Palladium-catalyzed cyclocarbonylation of o-iodophenols and 2-hydroxy-3-iodopyridine with heterocumulenes:: Regioselective synthesis of benzo[e]-1,3-oxazin-4-one and pyrido[3,2-e]-1,3-oxazin-4-one derivatives

Benzo[e]-1,3-oxazin-2-imine-4-ones (3) were synthesized by cyclocarbonylation of o-iodophenols with carbodiimides in the presence of a catalytic amount of a palladium catalyst and 1,4-bis(diphenylphosphino)butane under CO pressure. Product yields are dependent on the nature of the substrate, catalyst, solvent, and base as well as phosphine ligand. Reaction of o-iodophenols with unsymmetrical carbodiimides affords benzo[e]-1,3-oxazin-2-imine-4-ones (11a-g) in good yield and usually in a completely regioselective manner. Benzo[e]-1,3-oxazin-2,4-diones (5) were obtained in good to excellent yields using the same procedure and a 1:2 ratio of o-iodophenol/isocyanate. Pyrido[3,2-e]-1,3-oxazin-4-ones (16) were isolated in fine yield using 2-hydroxy-3-iodopyridine instead of an iodophenol as reactant. The reaction mechanism is believed to involve in situ formation of a carbamate eater followed by palladium-catalyzed carbonylative amidation.