Quinoxalino [2, 3-b][1, 4] benzothiazine 1 and its pyrazino-analogue 6 both give pairs of isomeric N-methyl derivatives. The N-methyl compounds 2 and 7 on oxidation with iodobenzene dichloride and other reagents give sulphoxides and sulphones (accompained by some ring substitution) but treatment of 6 with iodobenzene dichlorode gives benzothiazole-2-carboxamide 17. The action of sodium nitrite in acetic acid upon the ...