Practical One-Pot Multistep Synthesis of 2H-1,3-Benzoxazines Using Copper, Hydrogen Peroxide and Triethylamine

In this article, we describe simple one-pot syntheses of 2H-1,3-benzoxazines from ketones utilizing an imino-pyridine directing group (R¹R²-C=N-CH₂-Pyr), which promotes a Cu-directed sp² hydroxylation using H₂O₂ as oxidant and followed by an oxidative intramolecular C-O bond formation upon addition of NEt₃. This synthetic protocol is utilized in the gram scale synthesis of the 2H-1,3-benzoxazine derived from benzophenone. Mechanistic studies reveal that the cyclization occurs via deprotonation of the benzylic position of the directing group to produce a 2-azallyl anion intermediate, which is oxidized to the corresponding 2-azaallyl radical before the C-O bond formation event. Understanding of the cyclization mechanism also allowed us to develop reaction conditions that utilize catalytic amounts of Cu.