Sulfur-Mediated Synthesis of Indole-Fused Medium-Sized Rings

We disclose herein a transition-metal-free strategy to synthesize indole-fused medium-sized rings directly from parent indole molecules in one pot. The reaction design involves an interrupted Pummerer reaction between indoles and activated cyclic sulfoxides, followed by ring opening of the resulting sulfonium acetals, and finally cyclization. With trifluoroacetic anhydride (TFAA)-activated sulfoxides of five-membered N, S -acetals, indole-fused seven-membered rings were constructed with the sulfur atom attached to position C-2 of the indole rings; on the other hand, with the TFAA-activated sulfoxide of a six-membered S, S -acetal, the widely available 1,3-dithiane, indole-fused eight-membered rings were obtained with the sulfur atom attached to position C-3 of indoles.