Bridged Tricyclic Sesquiterpenes from the Tubercle Nudibranch Phyllidia coelestis Bergh

A new sesquiterpene, 1-formamido-10(1→2)-abeopupukeanane (1), was isolated from the tubercle nudibranch Phyllidia coelestis Bergh, along with 2-formamidopupukeanane (2), which is reported here as a natural product for the first time. A rearrangement pathway toward the unprecedented tricyclo­[4.4.0.02,8]­decane skeleton is proposed. Both compounds showed antiproliferative activity when targeting HeLa, MCF-7, KB, and HT-29 cancer cell lines in the range 0.05–10 μM.