Reductive transformations of 5,6‐dihydro‐4H‐1,2‐oxazines: Synthesis of 4‐hydroxy ketoximes, N‐hydroxypyrrolidine derivatives, and other nitrogen‐containing compounds

Abstract 6‐Siloxy‐substituted 1,2‐oxazines 1 are transformed into 4‐hydroxy ketoximes 2 by reduction with NaBH4 in ethanol. Reductive Beckmann rearrangement converts the oxime 2a into the 1,4‐amino alcohol 7. Diisobutylaluminum hydride (DIBAH) induces a novel reductive ring contraction of 1 to provide either N‐hydroxypyrrolidine derivatives 8 or nitrones 9. Other 1,2‐oxazines lacking the 6‐siloxy substituent are also studied under these reaction conditions. Catalytic hydrogenolysis either gives the acyclic amine 16 or it stops at the stage of the proline derivative 21. Mechanistic features of these synthetically valuable transformations are discussed.