Process for obtaining compounds usable in the production of Cefotetan, and new compounds obtained thereby

A process for obtaining a mono-, di- and/or tri-potassium salt of 4-carboxy-3-hydroxy-5-mercapto-isothiazole or a mono-, di- and/or tri-sodium salt of 4-carboxy-3-hydroxy-5-mercapto-isothiazole comprising formula (III):

• wherein
• R1, R2 and R3 are Na, or R1, R2 and R3 are K;
• R1 and R2 are Na and R3 is H, or R1 and R2 are K and R3 is H;
• or
• R1 is Na and R2 and R3 are H, or R1 is K and R2 and R3 are H;
• comprising:
• refluxing the disodium or dipotassium salt of 3-hydroxy-5-mercapto-4-isothiazole carbonitrile in an aqueous solution of between 3 to 6 equivalents of sodium hydroxide or potassium hydroxide for a time period of at least 15 h, to give an aqueous solution comprising the trisodium or respectively tripotassium salt of 4-carboxy-3-hydroxy-5-mercapto-isothiazole (III) in which R1, R2 and R3 are each Na or each K, and
• precipitating the salt from this solution by lowering the pH, by adding sufficient HCl to cause precipitation, wherein the pH is not lowered beyond pH 8.