Syntheses and Biological Evaluation of Costunolide, Parthenolide, and Their Fluorinated Analogues.

Inspired by the biosynthesis of sesquiterpene lactones (SLs), herein we report the asymmetric total synthesis of the germacrane ring (24). The synthetic strategy features a selective aldol reaction between β,γ-unsaturated chiral sulfonylamide 15a and aldehyde 13, as well as the intramolecular α-alkylation of sulfone 21 to construct a 10-membered carbocylic ring. The key intermediate 24 can be used to prepare the natural products costunolide and parthenolide (PTL), which are the key precursors for transformation into other SLs. Furthermore, the described synthetic sequences are amenable to the total synthesis of SL analogues, such as trifluoromethylated analogues 32 and 45. Analogues 32 and 45 maintained high activities against a series of cancer cell lines compared to their parent PTL and costunolide, respectively. In addition, 32 showed enhanced tolerance to acidic media compared with PTL. To our surprise, PTL and 32 showed comparable half-lives in rat plasma and in the presence of human liver microsomes.