An intramolecular C–O insertion of 2-(2-arylamidophenyl)-2-diazoacetate has been achieved using a catalytic amount of copper triflate under mild conditions to produce 2-aryl-4H-benzo[d][1,3]oxazine-4-carboxylate in good yields. In addition, 2-diazo-2-(2-arylthioamidophenyl)acetate affords the corresponding 2-aryl-4H-benzo[d][1,3]thiazine derivatives under similar conditions. This is the first example of the synthesis of benzoxazines and benzothiazines from ortho-amidophenyl diazoacetate and ortho-thioamidophenyl diazoacetate, respectively.