The structure of the rearranged polypropionate ester siserrone A (8), isolated from Siphonaria serrata, was investigated by standard spectroscopic methods and the relative stereochemistry determined by ROESY spectroscopy and chemical degradation studies. Base-catalysed rearrangement of denticulatin A (13) yields the polypropionate ester (16) while careful work-up of S. baconi yields the siphonarins, but not the baconipyrones, as earlier reported. The natural product status of polypropionate ester metabolites is discussed.