Ring Enlargement of Carbohydrate-Derived 1,2-Oxazines to Enantiopure 5-Bromo-1,2-oxazepines and Subsequent Palladium-Catalyzed Reactions

Dibromocarbene addition to D-glyceraldehyde-derived 1,2-oxazines syn-1 and anti-1 provided dibromocyclopropane intermediates syn-3 and anti-3, which smoothly reacted with methanol under ring enlargement to furnish 5-bromo-1,2-oxazepine derivatives syn-4 and anti-4. Related 1,2-oxazines such as arabinose-derived compounds furnished the 1,2-oxazepine derivatives syn-4e and anti-4f with fair efficacy. The alkenyl bromide moiety of 1,2-oxazepine derivatives syn-4 and anti-4 was then exploited for the introduction of new substituents via palladium-catalyzed C-C bond forming processes (Sonogashira, Suzuki, Stille, and Heck reactions). These transformations led to a series of new highly substituted 1,2-oxazepine derivatives syn-5 or anti-5-11 being of considerable interest for further synthetic elaborations.