Synthesis of 3‐ and 6‐Sulfonylindoles from ortho ‐Alkynyl‐ N ‐sulfonylanilines by the Use of Lewis Acidic Transition‐Metal Catalysts

Gold‐catalyzed reactions of ortho‐ alkynyl‐ N ‐sulfonylanilines produced the corresponding 3‐sulfonylindoles in good to high yields. For example, the reaction of N ‐mesyl‐ N ‐methyl‐2‐(1‐pentynyl)aniline, N ‐mesyl‐ N ‐methyl‐2‐(phenylethynyl)‐aniline, and 2‐ethynyl‐ N ‐mesyl‐ N ‐methylaniline in the presence of 10 mol % of AuBr 3 in toluene at 80 °C gave 3‐mesyl‐1‐methyl‐2‐propylindole, 3‐mesyl‐1‐methyl‐2‐phenylindole, and 3‐mesyl‐1‐methylindole in 95, 92, and 71 % yield, respectively. Furthermore, we found that the reactions of 2‐alkynyl‐6‐methoxy‐ N ‐sulfonyl‐anilines in the presence of indium catalyst (InBr 3 ) afforded the corresponding 6‐sulfonylindoles as the major product in good yields. For example, the reaction of 6‐methoxy‐ N ‐methyl‐2‐(1‐pentynyl)‐ N ‐tosylaniline in the presence of 5 mol % of InBr 3 in toluene at 80 °C gave an 87:13 mixture of 7‐methoxy‐1‐methyl‐2‐propyl‐6‐tosylindole and 7‐methoxy‐1‐methyl‐2‐propyl‐3‐tosylindole in 95 % yield. Most probably, the gold‐catalyzed reactions of ortho‐ alkynyl‐ N ‐sulfonylanilines proceed through a [1,3] sulfonyl migration, whereas the indium‐catalyzed cyclizations of 2‐alkynyl‐6‐methoxy‐ N ‐sulfonylanilines, which produce 6‐sulfonylindoles, proceed by an unprecedented [1,7] sulfonyl migration.