Chemical constitution and activity of bipyridyliium herbicides. Part VIII. 4‐Bromo‐6,7‐dihydrodipyrido[1,2‐α:2′,1′‐ c ]‐pyrazinediium dibromide and related compounds

6‐Chloro‐2,2′‐bipyridyl reacts with sodium alkoxides to afford the corresponding 6‐alkoxy‐2,2′‐bipyridyls, which form 6,7‐dihydro‐4‐oxodipyrido[1,2‐ a :2′,1′‐ c ]pyrazinium bromide on treatment with 1,2‐dibromoethane. 6‐Chloro‐2,2′‐bipyridyl with this reagent gives 4‐bromo‐6,7‐dihydrodipyrido[1,2‐ a :2′,1′‐ c ]pyrazinediium dibromide.