR in CSTR containing
ILs and selective extraction with scCO [123]. The method was based on the
2
different solubility of the unreacted alcohols and the product (e.g., (R)-24i or the
corresponding laurate).
An interesting two-step KR in the acetylation of trans-1,2-cyclohexanediol was real-
ized. First by continuous-flow lipase catalysis in scCO ,(R,R)-2-acetoxycyclohexane-
2
1-ol [122] was obtained with moderate enantiomer selectivity, while the second
acylation step proved to be highly enantiomer selective and gave pure (R,R)-24k.
Continuous-flow KR of further cycloalkanols such as trans-2-bromocyclohexan-1-
ol rac-23l and 2-methylene-substituted cycloalkanols rac-23o,p catalyzed by different
(commercial and in-house-made) lipases were performed [125]. Whereas the
selectivities (E) were similar in the continuous-flow and batch modes, the pro-
ductivities (r), were significantly higher in the continuous-flow mode.
Continuous-flow KR in PBRs proved to be synthetically efficient in the CaLB-
catalyzed acylation of 1,3,6-tri-O-benzyl-myo-inositol rac-23q resulting in 50%
conversion to l-(-)-24q with ee >99% [127]. Activity of Novozym 435-containing-
PBRs was retained over a long period of time. The similar continuous-flow KR of
6,7-dihydro-5H-pyrrolo[1,2-a]imidazol-7-ol rac-23r proved to be quite efficient as
well, providing the ester (R)-24r in good yields and excellent enantiomeric excess
[128].
The 50% maximal theoretical yield of KR processes may be overruled by DKR to
achieve 100% of the desired product if a fast racemization process for the substrate
can be coupled to the KR in situ. A mixture of CaLB immobilized onto IL-coated
particles and an acidic zeolite are allowed to carry out DKR in a ‘‘one-pot’’ under
◦
scCO flow conditions (50 C, 10 MPa) [133]. In this way, acylation of rac-23i with
2
vinyl propionate in a DKR gave the propionate of (R)-23i in 92% yield and >99.9% ee.
9.2.2.4 Effects of the Operation Conditions and the Mode of Enzyme Immobilization
Pressure and temperature [118] or the mode of lipase immobilization [113, 114]
may significantly influence lipase-catalyzed KR processes (Figure 9.10).
The study on the continuous-flow and batch mode KRs of 1-phenylpropan-2-ol,
1-cyclohexylethanol, and 1-phenylethanol (rac-23g,h,i respectively) indicated simi-
lar enantiomer selectivities (E) but higher productivities (r) in the corresponding
continuous-flow reaction in PBRs filled with different commercial lipase prepara-
◦
tions [118]. The effect of temperature (0–60 C) and pressure (1–120 bar) on the
continuous-flow acetylation of rac-23h in a CaLB-filled reactor was investigated

−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−→
Figure 9.10 The effect of (a) pressure and (a): (R)-12h;(b):(R)-8d; forms of CaLB:
temperature on the CaLB N435-catalyzed KR of N435, Novozyme 435, adsorbed on acrylate
an alcohol rac-11h and (b) the mode of immobi- beads; T2-150, covalently bound to polystyrol-
lization of CaLB and temperature on the CaLB- divinylbenzene beads; G250P, adsorbed on
catalyzed KR of an amine rac-7d in continuous- phenyl-grafted silica gel; SG10A, entrapped into
flow PBRs and the structures of the products; a hydrophobic sol–gel matrix.