Biocatalytic Asymmetric Synthesis of Diversely Substituted Dihydro-1,4-2H-Benzoxazines and Benzothiazines

Benzoxazine- and benzothiazine-containing compounds are known to exhibit diverse biological activities. They have been reportedly synthesized using transition metal catalysts or biocatalysts, albeit with limited substrate scope. In this work, we demonstrate the use of imine reductases (IREDs) for the synthesis of chiral benzoxazines and benzothiazines bearing bulky aryl substituents (16 examples), achieving high yields (up to 96%) and excellent enantiomeric excess (up to >99% ee). Notably, (S)-6,7-difluoro-3-phenyl-2H-benzo[b][1,4]oxazine, which is the analogue of a key precursor of the antimicrobial agent levofloxacin, was synthesized using Cf-IRED with outstanding enantioselectivity. A preparative-scale (0.5 g) biotransformation further highlighted the practicality of this biocatalytic strategy. Additionally, docking studies provided mechanistic insights into the origin of enantioselectivity in Cf-IRED and IR-5 with fluoro-substituted benzoxazines. Overall, this methodology expands the substrate scope of IREDs to include six-membered N-heterocycles containing oxygen and sulfur atoms.