Octacyclic and decacyclic ent‑abietane dimers with cytotoxic activity from Euphorbia fischeriana steud.

A novel Diels-Alder adduct possesses a 6/6/6/5/6/6/6/6 octacyclic skeleton featured with bicyclo[2.2.2]octane moiety, biseupyiheoid A (1), along with another decacyclic 6/6/6/3/5/6/5/6/6/6 fused diterpenoid dimer, bisfischoid C (2), were isolated from Euphorbia fischeriana. Their structures were determined by spectroscopic, X-ray crystallographic approaches, and quantum mechanical calculations. The structural features of 1 and 2 were hypothesized to involve intramolecular Diels-Alder reactions with different coupling patterns. Dimer 1 showed antiproliferative activity through apoptosis activation in LoVo cells.