Rare Caulamidine Hexacyclic Alkaloids from the Marine Ascidian Polyandrocarpa sp.

Two new caulamidines C () and D () and three isocaulamidines B, C, and D (, , and ) along with the known compound caulamidine B () were isolated from the marine ascidian sp. Their structures were elucidated by analysis of nuclear magnetic resonance (NMR) and electronic circular dichroism (ECD) data. Isocaulamidines have an altered pattern of -methyl substitution (N-15 vs N-13 in the caulamidines) with a concomitant double-bond rearrangement to provide a new C-14/N-13 imine functionality. Caulamidine C () and isocaulamidine C () are the first members of this alkaloid family with two chlorine substituents in the core 6-2,6-naphthyridine ring system.