Dimethyl l,2,5-thiadiazole-3,4-dicarboxylate

The highly complex reaction of tctrasulfur tetranitride with dimethyl acctylenedicarboxylale gives a mixture of dimethyl l,2,5-thiadiazole-3,4-dicarboxylate (67%), dimethyl 1,2,4-thiadiazolc-3,5-dicarboxylate (3 %), dimethyl l,3/.4,5.2,4-trithiadiazepine-6,7-dicarboxylate (le, 5%) and methyl l,37 4,5,2,4,6-trithiatriazepine-7-carboxylate (14%, see Section 4.5.).385... 

The only heterocyclic seven-membered ring system with maximum unsaturation containing six heteroatoms is 1,3A4,5,2,4,6-trithiatriazepine (1). The methoxycarbonyl derivative 2 is a minor product (14%) of the complex reaction of tetrasulfur tetranitride with dimethyl acetylenedicarboxylatc in refluxing toluene, which gives mainly dimethyl l,2,5-thiadiazole-3,4-dicarboxylate (3, 67%) (see Houben-Wcyl, Vol. E8d, pl54ff which includes an experimental procedure). Two other products are the trithiadiazepine 4 (5%, see Section 4.4.1.1.1.) and the 1,2,4-thiadiazole derivative 5 (3%).385... 

Nucleophiles attack the imide sulfur atom of trithiadiazepines. Trithiadiazepine (29, R = H) is converted by potassium t-butoxide and iodomethane into 3,4-dimethylthio-l,2,5-thiadiazole (30) by a rearrangement in which two of the ring sulfur atoms have become exocyclic with triphenylphosphine (29, R = C02Me) gives dimethyl l,2,5-thiadiazole-3,4-dicarboxylate (28) (87JCS(Pl)2i7). 

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