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

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.