Towards The Total Synthesis Of The Xiamycin Family Of Indolosesquiterpenes: Synthesis Of Oridamycins A And B

Abstract

The xiamycin family of indolosesquiterpenes comprises bioactive compounds isolated from several strains of Streptomyces. Several dimeric family members have shown strong activity against several strains of bacteria, including methicillin-resistant Staphylococcus aureus (MRSA) and vancomycin resistant Enterococcus (VRE). These promising initial results, coupled with the limited production of these compounds from their natural sources, prompted the design of a unified synthetic strategy capable of producing several family members from a common synthetic intermediate. Structurally, the xiamycin family members consist of a carbazole fused to a trans-decalin ring system. The difference between xiamycin A and oridamycin A is a single epimeric stereocenter at C16. It was envisioned that a key oxidative radical cyclization could produce both stereochemical patterns—with free-radical conditions generating the stereochemistry associated with oridamycin A, and chelated radical conditions producing the stereochemistry corresponding to xiamycin A. The total synthesis of oridamycins A and B was completed, utilizing a free-radical cyclization that correctly set three contiguous stereocenters, including two quaternary carbons. The fused carbazole was produced using a 6?-electrocyclization/aromatization sequence. Finally, oridamycin B was accessed through a palladium-catalyzed, oxime-directed C-H oxidation.