Meroterpenoids Composed of Flavone and Cyclic Sesquiterpene from Syzygium oblanceolatum

Phytochemical investigation of the Indonesian medicinal plant Syzygium oblanceolatum led to the isolation of three new meroterpenoids, syzygioblanes I–K (1–3). These compounds possess a unique flavanone-derived moiety fused with a sesquiterpenoid unit, further expanding the structural diversity of the syzygioblane family. Their structures and absolute configurations were established through comprehensive spectroscopic analyses. Syzygioblanes J (2) and K (3) represent new structural subclasses characterized by C-7′ ketone-deficient and C-5′ ketone-deficient frameworks, respectively, and plausible biosynthetic pathways for their formation are proposed. In addition, re-examination of NMR and nuclear Overhauser effect spectroscopy data led to the stereochemical revision of the previously reported syzygioblane E. Antiproliferative evaluation against a panel of human tumor cell lines, including the multidrug-resistant (MDR) DU145/TxR/CxR cell line, revealed that syzygioblane I retained collateral sensitivity toward MDR cells, although its selectivity was lower than that of syzygioblane A. These findings indicate that the structural features of the sesquiterpenoid moiety play an important role in modulating collateral sensitivity and provide further insight into the structure–activity relationships of this rare class of plant-derived spiromeroterpenoids.