A synthesis of 4-acetoxy-2,3-dihydro-3-oxo-4H-naphth[1,2-b][1,4]oxazine (4a) from 2-nitroso-1-naphthol is described. This compound (4a) is substituted by various nucleophiles at position 5, with loss of the N-acetoxy group; in contrast, it did not react at physiological pH with the biological nucleophile methionine. On heating, 4a rearranges to the 2- and 5-acetoxy isomers,