Two new meroterpenoids with 6/10/5/3 tetracyclic skeleton from fungus <i>Ampulloclitocybe clavipes</i>

Two new meroterpenoids, clavilactones R (1) and S (2) with 6/10/5/3 tetracyclic skeleton, and one known meroterpenoid clavilactone E were isolated from the fungus Ampulloclitocybe clavipes. Their chemical structures, including absolute configurations, were unambiguously determined through HR-ESI-MS, 1D/2D-NMR spectral data and ECD calculations. All three compounds were evaluated for their in vitro cytotoxic activities. Among them, clavilactone S (2) exhibited moderate cytotoxic activity against HGC-27 and A549 tumour cell lines, with IC50 values of 19.2 and 24.8 µM, respectively. Clavilactone R (1) showed weak cytotoxicity, while clavilactone E (3) displayed no obvious cytotoxic effect (IC50 > 50 μM) against both cell lines.