Abstract: The present invention relates to a process for the preparation of a 5-methyl-3-heteroaryl-2H,5H-1,3-oxazino[5,6-c] [1,2] benzothiazine-2,4-(3H)-dione 6,6-dioxide of the general formula I: ##STR1## in which R represents a heteroraryl radical, preferably substituted or unsubstituted pyrimidinyl or pyridyl, of the type consisting in reacting an amide of the general formula II: ##STR2## in which R.sup.1 represents a heteroarylamino radical, preferably substituted or unsubstituted pyrimidinylamino or pyridylamino, with a compound of the general formula III: ##STR3## in which R.sup.2 represents a C.sub.1 to C.sub.4 lower alkyl radical such as methyl or ethyl, an aryl radical such as phenyl, or an alkylaryl radical such as benzyl, wherein the reaction is carried out in an organic solvent selected from the group comprising pyridine, substituted pyridines such as 4-dimethylaminopyridine, 4-(pyrrolidin-1-yl)pyridine and 2,6-ditert.-butyl-4-methylpyridine, and mixtures thereof.