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.