Coupling of consecutive pyridine ring units for oligopyridine synthesis

The reaction of 2-ethylsulfinyl-6-(ethylthio)pyridine (2), 6-ethylsulfinyl- 6'-ethylthio-2,2'- -bipyridine (6), and 6-ethylsulfinyl-6"'-ethylthio-2,2': 6',2 ":6 ",2'''-quaterpyridine (10) with methylmagnesium bromide gave symme tric oligopyridines 6,6'-bis(ethylthio)-2,2'-bipyridine (4), 6,6'''-bis(eth ylthio)-2,2':6',2 ":6 ",2'''-quaterpyridine (8), and 6,6'''''''-bis(ethylth io)-2,2':6',2 ":6 ",2''':6''',2'''':6'''',2''''':6''''',2'''''':6'''''',2'' '''''-octipyridine (12) in respective yields of 70, 56, and 3%. On the othe r hand, the reaction of 6 and 10 with 2-(6-bromopyridinyl) lithium and 6-(2 ,2'-bipyridinyl)lithium gave the corresponding unsymmetric oligopyridines ( 15, 16, 13, and 19). The reaction with chiral (S)-2-{6-[1-(tert-butyldimeth ylsiloxy)-ethyl]pyridinyl}lithium gave optically active oligopyridines (17) and (18) in 78 and 58% yields, respectively.