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Abstract
The efficient multifunctionalization by one‐pot or cascade catalytic systems has developed as an important research field, but is often challenging due to incompatibilities or cross‐reactivities of the catalysts leading to side product formation. Herein we report the stereoselective preparation of cis‐ and trans‐4‐aminocyclohexanol from the potentially bio‐based precursor 1,4‐cyclohexanedione. We identified regio‐ and stereoselective enzymes catalyzing reduction and transamination of the diketone, which can be performed in a one‐pot sequential or cascade mode. For this, we identified regioselective keto reductases for the selective mono reduction of the diketone to give 4‐hydroxycyclohexanone. The system is modular and by choosing stereocomplementary amine transaminases, both cis‐ and trans‐4‐aminocyclohexanol were synthesized with good to excellent diastereomeric ratios. Furthermore, we identified an amine transaminase that produces cis‐1,4‐cyclohexanediamine with diastereomeric ratios >98 : 2. These examples highlight that the high selectivity of enzymes enable short and stereoselective cascade multifunctionalizations to generate high‐value building blocks from renewable starting materials.
Introduction
The aim of our research is a stereoselective synthesis development of 4-aminocyclohexanol by the application of a keto reductase (KRED) and an amine transaminase (ATA). 4-Aminocyclohexanol is a valuable precursor for active pharmaceutical ingredients, for example, lomibuvir (a HCV protease inhibitor), ambroxol (a secretolytic agent) and other bioactive molecules. Today, the trans-4-aminocyclohexanol is accessed via Ni-catalyzed synthetic procedure giving moderate yields. In our project we perform cis- and trans-4-aminocyclohexanol synthesis from 1,4-cyclohexanedione (a bio-based precursor) by an one-pot approach combining sequentially a KRED and an ATA as catalysts. For this, we envisaged two multistep enzymatic procedures. The route A would involve 4-hydroxycyclohexanone formation from 1,4-cyclohexanedione via a KRED-catalyzed monoreduction and a further transamination mediated by an ATA towards 4-aminocyclohexanol. The route B would consist of switching the steps of the previous sequential approach, that is, a monoamination of the diketone to yield 4-aminocyclohexanone, and the subsequent reduction of the remaining carbonyl group. Only route A turned out to be feasible, and we performed 4-aminocyclohexanol synthesis at the preparative scale in the sequential and tandem modes. Depending on the ATA, both isomers can be obtained.