Cylindricine alkaloids are unique in their structural makeup due to the unusual tricyclic pyrrolo[2,1-j]quinoline framework having a cis-fused azadecalin skeleton. Novel approaches to synthesising these complex molecules are useful due to the difficulty in isolating these compounds from natural sources.
Acetylenic sulfones have been used by our group in the synthesis of numerous nitrogen-containing heterocycles. We have applied our acetylenic sulfone methodology to the synthesis of 2,13-di-epi-cylindricine C and cylindricine C by employing a tandem conjugate addition and intramolecular cyclisation of a key intermediate β-amino ester. Key steps in our synthesis involve a Curtius rearrangement to install the nitrogen at the quaternary center and an electrophilic cyclisation to construct the tricyclic core. Desulfonylation and reduction of the resulting enaminone double bond moiety yielded 2,13-di-epi-Cylindricine C and Cylindricine C in a diastereomeric ratio of 3:1. A route amenable to an enantioselective approach was also achieved.