Synthesis of Phaitanthrin E and Tryptanthrin through Amination/Cyclization Cascade

Phaitanthrin E was biomimetically synthesized from methyl indole-3-carboxylate and methyl anthranilate or anthranilic acid using the ester group as an activating group. The reaction proceeds through NCS-mediated dearomatization/TFA-catalyzed protonation of indolenine/C(2) amination/Et₃N-promoted aromatization and cyclization in one-pot procedure. This method is capable of converting simple biomass materials to phaitanthrin E. The synthesis not only allows assessment of antiproliferative activity, but also affords experimental support for the hypothetical biosynthetic pathway of phaitanthrin E. The resulting phaitanthrin E derivatives were evaluated for in vitro antiproliferative activity against human colorectal cancer cells (DLD-1). The biogenetic intermediate of phaitanthrin E showed higher antiproliferative activity than the natural product, phaitanthrin E. Furthermore, a concise synthesis of tryptanthrin is also accomplished from indole-3-carbaldehyde and methyl anthranilate using the aldehyde group as an activating group.