Chemoenzymatic Synthesis of Chiral Piperidines Using the Immobilized Transaminase ATA‐117 in Continuous Flow Reactors

Nowadays, there is a growing need for streamlined and sustainable strategies to access chiral nitrogen heterocycles, whose stereodefined frameworks underpin their broad functional and pharmaceutical relevance. Herein, we report a robust continuous flow enzymatic protocol for the stereoselective synthesis of the natural alkaloid (–)-pinidinone and demonstrate its applicability to the preparation of a small family of structurally diverse chiral 2,6-disubstituted piperidines. The strategy relies on the enantioselective transamination of tailor-made α,β-unsaturated ketones catalyzed by an immobilized form of ATA-117, which triggers a spontaneous intramolecular aza-Michael reaction, enabling rapid access to chiral cis-(2R,6R)-piperidine scaffolds under mild conditions. The enzyme, successfully immobilized on Eupergit C, showed good operational and storage stability while a dual-flow feed configuration minimized the formation of imine and Michael-type by-products observed when a single stock solution of α,β-unsaturated ketone acceptor and isopropylamine (IPA) amino donor was employed. The optimized conditions (20 mM substrate, 200 mM IPA, 60 min residence time, 30 °C) allowed for complete conversion and high stereoselectivity, while an integrated in-line work-up improved automation and reduced manual handling. This study establishes a robust biocatalytic flow platform for the asymmetric synthesis of valuable nitrogen-containing heterocycles.