The condensation between pyruvic acid (I) and thiocarbohydrazide (II) provided 4-amino-3-mercapto-6-methyl-1,2,4-triazin-5(4H)-one (III). Further ring closure of (III) with (4-methylphenoxy)acetic acid (IV) by means of POCl3 yielded the required thiadiazolo triazinone.