[5C + 1N] Annulations: Two Novel Routes to Substituted Dihydrofuro[3,2-c]pyridines

Two novel routes based on [5C + 1N] annulations for the synthesis of 2,3-dihydrofuro[3,2-c]pyridines are described. Ammonium acetate (NH(4)OAc) is used as an ammonia source in both routes. The first route utilizes 1-acyl-1-[(dimethylamino)alkenoyl]cyclopropanes as a five-carbon 1,5-bielectrophilic species and combines the [5C + 1N] annulation and regioselective ring-enlargement of cyclopropyl ketone into one pot, whereas the second route utilizes 3-acyl-2-[(dimethylamino)alkenyl]-4,5-dihydrofurans as the five-carbon synthons, which involves a sequential intermolecular aza-addition, intramolecular aza-nucleophilic addition/elimination, and dehydration reaction.