Synthesis of the Alleged Structures of Fortisterol and Herbarulide and Structural Revision of Herbarulide

The alleged structures of 5,6-epoxy-5,6-secosteroids fortisterol and herbarulide differ only in the stereoconfiguration of C24. Applying insights into the hypothetical biosynthesis of this class of natural products, we devised a short synthetic access (four and eight steps, respectively) starting from commercial ergosterol and featuring an alkoxy radical rearrangement. The comparison of nuclear magnetic resonance spectroscopic data revealed herbarulide having the proposed structure of fortisterol, whereas synthesis of another two diastereomers could not conclusively prove the true structure of fortisterol. Along the way, a high-yielding and scalable access to the infamous Burawoy's ketone not requiring chromium(VI) reagents was developed.