N1-Phenoxycarbonyl-N2-thioacylureas (130), formed by addition of thioamides to phenoxycarbonyl isocyanate, in aqueous sodium carbonate undergo rapid cyclization to 6-substituted-1,3,5-thiadiazine-2,4(3H)-diones (131) in high yields (≃90%) (Equation (7)). Cyclization can also be achieve...