Sphenostylisins A–K: Bioactive Modified Isoflavonoid Constituents of the Root Bark of Sphenostylis marginata ssp. erecta

Sphenostylisins A–C (1–3), three complex dimeric compounds representing two novel carbon skeletons, along with an additional eight new compounds, sphenostylisins D–K (4–11), were isolated from the active chloroform-soluble extract of the root bark of Sphenostylis marginata ssp. erecta using a bioactivity-guided isolation approach. The structures were elucidated by means of detailed spectroscopic analysis, including NMR and HRESIMS analysis, and tandem MS fragmentation was utilized to further support the structures of 1–3. The absolute configuration of sphenostylisin C (3) was established by electronic circular dichroism analysis. Plausible biogenetic relationships between the modified isoflavonoids 1–11 are proposed, and a cyclization reaction of 9 was conducted to support one of the biogenetic proposals made. All of these pure isolates were evaluated against a panel of in vitro bioassays, and among the results obtained, sphenostylisin A (1) was found to be a very potent NF-κB inhibitor (IC50 = 6 nM).