Synthesis of the calothrixins, pentacyclic indolo[3,2-j]phenanthridine alkaloids, using a biomimetic approach

Oxidation of the indolo[2,3-a]carbazole 16, readily obtained in six steps from indigo, followed by deprotection results in formation of the indolo[3,2-j]phenanthridine quinone alkaloid calothrixin B 2, demonstrating the viability of the proposed biosynthetic route to this unique ring system.

Graphical abstractThe synthesis of the alkaloids calothrixins A and B is described; the key step is the biomimetic conversion of the indolo[2,3-a]carbazole ring system into the indolo[3,2-j]phenanthridine ring of the natural products.Figure optionsDownload full-size imageDownload as PowerPoint slide