Concise Synthesis of the ABC-Ring System of the Azafluoranthene, Tropoisoquinoline and Proaporphine Alkaloids: An Olefin Hydroacylation/Pomeranz–Fritsch Cyclization Approach

Abstract A straightforward approach toward a decorated cyclopenta[ ij ]isoquinoline embodying the ABC-ring system characteristic of the azafluoranthene (triclisine), tropoisoquinoline (pareitropone) and proaporphine (prodensiflorin B) alkaloids, is reported. The synthetic ­sequence entailed a novel 40% KF/Al 2 O 3 -mediated hydroacylation of a 2-allyl-benzaldehyde derivative, obtained in two steps from isovanillin, through O-allylation and Claisen rearrangement to assemble the AC-ring system. This was followed by an O-methylation and a reductive ­amination of the resulting indanone with aminoacetal. A modified Pomeranz–Fritsch cyclization was next implemented to install ring B, through sulfonamidation, followed by acid-promoted cyclization and ­final desulfonylation in situ .