On the bromination of thieno[c]-fused 1,5-naphthyridines and their isomeric N-oxides using dibromoisocyanuric acid in fuming sulfuric acid

Thieno2,3-c]-1,5-naphthyridine ( 3 ), thieno2,3-c]1,5-naphthyridine 5-oxide ( 7 ), thieno3,2-c]-1,5-naphthyridine ( 5 ) and thieno3,2-c]-1,5-naphthyridine 5-oxide ( 9 ) could conveniently be brominated at room temperature using dibromoisocyanuric acid in fuming sulfuric acid. Bromination occurred in good to moderate yields at the β position in the thiophene ring. Thieno2,3-c]-1,5-naphthyridine 9-oxide ( 12 ) and thieno3,2-c]-1,5-naphthyridine 9-oxide ( 13 ) also gave substitution in the thiophene ring at 95°. It was also found that 12 was deoxygenated under these reaction conditions. Direct oxidation of the brominated thienoc]naphthyridines with m-chloroperbenzoic acid gave the 5-oxides in high yield.