Elysiapyrones from <i>Elysia </i><i>d</i><i>iomedea</i>. Do Such Metabolites Evidence an Enzymatically Assisted Electrocyclization Cascade for the Biosynthesis of Their Bicyclo[4.2.0]octane Core?

[structure: see text] Biogenetically interesting polypropionate-derived metabolites 1 and 2, featuring an unprecedented skeleton, have been isolated from the sea slug Elysia diomedea. Their enantiomeric character indicates that the current spontaneous electrocyclization cascade biogenetic hypothesis for the bicyclo[4.2.0]octane core must be enzymatically aided. These compounds are isomeric with the 15-nor-9,10-deoxytridachione/15-norphotodeoxytridachione pair of metabolites and encourage speculation about their biosynthetic relationship.