The nitration of methyl-3-hydroxythiophene-2-carboxylate furnishes two products, the lower melting of which was previously thought to be the 4- (3) and the other the 5-isomer (2); these assignments have been reversed on the basis of carbon-13 NMR. data and the revised structures have been confirmed both by O to N acyl migrations and by the preparation of the first examples (20) and (23) of the thieno[3,4-b][1,4]oxazine ring system from derivatives of the 4-nitro isomer.
