The ring-opening and rearrangement reaction of 7-methyl-5H-1, 3, 4-thiadiazolo [3, 2-a] pyrimidin-5-ones (I) and the alternative isomeric 7-ones (XI) has been studied. I (R= H) and XI (R= H) gave quantitatively through N–N bond cleavage methylthiouracil (III) by treating with 5% NaOH, while they yielded 4-methyl-2-thiocyanato-6 (1H) pyrimidone (II) by treating with liquid ammonia. By treating I and XI (R= alkyl) with 5% NaOH, the S–C bond cleaved ...