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

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