Tubingensin B, a Cytotoxic Carbazole Alkaloid from the Sclerotia ofAspergillus tubingensis

Total synthesis of (-)-tubingensin B enabled by the strategic use of an aryne cyclization

Tubingensin B is an indole diterpenoid that bears a daunting chemical structure featuring a disubstituted carbazole unit, five stereogenic centres-three of which are quaternary-and a decorated [3.2.2]-bridged bicycle. We describe our synthetic design toward a concise and enantiospecific total synthesis of tubingensin B, which hinges on the strategic use of a transient aryne intermediate. Although initial studies led to unexpected reaction outcomes, we ultimately implemented a sequence of carbazolyne cyclization followed by Rh-catalysed fragmentation to install the seven-membered ring and vicinal quaternary stereocentres of the natural product. Coupled with a late-stage radical cyclization to construct the [3.2.2]-bridged bicycle, these efforts have enabled the total synthesis of tubingensin B. The design and evolution of our succinct total synthesis underscores the utility of long-avoided aryne intermediates for the introduction of structural motifs that have conventionally been viewed as challenging.