Aplysiasecosterol A: A 9,11-Secosteroid with an Unprecedented Tricyclic γ-Diketone Structure from the Sea Hare Aplysia kurodai

A new 9,11-secosteroid having an unprecedented tricyclic gamma-diketone structure, aplysiasecosterolA (1), was isolated from the sea hare Aplysia kurodai. The structure was determined by one-and two-dimensional NMR spectroscopic analysis, molecular modeling studies, a comparison of experimental and calculated ECD spectra, and a modified Mosher's method. Aplysiasecosterol A (1) exhibited cytotoxicity against human myelocytic leukemia HL-60 cells. A biosynthetic pathway for 1 from a known cholesterol was proposed and includes twice a-ketol rearrangements and an intramolecular acetalization.