3-(N-carbethoxymethyl-N-methylsulfamoyl)-thiophene-2-carboxylic acid methyl ester → 2H-Thieno(2,3-e)-1,2-thiazine-3-carboxylic acid, 4-hydroxy-2-methyl-, methyl ester, 1,1-dioxide

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13.2 G. of 3-(N-carbethoxymethyl-N-methyl-sulfamoyl)-thiophene-2-carboxylic acid methyl ester are suspended in 42 ml. of a 1-N methanolic sodium methylate solution in the cold and under a nitrogen stream. After stirring for 15 minutes, a clear solution results. The solution is heated to reflux for 20 minutes, then cooled, neutralized and evaporated in vacuo. The residue is taken up in methylene chloride, shaken once each time with water and a sodium bicarbonate solution, dried and evaporated. The residue is brought to crystallization with methanol. There is obtained 3-carbomethoxy-4-hydroxy-2-methyl-2H-thieno[2,3-e]-1,2-thiazine1,1-dioxide of melting point 193°-195° C.

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