A Convenient Synthesis of 2,2′,6,6′-Tetramethyl-4,4′-bipyridine and Its Oxidation to 2,2′,6,6′-Tetracarboxy-4,4′-bipyridine

By reaction of 2,6-dimethylpyridine (1b) with sodium the tetrahydro-4, 4′-bipyridine bis-sodium salt 2b is formed. Of different dehydrogenation reagents tested, only sulfur dioxide affords the title compound 3b, in 49 % overall yield. By oxidation of 3b with chromium trioxide, 2,2′, 6,6′-tetracarboxy-4,4′-bipyridine (3c, 70%) is produced, a valuable precursor for several 2,2′,6,6′-tetrasubstituted 4,4′-bipyridines, e.g., the corresponding acid chloride 3d and the carboxylic esters 3e and 3f.