Total Synthesis of (±)-Distomadines A and B

The total synthesis of distomadines A and B, two structurally unique tetracyclic quinolines, is described. The route features a three-step process to access the pyranoquinoline butenolide rings via a Suzuki cross coupling of a 5-bromo-4-methoxycarbonylmethoxyquinoline with a vinyl boronate, followed by an α-ketohydroxylation and double cyclization by intramolecular aldol condensation and lactonization. Subsequent manipulation of the side chain to introduce the guanidine fragment completed the synthesis of distomadine B, whereas the distomadine A congener resulted from decarboxylation of a late-stage intermediate.