The chemistry of 1,2,5‐thiadiazoles V. Synthesis of 3,4‐diamino‐1,2,5‐thiadiazole and [1,2,5] thiadiazolo[3,4‐b]pyrazines

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Abstract
The o‐diamine, 3,4‐diamino‐1,2,5‐thiadiazole (2), was synthesized from 3,4‐dichloro‐1,2,5‐thiadiazole (3) hy three methods. Aqueous glyoxal cyclized 2 into [1,2,5]thiadiazolo[3,4–6]‐pyrazine (14). 3,4‐Dichloro‐1,2,5‐thiadiazole 1,1‐dioxide (18) reaeted with 2 to give 1,3‐dihydro‐bis[1,2,5]thiadiazolo[3,4‐b:3′,4′‐e]pyrazine 2,2‐dioxide (19). The reaction of 2 with selenium oxyehloride led to [1,2,5]selenadiazolo[3,4‐c] [1,2,5]thiadiazole (12). Ring closure of 2,3‐diaminoquinoxaline (4) with thionyl chloride or selenium oxychloride gave [1,2,5]thiadiazolo‐[3,4‐b]quinoxaline (21) and [1,2,5]selenadiazolo[3,4‐b]quinoxaline (22), respectively. Sulfurous acid reduced 21 to the 4,9‐dihydro derivative 23, which was reoxidized to 21 with chloranil. Aqueous hase hydrolyzed 21 to 4 via the hydrated intermediate 24. Aqueous glyoxal cyclized 4 to the covalent hydrate of pyrazino[2,3‐b]quinoxaline (26), 27, which was dehydrated to 26. Compound 26 underwent rapid addition of two alcohols in a process analogous to covalent hydration.