Simple Synthesis of Fused 1,3,4-Thiadiazines Through Successive S_NAr Processes

A convenient method is described for the synthesis of novel 2-arylazo-1,3,4-thiadiazines fused with pyridine, pyrimidine or pyrazole ring. The synthetic strategy relies on a one-pot tandem cascade that proceeds through three consecutive steps without the isolation of intermediates: (1) nucleophilic aromatic substitution (SNAr) of the chlorine atom by the dithizone nucleophile, (2) intramolecular S-N-type Smiles rearrangement involving migration of the hetaryl fragment from nitrogen to sulfur, and (3) intramolecular nucleophilic substitution of the activated nitro group, which closes the fused thiadiazine ring. The use of ortho-chloronitro hetarenes is essential, as the nitro group serves both as an activating substituent for the initial SNAr step and as a leaving group in the final cyclization step. UV-Vis absorption spectra of the synthesized compounds were recorded.