Syntheses with isoxazoles. Part III. Rearrangement of 4,5,6,7-tetrahydro-4-hydroxyiminoisoxazolo[2,3-a]pyridinium salts into derivatives of pyrrolo[3,2-b]pyridin-2-one

When 4,5,6,7-tetrahydro-4-hydroxyiminoisoxazolo[2,3-a]pyridinium chloride (5) or bromide (6) was heated with acetic anhydride, rearrangement to the corresponding 4-acetyl-7-halogeno-5,6-dihydro-4H-pyrrolo[3,2-b]-pyridin-2(1H)-one (9) or (8) occurred. The bromo-derivative (8) was hydrogenated to give a debrominated dihydro-derivative (10), and hydrolysed to the deacetylated derivative (11). Mechanisms for the rearrangement are suggested.