Palladium-Mediated Oxidative Annulation of δ-Indolyl-α,β-Unsaturated Compounds toward the Synthesis of Cyclopenta[b]indoles and Heterogeneous Hydrogenation To Access Fused Indolines

The cyclopenta[b]indole moiety represents a key skeletal unit in several natural and synthetic compounds that exhibit diverse biological properties. We described herein a two-step sequence for synthesizing cyclopenta[b]indoles with great structural diversity in overall yields up to 37%. The key step was a palladium-catalyzed oxidative annulation of 3-alkylindoles (Fujiwara-Moritani reaction). The obtained cyclopenta[b]indoles were used as substrates in heterogeneous hydrogenation reactions to afford new fused indolines in moderate yields. An acid-catalyzed intramolecular cyclization of three such indolines gave tetracyclic lactams in 89, 90, and 61% yields.