Enantioselective and Step-Economic Synthesis of the Chiral Amine Fragment in the Tyrosine Kinase Inhibitor Repotrectinib by Direct Asymmetric Reductive Amination under Batch and Flow

A one-step and highly enantioselective synthesis of (R)-2-(1-aminoethyl)-4-fluorophenol ((R)-I), a key chiral intermediate toward preparation of the tyrosine kinase inhibitor repotrectinib, has been developed. Starting from the easily available substrate 1-(5-fluoro-2-hydroxyphenyl)ethan-1-one (2a), the target product (R)-I was prepared in an optically pure form through a Ru-catalyzed asymmetric reductive amination with NH₄OAc as the nitrogen source and H₂ as the reducing agent. The reaction can be carried out on a 20 g scale using a continuous-flow reactor instead of a traditional batch reactor. The flow technology enables higher reaction efficiency and does not require column chromatography for product purification. Compared to the known procedures, this method avoids the use of expensive chiral auxiliaries and protecting group operations, making it a step-economical and cost-effective alternative strategy for production of repotrectinib.