A multistep reaction of 5-halo-1,2,3-thiadiazoles and 1,2-phenylenediamines provides a new route to fused 1,3,6thiadiazepines. The overall process consists of the known stepwise formation of 5-[1-(2-aminophenyl)-1,2,3-triazol-5ylsulfanyl]-1,2,3-thiadiazole, and a novel ring transformation which involves the Smiles and Dimroth rearrangements followed by an intramolecular nucleophilic substitution of the thiol group, affording di[1,2,3]triazolo[1,5-a:5Ј,1Ј-d ]-[3,1,5]benzothiadiazepines. The influence of the substituents on the 1,2,3-thiadiazole and phenyl rings on this reaction was discussed.

Two alternative directions for thermal transformation of 6-amino-4-oxopyrano [3,4-d] [1,2,3]thiadiazoles 1, leading either to 6-hydroxy-4-oxo-[1,2,3]thiadiazolo[4,5-c]pyridines 2 or 2-cyano-2-(1,2,3-thiadiazol-5-yl)acetamide 4b, were observed. The first one represents a new, Dimroth-type rearrangement and proceeds by thermal opening of the pyrane ring, followed by the simultaneous rotational isomerization of the ketene intermediate 7 (s-cis) to 7 (s-trans) and its recyclization onto the amido group to form the pyridin-2-one cycle. The first step of the rearrangement has a calculated [B3LYP/ 6-31G(d)] activation barrier of 24-34 kcal/mol, the involve-

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