Synthesis and Unexpected Oxidization of the Tricyclic Core of Ugibohlin, Isophakellin, and Styloguanidine

A series of 4-substituted 5,6,7,9-tetra­hydro­pyrrolo­[2,3-f]­indolizin-9-ones, representing the tricyclic core skeleton
of ugibohlin, isophakellin, and styloguanidine, were synthesized via
an intramolecular Vilsmeier–Haack reaction. This reaction allows
the chemoselective C–C bond formation between the pyrrole C3
and proline C5 of N-[(pyrrol-2-yl)­carbonyl]­prolinamides
to construct the B-ring without the protection of the pyrrole nitrogen.
Unexpected oxidizative property of the tricyclic core skeleton was
observed, which could illuminate understanding of the biological formation
of these marine secondary metabolites.