Cyclopenta[b]indoles are indole-fused tricyclic rings. This structure is often found, e.g., in bioactive compounds. Methods for the synthesis of cyclopenta[b]indoles are, thus, interesting research targets. They can, for example, be prepared via annulation reactions using noble metal catalysts. Using lower-cost metals could be useful in this context.

Cheng-Yu He, Wen-Dao Chu, Quan-Zhong Liu, China West Normal University, Nanchong, and colleagues have developed a cycloaddition of vinyl diazoacetates with sulfonyl indoles catalyzed by low-cost Sc(OTf)₃ that gives cyclopenta[b]indoles (general product structure pictured). The team reacted different sulfonyl indoles that bear aryl substituents adjacent to the sulfonyl group with vinyl diazoacetates or β-methyl-substituted vinyl diazoacetates, using Sc(OTf)₃ as the catalyst, LiBr as an additive, and CHCl₃ as the solvent. The reactions were performed at 50 °C in the presence of a molecular sieve under an argon atmosphere.

The researchers obtained the desired cyclopenta[b]indoles in moderate to high yields. They propose that the reaction proceeds via indole-derived α,β-unsaturated imines that are generated from the sulfonyl indoles via the elimination of PhSO₂H.

• Synthesis of Cyclopenta[b]indoles via Sc(III)-Catalyzed Annulation of Vinyl Diazoacetates with Indole-Derived Unsaturated Imines,
  Qin Jiang, Ting Yang, Qi Li, Guang-Ming Liang, Yong Liu, Cheng-Yu He, Wen-Dao Chu, Quan-Zhong Liu,
  Org. Lett. 2023.
  https://doi.org/10.1021/acs.orglett.3c00192