Rare Caulamidine Hexacyclic Alkaloids from the Marine Ascidian Polyandrocarpa sp.

Two new caulamidines C ( 2 ) and D ( 4 ) and three isocaulamidines B, C, and D ( 1 , 3 , and 5 ) along with the known compound caulamidine B ( 6 ) were isolated from the marine ascidian Polyandrocarpa sp. Their structures were elucidated by analysis of nuclear magnetic resonance (NMR) and electronic circular dichroism (ECD) data. Isocaulamidines have an altered pattern of N -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 ( 2 ) and isocaulamidine C ( 3 ) are the first members of this alkaloid family with two chlorine substituents in the core 6 H -2,6-naphthyridine ring system.