Dimeric indole alkaloids are structurally diverse natural products that have attracted significant attention from the synthetic and biosynthetic communities. Here we describe the characterization of a P450 monooxygenase CnsC from Penicillium that catalyzes the heterodimeric coupling between two different indole moieties, tryptamine and aurantioclavine, to construct vicinal quaternary stereocenters and yield the heptacyclic communesin scaffold. We show, using biochemical characterization, substrate analogs, and computational methods, that CnsC not only catalyzes the C3-C3 carbon-carbon bond formation, but also controls the regioselectivities of the pair of subsequent aminal bond formations to yield the communesin core. The use of ω-N-methyltryptamine and tryptophol in place to tryptamine led to the enzymatic synthesis of isocommunesin compounds, which have not been isolated to date.

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.

We report the enantiospecific total synthesis of (+)-tubingensin A. Our synthesis features an aryne cyclization to efficiently introduce the vicinal quaternary stereocenters of the natural product and proceeds in only nine steps (longest linear sequence) from known compounds.

This minireview showcases the role indole diterpenoids have played in inspiring the development of clever synthetic strategies and new chemical reactions.