A cyclic reaction network that would allow concurrent enantioselective oxidation of achiral 3a and non-selective reduction of 4a was developed by using the cyclic deracemization of chiral analogue 1a as a model. Alcohol dehydrogenases (ADHs) have been used by Kroutil and coworkers in cyclic reaction networks leading to the deracemization of point-chiral alcohols, and have also been used in the enantioselective synthesis of chiral biaryls through both desymmetrization and dynamic kinetic resolution processes. Inspired by these studies, as well as Turner's seminal report of concurrent biocatalytic oxidation and chemical reduction pathways in the deracemization of benzylic amines, we screened a library of ADHs in conjunction with excess ammonia borane for the deracemization of 1a (Figure ). The ADHcatalysed oxidation of the benzylic alcohol moiety was realised using NADP as co-factor, an NADPH oxidase (PRO-NOX(001) or YcnD) as the co-factor recycling enzyme, and molecular oxygen as the terminal oxidant. We quickly identified ADH 159 as an effective catalyst for deracemization: addition of ten equivalents of ammonia borane at the outset of the ADH-catalysed oxidation led to a reaction profile consisting of almost exclusively 1a with an ee of 38% after 54 hours (Figure , Entry 4; the sense of enantioselectivity of the ADH was established from its action on a known substrate, see Supplementary Information Section 4 for details). Further optimisation of the recycling system, pH, and temperature provided (Ra)-1a in 95% ee after 24 hours (Figure , Entry 9; see Supplementary Information Section 5 for full optimisation details). Monitoring the reaction over time (Figure ) established that the ee increased over ~24 h as 1a underwent repetitive cycles of oxidation and reduction. oxidation and reduction pathways. In addition, this highly effective deracemization of 1a provides insight into the hierarchy of rates that characterises the cyclic reaction network: in order for deracemization to occur, it is necessary that the interconversion of the enantiomeric conformers of 2a (i.e., the enantiomerization of 2a) occurs faster than the reduction of 2a back to 1a. Both the reduction and enantiomerization of 2a must also occur faster than the oxidation of 1a. These kinetic constraints can be expressed as a hierarchy of rates, renant > rred > rox(Ra,Sa).