Structural Revision of Pseudocerosine and Validation of a Biosynthetic Proposal for E-ring Formation in Pyridoacridine Alkaloids

Pseudocerosine is the pigment responsible for the bright blue color of the rim on the marine flatworm Pseudoceros indicus. Compelling evidence is provided herein that pseudocerosine is actually a pyridoacridine, not an azepinoindole as initially proposed. This study also validates a biosynthesis proposal for E-ring formation in this revered class of alkaloids, and pseudocerosine (along with its intermediates described herein) is a new branch on the pyridoacridine family tree.