Cyclization reactions of tricarbonylmethane thiosemicarbazones: Formation of 1,3,4‐thiadiazole derivatives with concomitant CC bond cleavage

Under acylating conditions 3‐acetyl‐4‐hydroxy‐2H‐pyran‐2‐one thiosemicarbazones (1a, b and 2a, b) are transformed into 2‐(acylamino)‐5‐methyl‐1,3,4‐thiadiazoles 3b, c, e, f and 2H‐pyran‐2‐ones 1c and 2c via C–C bond cleavage. Similarly, the reaction of 1b with RCO2H/EtOH (R = Me, Et) affords 2‐anilino‐5‐methyl‐1,3,4‐thiadiazole (3d) and triacetic acid lactone 1c. Upon treatment with Ac2O/Et3N the dehydroacetic acid derivative 1b is transformed into the thiadiazoline 4, while the benzo derivative 2b is converted into thiadiazole 3e and 4‐acetoxycoumarin (2c).