Novel Furo-pyridine Derivatives via Sonogashira Reactions of Functionalized Pyridines.
T. Lechel, J. Dash, I. Brüdgam, H.-U. Reissig – 2008
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.