Total Synthesis of Bioactive Indolo[3,2-j]phenanthridine Alkaloid, Calothrixin B

Abstract

The total synthesis of bioactive calothrixin B (2) was completed which two kinds of carbazoles using three approaches. The common strategy was based on an allene-mediated electrocyclic reaction of a 6π-electron system involving one or two indole [b]-bonds for the construction of an appropriate 4-oxygenated 2,3,4-trisubstituted carbazole 28 or a 6-oxygenated 5-methylindolo[2,3-a]carbazole 39, respectively. Oxidation of the methyl group of 28 followed by reduction of the nitro group of 21 afforded the pentacyclic phenol 5, which was oxidized with CAN to give calothrixin B (2). In a biomimetic pathway, the fully protected 5-formylindolo[2,3-a]carbazole 40 with a methoxymethyl group provided calothrixin B (2) through N-methoxymethyl-calothrixin B 43.