New thiazolo[2,3-a] pyrimidinedione derivatives were synthesized by two different chemical methods.One method included the addition of a mixture of glacial acetic acid ,acetic anhydride (2 : 1), chloroacetic acid and anhydrous sodium acetate to mercapto-thieno[2,3-d]pyrimidinone derivatives 5a or 5b under reflux to give thieno[2,3-d]thiazolo[3,2-a]pyrimidinedione derivatives 6a or 6b which react w...