Continuous and Divergent Biotransformation of β‐Ketoesters Into Enantiomerically Pure β‐Hydroxyacids or Their Oligomers Through a Telescoped Multienzyme Cascade

Efficient, selective, and sustainable synthetic platforms are increasingly important in modern chemical manufacturing, yet chemical flexibility often remains limited. In this study, we develop a modular biocatalytic system for the divergent and continuous transformation of ethyl acetoacetate (EAA) into either enantiomerically pure ( R )‐3‐hydroxybutyric acid (( R )‐3HBA) or oligomers of ( R )‐poly(3‐hydroxybutyrate) (PHB). The cascade combines two telescoped packed‐bed reactors containing an immobilized, self‐sufficient Lactobacillus kefir ketoreductase ( Lk KRED) and commercial immobilized Candida antarctica lipase B (N435). By simply tuning the reaction conditions, the system selectively directs the substrate toward either product. Under aqueous conditions, it achieves quantitative conversion to ( R )‐3HBA with >99% enantiomeric excess and maintains excellent stability over 7 days of continuous operation. It also reaches a maximum space–time yield of 105 g L −1 h −1 , which is 33 times higher than the volumetric productivity of the sequential two‐pot process. Under anhydrous conditions, the same setup promotes polycondensation into PHB oligomers, as confirmed by matrix‐assisted laser desorption/ionization time‐of‐flight analysis. Overall, this platform combines productivity, stereoselectivity, and flexibility for the integrated synthesis of chiral building blocks or oligomers.