2-Aminothiophenol → 1,5-dihydro-2H-pyrimido[4,5-b][1,4]benzothiazine-2,4(3H)-dione

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Then, to a solution or suspension of a 5-hydroxypyrimidine-2,4(1H,3H)-dione derivative of formula (IV) in a reaction inert solvent is added N-bromosuccinimide portion-wise at room temperature. A preferred solvent is ethanol. The reaction mixture is stirred until the derivative of formula (IV) disappears. Then, a 2-aminobenzenethiol derivative of formula (V) is added and the mixture is heated at reflux. Certain 2-aminobenzenethiol derivatives of formula (V) are commercially available and, further, the derivatives of formula (V) can be prepared from the corresponding 2-aminobenzothiazoles by hydrolysis as described by Mital, R. L., et al., J. Chem. Soc. (c), 2148 (1969). When a precipitate results after reflux of the reaction mixture, the precipitate is collected to give a 1,5-dihydro-2H-pyrimido[4,5-b][1,4]benzothiazine-2,4(3H)-dione derivative of formula (I). When no precipitate is obtained after reflux, the derivative of formula (I) is obtained by chromatographic separation on silica gel. The foregoing method for reacting derivatives of formula (IV) with derivatives of formula (V) to yield derivatives of formula (I) is analogous to the method described by Sako, M., et al., Chem. Pharm. Bull., 32, 2474 (1984).

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