Oxygen → 2H-Pyrimido[4,5-b][1,4]benzothiazine-2,4(3H)-dione

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When derivative compounds of formula (II) are desired, a solution or suspension of 1,5-dihydro-2H-pyrimido[4,5-b][1,4]benzothiazine-2,4(3H)-dione derivative of formula (I) wherein R4 is hydrogen in acetonitrile is prepared. To that solution or suspension at room temperature is added, dropwise, an oxidant such as diethylazodicarboxylate, 1,4-benzoquinone or 2,3-dichloro-5,6-dicyano-1,4-benzoquinone. Alternatively, oxygen can be used as the oxidant. A preferred oxidant is diethylazodicarboxylate. The reaction mixture is stirred. When a precipitate results, the precipitate is collected an washed with diethylether to give a 2H-pyrimido[4,5-b][1,4]benzothiazine-2,4(3H)-dione derivative of formula (II). When no precipitate is obtained after oxidation, the solvent is removed from the reaction mixture, and the residue is triturated with diethylether and filtered to give a compound of formula (II).

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