Synthesis of the 2,2′:3′,3′':2′',2′'' tetraindole framework of the alkaloid bisindigotin

Abstract

Previous efforts towards the alkaloid bisindigotin using 3-hydroxyindole based precursors was dominated by rearrangement chemistry that destroys the desired tetraindole framework of the natural product. Herein, we report a synthesis of the 2,2′:3′,3′':2′',2′''-tetraindole core of the putative bisindigotin structure through a strategic ordering of oxidative dimerization, reduction and dehydration events. The instability of 2,2′:3′,3′':2′',2′''-tetraindole towards a variety of oxidants precluded the synthesis of bisindigotin by late-state oxidation. A p-TSA mediated rearrangement of a 2,2′:3′,3′':2′',2′'' tetraindole to the 3,2′:3′,3′':2′',3′'' regioisomer is also reported.