Gold(I)-Catalyzed and Iodonium Monochloride-Mediated Carboannulations of Indole/Benzofuran-Based Acetylenic Substrates: Easy Access to δ-Carbolines and Benzofuro[3,2-b]pyridines

Au(I)-catalyzed tandem cyclization of indole-/benzofuran-based N-propargylsulphonamides 3 and 4 led to the formation of dihydro δ-carboline and benzofuro[3,2-b]pyridine intermediates, which underwent a base (DBU) induced 1,2-elimination (-TsH) resulting in the formation of δ-carbolines 1a and benzofuro[3,2-b]pyridines 2a, respectively, with 62–97% yields. On the other hand, exposure of 3 and 4 to iodonium monochloride (ICl) and K₂CO₃ in refluxing DCM triggered a tandem iodocyclization followed by 1,2-elimination (-TsH), delivering 3-iodo δ-carbolines 1b and benzofuro[3,2-b]pyridines 2b, respectively, within 2 h with 44–86% yields. We propose plausible reaction mechanisms for the formation of products 1a–b and 2a–b. A photophysical study on a few selected products revealed their poor emissive property.