Synthesis of 2-(4H-1,2,6-thiadiazin-4-ylidene)malononitriles

The base-free TiCl4-mediated condensation of 3,5-disubstituted-4H-1,2,6-thiadiazin-4-ones 8 with malononitrile affords 20 difficult to access (3,5-disubstituted-4H-1,2,6-thiadiazin-4-ylidene)malononitriles 7. The reaction tolerates 3,5-diaryl, diphenoxy, dimethoxy and diphenylthio substituted thiadiazinones, but not diamino, monohydroxy or dihalo substituents. Nevertheless, asymmetrically substituted 3-halo-5-phenyl- and 3-chloro-5-methoxy-4H-1,2,6-thiadiazin-4-ones convert into the corresponding ylidenemalononitriles in good yield. Furthermore, the condensation works well with ethyl cyanoacetate and diethyl malonate, but not with Meldrum's acid, dimedone or nitromethane. Finally, 2-(3-chloro-5-phenyl-4H-1,2,6-thiadiazin-4-ylidene)malononitrile (7q) reacts with aniline to give 4,7-diphenyl-6-(phenylimino)-6,7-dihydropyrrolo[2,3-c][1,2,6]thiadiazine-5-carbonitrile (12) in moderate yield demonstrating the potential use of these ylidenes to prepare novel 6-5 fused 4H-1,2,6-thiadiazines.