An interesting synthesis of thieno[2,3‐d][1,2,3]thiadiazole via decomposition/recyclization of 3‐methoxycarbonyl‐1H‐thieno‐[2,3‐e][1,3,4]thiadiazine 4,4‐dioxide

Attempted hydrolysis of the ester of 3‐methoxycarbonyl‐1H‐thieno[2,3‐e][1,3,4]thiadiazine 4,4‐dioxide ( I ) under acidic conditions gave the ring‐contracted thieno[2,3‐d][1,2,3]thiadiazole ( V ) instead of the expected carboxylic acid. In addition to a discussion of the reaction, a plausible mechanism is presented.