AbstractTheo‐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 cyclized2into [1,2,5]thiadiazolo[3,4–6]‐pyrazine (14). 3,4‐Dichloro‐1,2,5‐thiadiazole 1,1‐dioxide (18) reaeted with2to give 1,3‐dihydro‐bis[1,2,5]thiadiazolo[3,4‐b:3′,4′‐e]pyrazine 2,2‐dioxide (19). The reaction of2with 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 reduced21to the 4,9‐dihydro derivative23, which was reoxidized to21with chloranil. Aqueous hase hydrolyzed21to4viathe hydrated intermediate24. Aqueous glyoxal cyclized4to the covalent hydrate of pyrazino[2,3‐b]quinoxaline (26),27, which was dehydrated to26. Compound26underwent rapid addition of two