Novel Furo‐pyridine Derivatives via Sonogashira Reactions of Functionalized Pyridines

A series of 4‐pyridyl nonaflates was coupled with several terminal alkynes to efficiently provide new 4‐alkynyl‐substituted 3‐alkoxypyridine derivatives. Apt conditions were developed for their conversion into furo[2,3‐ c ]pyridines. Sonogashira reactions of 4‐alkoxy‐substituted 3‐pyridyl nonaflates allowed an access to regioisomeric furo[3,2‐ c ]pyridines. For both types of alkynyl‐substituted alkoxypyridinesan alternative method for cyclization employing iodine monochloride furnished iodinated furo[2,3‐ c ]‐ or furo[3,2‐ c ]pyridines, which can undergo a second palladium‐catalyzed step. Iodination of 4‐hydroxypyridine derivative 24 with iodine afforded a pentasubstituted pyridine which after Sonogashira reaction immediately undergoes a cyclization to furo‐pyridine 25 . Thus, three different types of furo‐pyridines can be prepared starting from one precursor. Several compounds prepared are fluorescent and show strong Stokes shifts. (© Wiley‐VCH Verlag GmbH & Co. KGaA, 69451 Weinheim, Germany, 2008)