Synthesis of Coumarin Substituted Triazolothiadiazine Derivatives via Ring Transformation Reaction

A novel ring transformation reaction for the synthesis of 3‐(3‐aryl‐7H‐[1,2,4]triazolo[3,4‐b][1,3,4]thiadiazin‐6‐yl)‐2H‐chromen‐2‐ones has been described. Reaction of 3‐(2‐bromoacetyl)coumarins (1) with 5‐aryl‐1,3,4‐oxadiazole‐2‐thiol (2) gave ketones (4a–h). The in situ formed ketones (4a–h) were reacted with hydrazine hydrate to give 3‐(3‐aryl‐7H‐[1,2,4]triazolo[3,4‐b][1,3,4]thiadiazin‐6‐yl)‐2H‐chromen‐2‐ones (3a–h) and not 5 or 6. The compounds (3a–h) can also be prepared by the reaction of 3‐(2‐bromoacetyl)coumarins (1) with 5‐aryl‐1,3,4‐oxadiazole‐2‐thiol (2) in anhydrous ethanol to give corresponding 3‐(2‐(5‐aryl‐1,3,4‐oxadiazol‐2‐ylthio)acetyl)‐2H‐chromen‐2‐ones (4a–h). These on reaction with hydrazine hydrate in acetic acid gave corresponding 3‐(3‐aryl‐7H‐[1,2,4]triazolo[3,4‐b][1,3,4]thiadiazin‐6‐yl)‐2H‐chromen‐2‐ones (3a–h).