Poly-ε-caprolactone (PCL) is chemically produced on an industrial scale in spite of the need for hazardous peracetic acid as an oxidation reagent. Although Baeyer-Villiger monooxygenases (BVMO) in principle enable the enzymatic synthesis of ε-caprolactone (ε-CL) directly from cyclohexanone with molecular oxygen, current systems suffer from low productivity and are subject to substrate and product inhibition. The major limitations for such a biocatalytic route to produce this bulk chemical were overcome by combining an alcohol dehydrogenase with a BVMO to enable the efficient oxidation of cyclohexanol to ε-CL. Key to success was a subsequent direct ring-opening oligomerization of in situ formed ε-CL in the aqueous phase by using lipase A from Candida antarctica, thus efficiently solving the product inhibition problem and leading to the formation of oligo-ε-CL at more than 20 g L(-1) when starting from 200 mM cyclohexanol. This oligomer is easily chemically polymerized to PCL.
A three‐step enzymatic reaction sequence for the synthesis of poly‐ϵ‐caprolactone (PCL) was designed running in a fed‐batch operation. The first part of the cascade consisted of two oxidation steps starting with alcohol dehydrogenase catalyzed oxidation from cyclohexanol to cyclohexanone and further oxidation to ϵ‐caprolactone (ECL) by means of a Baeyer–Villiger monooxygenase. As a third step, lipase‐catalyzed hydrolysis of the lactone to 6‐hydroxyhexanoic acid (6‐HHA) was designed. With this biocatalytic multistep process reported herein, severe substrate surplus and product inhibition could be circumvented by the fed‐batch operation by adding the cyclohexanol substrate and by in situ product removal of ECL by hydrolysis, respectively. Up to 283 mm product concentration of 6‐HHA was reached in the fed‐batch operated process without loss in productivity within 20 h. After extraction and subsequent polymerization catalyzed by Candida antarctica lipase B, analysis of the unfractionated polymer revealed a bimodal distribution of the polymer population, which reached a mass average molar mass (Mw) value of approximately 63 000 g mol−1 and a dispersity (Mw/Mn) of 1.1 for the higher molecular weight population, which thus revealed an alternative route to the conventional synthesis of PCL.
A computational approach for the simulation and prediction of a linear three-step enzymatic cascade for the synthesis of e-caprolactone (ECL) coupling an alcohol dehydrogenase (ADH), a cyclohexanone monooxygenase (CHMO), and a lipase for the subsequent hydrolysis of ECL to 6-hydroxyhexanoic acid (6-HHA). A kinetic model was developed with an accuracy of prediction for a fed-batch mode of 37% for substrate cyclohexanol (CHL), 90% for ECL, and >99% for the final product 6-HHA. Due to a severe inhibition of the CHMO by CHL, a batch synthesis was shown to be less efficient than a fed-batch approach. In the fed-batch synthesis, full conversion of 100 mM CHL was 28% faster with an analytical yield of 98% compared to 49% in case of the batch synthesis. The lipase-catalyzed hydrolysis of ECL to 6-HHA circumvents the inhibition of the CHMO by ECL enabling a 24% higher product concentration of 6-HHA compared to ECL in case of the fed-batch synthesis without lipase.
Chiral polyesters in general can be employed for versatile biomedical purposes, but in vitro enzyme catalyzed biocatalytic routes by a multiple‐step cascade to make these functional biodegradable chiral polyesters have been hardly investigated. Recently, we developed an artificial three‐step enzymatic cascade synthesis by combining an alcohol dehydrogenase (ADH), a Baeyer–Villiger monooxygenase (BVMO) and a lipase (CAL‐A). Here, we extended this cascade for the synthesis of chiral methyl‐substituted oligo‐ɛ‐caprolactone derivatives to achieve both, the generation of chirality in a monomer and the subsequent polymerization. Several substrates were examined and provided access to functionalized chiral compounds in high yields (up to >99 %) and optical purities (up to >99 % ee). By subsequent enzymatic enantioselective ring opening of the enantiopure monomers, oligomeric lactones were successfully synthesized.
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