Chemistry of 1,2,4-triazines VIII: The oxidation of ethyl 3-methylthio-5-hydroxy-1,2,4-triazine-6-carboxylate and ethyl N^2-[5'(3)-methylthio-6'-ethoxycarbonyl)-1',2',4',-triazinyl]-3-methylthio-5- oxo-1,2,4-triazine-6-carboxylate and the substitution rea