A New Entry to Benzo(4,5)furo(3,2-b)pyridines via N-(Benzofuran-3-yl)iminophosphorane

Abstract: Mild thermal reaction of enones with N -(benzofuran-3-yl)iminophosphorane, newly prepared by Staudinger reaction of 3-azidobenzofuran with triphenylphosphine, provides a synthetic en-try to virtually unknown benzo[4,5]furo[3,2- b ]pyridines via a tan-dem aza-Wittig–electrocyclization process. Key words: iminophosphoranes, azides, aza-Wittig, electrocy-clization, benzofuropyridines Small heterocyclic molecules play an important role inmodern pharmaceutical and medicinal chemistry. 1 In par-ticular, benzo[4,5]furopyridines, isosteric of both carbo-lines and dibenzofuran, often display important biologicaland pharmacological activities. 2,3 Benzo[4,5]furo[ c ]pyridines and their tetrahydro deriva-tives are long-known compounds available through vari-ous reported methods. 4 In contrast, the benzo-[4,5]furo[ b ]pyridine congeners remain little known. In-deed, only scant examples of benzo[4,5]furo[2,3- b ]py-ridines were present in the literature 4b,5c,d until two veryrecent papers described the preparation of a variety ofthose tricyclic compounds through the unusual use of insitu generated 2-aminobenzofuran as the crucial interme-diate.