Efficient syntheses of thiadiazoline and thiadiazole derivatives by the cyclization of 3-aryl-4-formylsydnone thiosemicarbazones with acetic anhydride and ferric chloride

3-Aryl-4-formylsydnone 4′-phenylthiosemicarbazones 3a-d and 3-aryl-4-formylsydnone thiosemicarbazones 3e-h are effective precursors of sydnonyl-substituted heterocycles. The thiosemicarbazones 3a-d reacted with acetic anhydride (4a) to give 4-acetyl-2-phenylamino-5-(3-arylsydnon-4-yl)-4,5-dihydro-1,3,4]thiadiazoles 5a-d and 4-acetyl-2-(N-phenylacetamido)-5-(3-arylsydnon-4-yl)-4,5-dihydro-1,3,4]thiadiazoles 6a-d. However, under similar method, thiosemicarbazones 3e-h produced only 4-acetyl-2-acetamido-5-(3-arylsydnon-4-yl)-4,5-dihydro-1,3,4]thiadiazoles 6e-h in high yield. The sydnonyl-substituted thiadiazole derivatives 7a-h were also obtained successfully by the cyclization of 3-aryl-4-formylsydnone thiosemicarbazones 3a-h with ferric chloride (4b). In the cyclization, the thiosemicarbazones 3a-d are more reactive than the thiosemicarbazones 3e-h.