Fissisternoids A and B, two 2′,5′-quinodihydrochalcone-based meroterpenoid enantiomers with unusual carbon skeletons from Fissistigma bracteolatum

Abstract

Two pairs of meroterpenoid enantiomers, (±)-fissisternoids A (1) and B (2), along with their putative biogenetic precursor 3 were isolated from the branches and leaves of Fissistigma bracteolatum. Compound 1 represents an unprecedented meroterpenoid featuring a unique tricyclo [3,3,1,0^1′,5′] decane central framework, and compound 2 possesses a rare 6/6/5/4 tetracyclic carbon skeleton, both of which were derived from quinodihydrochalcone and monoterpenoid via a key [4 + 2] Diels–Alder cyclization and Prins reaction. Their structures and absolute configurations were established using spectroscopic data, X-ray diffraction, and electronic circular dichroism calculations. Compounds 1 and 2 exhibit anti-inflammatory activity (IC₅₀: 13.2 μM, 1; 9.8 μM, 2) via suppression of inflammatory cytokines IL-1β, IL-6, and iNOS.